Chapter 2 Common processes and techniques
Correctly used, these techniques will help to contain fire in the room where it
started, reducing damage.
Testing and commissioning procedures
Testing of installations is the first time we see whether the installation is
watertight. For pressure systems and sanitary systems, testing procedures are
set out in the relevant British Standards and Regulations.
Pre-testing checks
Before commissioning takes place:
● walk around the installation; check that you are happy that the installation is
correct and meets installations standards
● check that all open ends are capped off and all valves isolated
● check that all capillary joints are soldered and that all compression joints are
fully tightened
● check that sufficient pipe clips, supports and brackets are installed, and that
all pipework is secure.
INDUSTRY TIP
Testing procedures
You can access the Building
Testing procedures differ depending on the type of pipework installed. The Regulations 2010 Approved
process involves filling the system with water to a specific pressure, letting Document H: Drainage and
it stand for a period of time to temperature stabilise and then checking it for waste disposal at: www.gov.
pressure loss. Here, we will look at those different testing procedures. uk/government/uploads/
system/uploads/attachment_
Hot and cold water systems testing is detailed in BS 6700; central heating data/file/442889/BR_PDF_
systems testing is detailed in BS 5449; above-ground sanitation systems should AD_H_2015.pdf
be tested in accordance with Document H of the Building Regulations.
● Copper tubes and low carbon steel pipes: systems installed in copper
tube and low carbon steel pipes should be tested to 1.5 times normal
operating pressure. They should be left for a period of 30 minutes to allow
for temperature stabilisation and then left for a period of one hour with no
visible pressure loss.
● Plastic (polybutylene) pressure pipe systems: these are tested rather
differently to rigid pipes. There are two tests that can be carried out. These
are known as test type A and test type B and are detailed in BS 6700:
● Test type A: slowly fill the system with water and raise the pressure to
1 bar (100 kPa). Check and re-pump the pressure to 1 bar if the pressure
drops during this period, provided there are no leaks. Check for leaks.
After 45 minutes, increase the pressure to 1.5 times normal operating
pressure and let the system stand for 15 minutes. Now release the
pressure in the system to one-third of the previous pressure and let it
stand for a further 45 minutes. The test is successful if there are no leaks.
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1
Y
2
3
X
0
0 10 20 30 40 50 60
Key
1 Pumping X Time (minutes)
2 Test pressure 1.5 times maximum working pressure Y Pressure
3 0.5 times maximum working pressure
p Figure 2.61 Pressure test A chart
● Test type B: slowly fill the system with water, pump the system up to the
required pressure and maintain the pressure for a period of 30 minutes.
Note the pressure after this time. The test should continue without
further pumping. Check the pressure after a further 30 minutes. If the
pressure loss is less than 60 kPa (or 0.6 bar), the system has no visible
leakage. Visually check for leakage for a further 120 minutes. The test is
successful if the pressure loss is less than 20 kPa (0.2 bar).
1
Y 2
3 4
X
0
0 10 20 30 60 120 180
Key
1 Pumping X Time (minutes)
2 Pressure drop < 60 kPa (0.6bar) Y Pressure
3 Test pressure
4 Pressure drop < 20 kPa (0.2bar)
p Figure 2.62 Pressure test B chart
● Above-ground sanitation systems: these should be tested in accordance
with Document H of the Building Regulations. They should be tested to a
pressure of 38 mm water gauge (w/g) for a period of three minutes with no
pressure loss.
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Chapter 2 Common processes and techniques
Commissioning
Commissioning is the part of the installation where the system is filled and
run for the first time. It is when we see if it works as designed. The first task is
to fill the system and check for leaks at the appliances. This is best carried out
in stages so that sections of the installation, i.e. cold water, hot water, central
heating, can be filled and tested separately. At each stage of the filling process,
the system should be checked for leaks before moving on to the next section.
Once the systems have been filled they should be drained down and flushed
through with clean water, then refilled. The water levels in WC cisterns, cold
water storage cisterns, and feed and expansion cisterns (if fitted) should be
checked for compliance with the relevant regulations.
Gas installations should be checked for tightness, and central heating systems
should be run up to full operating temperature before being drained down while
they are still hot. Refill the system and add inhibitor before running the system
again.
Check the flow rates at all taps to see if they deliver the flow rates demanded by
the manufacturer’s literature, and check the operation of all controls, including
thermostats and motorised valves. Set the temperature of any cylinder
thermostats and let the water reach full temperature. Using a thermometer,
check the temperature of all radiators and the temperature of the hot water.
Benchmarking KEY TERM
At this stage of the installation, it is time to benchmark the system. Here, the Benchmarking: this is now
boiler and any hot water cylinder installed are checked for compliance with the a compulsory requirement
manufacturer’s instructions, including: to ensure that systems and
appliances are installed
● hot water flow rates in accordance with the
● flow and return temperatures regulations and the
● hot water temperature manufacturer’s instructions.
● operation and types of control It also safeguards any
guarantee against bad
● gas rates. workmanship.
The benchmark certificate should then be signed by the commissioning engineer.
Building Regulations Compliance certificates
Since 1 April 2005, the Building Regulations have demanded that all installations
must be issued with a Building Regulations Compliance certificate. This is to
ensure that all Building Regulations relevant to the installation have been
followed and complied with. This includes:
● the heating installation
● the sanitation system
● the hot and cold water systems
● the gas installation
● any electrical controls.
Certificates are issued by Local Authority Building Control.
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Handover to the customer
When the system has been tested, commissioned and benchmarked, it can then
be handed over to the customer. The customer will require all documentation
regarding the installation:
● all manufacturers’ installation and servicing instructions for boilers,
electrical controls, taps, sanitary ware and any other equipment fitted to the
installation
● the benchmarking certificate
● the Building Regulations Compliance certificate.
The customer must be shown around the system and guided as to how to use
any controls, thermostats and time clocks. Isolation points on the system for
gas, water and electricity should be pointed out and a demonstration given of
the correct isolation procedure in the event of an emergency. Explain to the
customer how the systems work and ask if they have any questions. Finally,
point out the need for regular servicing of the appliances and leave emergency
contact numbers.
Decommissioning of systems
Decommissioning a system or an appliance simply means taking it out of
service. This falls into two categories:
1 Temporary decommissioning: this is where a system or an appliance
is taken out of service for a period of time for repairs, replacement or
maintenance. The customer must be kept informed of when the system is
being shut down, the expected length of time of the decommission and the
expected reinstatement time. If the period of time is considerable, ensure
that the customer has access to vital services, i.e. gas, water and electricity.
2 Permanent decommissioning: this is where a system or appliance is
permanently disconnected and/or removed. This will involve disconnection
and making safe of any services. Pipes should be cut back and capped and, if
necessary, tested for soundness. All electrical disconnections should be made
by a qualified operative or an electrician.
VALUES AND BEHAVIOURS
With temporary decommissioning, the key to good customer service is
information: keep the customer informed and aware of any disruptions to
services such as water and electricity.
KEY TERM Maintenance activities
Maintenance: preserving Maintenance falls into two categories:
the working condition of
appliances and services. 1 Planned preventative maintenance: on larger installations, it may be
necessary to have a planned maintenance schedule so that systems and
equipment can be serviced and checked at regular intervals to ensure
optimum performance. Maintenance activities should be recorded in
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Chapter 2 Common processes and techniques
a logbook, together with the results of any tests performed. Planned
preventative maintenance operations include:
● checking and repairing float-operated valves and setting water levels in
cisterns Gate valve Stop valve
● cleaning out cold water cisterns of all sediment as required
● routine boiler maintenance
● checking and re-washering taps as required
● routine testing of above-ground drainage systems Service valve Double check valve
● checking the operation of any safety valves
● checking the operation of all external controls and isolation valves,
including:
– stop taps
– gate valves Single check Motorised zone
– isolation valves valve valve
– motorised valves
– thermostats.
2 Breakdowns, repairs and emergencies: these are unplanned maintenance
activities that can occur at any time and include: Safety/relief Drain valve
● burst pipes valve
● boiler breakdowns
● running overflows
● blockages
● dripping taps
● WC cistern problems.
Radiator 3-port motorised
Drawing symbols of plumbing valves and valve valve
appliances M
Working drawings for plumbing and heating installations often contain symbols
that represent pipes, valves and appliances. It is important that these symbols
are recognised for systems to be installed properly. All symbols shown will be in Pump Water meter
accordance with BS 1192:2007.
SUMMARY
During this chapter, we have explored the tools required, the materials we use
and the installation practices we need to master to enable us to install good, Expansion Float-operated
valve
vessel
working systems that not only meet the requirements of the regulations, but p Figure 2.63 Plumbing symbols
also satisfy the customer’s needs and expectations. Good working practices drawings
at the start of your plumbing career will serve you well as you broaden your
experience, gain knowledge and improve your skills.
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Test your knowledge
1 Which of the following would be the most suitable masonry drill bit to use
to make a hole in brickwork for a brown plastic plug?
a 5.5 mm HSS bit
b 7.0 mm HSS bit
c 5.5 mm SDS bit
d 7.0 mm SDS bit
2 What is the purpose of the tool shown below?
a To install a sacrificial anode
b To remove an immersion heater
c To remove a tap back-nut
d To tighten a compression nut
3 Which of the following is the British Standard for the manufacture of
copper pipes used in the plumbing and heating industry?
a BS EN 806
b BS 1710
c BS 1212
d BS EN 1057
4 What is the minimum total length of pipe required to machine bend
15 mm copper pipe to 90°?
a 60 mm
b 95 mm
c 100 mm
d 115 mm
5 LCS pipe is given a colour band to indicate its grade. What grade is
indicated by a blue band?
a Light
b Medium
c Heavy
6 Which of the following plastic pipe materials is commonly used for the
distribution of underground mains cold water supplies?
a Polybutylene
b ABS
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Chapter 2 Common processes and techniques
c PVCu
d MDPE
7 Which of the following is not a common size for PVCu soil pipes?
a 65 mm
b 82 mm
c 110 mm
d 160 mm
8 What is the maximum diameter of hole that can be drilled in a joist?
a 10% of the depth of the joist
b 20% of the depth of the joist
c 25% of the depth of the joist
d 30% of the depth of the joist
9 When installing 28 mm copper pipe in the horizontal plane, what is the
recommended clipping distance?
a 1.8 m
b 2.4 m
c 2.7 m
d 3.0 m
10 Which of the following is the maximum recommended clip distance for
40 mm plastic waste pipe in the vertical position?
a 0.5 m
b 0.9 m
c 1.2 m
d 2 m
11 Complete the table below to indicate which gauge of screw is suitable for
each plastic rawlplug type.
Yellow
Red
Brown
Grey
White
Blue
12 Calculate the maximum depth of notch if preparing to install copper pipe
in a joist that is 300 mm deep.
13 What grade of copper tube is most commonly supplied in coils and used
for microbore heating installations?
14 Give three statutory regulations relevant to the installation of a central
heating system.
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15 A joint made on copper pipe, which uses an electrically operated tool to
compress a fitting incorporating a rubber seal onto the pipe, is known as
what?
Practical activity
Practise your copper pipe fabrication by producing the pipe bends shown in
the diagram below, to the dimensions given.
400 mm centre to centre
100 mm end to centre
35 mm off-set at 30º
125 mm centre to start of bend
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SCIENTIFIC PRINCIPLES CHAPTER 3
CHAPTER 3
INTRODUCTION
Plumbing contains a lot of science. The laws of physics and chemistry are involved in one form or another in
almost everything that we do, from the installation of cold water systems and hot water systems to central
heating and drainage. It is the science behind these laws that gives us the theory to enable us to design and
install these systems correctly and efficiently. In this chapter, we will be investigating some of the laws of
physics and chemistry that we use in our day-to-day activities.
By the end of this chapter, you will have knowledge and understanding of the following:
l units of measurement used in the plumbing and heating industry
l properties of materials
l the relationship between energy, heat and power
l the principles of force and pressure, and their application in the plumbing and heating industry
l the mechanical principles in the plumbing and heating industry
l the principles of electricity in the plumbing and heating industry.
Before we begin, it is important that we familiarise ourselves with the SI units of
measurement so that we can use these as reference points during this chapter.
1 UNITS OF MEASUREMENT USED
IN THE PLUMBING AND HEATING
INDUSTRY: THE SI SYSTEM
The SI system of measurement is a universal, unified, self-consistent system of KEY TERM
measurement units based on the m/k/s (metre/kilogram/second) system. We Derived units: combinations
will use these measurement units as reference points throughout this chapter. of the seven base units by
The international system is commonly referred to throughout the world as SI a system of multiplication
after the initials of ‘Systeme International Unite’. The units can be categorised and division calculations.
into two main groups: There are 21 derived units
of measurement, some of
1 base units which have special names
2 derived units. and symbols.
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SI base units
Table 3.1 SI base units
Measure of: Base SI unit Symbol
Length metre m
Mass kilogram kg
Time second s
Electric current ampere A
Thermodynamic temperature kelvin K
SI derived units
Table 3.2 SI derived units
Measure of: Unit Symbol
Area (length × width) square metre m 2
Volume (length × width × height) cubic metre m 3
Volume of liquid (length × width × height × 1000) litre l
Velocity metre per second m/s
Acceleration metre per second squared m/s 2
Density kilogram per cubic metre kg/m 3
Specific volume cubic metre per kilogram m 3 /kg
Force (mass (kg) × acceleration (m/s 2 )) newton (kg/m/s 2 ) N
Pressure pascal Pa
Energy, work, quantity of heat joule J
Power watt W
Electric potential volt V
Electric resistance ohm Ω
Table 3.3 Copper pipe imperial Using unit conversion tables
and corresponding metric sizes
Despite efforts to adopt the metric system in the 1970s, it is obvious that there
Imperial Metric
are still many imperial units in use in the UK today. We still measure distances
½ inch 15 mm in miles rather than kilometres and often buy our food in pounds rather than
¾ inch 22 mm kilograms. It is therefore helpful to know how to convert from one type of
1 inch 28 mm unit to another.
1¼ inch 35 mm
In plumbing, we may come across many different imperial units that are still
1½ inch 42 mm
in use. An example of an imperial/metric conversion that we still use today is
2 inch 54 mm shown in Table 3.3.
Before 1973, copper pipe was manufactured in diameters by the inch and its
subdivisions.
Table 3.4 gives some of the common conversion factors that are still in use in
the UK.
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Chapter 3 Scientific principles
Table 3.4 Common conversions
Imperial Actual measurement Metric
1 inch [in] 2.54 cm
1 foot [ft] 12 in 0.3048 m
1 yard [yd] 3 ft 0.9144 m
1 mile 1760 yd 1.6093 km
1 int. nautical mile 2025.4 yd 1.853 km
1 sq inch [in 2 ] 6.4516 cm 2
1 sq foot [sq ft] 144 in 2 0.0929 m 2
1 sq yd [yd 2 ] 9 sq ft 0.8361 m 2
1 acre 4840 yd 2 4046.9 m 2
1 sq mile [mile 2 ] 640 acres 2.59 km 2
1 cu inch [in 3 ] 16.387 cm 3
1 cu foot [ft 3 ] 1728 in 3 0.0283 m 3
1 fluid ounce [fl oz] 28.413 ml
1 pint [pt] 20 fl oz 0.5683 l
1 gallon [gal] 8 pt 4.5461 l
1 ounce [oz] 437.5 grain 28.35 g
1 pound [lb] 16 oz 0.4536 kg
1 stone 14 lb 6.3503 kg
1 hundredweight [cwt] 112 lb 50.802 kg
1 long ton (UK) 20 cwt 1.016 t
ACTIVITY
There may be instances during our work when we have to convert from one unit to another. The following example
shows how to use the conversions in Table 3.4.
A plumber has to travel 25 miles to work every day but claims 35p per kilometre in travelling expenses. How
much does he claim?
Now, try these examples:
1 A plumber is asked to replace a cold water cistern in a roof space with a new like-for-like cistern. The capacity
of the cistern is quoted on the existing cistern as a 25-gallon nominal capacity. What size cistern in litres is
required?
2 A customer has requested that you quote for a new bathroom suite installation and sends you a plan of the
existing bathroom. The measurements are in feet and inches.
8 ft
11ft
a Convert the dimensions into metres.
b What is the area of the room in square metres?
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2 THE PROPERTIES OF MATERIALS
There are many materials that you, as a plumber, will deal with in your working
life. Each one will have different characteristics, such as weight, melting point,
density and strength. It is important that we know and understand the materials
we work with to ensure that the correct material is used for a given application.
Here, we will investigate some of the many different materials we use, together
with their working properties and their uses.
Relative density of solids, liquids and gases
Relative density is the ratio of the density of a substance to the density of
a standard substance under specific conditions. For liquids and solids, the
standard substance is usually distilled water at 4°C. For gases, the standard
is usually air at the same temperature and pressure as the substance being
measured.
When we talk about a material’s relative density, we are basically comparing
the mass of that material against water or air (see Table 3.5). In both cases the
water and air have a relative density (or specific gravity) of 1.
Table 3.5 Relative densities of common substances used in the plumbing industry
INDUSTRY TIP Solids
Substance Relative density Mass/m 3
Another phrase for relative Water (1 m 3 of water has a mass 1 1000 kg
density is ‘specific gravity’ and of 1000 kg at 4°C)
this usually refers to gases. Copper 9 9000 kg
Steel 7.48–8.0 (depending on the grade) 7480–8000 kg
Lead (milled) 11.34 11,340 kg
Table 3.6 Gases’ specific Lead (cast) 11.30 11,300 kg
gravity
Brass 8.4 8400
Gas uPVC 1.35 1350 kg
Gas Specific Polypropylene 0.91 910 kg
gravity
Air 1 Principal applications of solid materials
Natural 0.7 Lighter
gas than air The solid materials used in the plumbing industry can be classified into three
Propane 1.5 Heavier distinct groups:
than air
1 those made from metals
Butane 2.0 Heavier 2 those made from plastics
than air
3 those made from ceramics and fireclays.
Hydrogen 0.069 Lighter
than air
Metals
Metals are one of the main materials used in the plumbing industry. They can be
found in the form of pipes, tubes and fittings, and in the manufacture of boilers,
radiators and other heating appliances, as well as sundry items such as solder,
screws and nails.
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Chapter 3 Scientific principles
Metals can be subdivided into four specific groups, as described below.
1 Pure metals: these are the metals that are derived directly from the ore and
contain very little in the way of impurities. Table 3.7 lists the more common
metals and the ores from which they are extracted.
2 Alloys: an alloy is a mixture of two or more metals. This type of mixed
metal is used extensively in the plumbing industry. Alloys used include brass
(copper/zinc), bronze (copper/tin), gunmetal (copper/tin/zinc), lead-free
solders (nickel/tin or copper/tin) and steel (iron/carbon).
3 Ferrous metals: those metals that contain iron, such as steel and cast iron. KEY TERM
These corrode easily because of the formation of ferrous oxide, otherwise Corrosion: any process
known as rust. involving the deterioration
4 Non-ferrous metals: these metals do not contain iron and are known as or degradation of metal
pure metals. Non-ferrous metals include copper, lead, tin, zinc, aluminium components, where the
and nickel. Non-ferrous metals do not rust but can corrode over time. metal’s molecular structure
breaks down irreparably.
Table 3.7 Origin of common metals
Metal Ore Country Type
Iron Pyrite England Ferrous
Marcasite Mexico
Haematite Brazil
Magnetite Australia
Copper Copper North America Non-ferrous
Malachite Chile
Chalcopyrite Cyprus
Turquoise Canada
Azurite Germany
Aluminium Gibbsite Brazil Non-ferrous
Bauxite Jamaica
Cryolite India
Australia
Guinea
Lead Galina England Non-ferrous
Cerussite Germany
Australia
North America
Zinc Sphalerite Australia Non-ferrous
Zincite Canada
China
Peru
North America
Tin Cassiterite Malaysia Non-ferrous
Thailand
China
Indonesia
Bolivia
Russia
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Plastics
Just as plumbers should know their metals, they should also know their plastics
if mistakes during installation are to be avoided. There are many different
plastics that plumbers use in their day-to-day work for installing hot and cold
water supplies, central heating, guttering and rainwater pipes, and above-and
below-ground drainage systems.
There are two basic types of plastics: thermoplastics and thermosetting.
1 Thermoplastics: a thermoplastic is a type of plastic made from polymer
resins that becomes liquid-form when heated and hard when cooled. When
frozen, however, a thermoplastic becomes brittle and subject to fracture.
These characteristics are reversible and it can be reheated, reshaped and
frozen repeatedly. This quality also makes thermoplastics recyclable.
There are many different types of thermoplastics, some of which are used
extensively in plumbing systems. Each type varies in crystalline organisation
and density. Table 3.8 lists the plastics commonly used in the plumbing
industry and describes what they are used for.
2 Thermosetting: thermosetting plastics, such as polyester and epoxies, are
rigid plastics, resistant to higher temperatures than thermoplastics. Once it
has set, a thermosetting plastic cannot be remoulded. Its shape is permanent
and it does not melt when heated.
Table 3.8 Common plastics used in the plumbing industry
Type of plastic Uses Characteristics
uPVC Unplasticised polyvinyl chloride is used extensively for: Not suitable for hot water
CuPVC cold water mains installations.
cold water installations (chlorinated unplasticised polyvinyl chloride) Can be solvent welded.
solvent-welded and push-fit soil and vent pipes
solvent-welded waste and overflow pipes
underground drainage pipes
gutters and rainwater pipes.
Polyethylene MDPE (medium-density polyethylene) is used for: Cannot be solvent welded.
MDPE underground cold water mains (coloured blue) Degrades under direct sunlight.
HDPE cold water storage cisterns
underground gas pipes.
HDPE (high-density polyethylene) is used for:
underground cold water mains (coloured black).
Polypropylene Used for: Cannot be solvent welded.
push-fit waste and overflow pipe Slightly greasy to the touch.
cold water storage cisterns. Degrades under direct sunlight.
Polybutylene Used for: Cannot be solvent welded.
push-fit hot and cold water installations
central heating installations.
ABS Acrylonitrile butadiene styrene. Used for: Can be solvent welded.
water supply – potable water for apartments, offices, commercial Degrades severely under direct
installations sunlight.
solvent-welded waste and overflow pipes.
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Chapter 3 Scientific principles
Ceramics and fireclays
Ceramics and fireclays are used mainly for sanitary appliances and tiles. There
are three varieties that plumbers may use widely in their work:
1 Vitreous china: this is a clay material with an enamelled surface used to
manufacture bathroom appliances such as WCs and cisterns, wash hand
basins and bidets, as well as soap dishes and other sundry bathroom items. It
is made from very watery clay, known as ‘slip’, which is then spray enamelled
and fired in a kiln at high temperature.
2 Fireclay: this is used primarily for heavy-duty appliances, such as Belfast
sinks, London sinks, cleaners’ and butler’s sinks and shower trays, where
there is greater risk of damage and a higher water temperature may be
needed. Like other clays, this clay is highly malleable in its raw form. It can be
moulded, extruded and shaped by hand. It is also used in the manufacture of
building products such as chimney pots.
3 Ceramic tiles: these have many applications and are used extensively in
bathrooms, kitchens, floors and swimming pools. The origin of the tile can
be identified from looking at the reverse of the tile. This is known as the
‘biscuit’ of the tile. Tiles made in the UK usually have a white-coloured
biscuit, Italian tiles usually have biscuit that is cream in colour, and Turkish
and Spanish tiles have a dark red biscuit.
Principal properties of solid materials
Solid materials are made up of many molecules. How these molecules are
arranged and how they behave under certain conditions will determine their
properties. A solid material is assessed by its:
● strength – tensile, compressive and shear
● ductility
● malleability
● hardness
● conductivity – heat and electricity.
Tensile strength
Broadly speaking, the tensile strength of a material is a measure of how
well or badly it reacts to being pulled or stretched until it breaks. Some Tensional stress
materials, such as plastics, will stretch or elongate before breaking; others, p Figure 3.1 Tensile strength
such as metals, will also deform in a similar way but not by as much, and
hard materials such as concrete and brick will not deform at all but will IMPROVE
simply snap. YOUR MATHS
A tensile strength test is also known as a tension test and is the most Tensile strength is
fundamental type of mechanical test that can be performed on a material. measured in units of force
The tests are simple and relatively inexpensive. By simply pulling on a material per unit area. In the SI
under specific conditions, how the material will react to being pulled apart will system, the unit is newton
quickly become apparent. The point at which the material fractures is its tensile per square metre (N/m²
or Pa – pascal).
strength.
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IMPROVE Compressive strength
YOUR MATHS Compressive strength is the maximum stress a material can sustain when
In the SI system, being crushed. Hard materials, such as concrete or cast iron, will shatter under
compressive strength is compressive stress, while others, like plastics and some metals, may distort in
measured using the unit shape. This is called barrelling.
newton per square metre
(N/m² or Pa – pascal). Compressive strength is calculated by dividing the maximum load by the
original cross-sectional area of a specimen in a compression test, and is
measured in units of force per unit area.
Shear strength
Shear strength is the stress state caused by a pair of opposing forces acting
along parallel lines of action through the material. In other words, the stress
caused by sliding faces of the material relative to one another – for example,
cutting paper with scissors or ripping a substance apart.
Compressional stress Ductility of a material
p Figure 3.2 Compressive strength
Ductility is a mechanical property that describes by how much solid materials
can be pulled, pushed, stretched and deformed without breaking. It is often
described as the toughness of a material to withstand plastic deformation. In
materials science, ductility specifically refers to a material’s ability to deform
under tensile stress. This is often characterised by the material’s ability to be
stretched into a wire. Copper is one of the most ductile materials a plumber
will use because it is easily bent and softened into various shapes.
Malleability of a material
Malleability can be defined as the property of a material, usually a metal, to
be deformed by compressive strength without fracturing. If a metal can be
hammered, rolled or pressed into various shapes without cracking or breaking,
or other detrimental effects, it is said to be malleable. This property is essential
Shear stress
in sheet metals, such as lead, that need to be worked into different shapes.
p Figure 3.3 Shear strength
Hardness
Table 3.9 The Mohs hardness
scale Hardness is the property of a material that enables it to resist bending,
Material Hardness scale scratching, abrasion or cutting.
Talc 1 Hardness of minerals can be assessed by reference to the Mohs scale, which
Gypsum 2 ranks the ability of materials to resist scratching by another material. There
Calcite 3 is a good reason for grouping materials this way. If an unknown material is
Fluorite 4 discovered, it is one way how to find out what it is by seeing how hard it is.
Apatite 5 The Mohs hardness scale starts at 1 for the softest material and goes up to 10
Feldspar 6 for the hardest.
Quartz 7
Diamond is the hardest material, which explains why it is used on many cutting
Topaz 8 edges.
Corundum 9
Diamond 10
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Conductivity IMPROVE
Conductivity is the property that enables a metal to carry heat (thermal YOUR MATHS
conductivity) or electricity (electrical conductivity). Thermal conductivity is
measured in watts per metre
● Thermal conductivity: here, heat is transferred from molecule to molecule kelvin (W/mK). Electrical
through the substance. How fast or how well the heat travels will determine conductivity is measured
the material’s thermal conductivity. For example, metals, such as copper, in ohms (Ω).
transfer the heat quickly and are said to be good conductors of heat, whereas
other materials, such as polyurethane, allow the passage of heat only very
slowly and so are poor conductors of heat. The inability of polyurethane to
allow the passage of heat makes it a very good insulator with the ability to keep
heat in. Thermal conductivity is measured in watts per metre kelvin (W/mK).
● Electrical conductivity: this is the ability of a material to allow an electrical
charge or current to pass through it. It is measured in ohms (Ω). Materials
that allow an electrical current to flow freely, such as copper and gold, are
known as good conductors, whereas those that do not allow the passage of an
electrical current, such as wood, ceramics and PVC, are known as insulators.
Oxidation, corrosion and degradation
of solid materials
All solid materials will corrode or degrade over time. The amount that
materials corrode or degrade will depend upon the material’s resistance and the
environment in which the material exists. In this section of the chapter, we will
investigate these three processes and how they affect plumbing materials.
Oxidation of metals
Metals are oxidised by the presence of oxygen in the air. This process is more
commonly called corrosion. Electrons jump from the metal to the oxygen
molecules. The negative oxygen ions that are formed penetrate into the metal,
causing the growth of an oxide on the metal’s surface. As the oxide layer
increases, the rate of electron transfer decreases. Eventually, the corrosion stops
and the metal becomes passive. However, the oxidation process may possibly
continue if the electrons succeed in entering the metal through cracks, pits or
impurities in the metal, or if the oxide layer is dissolved.
Corrosion
Corrosion is the main reason for metals deteriorating. Most metals will corrode
on contact with water (and moisture in the air), acids, salts, oils, and other solid
and liquid chemicals. Metals will also corrode when exposed to some gases,
such as acid vapours, ammonia gas and any gas containing sulphur.
Corrosion specifically refers to any process involving the deterioration or
degradation of metal components. The best-known case is that of the rusting
of steel and iron where the formation of ferrous oxide occurs. The corrosion
process is usually electrochemical.
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When rusting occurs, the metal atoms are exposed to an environment containing
KEY POINT
water molecules. Here, they give up electrons and become positively charged ions.
The electrochemical
process involves the Metal Air Rust Water
passage of a small
electrical charge between
two metals that are at Oxygen (O 2 )
opposite ends of the
electromotive series of
metals. The stronger, 2+ −
noble metal is called Fe + 2OH → Fe(OH) 2 Fe 2+
−
the cathode and the O + 4e + H O → 4OH −
2
2
weaker metal is known
as the anode. When Cathode area
these two dissimilar 2Fe → 2Fe + 4e −
2+
metals are placed in an
electrolyte such as water, Anode area
an electric charge is p Figure 3.4 How rust is formed
generated and the anodic
metal is ‘eaten’ away This effect can occur locally to form a pit or a crack, or it can extend across a
by the cathodic metal. wide area to produce general corrosion.
A by-product of this
reaction is the generation
of hydrogen gas. The Other forms of metal corrosion that occur
process accelerates
when heat is present. in plumbing and heating systems
There are many forms of metal corrosion that can occur within plumbing and
heating systems, including:
● de-zincification
l galvanic corrosion
l erosion corrosion
l pitting corrosion.
De-zincification of brass
Brass is an alloy mixture of copper and zinc. De-zincification of brass is a form
of selective corrosion (often referred to as de-alloying) that happens when zinc
is leached out of the alloy, leaving a weakened brittle porous copper fitting.
This commonly occurs in chlorinated tap water or in water that has high levels
of oxygen. Signs of de-zincification are a white powdery zinc oxide coating
the surface of the fitting, or if the yellow brass turns a shade of red. Selective
p Figure 3.5 De-zincification and corrosion can be a problem because it weakens a fitting, leaving it vulnerable
its effects to possible failure and eventual leaks.
KEY TERM Galvanic corrosion
Electrolyte: a fluid that allows Galvanic corrosion (also called galvanic action, ‘dissimilar metal corrosion’ and
the passage of electrical often wrongly termed ‘electrolysis’) occurs when two dissimilar metals are in
current, such as water. The contact with each other through the presence of an electrolyte. Metals are
more impurities (such as salts
and minerals) there are in the graded through the electromotive series (also known as the electrochemical
fluid, the more effective it is series) of metals. The further the metals are apart in the series, the greater the
as an electrolyte. chance of galvanic corrosion.
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For galvanic corrosion to occur, three conditions must be present:
Copper
1 electrochemically opposed metals must be present Lead
2 these metals must be in electrical contact
3 the metals must be exposed to an electrolyte. Tin
Nickel
One of the metals is the most noble, cathodic metal and the other is the weaker, Iron
least noble anodic metal. When an electrolyte is introduced, such as water, a
small electrical direct current (DC) is generated between the two metals. The Chromium
stronger of the two metals will destroy the weaker metal, with hydrogen being Zinc
produced as a by-product. Manganese
Aluminium
Erosion corrosion Magnesium
Erosion corrosion occurs in tubes and fittings because of the fast-flowing
effects of fluids and gases. The increased turbulence caused by pitting on ANODIC
the internal surfaces of a tube can result in rapidly increasing erosion rates (least noble)
and eventually a leak. Erosion corrosion can also be encouraged by poor p Figure 3.6 Electromotive series
workmanship. For example, burrs left at cut tube ends can cause disruption of metals
to the smooth water flow, and this can cause localised turbulence and high flow
velocities, resulting in erosion corrosion.
p Figure 3.7 Erosion corrosion
Pitting corrosion
Pitting corrosion is the localised corrosion of a metal surface and is confined to
a point or small area that takes the form of cavities and pits. Pitting is one of
the most damaging forms of corrosion in plumbing, especially in central heating
radiators, as it is not easily detected or prevented.
p Figure 3.8 Pitting corrosion
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Degradation of plastics
The use of plastics is becoming common in the plumbing industry. Everything
from hot and cold water services to central heating and drainage can now be
installed in some form of plastic material. Problems, however, can occur with
plastics under certain conditions. Degradation of plastics can occur from a
variety of causes such as:
l heat
l light
l oxygen
l ultraviolet (UV) degradation.
Heat (thermal degradation)
One of the limiting factors when using plastics in high temperature applications
is their tendency to not only soften but also to thermally degrade. In some
instances, thermal degradation can occur at temperatures much lower than
those at which mechanical failure is likely to occur.
All plastics experience some form of degradation during their life. The chemical
reactions that occur with thermal degradation lead to both physical and optical
changes, such as:
l reduced ductility and embrittlement
l chalking
l colour changes
l cracking.
KEY POINT Light (photodegradation)
Photodegradation takes This occurs due to the action of light, whether from natural sunlight or electrical
place in direct light, fluorescent lighting, and generally causes a yellowing of the plastic material. It is
even electric light,
whether heat is present usually more pronounced on light-coloured plastics but can affect all colours.
or not. UV degradation
takes place in daylight, Oxygen (oxidative degradation)
whether the Sun is This is decomposition of the plastic due to the presence of oxygen, which
present or not. Its effects alters the plastic’s properties. Colour change is often the first sign of oxidative
occur even on cloudy degradation, coupled with a change in flow, mechanical and electrical properties
days and as such it is of the plastic, even if the colour change is not noticeable. Polypropylene,
generally down to the
climate. polyethylene and ABS are the plastics most severely affected. PVC, however, is
unaffected by oxidative degradation.
UV degradation
Most plastics are vulnerable to degradation by the effects of direct exposure to
the UV part of the daylight spectrum. UV solar radiation is present even on cloudy
days. When UV attack occurs, the colour of the plastic may change and its surface
will become brittle and chalky. This can happen over a very short time period and
will lead to cracking and eventual failure.
Polypropylene waste pipes and MDPE water pipes are adversely affected by UV
degradation, with ABS pipework and fittings being severely compromised by
prolonged exposure to the UV daylight spectrum.
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Preventing corrosion
Corrosion is one of the most destructive processes to plumbing and heating
systems, but there are methods we can employ to prevent and protect from
corrosion:
l Galvanisation is one method of protecting steel from rusting by coating with
a thin layer of zinc. Galvanising is a process by which the steel is dipped in a
bath of molten zinc.
l Greasing and oiling are some commonly used methods to prevent rusting.
The grease and oil prevent water and moisture penetration. KEY TERM
l Chrome plating and anodising prevent corrosion of metal by coating the Anodising: coating one
metal, creating a barrier between it and the corrosive environment. metal with another by
l Wet central heating systems can be protected from corrosion by the use of electrolysis to form a
corrosion inhibitors mixed with the system water. protective barrier against
corrosion.
l Plastics can be protected from the effects of UV light by painting.
l Sacrificial anodes (magnesium rods) placed inside hot water storage cylinders
protect the cylinder from electrolytic corrosion.
l Metals can be coated with enamel for protection. Enamel consists of a thin
layer of glass heated to a high temperature which then fuses on to the
surface of the metal.
The properties of liquids
The plumbing industry is primarily concerned with liquids in one form or
another, with water being the most common fluid we deal with. Liquids you may
come across in your working life include:
l water
l refrigerants
l glycols and anti-freeze
l fuel oils
l lubricants.
Here, we will investigate these liquids and their uses within the building services
industry.
Water
Water is the most abundant compound on earth. It covers seven-tenths of the
Earth’s surface and is the key to life on Earth. Water has many uses, including Table 3.10 The energy of sensible
and latent heat of water from
hot and cold water supplies and wet central heating systems. Yet, what do we 0°C of water to 100°C of steam
actually know about water?
Boiling point of
The properties of water Pressure the water
bar kPa °C kJ/kg
l Water is a colourless, odourless and tasteless liquid: any taste it does 0 0 100.00 419.06
have comes from the minerals that may be dissolved in it, and this can often
explain why water tastes different in different parts of the country. 1 100.0 120.42 505.6
l Water can exist in all three states of matter: liquid (water), solid (ice) 2 200.0 133.69 562.2
and gas (steam). 3 300.0 143.75 605.3
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l Water has a maximum density of 1000 kg per cubic meter (m ) at 4°C:
3
KEY POINT at this temperature, water is at its densest. When the temperature of water is
The effects of the either raised or lowered from 4°C, water loses density. This peculiar behaviour
changes in density of
water can benefit water is known as the ‘anomalous expansion’ of water. At 100°C, water has a density
3
3
heating by creating heat of 958 kg/m and at 0°C, its density is 915 kg/m . This can be expressed
circulation by convection. as a percentage. When heated, water expands by 4 per cent; when cooled
We will deal with heat it expands by 10 per cent. When water is turned to steam, it expands by
transfer through water 1600 times, so 1 m of water will transform into 1600 m of steam!
3
3
later in the chapter.
l The boiling point of water at sea level is 100°C: if the pressure is raised
from this, the boiling point increases. At 1 bar pressure, the boiling point of
water is 120°C. Similarly, if the pressure is lowered, then the boiling point
decreases. At the top of Mount Everest, the boiling point of water is 69°C.
l Water freezes at 0°C: again, pressure can affect this. If the pressure
increases then the freezing point is lower. Dissolved minerals can also affect
the freezing point.
l The relative density of water is 1: this is the measurement that all other
solids and liquids are measured against.
l The specific heat capacity of water is 4.187 kJ/kgK: the specific heat
capacity of a substance is the amount of heat required to raise the
temperature of 1 g of the substance by 1°C (or by 1 K). In the case of water,
it takes 4.187 kJ of heat to raise 1 kg of water by 1°C.
l Water itself is a poor conductor of electricity: it is the presence of
dissolved minerals that makes water a good conductor of electricity. Sea
water, for example, is a very good conductor of electricity because of the
dissolved salts and minerals it contains.
l Water is a poor conductor of heat: compared to most metals, water is a
poor conductor of heat. In fact, water is a better insulator of heat than it is
conductor. That is why it takes so much energy to raise the temperature of
water by 1°C (see specific heat capacity, above).
l Water is known as the ‘universal solvent’: almost all substances dissolve
in water to a certain extent. Because of this, it is almost impossible to get
chemically pure water on Earth.
l Water is classified as being hard or soft: the hardness and softness of water
affects its pH value (see Table 3.11).
l Water goes through several stages to be turned into steam: at
atmospheric pressure, the boiling point of water is 100°C. To raise the
temperature of the water from 0°C to 100°C takes 419 kJ/kg of energy (hf).
To turn the boiling water at 100°C to steam at 100°C takes a further 2257
kJ/kg of energy (hfg). At this point, the steam is said to be saturated steam.
In other words, it is saturated with heat. The total heat, therefore, to turn
water at 0°C to steam at 100°C takes 2676 kJ/kg of heat energy. Any further
heat added after this does not increase the temperature of the steam; it
remains at 100°C and the steam is known as ‘superheated’ steam because of
the extra heat energy. To increase the temperature of the steam, the initial
pressure of the water will have to be increased.
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Table 3.11 Classification of water
Type of water pH value Base Notes
Neutral 7 N/A Neutral water is neither soft nor hard.
Soft Below 7 Acidic Water is made soft by the presence of carbon dioxide (CO 2 ). It is particularly destructive
to plumbing systems containing lead as it can dissolve the lead, making the water
contaminated. Because of its lead-dissolving capability, soft water is known as ‘plumbo-
solvent’. Soft water lathers soap easily.
Temporary Above 7 Alkali Temporary hard water contains calcium carbonate (CaCO 3 ), otherwise known as limestone.
hard water This kind of water can be softened by boiling but leaves behind limescale residues, which
can block pipes and other plumbing fittings and appliances. When water reaches 65°C, the
calcium in the water re-forms in a process known as precipitation, causing scaling within
plumbing systems. Lathering of soap is difficult.
Permanently Above 7 Alkali Permanently hard water contains magnesium and calcium chlorides, and sulphates in the
hard water solution. It cannot be softened by boiling.
Capillary attraction
Capillary attraction is the process where water (or any fluid) can be drawn
upwards through small gaps against the action of gravity. The wider the
gap, the less capillary attraction takes place. It is of particular interest to
plumbers as it has the ability to cause problems within some plumbing
systems, such as:
l it can cause water to be drawn up underneath tiles and roof weatherings,
resulting in water leaks inside the building
l it can initiate water trap seal loss in above-ground drainage systems; in this
instance, there are two forces at work – capillary attraction and siphonic
action.
Conversely, it is also the process we use to make soldered capillary joints on
copper tubes and fittings.
p Figure 3.9 Capillary attraction
Before capillary attraction can take place, two processes need to be present.
These are adhesion and cohesion.
Adhesion and cohesion KEY TERMS
Water is fluid because of cohesion. The cohesive quality gives water a slight film Cohesion: the way in which
on its surface, which is known as the surface tension. the water molecules ‘stick’
Water is also attracted to other materials, and so it tends to stick to whatever to one another to form a
it comes into contact with. This is known as adhesion. When water is placed in mass rather than staying
individual. This is because
a vessel or a glass, the adhesion qualities of the water give it a slightly curved water molecules are attracted
appearance. This is known as the meniscus and can be convex (outward curve) to other water molecules.
or concave (inward curve). Adhesion: the way in which
water molecules ‘stick’ to
Refrigerants other molecules they come
Refrigerants are fluorinated chemicals that are used in both liquid and gas states. into contact with.
They can, therefore, be classified as both liquid (when compressed) and gas
(vapour). All refrigerants boil at extremely low temperatures, well below 0°C.
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When a refrigerant gas is compressed, it changes its state to a liquid. During this
process a lot of heat and pressure are generated. When the pressure is released
quickly, it generates cold. Refrigerants’ ability to change their state quickly with
such wide temperature changes allows them to be used in refrigeration plants,
air conditioning systems and heat pumps. The process is known as the vapour
compression refrigeration cycle.
Vapour Vapour
Compressor
Evaporator Fan Condenser
Liquid & vapour Liquid
Expansion valve
p Figure 3.10 The vapour compression refrigeration cycle
The refrigerant vapour enters the compressor, which compresses it, generating
heat. The compressed vapour then enters the condenser, where the useful heat
is removed and the vapour condenses to a liquid refrigerant. From here, the
liquid refrigerant then passes into the expansion valve, where rapid expansion
takes place, converting the warm liquid into a super-cold vapour/liquid mix,
which creates the refrigeration effect. The vapour/liquid mix passes through
the evaporator, where final expansion to a vapour takes place. This then
enters the compressor for the cycle to begin again.
Glycol
Glycol is the name used for solar hot water system anti-freeze solution. It
is used for protecting solar panels from freezing during the winter when
mixed with water in the sealed solar panel circuit. It is available in two forms:
propylene glycol and ethylene glycol. Propylene glycol is the preferred chemical
for solar panels as ethylene glycol is highly toxic. The anti-freeze should be
checked regularly as its anti-freezing capability diminishes with time and the
solution can become corrosive with age.
Fuel oils (kerosene)
Kerosene is a fuel oil that is used with most domestic oil-fired boilers (see
Chapter 7, Central heating systems, page 461). Kerosene is a thin, clear liquid
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Chapter 3 Scientific principles
formed from hydrocarbons, and has a density of 0.78–0.81 g/cm . It is made
3
from the distillation of petroleum at temperatures between 150°C and 275°C.
The flashpoint of kerosene is between 37°C and 65°C, and it will spontaneously
combust at 220°C. The heat of combustion of kerosene 43.1 MJ/kg, and its
higher heating value is 46.2 MJ/kg.
Lubricants
A lubricant is a substance, often a liquid or grease, introduced between
two moving surfaces to reduce friction, thus improving efficiency and
reducing wear. There are many types of lubricant in use in the plumbing
industry:
l Silicone grease and spray: used for general lubrication of plumbing parts
for water and drainage systems. It is also used when jointing push-fit plastic
pipe systems to lubricate the rubber seals.
l Graphite paste: used for lubrication of gas taps.
l Cutting oils: used when threading low carbon steel pipe. They help to
prevent overheating of the cutting dies.
l Penetrating oils: used to help loosen tight and rusted joints.
The principal applications of gases
In this section of the chapter, we will look at the principal uses of gases in the
building services industry, together with their properties and the scientific laws
that apply to them.
Types of gases
The principal gases in the building services industry are listed below.
l Air: this has limited uses within the plumbing industry.
l It can be used as a heating medium in warm-air heating systems. Here,
the air is warmed by a warm-air heater, usually fired by gas. The warm air
is distributed to the property by means of a fan.
l It can be used as a pressure charge in expansion vessels. These are usually
installed in sealed heating systems and some unvented hot water storage
vessels.
l Air at high pressure can be used to clear blocked drains.
l Steam: once the preferred method of heating, the use of steam has declined
over recent years. However, because of new, more efficient system designs,
steam is being used as a heating medium for:
l new combined heat and power applications – steam can be used to
generate electricity and warm properties in district heating systems
l electricity generation
l hot water production using large hot water calorifiers
l heating systems – the steam is used instead of water in the heat
emitters.
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l LPG: liquid petroleum gas (LPG) can be used for heating appliances such
as boilers, cookers and fires. It is also used with plumbers’ blowtorches for
soldering capillary fittings. There are two basic types:
1 butane – used mainly as a camping gas
2 propane – the most widely used LPG in the building services industry.
l Natural gas: the most widely used fuel in the UK, natural gas has many
applications, both domestic and industrial. It is used as a fuel for:
l gas fires
l cookers
l room heaters
l condensing central heating boilers
l water heaters
l electricity generation
l industrial heating and processes.
l Carbon dioxide: used as a freezing agent with pipe-freezing kits, and is also
used in fire extinguishers.
l Refrigerant gas: see the section on refrigerants (pages 161–2).
Gas laws
Gases behave very differently from the other two states of matter we have
studied so far: solids and liquids. Gases, unlike solids and liquids, have neither a
fixed volume nor a fixed shape. They are moulded completely by the container
in which they are held. There are three variables by which we measure gases.
These are as follows.
KEY POINT Pressure
Pressure is measured as This is the force that the gas exerts on the walls of its container; it is equal on
force per unit area. The all sides of the container. For example, when a balloon is inflated, the balloon
standard SI unit for pressure expands because the pressure of air is greater on the inside of the balloon
is the pascal (Pa). However, than the outside. The pressure is exerted on all surfaces of the balloon equally
in plumbing it is more and so the balloon inflates evenly. If the balloon is released, the air will move
likely that pressure will be from the area of high pressure (inside the balloon) to the area of low pressure
measured in bar pressure (1 (outside the balloon).
bar = 100 kPa) or millibar
(1 mbar = 100 Pa). Volume
The volume of gas in a given container is affected by temperature and pressure.
Pressure is constant if temperature is constant. If temperature is increased, then
both the volume and pressure increase.
Temperature
An important property of any gas is its temperature. The temperature of a gas
is a measure of the mean kinetic energy of the gas. The gas molecules are in
constant random motion (kinetic energy). The higher the temperature, then
the greater the kinetic energy and greater the motion. As the temperature falls,
the kinetic energy decreases and the motion of the gas molecules diminishes.
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Chapter 3 Scientific principles
Charles’s law
Charles’s law was discovered by Jacques Charles in 1802. It states that the
volume of a quantity of gas, held at constant pressure, varies directly with the INDUSTRY TIP
kelvin temperature. But what does that mean?
Charles’s law can be
It relates to how gases expand when they are heated up and contract when they
are cooled. In other words, as the temperature of a quantity of gas at constant explained with the following
analogy.
pressure increases, the volume increases. As the temperature goes down, the
volume decreases. If a sealed copper pipe were
pressurised to 20 mb at
Boyle’s law room temperature and then
placed in direct sunlight
Boyle’s law states that the volume of a sample of gas at a given where the pipe could warm
temperature varies inversely with the applied pressure. In other words, up, then the pressure inside
if the pressure is doubled, the volume of the gas is halved. Table 3.12 the pipe would rise. The rise
illustrates this point. in pressure would be directly
proportional to the rise in
IMPROVE YOUR MATHS temperature. If the pipe were
Boyle’s law can also be expressed as: allowed to cool down to room
temperature, then it would
‘Pressure multiplied by volume is constant for a given amount of gas at constant return to its original pressure.
temperature.’
To put this in mathematical terms:
IMPROVE
P × V = constant (for a given amount of gas at a fixed temperature) YOUR MATHS
Since P × V = K, then:
The mathematical
P × V = P × V expression for Charles’s
i i f f
Where: law is shown below:
÷ T = V ÷ T
V 1 1 2 2
= initial volume
V i
Where:
P = initial pressure
i
V = final volume V = volume
f
P = final pressure T = temperature
f
K = constant
INDUSTRY TIP
Table 3.12 Sample of gas at constant temperature and varying pressure
Test Pressure Volume Formula Calculation The principle of Boyle’s law
1 100 kPa 50 cm 3 P × V = K 100 × 50 = 5000 applies to a child’s balloon.
2 50 kPa 100 cm 3 P × V = K 50 × 100 = 5000 If the balloon is inflated
to a set pressure and then
3 200 kPa 25 cm 3 P × V = K 200 × 25 = 5000 squeezed, the pressure inside
4 400 kPa 12.5 cm 3 P × V = K 400 × 12.5 = 5000 increases as the space inside
5 25 kPa 200 cm 3 P × V = K 25 × 200 = 5000 the balloon decreases. If the
space inside the balloon were
halved, then the pressure
would double.
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3 THE RELATIONSHIP BETWEEN
ENERGY, HEAT AND POWER
The relationship between energy, heat and power is such that it is almost
impossible to have one without the other two. Below is a list of units for
energy, heat and power.
l The unit of power: the watt is the SI unit for power. It is equivalent to one
joule per second (1 J/s) or, in electrical units, one volt ampere (1 V·A).
l The unit of heat: the joule is the unit of heat; 4.186 joule of heat energy
(which equals one calorie) is required to raise the temperature of 1 g of water
from 0°C to 1°C.
l The unit of energy: also the joule (see above).
l Specific heat capacity: the specific heat capacity of a substance is the
amount of heat required to change a unit mass of that substance by one
degree in temperature. It is measured in kilojoules per kilogram per degree
celsius (kJ/kg/°C).
Heat energy is transferred because of temperature difference – for example,
heat passes from a warm body with high temperature to a cold body with low
temperature. The transfer of energy as a result of the temperature difference
alone is referred to as heat flow. The watt, which is the SI unit of power, can be
defined as 1 joule per second (J/s) of heat flow.
In this part of the chapter, we will investigate the energy/heat/power/temperature
DEGREES
relationship, and its implications for the building services industry.
Celsius Kelvin Farenheit
100 373 212
Temperature
Temperature is simply the degree of hotness or coldness of a body or environment,
and is expressed in terms of units or degrees designated on a standard scale,
0 273 32
usually celsius (centigrade) (°C) or kelvin (K).
Celsius (°C)
This scale, using increments of 1 degree (1°), is the most widely used by the
- 100 173 - 148
building services industry. In simple terms, it has a zero point (0°C), which
corresponds to the temperature at which water will freeze. When this scale is
used, the degree symbol (°) should accompany it, i.e. 21°C.
- 200 73 - 328
Kelvin (K)
This has the same increments as the Celsius scale, but has a minimum temperature
- 273 0 - 460 that corresponds to the point at which all molecular motion will stop. This
temperature is often called absolute zero and is equal to −273°C. Therefore:
C K F
p Figure 3.11 The relationship l −273°C = 0K, or
between celsius, kelvin and
fahrenheit l temperature K = temperature °C + 273.
The degree symbol (°) is not used when using the Kelvin scale, i.e. 21 K. The
two scales (C and K) are, for the most part, interchangeable. The SI unit of
temperature is the kelvin; however, when discussing temperature difference,
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Chapter 3 Scientific principles
celsius or kelvin may be used and, since both scales correspond with each other,
temperature difference is uniform. In other words, a 1°C temperature difference INDUSTRY TIP
is equal to a 1 K temperature difference.
Celsius is named after the
Measuring temperature Swedish astronomer, Anders
Celsius (1701–1744). The
Many methods have been developed for measuring temperature. Most rely Kelvin scale is named after
on measuring some physical property of a working material that varies with the Belfast-born engineer and
temperature. Temperature measuring devices include the following. physicist William Thomson,
l Glass thermometer: one of the most common devices for measuring First Baron Kelvin (1824–1907).
temperature. This consists of a glass tube filled with mercury or some other
liquid. Temperature increases cause the fluid to expand, so the temperature
can be determined by measuring the volume of the fluid. These thermometers
are usually calibrated so that the temperature can be read by observing the
level of the fluid in the thermometer.
l Gas thermometer: this measures temperature by the variation in volume or
pressure of a gas.
l Thermocouple: this device is a connection between two different metals
that produces an electrical voltage when subjected to heat. This senses a
temperature difference. Thermocouples are a widely used type of temperature
sensor for measurement and control when used with digital thermometers
(see below). They can also be used to convert heat into electrical power.
l Thermistor: thermistors are resistors that vary with temperature. They are
constructed of semiconductor material with a resistivity that is especially
sensitive to temperature. When the temperature is measured, the resistance
of the thermistor responds in a predictable way.
l Infrared thermometers: these use infrared energy to detect temperatures.
They detect actual energy levels by the use of an infrared beam and so p Figure 3.12 Glass thermometer
the thermometer does not need to actually touch the surface to take
an accurate temperature measurement.
l Digital thermometers: these are probably the most common thermometer
used in the plumbing industry. Dual digital thermometers can read two
temperatures simultaneously, instantly giving the temperature difference
between two points, which is essential when benchmarking central heating
boilers for reading the temperature of both flow and return pipes.
p Figure 3.13 Digital thermometer
States of matter
Everything around us is made up of matter, which can exist in three classic
states: solid, liquid and gas. Each of the phase changes is associated with either
an increase or decrease in temperature. For example, if heat energy is applied
to ice, it melts to form water and, if more heat energy is applied to the water, it
reaches its boiling point, where it vaporises, evaporating to steam. The process
can also work in reverse. When the heat is given up by the steam, it condenses
back to water. Each of these phase changes is given a name: p Figure 3.14 Infrared thermometer
l ice (solid) to water (liquid) is called melting
l water (liquid) to steam (gas) is called evaporation/vaporisation
l steam (gas) back to a water (liquid) is called condensation
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l water (liquid) to ice (solid) is called freezing (solidification)
l ice (solid) to steam (gas) is known as sublimation
l steam (gas) to ice (solid) is known as deposition.
Steam
Sublimation (ice directly to steam) Water Evaporation Energy Condensing
Deposition (steam directly to ice)
Energy
Melting Energy Freezing Energy
Ice
p Figure 3.15 States of matter
Sensible and latent heat of liquids
and gases
Sensible heat of liquid and gases
When heat is applied to a liquid, its temperature will rise as heat is added
without a change of state. The resulting increase in heat is known as sensible
heat. This process can be reversed. When heat is removed from the liquid and
its temperature decreases, the heat that is removed is also called sensible heat.
Therefore, any heat that causes a change in temperature without a change of
state is known as sensible heat.
Latent heat of liquid and gases
Changes of state, as we have already seen, are the result of a change in temperature.
Solids can become liquids, liquids can become gases and each change of state is
reversible. The heat that causes any change of state is known as latent heat. Latent
heat, however, does not affect the substance’s temperature. For example, water
boils at 100°C. The heat required to raise the water to its boiling point of 100°C is
called sensible heat. The heat required to keep it boiling at 100°C is latent heat.
Steam
Evaporation Energy at 100ºC. A change in temperature
Water is heated from 0ºC to water
but no change of state. This is
Water sensible heat.
The ice remains at 0ºC and
melts to become water at 0ºC.
A change of state without a change Melting
in temperature. This is latent heat. Energy
Ice
p Figure 3.16 How sensible and latent heat work together
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Chapter 3 Scientific principles
Methods of heat transfer
So far we have investigated temperature and heat, and how these affect the
different states of matter. Now, we will consider the methods of heat transfer.
There are three methods by which heat can be transferred through a substance
or from one substance to another. These are:
1 conduction
2 convection
3 radiation.
We will look at each one in turn.
Conduction
Conduction happens when heat travels through a substance, with the heat being
transferred from one molecule to another.
Consider a piece of copper tube. If heat is applied to one end, before long the
heat will have travelled through the material so that the effects of the heat will
be felt at the other end. This occurs because kinetic energy in the form of heat
is being passed from one copper molecule to another very quickly. When the
copper is cold, the atoms move very slowly. As heat is applied, these atoms gain
speed and collide with the slower, cooler atoms. In this way, some of the kinetic Table 3.13 Coefficient thermal
energy is passed through the material, the slow atoms becoming faster and conductivity of common
colliding with other slow atoms, and so on. substances
Not all substances, however, transfer heat at the same rate. Some materials, Thermal
such as plastic or wood, are very poor at transferring heat, with little or no heat conductivity
transference occurring at all. Material W/m/K
Silver 406.0
Most metals are very good conductors of heat and, because of this, they are
also very good at conducting electricity. Materials that do not transfer heat Copper 385.0
well, such as plastic, are known as insulators. Gold 310
Aluminium 205.0
IMPROVE YOUR MATHS Brass 109.0
The rate at which a material will transfer heat is known as the coefficient of Steel 50.2
thermal conductivity, which is measured in W/m/K. It can be found using the Lead 34.7
following equation: Concrete 0.8
heat × distance
Thermal conductivity = Polyethylene 0.5
area × temperature difference HD
Wood 0.12–0.04
Table 3.13 lists some common substances, together with their coefficient of Polystyrene 0.03
thermal conductivity. expanded
From Table 3.13, it can be seen that silver is the best conductor of heat, with
copper coming a close second.
The poorest conductor of heat is expanded polystyrene, which is an excellent
insulator of heat.
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The coefficient of linear expansion
Most materials expand when they are heated. When copper pipework expands
it can often be heard as a ticking when the central heating is on. The copper
expands in length by 0.000018 mm/°C. This may not seem a lot, but when it is
considered that this figure is for every degree rise in temperature, then the length
of expansion can be significant. On larger installations, it may mean the use of
expansion joints to accommodate the amount of expansion so that damage to
the pipework is eliminated. PVCu expands by a greater amount of 0.0005 mm/°C.
IMPROVE YOUR MATHS
Let’s see by how much copper expands.
20 m of 22 mm copper pipe contains water that rises from 4°C to 85°C. By how
much does the copper expand?
There is 20 m of copper pipe, an 81°C temperature difference and a 0.000018
coefficient of expansion of copper, so:
20 m × 81°C × 0.000018 mm/°C = 29.16 mm
Convection
Convection is heat transfer through a fluid substance, which can be water or air.
Convection occurs because heated fluids, due to their lower density, rise and
cooled fluids fall.
As water or air is heated it expands, which makes it less dense and therefore
lighter. If a cooler, denser material is above the warmer layer, the warmer
material will rise through the cooler material. The lighter, rising material will
release its heat into the surrounding environment, become denser (cooler),
and will fall because of the effect of gravity, to start the process over again.
In a hot water system, this process is known as gravity circulation.
Hot, less dense water
rises through the
water to the top of
the cylinder.
Cooler, dense water
falls back towards the
heat source to be
reheated and the
process starts again.
p Figure 3.17 Gravity circulation in a hot water system
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Chapter 3 Scientific principles
Modern radiators in central heating systems use two methods of heat transfer,
with convection being the main heat transfer method. The other is radiation.
Radiation
The third method of heat transfer is radiation. Radiation heat transfer is thermal
radiation from infrared light, visible or not, which transfers heat from one body
to another without heating the space in between. Like all forms of light, thermal
radiation travels in straight lines.
Consider the heat from the Sun, which travels millions of miles through the
vacuum of space to heat the Earth. The heat can be felt from a distance because
it travels in waves, which are emitted from the heat of the Sun. Radiation is the
heat transfer method that makes solar hot water collectors in solar hot water
systems so effective.
Radiation heat can also be felt from a hot radiator, even though there is no
visible heat source or flame. This is because the heat is being radiated as
thermal energy.
Radiated heat is better absorbed by some materials than others. The colour and
texture of a surface can also affect the heat absorption. A dull matt surface will
absorb heat more effectively than a shiny polished surface. This is the reason
that solar thermal panels are dark and dull, to allow them to absorb the Sun’s
heat more effectively. This is also why a lot of cars in hot countries are coloured
white, to reflect the heat.
Solar thermal radiation
Sun Earth
Solar thermal radiation
Figure 3.18 Thermal radiation from the Sun
Energy, heat and power calculations
In this part of the chapter, we will look at simple energy, heat and power
calculations using information we have previously discovered. To recap, the
SI units of measurement of energy, heat and power are:
l energy – the joule (J)
l heat – the joule (J)
l power – the watt (W)
l specific heat capacity – kilojoules per kilogram per degree celsius (kJ/kg/°C).
Calculations using the specific heat capacity of water KEY POINT
Example 1 Remember: the specific
heat capacity of water is
How many kilojoules would it take to heat 100 litres of water from 30°C to 80°C? 4.186 kJ/kg/°C.
The formula for this is:
L × Δt × SHC of water
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ACTIVITY Where:
Using the formula shown in L = litres
Example 1, calculate how Δt = temperature difference
many kilojoules it would
take to heat 140 litres of SHC of water = 4.186
water from 4°C to 65°C.
Therefore:
100 × (80−30) × 4.186 = 20930 kJ
Example 2
We can develop this concept further to calculate how many kilowatts it would
take to raise the temperature of the 100 litres of water by 50°C. To do this,
we need to state a time frame. Let us assume that the 100 litres of water is
required in one hour. The calculation would then become:
L × Δt × SHC of water
Time (in seconds)
Where:
ACTIVITY L = litres
Using the formula shown Δt = temperature difference
in Example 2, calculate SHC of water = 4.186
how many kilowatts it
would take to raise the 1 hour in seconds = 3600
temperature of the 140
litres of water from 4°C Therefore:
to 65°C in two hours. 100 × (80–30) × 4.186
= 5.81 kW
3600
Example 3
KEY POINT How many seconds would it take for 20 kg of water to be heated by 15°C using
Remember: water has a a 3 kW heating element?
specific heat capacity of
4.186 kJ/kg/°C and that The formula for this is:
1 W = 1 J/s. kg × t × SHC
kW
Where:
ACTIVITY kg = kilograms
Using the formula shown t = temperature
in Example 3, calculate kW = kilowatts
how many seconds it
would take for 42 kg SHC = specific heat capacity
of water to be heated
by 30°C using a 3 kW Therefore:
heating element. 20 × 15 × 4.186
= 418.6 s or 6.976 minutes
3
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Chapter 3 Scientific principles
4 THE PRINCIPLES OF FORCE
AND PRESSURE, AND THEIR
APPLICATION IN THE PLUMBING
AND HEATING INDUSTRY
In this part of the chapter, we will look at the scientific principles of force and
pressure, and investigate how they apply to the building services industry.
The SI units of force and pressure
Table 3.14 SI units of force and pressure
Velocity metres per second m/s
Acceleration metres per second squared m/s 2
Flow rate metres cubed per second m 3 /s
Force newton (equal to kg m/s 2 ) N
Pressure, stress pascal (equal to N/m 2 ) Pa
Velocity and acceleration
l Velocity is the measurement of the rate at which an object changes its
position. In order to measure it, we need to know both the speed of the
object and the direction in which it is travelling. It is measured in metres
per second (m/s).
l Acceleration is a measure of the rate at which an amount of matter
increases its velocity. It is measured in a change of velocity over a period
of time and, as such, is directly proportional to force. It will increase and
decrease linearly with an increase or decrease in force if the mass remains
constant. It is measured in metres per second squared (m/s ).
2
l Acceleration due to gravity is the rate of change of velocity of an object
due to the gravitational pull of the Earth. If gravity is the only force acting on
an object, then the object will accelerate at a rate of 9.81 m/s downwards
2
towards the ground.
Flow rate
In plumbing, flow rate is defined as an amount of fluid that flows through
a pipe or tube over a given time. It is usually measured in metres cubed per
3
second (m /s). However, in plumbing systems, flow rate is usually measured in
litres per second (l/s).
IMPROVE YOUR MATHS
To convert from m /s to l/s, multiply m /s by 1000.
3
3
To convert from l/s to m 3 /s, multiply l/s by 0.001.
Flow rate can also be measured in kilograms per second (kg/s). Since 1 litre of
water has a mass of 1 kilogram, then 1 litre per second (l/s) = 1 kilogram per
second (kg/s).
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Force
Force is an influence on an object at rest that, acting alone, will cause the motion
of the object to change. If the object at rest is subjected to a force, it will start
to move. For example, consider water in a pipe connected to a cistern at one end
and a tap at the other. When the tap is closed, the water is not moving and so
is said to be at rest. When the tap is opened, the force of gravity will move the
water out of the tap, causing water to flow. It is measured in newtons (N).
IMPROVE YOUR MATHS
The unit of the force of gravity is the newton. It is the force required to
accelerate a mass of 1 kg at 1 metre per second, every second. On Earth, that
force of acceleration (known as gravitational pull) is 9.81 metres per second per
second, or 9.81 m/s . Therefore, if we multiply the mass of an object (in kg) by
2
2
9.81, the result is measured in newtons (kgm/s ).
ACTIVITY
Calculating force
When the tap is When the tap is opened,
Consider the cistern in closed, the body of the force of gravity
Figure 3.19. If it contained water is at rest pushes the water down
a mass of water equal to the pipe and out of the
tap causing a flow of
40 kg, then by multiplying water
the mass by the force of
gravity, the force of the
cistern acting downwards
can be calculated:
40 × 9.81 = 392.4 N
If a cistern in a roof space
contains a volume of
100 litres of water and
1 litre = 1 kg, what is
the force acting on the
platform it is standing on?
p Figure 3.19 The force of gravity on a cold water system
Pressure
In physics, pressure is defined as force per unit area. For an object sitting on a
surface, the force pressing on the surface is the weight of the object measured
2
in newtons per square metre (N/m ). However, in different orientations it
might have a different area in contact with the surface and will therefore exert
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Chapter 3 Scientific principles
a different pressure. For example, if a cistern measuring 1 m long × 0.5 m
wide × 0.7 high was placed in a roof space, then what pressure would it exert if:
l it was placed on its bottom
l it was placed on its side
l it was placed on its end?
IMPROVE YOUR MATHS
Before we can attempt these calculations, we must first find the mass of the
cistern in kg. The formula for this is:
length × width × height = volume in m 3
From earlier calculations, we know that to find the force of an object we use the
formula:
kg × gravity = N
Since 1 litre of water has a mass of 1 kg, a cistern measuring 1 m × 0.5 m × 0.7 m
has a force of 3433.5 N. ACTIVITY
The formula for finding pressure is:
Pressure
force
= N/m 2 What is the pressure
area exerted by a block of lead
From these calculations we can see that the greater the surface area for a given with a cross-sectional area
mass, the less force will be exerted by that mass. This is of particular importance of 4 m and a mass of
2
when placing large cisterns in roof spaces since the greater the surface area we 4000 kg?
can rest the cistern on, the more we can spread the load of the cistern.
Static pressure of water (head)
The unit of water pressure is the pascal. The pressure exerted by water is
due to its mass and is determined by the height of the column of water.
For instance, if the pressure exerted by a water main is 300 kilopascals
(kPa) it will balance a column of water about 30 m high. This pressure is
equivalent to a head of
water of 30 m. Therefore, 10 m of head = 100 kPa.
Water pressure in plumbing systems is usually measured in bar pressure.
Static head measured from
Static head of water in plumbing systems is measured from the bottom of the bottom of the cistern
exerts a pressure of 10 kpa
the water source, i.e. the cistern, to the outlet, as shown in Figure 3.20. per metre of head.
10 m
ACTIVITY 10 kpa = 0.1 bar = 1 m
Static pressure of water
If the vertical distance between the bottom of a cold water cistern
and the tap is 16 m, what is the pressure at the tap in:
a kilopascals
b bar?
Static head at the tap is
100 kpa = 1 bar = 10 m
p Figure 3.20 The head of pressure on a cold
water system
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Table 3.15 Conversions for Table 3.15 shows the conversions for common units of head of pressure.
common units of head of pressure
Dynamic pressure
Metres
Kilopascals head Also called working pressure, dynamic pressure is the pressure of water while it
(kPa) Bar of water is in motion. In other words, it is the pressure of flowing water. If the pressure of
10 0.1 1 the water is increased, the velocity and flow rate will also increase.
20 0.2 2 Atmospheric pressure
30 0.3 3
Atmospheric pressure is the amount of force or pressure exerted by the
40 0.4 4 atmosphere on the Earth and the objects located on it. The more pressure
50 0.5 5 there is, the stronger that force will be; at sea level, the atmospheric pressure
100 1 10 is 101.325 kPa. This is known as 1 atmosphere (atm). Atmospheric pressure
150 1.5 15 decreases with height.
200 2 20
250 2.5 25 The principle of a siphon (siphonic action) due to atmospheric
300 3 30 pressure
The principle of a siphon is to discharge water from a high vessel to a lower
350 3.5 35
vessel using atmospheric pressure and the cohesive properties of water.
400 4 40
450 4.5 45 The principle of a siphon can be understood with reference to the diagram (see
500 5 50 Figure 3.21). The two beakers are both at atmospheric pressure, but they are at
different levels. The pressure at beaker ‘B’ is greater because it is lower. The
outlet from the hose at ‘B’ must be lower than the inlet of the hose at ‘A’ for
flow to take place. When suction is applied to the end of the hose at ‘B’, the
water will flow upwards over the top of beaker ‘A’, where the atmospheric
pressure is slightly lower. Here, gravity and the cohesive nature of water will
empty the contents of beaker ‘A’ into beaker ‘B’.
The relationship between
velocity, pressure and flow
A rate in plumbing systems
As we have already discovered, if pressure is
applied to a pipe full of water, the effect is to
increase the velocity and therefore the flow rate of
the water. The more pressure that is applied, the
greater the velocity and flow rate becomes.
A similar effect occurs when a pipe is suddenly
Water from beaker A flows backwards reduced in size; this can be seen in a hosepipe. If
to beaker B when a negative pressure is B
applied at point C, emptying beaker A. the end of a flowing hosepipe is suddenly reduced,
then the speed increases and the water shoots
This process is known as siphonic
action. further away, but the pressure and flow rate will be
C reduced. This is called the Bernoulli effect.
p Figure 3.21 Siphonic action
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Chapter 3 Scientific principles
This describes the result of a reduction in pipe size, where the speed of fluid
increases at the same time as the pressure or the fluid’s potential energy
decreases.
Increased fluid speed,
decreased internal pressure
p Figure 3.22 The Bernoulli effect
Similarly, if the pipe suddenly increases in size, then the velocity of the water
will decrease but the pressure will increase slightly. The flow rate remains
constant.
Factors affecting flow rate
As we have seen, flow rate is unaffected by sudden increases in pipe size but,
as described below, there are elements in plumbing systems that can severely
affect the flow rate.
l Changes in direction: any change in direction of a pipe will offer resistance
to the flow of the water. That resistance will, in effect, be an increase in
the overall length of the pipe. For example, an elbow installed in the run
of copper pipe will offer resistance equivalent to 0.37 m of pipe. So, if 10
elbows are used, then the length of the pipe has, theoretically, increased
by 3.7 m. Machine-made bends offer slightly less resistance at 0.26 m
of pipe. This will also vary with the material of the pipe (see ‘Frictional
resistance of the internal bore of the pipe’ below).
Table 3.16 Resistances in the form of equivalent lengths of common fittings
Nominal pipe size* (mm)
8 10 12 15 22 28
Type of fitting Equivalent length (m)
Capillary elbow 0.16 0.21 0.28 0.37 0.60 0.83
Compression elbow 0.24 0.33 0.42 0.60 1.00 1.30
Square tee piece 0.27 0.37 0.49 1.00 1.6 2
Swept tee piece 0.22 0.29 0.38 0.60 0.75 1
Manifold connection 0.60 1.00 1.20 n/a n/a n/a
Minimum radius (machine) bend 0.12 0.16 0.20 0.26 0.41 0.58
* Copper tubes to BS EN 1057 R250
l Size of pipe: the greatest factor in the flow rate of any system is the size of
the pipe itself. The bigger the bore of the pipe, the better the flow rate will be.
l Pressure: pressure increases flow rate. The greater the pressure, the greater
the flow rate.
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l Length of the pipe: flow rate diminishes with length because of the frictional
resistance of the wall of the pipe. Water flows faster down the centre of
the pipe than it does at the pipe wall. The nearer the water is to the wall
of the pipe, then the greater the frictional resistance and so the slower the
water becomes. The frictional resistance of the pipe is slowing the flow rate
constantly. The greater the length, the more frictional resistance, the greater
the loss of flow rate. To counter this effect, the pipe size should be increased
initially at the start of the pipe run and then reduced as length increases.
l Frictional resistance of the internal bore of the pipe: different materials
offer different frictional resistance. Polybutylene pipe, for instance, has the
smoothest bore of all common pipe materials and low carbon steel the
roughest. Therefore, low carbon steel at like-for-like sizes will have a much
lower flow rate than polybutylene pipe.
l Constrictions such as valves and taps: taps and valves offer a lot of
resistance to the flow of water. Some stop taps can increase pipe length by
up to 6 m per valve.
5 THE MECHANICAL PRINCIPLES
IN THE PLUMBING AND HEATING
INDUSTRY
Simple machines are those that aid with the lifting and moving of loads that are
too heavy to lift or move on their own. There are four main types:
1 levers
2 wheel and axles
3 pulleys
4 screws.
IMPROVE YOUR These machines give a mechanical advantage (velocity ratio) to human effort,
MATHS meaning they multiply the force that is put into them. There are two types of
The calculation for finding out mechanical advantage:
how a lever functions is:
1 Ideal mechanical advantage (IMA): purely theoretical, based upon an
Load
Mechanical advantage = ‘ideal machine’, which does not exist.
Effort
2 Actual mechanical advantage (AMA): this is the mechanical advantage
of a real machine such as a wheelbarrow (lever). AMA takes into
consideration real-world factors such as energy lost because of friction.
Simple machines
Here, we will look at the machines themselves and their possible uses in
everyday working life.
Levers
In physics, a lever is a rigid object that can be used with a pivot point or fulcrum
to multiply the mechanical force that can be applied to another, heavier object.
Levers are examples of mechanical advantage.
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There are three classes of lever, as follows.
l First class lever: a simple see-saw arrangement where the long arm
(force effort) is proportional to the short arm (load). Examples of this
are:
l the lever arm of a float-operated valve
l claw hammer
l water pump pliers (double lever). F
W
l Second class lever: a variation on the first class lever. Examples of this
are: p Figure 3.23 First class lever
l wheelbarrow
l crowbar.
l Third class lever: examples of this are:
l the human arm
l tools, such as a hoe or scythe
l spades and shovels.
Wheel and axles F W
The wheel and axle is composed of a wheel, which is larger than the p Figure 3.24 Second class lever
diameter of the axle. Either of these can be used as the effort arm and F
the resistance arm, and this depends where the force is applied. The force
is usually applied to the wheel rather than the axle to gain the maximum
output. The point where the axle joins the wheel is known as the fulcrum
and this acts as the point where the force from the larger wheel is
transferred to the smaller axle.
The wheel and axle multiplies the ‘torque’ during the turning motion.
Both the wheel and the axle have ropes wound around them. The load is W
lifted by pulling on the rope around the wheel so that the wheel and axle p Figure 3.25 Third class lever
is rotated once, therefore:
Radius of the wheel R
Mechanical advantage = = KEY TERM
Radius of the axle r
Spanners and screwdrivers use the principle of wheel and axle. Wheel and axle: a
mechanical device used to
Pulleys wind up weight; includes a
grooved wheel, turned by a
A pulley is a collection of one or more wheels over which a rope or chain is cord/chain, and a rigid axle.
looped to aid lifting heavy objects. Pulleys are examples of simple machines. In
other words, they multiply the lifting forces.
How do pulleys work?
A single pulley reverses the direction of the lifting force. When the rope is pulled
down, the weight lifts up. If a lift of 100 kg is needed, an equal force of 100 kg r
must be exerted. A lift of 1 m high needs to be pulled downwards 1 m.
If more ropes and wheels are added, the effort needed to lift the weight is R
reduced. The 100 kg weight is now supported by two ropes instead of one, so
the lift effort is halved. This gives a positive mechanical advantage. The bigger
the mechanical advantage, the less force is needed.
p Figure 3.26 The pulley wheel
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100 kg lifting force 50 kg lifting force 25 kg lifting force
100 kg load 100 kg load 100 kg load
p Figure 3.27 Single pulley system p Figure 3.28 Two pulley system p Figure 3.29 Four pulley system
If four wheels are used and held together by a long rope or chain that loops over
them, the 100 kg weight is now supported by four ropes, which means that each
rope is supporting a quarter of the total 100 kg weight, or 25 kg. This means
that only a quarter of the force (25 kg) is needed to lift the weight (100 kg). This
system is known as a block and tackle.
Screws
In terms of simple machines, a screw is a machine that converts rotation into
a straight-line motion that can be placed vertically, horizontally or at an angle.
It is basically a cylinder or wedge with an incline plane wrapped around it.
It was originally designed as a simple water pump (the Archimedes screw), a
task for which it is still used today. It can be found in many objects, such as
screw fixings, bolts and threads on pipe. It can also be seen on drills and auger
bits, and as a means of moving solid fuel, such as coal, towards a boiler by its
rotary motion.
p Figure 3.30 The Archimedes screw IMPROVE YOUR MATHS
The following formula is used to calculate the mechanical advantage of a screw:
π × D
MA =
L
Where:
MA = mechanical advantage
π = 3.142
D = diameter
p Figure 3.31 The Archimedes L = length
screw in action as a water lifter
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Chapter 3 Scientific principles
Basic mechanics: moments of a force
(torque)
In physics, the moment of a force is the measure of the turning effect
(or torque) produced by a force acting on a body. It is equal to the applied force
and the perpendicular distance from its line of action to the pivot, about which
the body turns. The turning force around the pivot is called the moment. Its
unit of measurement is the newton.
IMPROVE YOUR MATHS
The moment of a force can be worked out using the formula:
Moment = force applied × perpendicular distance from the pivot
If the magnitude of the force is F and the perpendicular distance is d, then:
Moment = F × d
An example of this would be a spanner turning a bolt. It is
much easier to turn the bolt using a long spanner than it is
using a short spanner. This is because more torque (turning
force) can be applied at the bolt (pivot) for less effort. A The moment of a
force - the pivot
long spanner is an example of a force multiplier.
Centre of gravity Distance from
In physics, the centre of gravity of an object is the the pivot
imaginary point where all of the weight of the object is
concentrated. This concept is especially important when
designing large structures such as multi-storey buildings and
bridges, or making a prediction of the gravitational effect on Force applied
a moving object or body. Another term for it is the ‘centre
of mass’.
The centre of gravity will vary from object to object. In
symmetrically shaped objects, it will coincide with the Moment = Force applied × Distance from the pivot
geometric centre. = Newtons
p Figure 3.32 The moment of force
In irregularly (asymmetrically) shaped objects, the centre of
gravity may be some distance away from the centre of the
object; in hollow objects, such as a ball, it may be in free space, away from the
object’s physical form.
KEY POINT
For many solid objects, the location of the geometric centre follows the
object’s symmetry. For example, the geometric centre of a cube is the point
of intersection of the cube’s diagonals.
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The City & Guilds Textbook: Plumbing Book 1
Action and reaction: Newton’s third law
of motion
A push or a pull (action) on an object can often result in movement (reaction)
when the pull or push is greater than the weight of the object. If both action
and reaction are equal, then no movement takes place because the object is
pushing or pulling against the action with equal force. This is known as
contact force and is a result of contact interactions (normal, frictional,
tensional, and applied forces are all examples of contact forces). Other forces
are a result of ‘actions-at-a-glance’ interactions (gravitational pull, electrical
and magnetic). These two types of force have one thing in common: for every
force applied there is an equal opposing force and as such is subject to action
and reaction.
There are many ways in which this can be seen. For example, when a person sits
on a chair (action), the downward force of the person provokes an upward force in
the chair (reaction). The person and the chair have equal force and so equilibrium
50N
exists. If the person were too heavy for the chair, then the chair
would collapse (reaction).
This is Newton’s third law of motion, which states:
Every action has an equal but opposite reaction.
50N 50N
This means that, for every force that an object is subjected to:
1 there is an opposing force from the object
2 both action and reaction forces are equal
p Figure 3.33 Action and reaction 3 forces always come in pairs (points 1 and 2).
Equilibrium 50N
When all the forces acting on a stationary object are balanced, the object is
said to be in a state of equilibrium. The forces are balanced when all forces (left,
right, front, back, up and down) are the same. In Figure 3.34 (left), all forces are
50 N and are therefore equal forces in equilibrium.
50N 50N
50N 50N 30N 30N
50N 50N
p Figure 3.34 Balanced forces in equilibrium (left) and unbalanced forces in equilibrium (right)
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30N 30N
50N
Chapter 3 Scientific principles
The same can apply for unequal forces. They, too, can be in a state of equilibrium
provided left and right forces are equal but not necessarily the same as the equal
up and down forces.
The key word here is balanced. All forces, whether equal or not, must be
balanced. The forces cancel each other out and so add up to zero. In other
words, for an object to be in equilibrium, the sum of the forces on each part
of the system must be zero. Look at Figure 3.35.
Upward/downward forces are equal
so no movement takes place
50 N 50 N
Unequal horizontal forces resulting
in movement =
50 – 30 = 20
50 N 50 N 30 N 50 N Movement of 20 N
50 N 50 N
Vertical/horizontal forces are equal
so no movement takes place.
Forces are zero because they cancel
each other out.
p Figure 3.35 Forces acting on an object
6 THE PRINCIPLES OF ELECTRICITY
IN THE PLUMBING AND HEATING
INDUSTRY
Electricity is a vital part of everyday life. It powers lighting, household appliances and
heating systems, but its danger cannot be overstated. We cannot see it, hear it or
smell it, yet if we touch it, it can kill. Because of the obvious dangers, it is necessary
for us to have a better understanding of what electricity is and how it works.
In this section, we will find out about electricity, its scientific laws and basic
circuitry.
Electrical units of measurement
Table 3.17 Electrical units of measurement
Parameter Measuring unit Symbol Description
Voltage volt V or E Unit of electrical potential
V = I × R
Current ampere I or i Unit of electrical current
I = V ÷ R
➜
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The City & Guilds Textbook: Plumbing Book 1
Resistance ohm R or Ω Unit of DC resistance
R = V ÷ I
Conductance siemen G Reciprocal of resistance
G = 1 ÷ R
Capacitance farad C Unit of capacitance
C = Q ÷ V
Charge coulomb Q Unit of electrical charge
Q = C × V
Power watts W Unit of power
P = V × I
KEY TERMS The basic principles of electron flow
Molecule: the smallest Everything is made up of molecules, and these in turn are made up of atoms.
particle of a specific
element or compound Atoms consist of even smaller particles. At the centre of every atom is a nucleus,
that retains the chemical which is made up of protons and neutrons. Protons have a positive (+) electrical
properties of that element charge whilst neutrons do not have an electrical charge – they are neutral.
or compound. Revolving in orbit around the nucleus is the electron. This has a negative (−)
Atom: a fundamental electrical charge.
piece of matter made up
of three kinds of particles
called subatomic particles E
– protons, neutrons and
electrons.
P
E N
N
N E
P
P
Nucleus
E Electron
P Proton
N Neutron
.
p Figure 3.36 Protons, neutrons and electrons
Normally atoms possess equal numbers of positively charged protons and
negatively charged electrons, and these effectively cancel one another out,
leaving the atom electrically neutral. It is possible in some cases, however,
to add or remove an electron to/from an atom to make it either positively or
negatively charged. In that case, the atom is known as an ion.
As can be seen from Figure 3.36, the atom is like a micro-solar system whereby
the electrons orbit the nucleus in the same way as the planets orbit the Sun.
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Chapter 3 Scientific principles
Electrons are arranged in varying distances from the nucleus – the further they
are away, the less they are attracted to the atom and are easily deflected from
their orbits to be attracted by other atoms. This constant to-ing and fro-ing
of electrons from one atom to another is the structure that makes electricity
possible. Materials that allow the movement of free electrons are known as
conductors and those that restrict their movement are known as insulators.
The measurement of electrical flow
Electricity is measured in two ways:
1 by the amount of current – this is the number of electrons flowing, measured
in amperes
2 by the voltage – this is the push, or pressure, which causes electrons to flow,
measured in volts.
The push or pressure that causes electrons to flow is also known as ‘potential
difference’. In a conductor the path is clear for electrons to move, and it is the
voltage (pressure) that makes them do so.
The units of electrical measurement
When we think about electricity we think in terms of voltage, amperage,
resistance and power, but what do these terms mean and what do they do?
Here, we will investigate the various electrical units, their interaction with one
another and how we can calculate one if two others are known (Ohm’s law).
Voltage
When a potential difference or voltage is applied across a circuit, electrons will
flow. The higher the potential difference or voltage, the greater the ‘pressure’ on
each electron. If the resistance in a circuit stays the same, then the larger the
potential difference the greater the current or flow (amps) in the circuit.
IMPROVE YOUR MATHS
Voltage can be calculated by:
Current (I) × Resistance (R)
Resistance
Resistance is the movement of electrons through a conductor. All electrical
circuits will have resistance but some will have more than others. Resistance in
some circuits is necessary to ensure that not too many electrons flow and, in
others, as little resistance as possible is required so that high current will flow.
There is a definite interaction between current (electron flow), voltage (current
flow) and resistance. As the electrical pressure (voltage) increases, more electrons
flow. Increasing the voltage also increases the amperes of current, but if resistance
is also increased this decreases the flowing current thus reducing the amperes.
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The City & Guilds Textbook: Plumbing Book 1
These relationships between current, voltage and resistance are the theory
behind Ohm’s law, which will be looked at in detail later in this section.
IMPROVE YOUR MATHS
Resistance can be calculated by:
Voltage (V) ÷ Current (I)
Amperage
In the UK, voltage is supplied at 230 V, but different appliances need different
amounts of electricity in order to work effectively. The rate at which electricity flows
through an appliance is known, in electrical units, as amperage, often shortened to
amps. If we consider that water at a certain pressure with a certain size pipe will
deliver a set amount of water, if we increase the pipe size, then the pressure stays
the same but the flow rate increases. In electrical terms, if voltage is the pressure
then amps is the flow rate; the bigger the cable, the bigger the flow rate or amperage.
The ampere, symbol I, is the SI unit of electric current, and is defined in terms of
the coulomb: 1 ampere is the amount of electric current (flow rate of electricity)
carried by a charge of 1 coulomb flowing for 1 second.
IMPROVE YOUR MATHS
Amps can be calculated by:
Voltage (V) ÷ Resistance (R)
Power
The rate at which electric energy is converted to other forms of energy, such as
heat, light or mechanical, is called power (P) and is equal to the sum of the current
and the voltage. An electrical shower that is rated at, say, 8 kW simply means that
the electrical power of 8 kW is converted into heat to heat the water. Electrical
power is, therefore, the rate at which electricity is produced or consumed, and can
be defined as the amount of electric current flowing due to the voltage.
IMPROVE YOUR MATHS
Electrical power is measured in watts (W). The formula is:
Current (I) × Voltage (V) = Power (W)
The types of electrical current
There are two types of electrical current. These are:
1 direct current
2 alternating current.
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Chapter 3 Scientific principles
Direct current
In a direct current (DC) circuit the electrons always flow from the negative (−) p Figure 3.37 Direct current symbol
pole towards the positive (+) pole. The polarity, or direction, of the electrons
never reverses. DC can be produced from a number of sources, including
electrochemical, photovoltaic cells and batteries. DC can be stored in batteries
and cells.
Alternating current
Alternating current (AC), unlike DC, does not travel in a constant direction.
It alternates. In other words, it reverses its direction of travel constantly and
uniformly throughout the circuit at 50 times a second. This rapid movement _ +
(50 times/second) is called the frequency and is measured in Hertz (Hz). In the Battery
UK, the frequency of AC power is 50 Hz.
p Figure 3.38 Simple direct
1 cycle = 1/50 of a second current circuit
50 cycle/second = 50Hz
KEY TERM
+ Hertz (Hz): the SI unit
of frequency, measuring
the number of cycles
per second in alternating
current.
−
p Figure 3.39 The AC sine wave p Figure 3.40 Alternating current symbol
The advantage that AC has over DC is that AC can easily be transformed to
higher or lower voltages. DC voltages are difficult to transform. Changing AC
voltages is done by the use of a transformer, which uses the properties of AC
electromagnets to change the voltages. AC generator
Another advantage is that AC can easily be transported over long distances
without excessive voltage loss and is, therefore, much more efficient than DC. p Figure 3.41 Simple alternating
current circuit showing the
AC is generated at power stations and portable electricity generators. It cannot alternating direction of
be stored. electron flow
Material conductivity and resistance
As we have already seen, the atom is orbited by electrons. Electrons carry a
negative charge and can move from atom to atom. The direction of movement
between atoms is random unless a force causes the electrons to move in one
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The City & Guilds Textbook: Plumbing Book 1
direction. This directional movement of electrons due to an electromotive
force (EMF) is known as electricity. How well a material allows electron
movement is called conductivity, and how well it resists electron flow is
Table 3.18 Density of common
conductors known as resistivity.
Element Density Here, we will investigate these two properties.
Silver 10,490 kg/m 3 Conductivity
Copper 8960 kg/m 3
Electrical conductivity is a measure of how well a material accommodates the
Gold 19,300 kg/m 3
movement of an electric charge. This means that any electrical conductor is
Aluminium 2700 kg/m 3
Iron 7150 kg/m 3 one that has many free electrons. A good conductor allows the free movement
Chromium 7860 kg/m 3 of electrons, whereas a poor conductor (known as an insulator) restricts this
free movement. As a general conductor, copper is the most commonly used
Lead 11,340 kg/m 3 because it is cheap, reasonably flexible, reasonably light, the second best
Titanium 4506 kg/m 3 conductor (in terms of electrical resistance) and the best conductor per unit
weight.
Resistivity
Electrical resistivity is the opposite of conductivity. It is the opposition of a material
KEY POINT to the flow of electrical current through it, resulting in a change of electrical energy
Remember: (I) current into heat, light or other forms of energy. For example, when electricity passes
is what flows in a wire through the heating element of an immersion heater, the element resists the flow
or conductor. Current is of electrical current, thus generating heat. The same effect occurs in a light bulb.
measured in (A) amperes The lighting filament offers resistance to the flow of electricity and ‘glows’ with the
or amps. (V) voltage is heat generated. By including an electronic variable resistor in the light switch,
the difference in electrical the brightness can also be resisted, creating a dimmer switch.
potential between two
points in a circuit. It is The amount of resistance depends on the type of material.
the push, or pressure,
behind current flow and Ohm’s law
is measured in volts.
(R) resistance determines So far we have looked at voltage, current, resistance and power. Here, we will
how much current investigate how these are related to one another by the use of Ohm’s law.
will flow through a
component. Resistors are Ohm’s law states that:
used to control voltage The current through a conductor between two points is equal to the
and current levels. voltage across the two points, and inversely proportional to the resistance
Resistance is measured between them.
in ohms (Ω).
(P) power is the amount It defines the relationships between power, voltage, current and resistance. One
of current multiplied by ohm is the resistance through which one volt will maintain a current of one
the voltage at a given ampere.
point. It is measured
in watts. Before we look at Ohm’s theory, let us first refresh ourselves on power, voltage,
current and resistance, and their units of measurement.
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