SOLUTIONS HW9
1.
The air conditioner in a car uses R134a and the compressor power input is 2 kW,
bringing the R134a from 165kPa to 1200kPa by compression. The cool space is a
heat exchanger that cools 35°C atmospheric air from the outside down to 15°C and
blows it into the car. What is the mass flow rate of the refrigerant and what is the
low temperature heat transfer rate? Also, find the mass flow rate of air at 15°C.
P
1
= P
4
= 165 kPa
P
2
= P
3
= 1200kPa.
The fluid that enters the turbine is saturated vapor. Hence s1 = sg and h
1
= h
g
s
1
=
s
2
=
1.7354 kJ/kgK
We know P and s at state 2. Hence we get h
2
= 430.7 kJ/kg (interpolate.)
The fluid that leaves the condenser is saturated liquid.
Hence h
3
= h
f
= 266 kJ/kg (Interpolation)
Isenthalpic throttling. Hence h
4
= h
3
Hence Wc = m (h
2
 h
1
)
So, mass flow rate m = 2/ (430.7389.2) =
0.048 kg/s
Q
L
= m (h
1
– h
4
) = 0.048(389.2266.05) = 5.94 kW
But Q
L
is also mair*Cp*(Tout – Tin)
Hence mass of air = 5.94/(1.004*20) =
0.2949 kg/s
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In an actual refrigeration cycle using R134a as the working fluid, the flow rate is
0.02kg/s. Vapor enters the compressor at 100kPa and 10°C and leaves at 1MPa
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 Spring '09
 KATZ
 Diesel engine, Mass flow rate, compression ratio, Pmeff

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