78HE-1
Sr. No. 10
EXAMINATION OF MARINE ENGINEER OFFICER
HEAT ENGINES
CLASS I
(Time allowed - 3 hours)
INDIA (2001)
Afternoon Paper
N.B. -
Total Marks 100
(1) Attempt SIX questions only.
(2) All questions carry equal marks.
(3) Neatness in handwriting and clarity in expression carries weightage
1.
In a Freon-12 refrigerating plant the compressor takes the refrigerant in at 0.6417 bar and discharges
at 13.66 bar and 70OC. At condenser outlet the Freon is saturated liquid at 13.66 bar without undercooling. The flow of the refrigerant is 15 kg/min. Calculate the refrigerating effect and the coefficient
of performance: (a) when the compression is isentropic,
(b) when the isentropic efficiency of compression is 0.85.
2.
The ratio of compression of an engine working on the constant volume cycle is 9.6: 1. At the
beginning of compression the pressure is 1.013 bar and temperature is 30OC. At the end of heat
supply at constant volume the temperature is 1700OC. Taking index of compression and expansion as
1.4. Calculate: (a) the pressure and temperature at the end of compression,
(b) the pressure at the end of constant volume heat supply,
(c) the pressure and temperature at the end of expansion,
(d) the theoretical thermal efficiency.
3.
The internal volume of a starting air receiver of an auxiliary diesel engine is 15 m 3. When filled it
contains 2.0 m3 of free air at engine room condition of 1.03 bar and ambient temperature of 25OC.
The air is compressed according to the law PVl.3 = constant. Find: (a) the final temperature and pressure of the air in the receiver, neglecting any cooling of air and loss
of heat in the process,
(b) the work done during this compression,
(c) the reduction in pressure in the air receiver, if the contents of the receiver now cool down to the
ambient temperature of engine room.
4.
Steam is expanded isentropically in the first stage of a turbine from 50 bar 450OC to 2.0 bar. It is then
passed through a re-heater and its temperature is raised to 300OC at constant pressure of 2.0 bar. The
steam now passes through the final stage and is expanded isentropically to 0.055 bar. Assume the reheat cycle to be ideal. Calculate the Rankine efficiency. If the isentropic efficiencies in each of the
turbines is 85%, calculate the thermal efficiency of the system.
5.
A waste-heat boiler is placed in the exhaust system of a gas engine which develops 3000KW, using
gas whose lower or net calorific value is 17880 KJ/m3 at 0OC and 1.01325 bar. The residual waste of
combustion is 0.3 kg/m3 and the relative density of the gas is 0.45 referred to air. The overall
efficiency of the engine is 25 per cent and the air-gas volume ratio is 4.2. The boiler is fed with jacket
water as feed at 65OC and generates dry saturated steam at 1.4 bar. The exhaust gases enter the boiler
at 415OC and leave at 150OC.
Calculate: (a) the weight of steam raised per hour,
(b) the percentage of the heat supplied to the engine which is recovered by this means.
The mean specific heat at constant pressure for the dry-exhaust gases may be taken as 1.05 KJ/kgK
and for steam as 2.0 KJ/kgK.
6.
In a single acting air compressor, the clearance volume is 5% of the swept volume. Diameter of the
cylinder is 220 mm, stroke 250 mm, and it runs at 2.5 rev/s. It receives air at 1 bar and delivers it at
5.5 bar, the index of compression and expansion being 1.28. Calculate: (a) the indicated power,
(b) the mean indicated pressure,
(c) the volumetric efficiency,
7.
The wall of a cold room consists of a layer of cork sandwiched between outer and inner walls of
wood, the wood walls being each 32mm thick. The inside atmosphere of the room is maintained at –
22OC when the external atmospheric temperature is 27OC, and the heat loss through the wall is 45
W/m2. Taking the thermal conductivity of wood and cork as 0.2 W/mK and 0.05 W/mK respectively.
The rate of heat transfer between each exposed wood surface and their respective atmospheres as
15W/m2K, calculate: (a) the temperatures of the exposed surfaces,
(b) the temperatures of the interfaces, and
(c) the thickness of the cork.
8.
The following particulars relate to a two-row velocity compounded impulse wheel Steam velocity at
nozzle outlet = 650 m/s
Mean blade velocity = 125 m/s
Nozzle outlet angle = 16O
Outlet angle, first row of moving blades = 18O
Outlet angle, fixed guide blades = 22O
Outlet angle, second row of moving blades = 36O
Steam flow = 2.5 kg/s.
The ratio of the relative velocity at outlet to that at inlet is 0.84 for all blades. Determine the following
(a) the -inlet angles of the first moving row, fixed, and second moving row of blades,
(b) the axial thrust on the blades,
(c) the power developed,
(d) the efficiency of the wheel.
9.
In an open circuit, continuous combustion, constant pressure gas turbine takes in air at 20OC and
compresses it to 6 times the intake pressure in a rotary compressor whose isentropic efficiency is
82%. A heat exchanger then transfers 70% of the heat available for this purpose, from the turbine
exhaust to the compressed air. The compressed air then passes through a combustion chamber at
constant pressure in which its temperature is raised to 900OC. The air is then expanded through a gas
turbine whose isentropic efficiency is 85%. The pressure ratio through the turbine is 6. From the
turbine the air passes through the heat exchanger to exhaust. The turbine is coupled to the
compressor. Neglect the mass of fuel and take = 1.391, CP = 1.006 KJ/kgK upto entry to the turbine.
 = 1.34, CP = 1.01 KJ/kgK through the turbine and heat exchanger to exhaust.
Determine: (a) the thermal efficiency of the arrangement,
(b) the percentage increase in fuel required if the heat exchanger were removed and all other
conditions remained the same.
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78HE-1
Sr. No. 10
EXAMINATION OF MARINE ENGINEER OFFICER
HEAT ENGINES
CLASS I
(Time allowed - 3 hours)
INDIA (2001)
Afternoon Paper
N.B. -
Total Marks 100
(1) Attempt SIX questions only.
(2) All questions carry equal marks.
(3) Neatness in handwriting and clarity in expression carries weightage
Answers
Answer for Question No. 1
(a) 19.145 kJ/s (kw) 1.428
(b) 16.78 kJ/s (kw) 1.0639
Answer for Question No. 2
24.03 bar, temp = 475.80C
63.32 bar
525.80C 2.67 bar
Thermal Efficiency = 59.5%
Answer for Question No. 3
(a) 375.150C 29.37 bar
(b) –793.17 kJ
(c) 15.87 bar
Answer for Question No. 4
37.83%
33.15%
Answer for Question No. 5
(a) 1744.2 kg/h
(b) 9.76%
Answer for Question No. 6
(a) 4.22 kw
(b) 1.78 bar
(c) 86.05%
Answer for Question No. 7
(a) 240C , - 190C
(b) 16.80C , - 11.80C
(c) 31.8 mm
Answer for Question No. 8
(a) 19.70C 38.240C 24.730
(b) 156.1N
(c) 365.234 kw
(d) 69.15%
Answer for Question No. 9
(a) 29.57%
(b) 44.11%