IMD

Compiled by J.S. Crozier, MSAIRAC.

This article describes the general principles of the vapour compression refrigeration cycle and how one refrigerant, R134a for example, can be used to provide the widely disparate temperatures required for both refrigeration and air- conditioning applications. 

The compressor power and displacement for both refrigeration and air- conditioning applications is also touched on.
(Since the article was complied in 1998, as part of an in-company training course, R134a has turned out to be not as environmentally friendly as was first thought and will be discontinued in Europe for new car air- conditioning on 1 January 2011. See EU directive 2006/40/EC. )

Liquid Refrigerants
The ability of liquids to absorb enormous quantities of heat as they vaporise is the basis of the modern vapour compression, mechanical refrigerating system.  When vaporising liquids, the refrigerating effect can be started and stopped at will, the rate of cooling can be predetermined within small limits, and the vaporising temperature of the liquid can be governed by controlling the pressure at which the liquid vaporises.

Moreover, the vapour can be readily collected and condensed back into the liquid state so that the same liquid can be used over and over again to provide a continuous supply of liquid for vaporisation.

Only a relatively few liquids have properties that make them desirable as refrigerants, and most of these have been compounded specially for that purpose.

There is no one refrigerant that is best suited for all the different applications and operating conditions.  For any specific application the refrigerant selected should be the one whose properties most closely fit the particular requirements of the application.

Of all the fluids presently in use as refrigerants, the one that most nearly meets all the qualifications of the ideal general-purpose refrigerant is one called tetrafluoroethane or R134a.  It is used in air-conditioning, cold rooms and medium temperature refrigeration; down to about -18C. 

R134a has a saturation (boiling) temperature of -26, 2 .C at sea level atmospheric pressure.  Because of this R134a can only be stored as a liquid at normal temperatures if kept under pressure in suitably designed steel cylinders.

Vaporising the Refrigerant
An insulated space can be adequately refrigerated by merely allowing liquid refrigerant to vaporise in a container vented to the outside as shown in Fig. 5.  Since the R134a refrigerant is under standard atmospheric pressure, its saturation temperature is -26, 2 .C.  Vaporising at -2 .C this low temperature, the refrigerant readily absorbs heat from the 5C space through the walls of the containing vessel.  The heat absorbed by the vaporising liquid leaves the space in the vapour escaping through the open vent.  Since the temperature of the liquid remains constant during the vaporising process, refrigeration will continue until all the liquid is vaporised.

Any container, such as the one in Fig.1, in which a refrigerant is vaporised during a refrigerating process, is called an evaporator.  An evaporator is one of the essential parts of any mechanical refrigerating system.  

The heat taken in by the refrigerant leaves the space in the vapour escaping through the vent.

Fig.1. The refrigerant liquid vaporises as it takes in heat from the 5˚C space.

Controlling the Vaporising Temperature
The temperature at which the liquid vaporises in the evaporator can be controlled by controlling the pressure of the vapour over the liquid, which in turn is governed by regulating the rate at which the vapour escapes from the evaporator.  For example, if a hand valve is installed in the vent line and the vent is partially closed off so that the vapour cannot escape freely from the evaporator, vapour will collect over the liquid, causing the pressure in the evaporator to rise with a corresponding increase in the saturation temperature of the refrigerant (Fig. 2).

Fig.2. The boiling or saturation temperature of the liquid refrigerant in the evaporator is controlled by controlling the pressure of the vapour over the liquid with the throttling valve in the vent.

By carefully adjusting the vent valve to regulate the flow of vapour from the evaporator, it is possible to control the pressure of the vapour over the liquid and cause the R134a to vaporise at any desired temperature between -26,2C and the space temperature.  Should the vent valve be completely closed so that no vapour is allowed to escape from the evaporator, the pressure in the evaporator will increase to a point such that the saturation temperature of the liquid will be equal to the space temperature; 5C.  When this occurs, there will be no temperature differential and no heat will flow from the space to the refrigerant.  Vaporisation will cease and no further cooling will take place.

Fig.3. Pressure of refrigerant in the evaporator is reduced below atmospheric pressure by the action of a vapour pump.

When vaporising temperatures are required that fall below the saturation temperature of the refrigerant corresponding to atmospheric pressure, it is necessary to reduce the pressure in the evaporator to some pressure below atmospheric.  This can be accomplished through the use of a vapour pump as shown in Fig.3.  By this method, vaporisation of the liquid R134a can be brought about at very low temperatures in accordance with the pressure-temperature relationships given in the saturation tables.

Maintaining a Constant Amount of Liquid in the Evaporator
Continuous vaporisation of the liquid in the evaporator requires that the supply of liquid be continuously replenished if the amount of liquid in the evaporator is to be maintained.  One method of replenishing the supply of liquid in the evaporator is through the use of a float valve assembly, as illustrated in Fig. 4. 

Fig. 4. Float valve assembly maintains the liquid level in evaporator.  The pressure of the refrigerant is reduced as the refrigerant passes through the needle valve.

The action of the float assembly is to maintain a constant level of liquid in the evaporator by allowing liquid to flow into the evaporator from the storage tank or cylinder at exactly the same rate that the supply of liquid in the evaporator is being depleted by vaporisation.  Any increase in the rate of vaporisation causes the liquid level in the evaporator to drop slightly, thereby opening the needle valve wider and allowing liquid to flow into the evaporator at a higher rate.  Likewise, any decrease in the rate of vaporisation causes the liquid level to rise slightly, thereby moving the needle valve in the closing direction to reduce the flow of liquid into the evaporator.  When vaporisation ceases entirely, the rising liquid level will close the float valve tightly and stop the flow of liquid completely.  When vaporisation is resumed, the liquid level will fall allowing the float valve to open and admit liquid to the evaporator.

The liquid refrigerant does not vaporise in the storage cylinder and feed line because the pressure in the cylinder is such that the saturation temperature of the refrigerant is equal to the temperature of the surroundings.  The high pressure existing in the cylinder forces the liquid to flow through the feed line and the float valve into the lower pressure evaporator.  In passing through the float valve, the high pressure refrigerant undergoes a pressure drop that reduces its pressure to the evaporator pressure, thereby permitting the refrigerant liquid to vaporise in the evaporator at the desired low temperature.

Any device, such as the float valve illustrated in Fig.4, used to regulate the flow of liquid refrigerant into the evaporator is called a refrigerant metering device.  The refrigerant metering device is an essential part of every mechanical refrigerating system.

There are various different types of refrigerant metering devices in use at the present time but will not be discussed in any detail here.  The float type of control illustrated in Fig.4 has some disadvantages, mainly bulkiness, which tends to limit its use to some few special applications.  One widely used type of metering device is the thermostatic expansion valve.  A flow diagram illustrating the use of a thermostatic expansion valve to control the flow of refrigerant into a serpentine coil type evaporator is shown in Fig. 5.

Fig.5. Serpentine coil evaporator with thermostatic expansion valve refrigerant control.

Salvaging the Refrigerant
As a matter of convenience and economy, it is not practical to permit the refrigerant vapour to escape to the outside and be lost by diffusion into the air.  The vapour must be collected continuously and condensed back into the liquid state so that the same refrigerant is used over and over again, thereby eliminating the need for ever replenishing the supply of refrigerant in the system.  To provide some means of condensing the vapour, another piece of equipment, a condenser, must be added to the system (Fig. 6).  The condenser is an essential part of every mechanical refrigerating system.

Fig.6. Collecting and condensing the refrigerant vapour.  Refrigerant absorbs heat in the evaporator and gives off heat in the condenser.

Since the refrigerant vaporises in the evaporator because it absorbs the necessary heat from the refrigerated space, all that is required in order to condense the vapour back into the liquid state is that the heat be caused to flow out of the vapour into another body.  The body of material employed to absorb the heat from the vapour, thereby causing the vapour to condense, is called the condensing medium.  The most common condensing media are air and water.  The water used as a condensing medium is usually supplied from a cooling tower.  The air used as a condensing medium is ordinary outdoor air at normal temperatures.

For heat to flow out of the refrigerant vapour into the condensing medium the temperature of the condensing medium must be below that of the refrigerant vapour.  However, since the pressure and temperature of the saturated vapour leaving the evaporator are the same as those of the vaporising liquid, the temperature of the vapour will always be considerably below that of any normally available condensing medium.  Therefore, heat will not flow out of the refrigerant vapour into the air or water used as the condensing medium until the saturation temperature of the refrigerant vapour has been increased by compression to some temperature above the temperature of the condensing medium.  The vapour pump introduced in fig.4 serves this purpose in fig.6 as a compressor.

Before compression, the refrigerant vapour is at the vaporising temperature and pressure.  Since the pressure of the vapour is low, the corresponding saturation temperature is also low.  During compression the pressure of the vapour is increased to a point such that the corresponding saturation temperature is above the temperature of the condensing medium being employed.  At the same time, since mechanical work is done on the vapour in compressing it to the higher pressure, the internal energy of the vapour is increased with a corresponding increase in the temperature of the vapour.

After compression, the high-pressure, high-temperature vapour is discharged into the condenser where it gives up heat to the lower temperature condensing medium.  Since a vapour cannot be cooled to a temperature below its saturation temperature, the continuous loss of heat by the refrigerant vapour in the condenser causes the vapour to condense into the liquid phase at the new, higher pressure and saturation temperature.  The heat given off by the vapour in the condenser is carried away by the condensing medium.  The resulting condensed liquid is normally cooled by a few degrees below its condensing temperature before flowing out of the condenser at condenser pressure into the liquid storage tank ready to be recirculated to the evaporator.

Notice that the refrigerant, sometimes called the working fluid, is merely a heat transfer agent which carries the heat from the refrigerated space to the outside.  The refrigerant absorbs heat from the refrigerated space in the evaporator, carries it out of the space, and rejects it to the condensing medium in the condenser.

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Main Source of Reference:

Title: Principles of Refrigeration (SI Version)
Author: Roy J. Dossat
Publisher: John Wiley & Sons