Lesson 6 - Refrigeration Theory and Cycle

To keep the refrigeration plant operating as efficiently as possible, we should follow a few rules:

• Keep the evaporation temperature as high as possible. Why, you may ask? Because the higher the evaporation temperature, the higher the evaporation pressure, which also increases the gas density. The increase in gas density allows a greater capacity of the compressor that follows the evaporator. (A compressor is a constant volume machine. Therefore, at a higher inlet pressure, each cylinder of the compressor will contain a greater mass of gas). This has the profound effect of increasing the output of the refrigeration plant. However, there is a slight increase in compressor input power; this is of minor significance when compared with the improvements in output.

• Keep the condensation temperature as low as possible. Why? Because the lower the condensation temperature, the more heat we have removed, and the cooler the refrigerant becomes. Lowering the condenser pressure will inherently demand less work from the condenser, and therefore less energy is needed.

• A couple of other points to be considered:

  • Subcooling the refrigerant at the condenser exit will move points 3 & 4 to the left, and increase the cooling load per unit of mass refrigerant flow. Point 4 can still be a mix of liquid and vapour, or just liquid.

  •  A small amount of superheating of the refrigerant at point 1 – the entry to the compressor – is required to prevent damage to the compressor, which might occur if liquid was allowed to enter.

Sometimes, the relationship of these values is called the ‘stretch’ of a compressor and is comparable to the ‘compression ratio’ of automotive engines. A basic rule is that the performance is improved by reducing the ‘stretch.’

As we move towards the end of this lesson, let’s discuss heat pumps and their performance measures.

Heat Pumps

A heat pump uses the exact same technology described in this lesson, however, the goal of a heat pump is the rejection of heat into a fluid. The main difference in discussing and determining the COP of a heat pump is the numerator. Instead of the cooling load rate, we want to measure the heat released from the condenser, Q_c.

Therefore, the COP for a heat pump is the ratio of the heat released from the condenser (Q_c) to the work done by the compressor (Q_w). Mathematically, this looks like:

If you remember, the equation we used for the COP for a refrigeration plant was:

In comparison, the equation we would use for the same system as a heat pump would be written as:

Since we have:

Then by manipulating these equations we can say:

The next lesson will discuss the performance and types of condensers, and the compressors used in refrigeration and heat pump systems.