The Vapour Compression Cycle

Cycle on p-h format and system

Here are two diagrams of the vapour compression cycle. Try your mouse over the diagrams to commence the activatation of each process in turn. The four processes are Vaporization (Evaporation), Compression, Condensation and Expansion.

The upper diagram shows the refrigeration circuit, and the lower one is the corresponding P-h diagram The process starts with vaporisation of the refrigerant in the evaporator. This is complete at point 2. Compression is used to raise the pressure of the refrigerant, point 3, so that it can condense at a higher temperature. When all the vapour has condensed, point 4, the pressure is reduced in an expansion device, and the refrigerant is returned to its original condition 1.

Notice that Expansion is a constant enthalpy process. It is drawn as a vertical line on the P-h diagram. No heat is absorbed or rejected during this expansion, the liquid just passes through a valve, like water coming out of a tap. The difference is, that because the liquid is saturated at the start of expansion by the end of the process it is partly vapour. Point 1 is inside the curve and not on the curve as described in the Evaporation process. The refrigerant at the beginning of the vaporization is already partly evaporated! How much? This depends on the shape of the curve, and the start and end pressures.

On the other hand the Compression process is shown as a curve. It is not a constant enthalpy process. The energy used to compress the vapour turns into heat, and increases its temperature. This tends to raise the temperature of the vapour, making point 3 move further and further into the superheated part of the diagram as compression progresses. Point 3 is outside the curve and not on the curve as described in the Compression process. This means that before Condensation can start, the vapour has to be cooled down.

There are still a few more tricks to learn before this cycle can be turned into a practical ready-to-go refrigeration machine. Find out what they are by looking at the Simple Practical Cycle.

Maximum Efficiency of the Vapour Compression Cycle
It takes a lot of heat to evaporate liquid. In other words a small amount of liquid circulating in a refrigerator can perform a large amount of cooling. This is one reason why the vapour compression cycle is widely used. The refrigeration system can be small and compact. Also from a practical point of view heat exchange is much better when using change of state - evaporation and condensation. However the expansion of the high pressure liquid, process 4 - 1 above is non reversible. And so the efficiency of this cycle can never even approach Carnot effciency.

Learn more about a Reversible Process and the Carnot Cycle

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