SECTION A
Answer all questions, this Section carries 40 Marks
QUESTION 1
a) With the aid of well labelled sketch and appropriate equations where necessary, explain the
working principles of the following auxiliary equipment
i)
Steam traps,
[2 marks]
ii)
Quick Start exhauster,
[3 marks]
iii)
Condenser flash Box and
[3 marks]
iv)
Cooling tower.
[3 marks]
b) Explain three main factors affecting the economics in power plants in the following key areas
of decision making for new and existing plants,
i) Plant selection,
[3 marks]
ii) Generation of power and
[3 marks]
iii) Distribution of power.
[3 marks]
c) The following table 1.1 gives scaled average readings data of a stage 2 boiler taken for a period
of 3 months for Hwange Power station unit 6 boiler.
Item
Value
Feed water rate
1400kg/hr at 25ºC
Steam generated
15Bar at 300ºC
Dryness fraction
0.89
Coal used
100kg/hr
CV of coal
25MJ/kg
Ash and unburnt coal collected from beneath the
7.25kg/hr
grate
CV of unburnt fuel
Flue gasses formed/kg fuel
2MJ/kg
17.3kg/kg
Flue gas temperature
325ºC
Temperature of air
16ºC
Cp of flue gas
1.025kg/kgK
Determine:
i) Equivalence Evaporation
[2 marks]
ii) Heat balance in a minute bass,
[4 marks]
iii) Draught produced in mmHg
[2 marks]
iv) Comment on the performance of the plant assuming all boilers are represented.
[2 marks]
d) Power plant chemistry is an important section in power plants, the operation, verification and
continued operation of plants entirely rests within the department. Various tests are carried out
to ensure continued generation and reliability of these station. In each of the following key
activities done by the department, explain any three items that are covered by each of the
following.
i) Coal tests
[3 marks]
ii) Water tests.
[3 marks]
e) A nozzle of an impulse turbine of 1MW of capacity has steam entering at 20bar, 300⁰C
(superheated states) and steam consumption is 8kg/kWh. Steam leaves at 0.2 bar and 10% of
heat drop is lost in overcoming friction in diverging portion of the nozzle. If throat diameter of
each nozzle is 1 cm. Find
i) Number of nozzles required
[2 marks]
ii) Exit diameter of each nozzle, taking n as 1.3.
[2 marks]
SECTION B (TOTAL 60MARKS)
ANSWER ANY 3 QUESTIONS FROM THIS SECTION
QUESTION 2
A. Explain in brief how each of the following factors affect the efficient operation of a power
system network interlocked to a power pool
i) under-voltage
[2 marks]
ii) reactors
[2 marks]
iii) malfunctioning breakers
[2 marks]
iv) limited protection
[2 marks]
B. Describe in brief the methods used to classify power plant boilers.
[6 marks]
C. Air at temperature of 15ºC passes through a heat exchanger at velocity of 30m/s were its
temperature is raised to 800ᵒC. it then enters a turbine with the same velocity of 30m/s and
expands until the temperature falls to 650C on leaving the turbine, the air is taken at 60m/s to a
nozzle where it expands until temperature falls to 500ᵒC. if the air flow rate is 2kg/s. Taking
specific enthalpy of air, h = CpT, Cp =1.005kJ/kgK
Calculate:
i) Rate of heat transfer to air
[2 marks]
ii) Power output from turbine assume no heat loss
[2 marks]
iii) Velocity of air at exit from nozzle,
[2 marks]
QUESTION 3
A. Explain the following as used in Power Plant Engineering.
i) Fourier law of heat conduction
[3 marks]
ii) Newton’s law of cooling
[3 marks]
iii) Stephan-Boltzmann’s constant of radiative heat
[3 marks]
b. Table 3.1 shows values for scaled monthly averaged values obtained on a log sheet
completed by operators during the month of July 2014 on a vacuum condenser at Munyati
Power Station TA7.
Table 3.1: Condenser Readings scaled for a month.
Parameter
Value
Recorded vacuum
11cm Hg
Barometer reading
76.5cm of Hg
Mean condenser temperature
34ºC
Condensate collected
1800kg/hr
Mass of cooling water
57700kg/hr
Cooling water temperature rise
17.5ºC
Cooling water inlet temperature
8.5ºC
Calculate:
i) Vacuum efficiency
[3 marks]
ii) Condenser efficiency
[3 marks]
iii) State of steam entering the condenser and
[3 marks]
iv) Hence Comment on the results obtained
[2 marks]
QUESTION 4
A. With the aid of well labelled T-s diagrams and necessary equations where necessary, show
how the efficiency of the Rankine Cycle is:
i) Increased
[2 marks]
ii) decreased.
[2 marks]
B. Describe with aid of a sketch and necessary equations where the main components of the
following types of boilers’
i) Fluidised bed,
[4 marks]
ii) Pulverised fuel and
[4 marks]
iii) Chain grate.
[4 marks]
C. Boiler trial at Bara, Soweto Municipal Boiler gave the following results presented in table
Q4.1. over a period of one year. The, dry fuel as burnt contained 84% C and 3 % free
hydrogen. The flue gas analysis by volume gave CO2 -11.5%, Oxygen -8.4% and Nitrogen
80.1%.
Table Q4.1: Flue Gas Analysis and Fuel Burnt
Parameter
Value(%)
Dry Fuel Burnt: Carbon, C
84
Free hydrogen, H
3
Flue Gas as analysed by volume:
CO2
11.5
O2
8.4
N,
80.1
Calculate per kg of dry fuel the weight of air supplied and the weight of excess air.
[4 marks]
QUESTION 5
A) Briefly explain the effects of the following terms in the combustion of fossil fuels in
balanced draught boiler systems.
i) Turbulence
[2 marks]
ii) Time
[2 marks]
iii) Temperature
[2 marks]
B) Explain the difference between direct contact and surface contact feed water heaters with
the aid of sketch showing clearly the flow around the feed water heater.
[2 marks]
C) The velocity of steam leaving the nozzle of an impulse turbine is 900m/s and nozzle angle
is 20, the blade velocity is 300m/s with a blade velocity coefficient of 0.7. Calculate for
unit mass flow rate and symmetry blading
i) Blade angle at inlet
[3 marks]
ii) Driving force on the wheel,
[3 marks]
iii) Axial thrust and
[3 marks]
iv) Power developed
[3 marks]
*****************End of main exam @ 2016 November**********************