Subject : Refrigeration and Air Conditioning

Topic : Duct Design, Controls

Difficulty : Medium

The natural gases must be liquefied in order to produce low temperature for refrigeration purposes. the following methods of expansion of gases may be used for producing the low temperature of gas or to produce refrigeration.

The principle of the Joule's Thompson effect is a shown in Fig 1. Its fundamental principle is that of using internal work to cool a gas. this work results from the fact that the molecules separate further as the gas expands. in separating, they are moving against the Vander Waal's force of attraction between them; the energy for this motion is obtained at the expense of their kinetic energy. the loss in kinetic energy results in a decrease in internal energy of the gas and a drop in temperature.

But the assumption is the molecules, are dimensionless points, disregards their effect volumes . the effect of the volume of the molecules is to cause the gas to heat instead of cooling during expansion thus the net temperature effect of the expansion. thus the net temperature effect if the expansion is the result of two processes, one of that tends to cool the gas and another that tends to heat it. since the changes in temperature in the two processes are not the same, there is inversion temperature at which the effect is zero. below this temperature the gas cools on expansion while above inversion temperature it gets heated.

Fig.1: liquefaction process of natural gases.