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A refrigeration system using NH_3 works on standard vapour compression cycle.The condensing temperature

is 30°C. investigate the performance of the system based on:-

i)COP

ii)Mass of refrigerant circulated

iii)Volume of refrigerant handled by compressor

iv)Temperature of refrigerant at compressor delivery. The system operates at temperature in the evaporator of -10°C and 16°C respectively. Capacity of the system is 10TR. Use properties of saturated NH3 as:

enter image description here

Marks: 12M,

Year: Dec2014, Dec2015, Dec2013, May2014

Difficulty: Medium

1 Answer
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T_E=T_{sat_{Pe}}=-10℃,T_C=T_{sat_{Pc}}=30℃

Capacity = 10TR

Q ̇_a=3.5×10=35 kJ/sec

enter image description here

Assuming that superheating of refrigerant at inlet to compressor takes place in evaporator and adds to refrigeration effect.

s1=s2

s_{g_{-10℃}}+Cpg×ln(\frac{Tsup}{Tsat})=s_{g_{30℃}}+Cpg×ln(\frac{Tsup}{Tsa}t)

5.5789+2.8×ln⁡(\frac{16+273}{-10+273})=4.9842+2.8×ln⁡(\frac{T2}{30+273})

T2=411.74 K=138.74 °C

h1=h_{g_{-10℃}}+Cpg(Tsup-Tsat)

=1426.6+2.8(16-(-10))

h1=1499.4 kJ/kg

h3=h_{f_{30℃}}=323.1 kJ/kg

h3=h4=323.1 kJ/kg

h2=h_{g_{30℃}}+Cpg(Tsup-Tsat)

=1468.9+2.8(138.74-30)

=1773.372 \frac{kJ}{kg}

COP of given system = \frac{R.E.}{Wc}=\frac{h1-h4}{h2-h1}

COP=4.294 …

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