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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:

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Marks: 12M,

Year: Dec2014, Dec2015, Dec2013, May2014

Difficulty: Medium

1 Answer
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TE=TsatPe=10,TC=TsatPc=30

Capacity = 10TR

Q̇a=3.5×10=35kJ/sec

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

s1=s2

sg10+Cpg×ln(TsupTsat)=sg30+Cpg×ln(TsupTsat)

5.5789+2.8×ln(16+27310+273)=4.9842+2.8×ln(T230+273)

T2=411.74 K=138.74 °C

h1=hg10+Cpg(TsupTsat)

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

h1=1499.4 kJ/kg

h3=hf30=323.1kJ/kg

h3=h4=323.1 kJ/kg

h2=hg30+Cpg(TsupTsat)

=1468.9+2.8(138.74-30)

=1773.372 kJkg

COP of given system = R.E.Wc=h1h4h2h1

COP=4.294

Also, Q̇a=ṁR×(h1h4)

mass flow rate,ṁR=0.02975kgsec

Volume of refrigerant handled by compressor, V1

v1=vg10×TsupTsat=0.4189×289263=0.4603m3kg

V1=ṁR×v1=0.01369m3s=0.8216m3min

Temperature of refrigerant at compressor delivery = T2=138.74°C

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