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Improving the Vapor-Compression Refrigeration Cycle

Process flow diagram for a vapor-compression refrigeration cycle with the condenser and evaporator highlighted.

In the previous lesson, we discussed the importance of selecting the appropriate refrigerant. We stressed the importance of the P*-T relationship. This relationship determines the operating pressures of the condenser and the evaporator.

Recall from Lesson 10B:

The

vapor pressure

of the

refrigerant

at the temperature of the condenser should not be too high, P*(Tcond) < 2 MPa, to keep equipment costs low.

The

vapor pressure

of the

refrigerant

at the temperature of the evaporator should be greater than 1 atm, P*(Tevap) > 1atm, to prevent air leakage into the system.

The combination of these two constraints puts a firm limit on the range of temperatures between which a simple

vapor-compression refrigerator

can operate.

How can we make the cold space even colder (reduce TC) and reject heat at an even higher temperature (increase TH) without dramatically increasing the cost to build the refrigeration system ?

Lesson 10C: page 1 of 9

Previous LESSON

The key to the improvements to the simple vapor-compression refrigeration cycle is the vapor pressure of the refrigerant.
The goal is the keep the operating pressure of the condenser below about 2 MPa and to keep the operating pressure of the evaporator above atmospheric pressure.
Meeting these goals will reduce the cost of construction of the system.
But, these two goals put a tight constraint on how cold we can make the refrigerated space and how hot the high temperature reservoir can be as well.
In this lesson, we will discuss two modifications to the simple V-C cycle that improve the COP while keeping the operating pressures of the evaporator and condenser in their optimal ranges.
Let’s see how they work.

Improving the Vapor-Compression Refrigeration Cycle

vapor pressure

refrigerant

vapor pressure

refrigerant

vapor-compression refrigerator

Ch10, Lesson C, Page 1 - Improving the Vapor-Compression Refrigeration Cycle