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TaPbalesotf CExamination Papers
AUGUST 2014
NATIONAL CERTIFICATE
POWER MACHINES N5
(8190035)
5 August 2014 (Y-Paper)
13:00 – 16:00
REQUIREMENTS:
Steam Tables (BOE 173)
Superheated Steam Tables (Appendix to BOE 173)
Drawing instruments are required.
Calculators may be used.
This question paper consists of 5 pages and a formula sheet of 3 pages.
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DEPARTMENT OF HIGHER EDUCATION AND TRAINING
REPUBLIC OF SOUTH AFRICA
NATIONAL CERTIFICATE
POWER MACHINES N5
TIME: 3 HOURS
MARKS: 100
__________________________________________________________________
INSTRUCTIONS AND INFORMATION
1. Answer ALL the questions.
2. Read ALL the questions carefully
3. Number the answers according to the numbering system used in this question
paper.
4. Write neatly and legibly.
___________________________________________________________________
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QUESTION 1:
1.1 The blades of a single-stage, impulse turbine has a mean diameter of 960 (3)
mm. 100 kW of power develops at a speed of 4 755 r/min. The gas
discharges in an axial direction and the coefficient of friction is 0,87. Both
blade angles are 30° each to the plane of rotation.
1.1.1 Calculate the blade velocity in m/s (8)
1.1.2 Use a scale of 1 cm = 50 m/s and construct a velocity diagram in (8)
the ANSWER BOOK. Enter ALL the values (m/s) onto the
diagram.
HINT: Use the answer page in the landscape format to construct the
diagram.
NB: NO marks will be awarded if the values (m/s) are NOT entered
onto the diagram and if the diagram is NOT constructed to the
given scale
1.1.3 Determine the following from the diagram: (1)
a) The nozzle angle (3)
b) The mass flow of gas in kg/s (3)
c) The diagram efficiency
1.2 State the volume and enthalpy of 1 kg of steam which has a temperature (2)
of 500 ºC and a pressure of 0,5 MPa.
[20]
QUESTION 2:
2.1 With the aid of steam tables, calculate the following:
2.1.1 The enthalpy found in 2 kg of steam which has a pressure of 480 (3)
kPa and is 7,3% wet.
2.1.2 The enthalpy of a kilogram of steam which has a pressure of 1 ,5 (3)
MPa, a temperature of 396 ºC and a specific heat capacity of 2,1
kJ/kg.K.
2.1.3 The dryness fraction of wet steam which has an enthalpy of 2 650 (3)
kJ/kg at a pressure of 1,3 MPa.
2.1.4 The heat required to change 2,3 kg of water from a temperature of (3)
31 ºC to dry saturated steam at a pressure of 1 000 kPa.
2.2 Name THREE apparatus used to determine the dryness fraction of steam. (3)
2.3 A boiler plant supplies 5 600 kg of wet steam every hour at a pressure of (5)
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800 kPa. The steam is 9,5% wet and the supply water enters the boiler at
29 ºC. The boiler consumes 11 kg of coal every minute, which has a heat
value of 35 MJ/kg.
Calculate the efficiency of the boiler.
[20]
QUESTION 3:
1 kg of fuel oil which has the formula C12 H4 completely burnt with the aid
of 40% excess air. The atomic mass of carbon, oxygen and hydrogen is
12,16 and 1, respectively.
Calculate the following:
3.1 The theoretical mass. of air required to burn the fuel (8)
3.2 The actual mass of air used to burn the fuel (2)
3.3 The mass of each product of combustion (6)
3.4 The percentage analysis, by mass, of the combustion products (4)
[20]
QUESTION 4:
A single cylinder, single-acting compressor takes in 56,4 m3 of air every
hour. The air is delivered at a Pressure of 900 kPa after it is received at a
pressure of 103 kPa and 22 ºC. The compressor does not have a
clearance volume and the law of compression is PV1,35 = C. The stroke to
bore ratio is 1,6 to 1 and the speed is 350 r/min.
The electric motor experiences a power loss of 10% and the compressor
experiences a power loss of 12%. Air has a specific heat capacity of
0,287 kJ/kg.K.
Calculate the following:
4.1 The indicated power of the compressor (4)
4.2 The bore diameter in mm (8)
4.3 The stroke length in mm (2)
4.4 The power rating of the motor used to drive the compressor in kW (6)
[20]
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QUESTION 5:
Name TWO types of compressors. (2)
5.1 State Boyle's law. (4)
5.2 State Charles' law. (5)
5.3 A constant volume process was used to heat 3,3 kg of gas from a
temperature of 21 ºC and 0,87 m3 to a temperature of 137 ºC. The
specific heat capacity of the gas at constant volume is 0,718 kJ/kg. and
the gas constant is 0,289 kJ/kg.K.
Calculate the following:
5.3.1 The quantity of heat transferred in kJ (3)
5.3.2 The final pressure of the gas after heating (6)
[20]
TOTAL: 100
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Marking Guidelines
AUGUST 2014
NATIONAL CERTIFICATE
POWER MACHINES N5
(8190035)
5 August 2014 (Y-Paper)
13:00 – 16:00
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TaPbalesotf CExamination Papers
NOVEMBER 2012
NATIONAL CERTIFICATE
POWER MACHINES N5
(8190035)
15 November 2012 (X-Paper)
09:00 – 12:00
REQUIREMENTS:
Steam Tables (BOE 173)
Superheated Steam Tables (Appendix to BOE 173)
Calculators may be used.
Candidates will require drawing instruments, pens and a
ruler.
This question paper consists of 6 pages and a 3-page formula sheet.
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Power Machines N5
DEPARTMENT OF HIGHER EDUCATION AND TRAINING
REPUBLIC OF SOUTH AFRICA
NATIONAL CERTIFICATE
POWER MACHINES N5
TIME: 3 HOURS
MARKS: 100
__________________________________________________________________
INSTRUCTIONS AND INFORMATION
1. Answer ALL the questions.
2. Read ALL the questions carefully
3. Number the answers correctly according to the numbering system used in this
question paper.
4. Write neatly and legibly.
___________________________________________________________________
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QUESTION 1: (2)
1.1 What is meant by the specific heat capacity of an object? (2)
1.2 What is the purpose of governors? (2)
1.3 Name TWO types of steam turbines.
1.4 What is the purpose of an Orsat apparatus? (2)
1.5 Name TWO types of steam boilers. (2)
[10]
QUESTION 2:
0,6 kg of a gas is at a pressure of 1 ,5 MPa and a temperature of 300 ºC.
This gas is expanded reversibly and politropically to a pressure of 150
kPa. The following data is applicable to this gas:
- Index of expansion = 1,24
- Gas constant= 0,287 kJ/kg.K
- Specific heat capacity at constant pressure= 1,005 kJ/kg.K
Calculate the following:
2.1 The temperature of the gas after expansion (3)
2.2 The work done (5)
2.3 The heat flow (4)
2.4 The change in entropy during the expansion for 1 kg of steam (3)
[15]
QUESTION 3:
4 637,5 kJ of heat is dissipated from 1,75 kilograms of wet steam which is
at a pressure of 2 000 kPa. The supply water which is used to form the
steam enters the evaporator at 26,5 ºC. The wet steam is then
superheated to 350 °C. The specific heat capacity of the water is 4 187
J/kg. K.
Calculate the following:
3.1 The dryness fraction of the wet steam (7)
3.2 The change in enthalpy from wet steam to superheated steam for 1 kg of (3)
steam
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3.3 The increase in the specific volume of the steam for 1 kg of steam (5)
[15]
QUESTION 4:
10 800 kg of steam is generated every hour in a steam plant which
consists of an evaporator, superheater and an economiser. The feed water
enters the economiser at 29 ºC and the evaporator at 72 ºC. The steam
which leaves the evaporator is 50% wet. The pressure and temperature of
the superheated steam is 3 000 kPa and 250 ºC. Coal, which has a heat
value of 35 MJ/kg, is used at a rate of 15 kg/min.
Calculate the following for 1 kg of fuel:
4.1 The quantity of heat transferred to the water in the economiser (4)
4.2 The quantity of heat transferred to the water in the evaporator (4)
4.3 The quantity of heat transferred to the water in the superheater (3)
4.4 The efficiency of the steam plant (2)
4.5 The equivalent evaporation from and at 100 ºC (2)
[15]
QUESTION 5:
5.1 A certain sample of fuel contains the following elements by mass:
- Oxygen 3,7%
- Hydrogen 9,2%
- Carbon 84,6%
- Sulphur 2,5%
The calorific value of sulphur, carbon and hydrogen is 9,5 MJ/kg, 35
MJ/kg and 145 MJ/kg respectively. The partial pressure of the steam is
6,5 kPa in the exhaust gases.
Calculate the following:
5.1.1 The higher calorific value of the fuel (4)
5.1.2 The lower calorific value of the fuel (4)
5.2 A fuel was tested, using a bomb calorimeter, and the following data was
recorded:
- Mass of fuel used 0,73 g
- Water equivalent of the bomb 50 g
- Mass of the water in the calorimeter
2,5 kg
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- Minimum temperature of the water 24,767ºC
- Maximum temperature of the water 28,482ºC
- The quantity of hydrogen in the fuel 15,4%
- The partial pressure of the steam in the exhaust gases 5,5 kPa
- The specific heat capacity of water 4,187 kJ/kg.ºC
Calculate the following:
5.2.1 The higher calorific value of the fuel in MJ (4)
5.2.2 The lower calorific value of the fuel in MJ (3)
[15]
QUESTION 6:
0,04 m% of air is drawn into a single-acting, single-cylinder, reciprocating
air compressor, at a pressure of 105 kPa. The air is then delivered to an
air receiver at a pressure of 625 kPa, according to the law PV1,32 = C. The
compressor has a mechanical efficiency of 82% and a motor transmission
efficiency of 88%. The stroke to bore ratio is 1,4 to 1 and the compressor
runs at 8 r/s.
Calculate the following:
6.1 The diameter of the piston (6)
6.2 The indicated power (3)
6.3 The shaft power (3)
6.4 The power of the electric motor required to drive the compressor (3)
[15]
QUESTION 7:
The nozzles of an impulse turbine supply 5 kg of gas at an angle of 19°,
every second. The gas leaves the nozzle at a speed of 1 000 m/s. The
mean blade speed is 475 m/s and the outlet angle of the moving blades is
23°. There is a 9,5% frictional loss in the relative part of the blades.
7.1 Use a scale of 1 mm = 5 m/s and construct a velocity diagram in the (5)
ANSWER BOOK (landscape) and insert ALL the values (m/s) on the
diagram.
7.2 Determine the following from the diagram:
7.2.1 The inlet angle of the moving blade (2)
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7.2.2 The exit angle of the gas Power Machines N5
7.2.3 The driving force on the wheel in kN
7.2.4 The axial thrust on the wheel in N (2)
7.2.5 The power developed by the turbine in MW (2)
(2)
(2)
[15]
TOTAL: 100
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Marking Guidelines
NOVEMBER 2012
NATIONAL CERTIFICATE
POWER MACHINES N5
(8190035)
15 November 2012 (X-Paper)
09:00 – 12:00
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