Page 94 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

For an isothermal process

Work input,

1
W = mRTln
2
1.01
= 0.4 x 0.260 x (22 + 273)
5.5
= 52 kJ

In an isothermal process all the work input is rejected as heat. Therefore, heat rejected,

Q = W = 52 kJ

Example 3.2

o
In a thermally insulated reciprocating air compressor, air at 0.98 bar and 20 C is

compressed into one sixth of its original volume. Determine the pressure and

temperature of the air after compression. If the compressor cylinder contains 0.05 kg

of air, calculate the required work input. For air, take  = 1.4 and cv = 0.718 kJ/kgK.

Solution to Example 3.2

Data : p1 = 0.98 bar; T1= 20 + 273 = 293 K

V 1
2 = ; m = 0.05 kg; W = ?
V 1 6

As the cylinder is well insulated the heat transfer is negligible and the process may be

treated as adiabatic.

Considering air as a perfect gas

p V  
From equation 3.6, 2 =  1 
V
p 1  2 
p2 = 0.98 x 6 1.4

= 12 bar

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