Refrigerator Troubleshooting Diagram

Refrigerator Dry Expansion Circuit Schematic

Oil bleed and rectifier for R.22 flooded evaporator

Many of the new, alternative refrigerants are ‘blends’, which have two or three components, developed for existing and new plants as comparable alternatives to the refrigerants being replaced. They are ‘zeotropes’ with varying evaporating or condensing temperatures in the latent heat of vaporization phase, referred to as the ‘temperature glide’. Figure 3.3 shows the variation in evaporating and condensing temperatures.

To compare the performance between single component refrigerants and blends it will be necessary to specify the evaporating temperature of the blend to point A on the diagram and the condensing temperature to point B.

The temperature glide can be used to advantage in improving plant performance, by correct design of the heat exchangers. A problem associated with blends is that refrigerant leakage results in a change in the component concentration of the refrigerant. However, tests indicate that small changes in concentration (say less than 10%) have a negligible effect on plant performance.

The following recommendations apply to the use of blends:

• The plant must always be charged with liquid refrigerant, or the component concentrations will shift.
• Since most blends contain at least one flammable component, the entry of air into the system must be avoided.
• Blends which have a large temperature glide, greater than 5K, should not be used for flooded-type evaporators.

It will be useful to remind ourselves of the requirements for a fluid used as a refrigerant.

• A high density of suction gas
• Non-corrosive, non-toxic and non-flammable
• Critical temperature and triple point outside the working range
• Compatibility with component materials and lubricating oil
• Reasonable working pressures (not too high, or below atmospheric pressure)
• High dielectric strength (for compressors with integral motors)
• Low cost
• Ease of leak detection
• Environmentally friendly

No single fluid has all these properties, and meets the new environmental requirements, but this chapter will show the developments that are taking place in influencing the selection and choice of a refrigerant.

The passage of an electric current through junctions of dissimilar metals causes a fall in temperature at one junction and a rise at the other, the Peltier effect. Improvements in this method of cooling have been made possible in recent years by the production of suitable semiconductors. Applications are limited in size, owing to the high electric currents required, and practical uses are small cooling systems for military, aerospace and laboratory use (Figure 2.13).

Any gas, when compressed, rises in temperature. Conversely, if it is made to do work while expanding, the temperature will drop. Use is made of the sensible heat only (although it is, of course, the basis of the air liquefaction process).

The main application for this cycle is the air-conditioning and pressurization of aircraft. The turbines used for compression and expansion turn at very high speeds to obtain the necessary pressure ratios and, consequently, are noisy. The COP is lower than with other systems.

The normal cycle uses the expansion of the air to drive the first stage of compression, so reclaiming some of the input energy (Figure 2.12).

The low pressures (8–22 mbar) required to evaporate water as a refrigerant at 4–7°C for air-conditioning duty can be obtained with a steam ejector. High-pressure steam at 10 bar is commonly used. The COP of this cycle is somewhat less than with the absorption system, so its use is restricted to applications where large volumes of steam are available when required (large, steam-driven ships) or where water is to be removed along with cooling, as in freeze-drying and fruit juice concentration.

Vapour can be withdrawn from an evaporator by absorption (Figure 2.11) into a liquid. Two combinations are in use, the absorption of ammonia gas into water and the absorption of water vapour into lithium bromide. The latter is non-toxic and so may be used for airconditioning. The use of water as the refrigerant in this combination restricts it to systems above its freezing point. Refrigerant vapour from the evaporator is drawn into the absorber by the liquid absorbant, which is sprayed into the chamber. The resulting solution (or liquor) is then pumped up to condenser pressure and the vapour is driven off in the generator by direct heating. The high-pressure refrigerant gas given off can then be condensed in the usual way and passed back through the expansion valve into the evaporator. Weak liquor from the generator is passed through another pressure reducing valve to the absorber. Overall thermal efficiency is improved by a heat exchanger between the two liquor paths and a suction-toliquid heat exchanger for the refrigerant. Power to the liquor pump will usually be electric, but the heat energy to the generator may be any form of low-grade energy such as oil, gas, hot water or steam. (Solar radiation can also be used.) The overall energy used is greater than with the compression cycle, so the COP (coefficient of performance) is lower. Typical figures are as shown in Table 2.2.

The absorption system can be used to advantage where there is a cheap source of low-grade heat or where there are severe limits to the electrical power available. A modified system of the ammonia–water absorption cycle has been developed for small domestic refrigerators.

Some volatile fluids are used once only, and then escape into the atmosphere. Two of these are in general use, carbon dioxide and nitrogen. Both are stored as liquids under a combination of pressure and low temperature and then released when the cooling effect is required. Carbon dioxide is below its critical point at atmospheric pressure and can only exist as ‘snow’ or a gas. Since both gases come from the atmosphere, there is no pollution hazard. The temperature of carbon dioxide when released will be – 78.4°C. Nitrogen will be at – 198.8°C. Water ice can also be classified as a total loss refrigerant.

The requirements for the working fluid are as follows:
1. A high latent heat of vaporization
2. High density of suction gas
3. Non-corrosive, non-toxic and non-flammable
4. Critical temperature and triple point outside the working range
5. Compatibility with materials of construction, with lubricating oils, and with other materials present in the system
6. Convenient working pressures, i.e. not too high and preferably not below atmospheric pressure
7. High dielectric strength (for compressors having integral electric motors)
8. Low cost
9. Ease of leak detection
10. Environmentally friendly

No single working fluid has all these properties and a great many different chemicals have been used over the years. The present situation has been dominated by the need for fluids which are environmentally friendly.