REFRIGERATION CYCLES 775
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- An ammonia vapour-compression refrigerator operates between an evaporator pressure of 2.077 bar and
a condenser pressure of 12.37 bar. The following cycles are to be compared ; in each case there is no
undercooling in the condenser, and isentropic compression may be assumed :
(i) The vapour has a dryness fraction of 0.9 at entry to the compressor.
(ii) The vapour is dry saturated at entry to the compressor.
(iii) The vapour has 5 K of superheat at entry to the compressor.
In each case calculate the C.O.P.(ref.) and the refrigerating effect per kg.
What would be the C.O.P.(ref.) of a reversed Carnot cycle operating between the same saturation tempera-
tures? [Ans. 4.5 ; 957.5 kJ/kg ; 4.13 ; 1089.9 kJ/kg ; 4.1 ; 1101.4 kJ/kg] - A refrigerator using Freon-12 operates between saturation temperatures of – 10°C and 60°C, at which
temperatures the latent heats are 156.32 kJ/kg and 113.52 kJ/kg respectively. The refrigerant is dry
saturated at entry to the compressor and the liquid is not undercooled in the condenser. The specific heat
of liquid freon is 0.970 kJ/kg K and that of the superheated freon vapour is 0.865 kJ/kg K. The vapour is
compressed isentropically in the compressor. Using no other information than that given, calculate the
temperature at the compressor delivery, and the refrigerating effect per kg of Freon.
[Ans. 69.6°C ; 88.42 kJ/kg] - A heat pump using ammonia as the refrigerant operates between saturation temperatures of 6°C and
38°C. The refrigerant is compressed isentropically from dry saturation and there is 6 K of undercooling in
the condenser. Calculate :
(i) C.O.P.(heat pump) (ii) The mass flow of refrigerant
(iii) The heat available per kilowatt input. [Ans. 8.8 ; 25.06 kg/h ; 8.8 kW] - An ammonia vapour-compression refrigerator has a single-stage, single-acting reciprocating compressor
which has a bore of 127 mm, a stroke of 152 mm and a speed of 240 r.p.m. The pressure in the evaporator
is 1.588 bar and that in the condenser is 13.89 bar. The volumetric efficiency of the compressor is 80% and
its mechanical efficiency is 90%. The vapour is dry saturated on leaving the evaporator and the liquid leaves
the condenser at 32°C. Calculate the mass flow of refrigerant, the refrigerating effect, and the power
ideally required to drive the compressor. [Ans. 0.502 kg/min ; 9.04 kW ; 2.73 kW] - An ammonia refrigerator operates between evaporating and condensing temperatures of – 16°C and 50°C
respectively. The vapour is dry saturated at the compressor inlet, the compression process is isentropic and
there is no undercooling of the condensate.
Calculate :
(i) The refrigerating effect per kg,
(ii) The mass flow and power input per kW of refrigeration, and
(iii) The C.O.P.(ref.).[Ans. 1003.4 kJ/kg ; 3.59 kg/h ; 0.338 kW ; 2.96] - 30 tonnes of ice from and at 0°C is produced in a day of 24 hours by an ammonia refrigerator. The
temperature range in the compressor is from 298 K to 258 K. The vapour is dry saturated at the end of
compression and expansion valve is used. Assume a co-efficient of performance of 60% of the theoretical
and calculate the power in kW required to drive the compressor. Latent heat of ice is 334.72 kJ/kg.
Temp. Enthalpy Entropy of liquid Entropy of vapour
K kJ/kg kJ/kg K kJ/kg
Liquid Vapour
298 100.04 1319.22 0.3473 4.4852
258 – 54.56 1304.99 – 2.1338 5.0585
[Ans. 21.59 kW]- A refrigerant plant works between temperature limits of – 5°C (in the evaporator) and 25°C (in the
condenser). The working fluid ammonia has a dryness fraction of 0.6 at entry to the compressor. If the
machine has a relative efficiency of 50%, calculate the amount of ice formed during a period of 24 hours.