Mumbai University > Mechanical Engineering > Sem8 > Refrigeration and Air Conditioning

Marks: 5M

Year: Dec 2012

The temperature of air and other gases can be reduced by expansion in an adiabatic process. In this case there is no change of phase of gas involved. According to first law of thermodynamics, Q= W+ ΔU

Since, Q=0 (process is adiabatic)

Therefore, W= $- ΔU= mC_v (T_1-T_2)$

As work done by system is positive, and ΔU= f(T) = f(T2)- f(T1), therefore -ΔU means T2 is less than T1.

Hence, due to expansion work the temperature of gas is reduced from $T_1$ to $T_2$. The relation between temperature and pressure is given by,

$$P∝T$$

So if air is expanded from pressure p1 to p2 then the reduced temperature achieved i.e.T2, is below atmospheric temperature in many cases, depending upon the expansion pressure range. Therefore, very low temperature required for producing refrigerating effect or cooling can be obtained by expansion of gas. This concept is used in air refrigeration cycles.

When a gas passes through a restriction such as capillary tube or restricted valve, its pressure decreases while enthalpy remains constant. Such a process is similar to expansion process and is called throttling. And therefore, this process can also be used for refrigeration.