THE VAPOUR COMPRESSION REFRIGERATION CYCLE BASICS AND TUTORIALS


Almost all domestic and industrial refrigeration plants are of the vapour compression type in which work input is required. In the vapour absorption cycle there is, instead, heat energy input. We are looking here only at the vapour-compression cycle.

In this cycle, heat energy is extracted from the low temperature area by evaporation of a fluid (refrigerant), requiring latent heat, and rejection to the higher temperature area by condensation of the vapour.

It is therefore necessary to use refrigerants which will evaporate at low temperatures, i.e. have low boiling points or saturation temperature, because we are making use of the heat demanded by evaporation to produce the refrigeration effect. Choice of refrigerant is also determined by considerations such as corrosivity, inflammability and ease of leak detection.

By increasing the operating pressure, the saturation temperature is raised, and in the case of carbon dioxide plant where the boiling point is about –78°C, a high pressure system is required in order to achieve evaporation at temperatures more usually required in refrigeration plant.

This represents a major disadvantage to its use. The main refrigerants are:

Tetrafluoroethane (CH2F–CF3) Refrigerant 134a
Freon (CF2Cl2)
Ammonia (NH3)
Methyl chloride (CH3Cl)
Carbon dioxide (CO2)
Freon (a trade name) is a CFC, and therefore is being phased out because of environmental concerns.

Figure 2.7.3 shows a diagrammatic arrangement of items in the basic cycle, and Figure 2.7.4 gives an idea of the actual plant. Starting at point 1, vapour is drawn into the compressor from the low pressure side and compressed to form, usually, a dry or superheated vapour.

The vapour passes through the condenser coils where heat energy is extracted by air circulation (e.g. domestic refrigerator) or by circulating water around the coils (e.g. some industrial and marine plant), to produce a saturated or sub-cooled liquid at point 3.

The compressed liquid is then expanded through a regulating valve (throttle), or expansion valve, to form a very wet vapour at 4. Because this is a throttling process, from the SFEE, the enthalpy before and after the expansion is the same.

The wet vapour passes through the evaporator coils where it absorbs heat energy from the warmer surroundings. In so doing, the vapour becomes drier, i.e. its dryness fraction increases as the latent heat energy is absorbed.

The evaporator coils are situated around the freezer cabinet in a domestic refrigerator, and in large industrial and marine plants they are arranged in ‘batteries’ with a fan to provide chilled air circulation. Looking at the basic cycle, we can see that there are only two pressures to consider – a high pressure on one side of the compressor and a lower pressure on the other.

It is clear that the mass flow of refrigerant around the circuit is constant at all points. The main refrigerant effect occurs through the evaporator, but because a very wet vapour is produced at the regulating valve (also called the expansion valve), a small refrigeration effect is created, and inspection of the plant would show this pipe ice covered if it was not insulated.

We have quoted the reversed Carnot cycle as the ideal refrigeration cycle. In the practical refrigeration cycle, the major departure from this is that the expansion cannot be isentropic, and in fact occurs by throttling through the expansion valve giving a constant enthalpy process.

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