Page 100 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

The mass of air can be found using equation pV = mRT

p v . 1 02 x10 5 x 015
. 0
 m = 1 1 = = . 0 018 kg
RT 1 . 0 287 x10 3 195

i.e. Total work done = 0.0181 x 152.8 = 2.76 kJ

n
c) Polytropic process (pV = C)

This is the most general type of process, in which both heat energy and work energy cross

the boundary of the system. It is represented by an equation in the form

n
pV = constant (3.13)

If a compression or expansion is performed slowly, and if the piston cylinder assembly is

cooled perfectly, then the process will be isothermal. In this case the index n = 1.

If a compression or expansion is performed rapidly, and if the piston cylinder assembly is

perfectly insulated, then the process will be adiabatic. In this case the index n =  .

If a compression or expansion is performed at moderate speed, and if the piston cylinder
assembly is cooled to some degree, then the process is somewhere between those

discussed above. Generally, this is the situation in many engineering applications. In this

case the index n should take some value, which is between 1 and  depending on the

degree of cooling.

Some practical examples include:

compression in a stationary air compressor (n = 1.3)

compression in an air compressor cooled by a fan (n = 1.2)

compression in a water cooled air compressor (n = 1.1)

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