(a) A mixture of air and water vapour at 1.01325 bar 16 0C has dew point of 5 0C. Calculate the relative humidity and specific humidity. 20
(b) A refrigerating plant of 10 kW capacity operates in the quasiideal vapour compression cycle. The refrigerant is R 134 a. The saturation temperatures in the evaporator and condenser are respectively –20 0C and 40 0C. The plant includes a heat exchanger in which the saturated liquid leaving the condenser is subcooled to 30 0C by the saturated vapour leaving the evaporator. The subcooled liquid is then throttled to –20 0C before entering the evaporator and the vapour leaving the heat exchanger is compressed isentropically to the condenser pressure. Sketch the schematic diagram and the cycle on p-h and T-s diagrams, numbering the important point. Calculate the mass flow rate of the refrigerant and power to be supplied to the compressor. The mechanical efficiency of the compressor may be assumed 80%.
The compressor used by the plant is a single-stage, single-acting reciprocating compressor having stroke-diameter ratio of 1.2 and a speed of 240 rev/min. Assuming compressor volumetric efficiency to be 80% determine the diameter and stroke of the compressor.
Properties of refrigerant may be obtained from table below.
|
t
|
Ps
|
vg
|
Enthalpy kJ/kg
|
Entropy kJ/kg-K
|
Specific heat kJ/kg-K
|
|
0C
|
bar
|
m3kg
|
Liquid
|
Vapour
|
Liquid
|
Vapour
|
Liquid
|
Vapour
|
|
–20
|
1.3268
|
0.1474
|
173.82
|
386.66
|
0.9009
|
1.7417
|
1.282
|
0.805
|
|
40
|
10.165
|
0.0199
|
256.35
|
419.58
|
1.1903
|
1.7115
|
1.5
|
1.12
|