Refrigeration

In general, refrigeration may be defined as any process of heat removal. More specifically, refrigeration is defined as that branch of science that deals with the process of reducing and maintaining the temperature of a space or body below the temperature of its surroundings.

If a space or body is to be maintained at a temperature lower than its surrounding ambient temperature, heat must be removed from the space or body being refrigerated and transferred to another body or substance whose temperature is below that of the refrigerated body.

Mechanical refrigeration is primarily an application of thermodynamics wherein the cooling substance goes through a cycle in which it is recovered for re-use. A thermodynamic cycle can be operated in the forward direction to produce mechanical power from heat energy, or it can be operated in the reverse direction to produce heat energy from mechanical power. The reversed cycle is essentially utilised for the cooling effect that it produces during a portion of the cycle and is thus called a refrigeration cycle.

Vapour – Compression Refrigeration Cycles

The most widely used domestic refrigerators function on a vapour-compression cycle which operates between two pressure levels using a two-phase working substance or refrigerant which alternates cyclically between the liquid and vapour phases in continuos circulation.

In order to convert a liquid into a vapour, an energy transfer is required. This energy is acquired by the vapour molecules in the evaporation process and is termed the enthalpy of evaporation. If this energy is subsequently transferred from the vapour, the energy of the molecules is diminished and liquid is formed during the process of condensation.

The evaporation and condensation processes take place when the refrigerant is absorbing and rejecting heat, and these are essentially constant temperature and constant pressure processes.

Commonly, the vapour compression cycle system within a refrigerator comprises four main devices, namely:

• Evaporator
• Compressor
• Condenser
• Expansion or throttle valve

These individual elements are illustrated and have their functions examined with reference to the following systems diagram of a refrigeration unit.

Referring to the diagram below, a wet low-pressure, low temperature refrigerant enters the evaporator at point 1 and boils (evaporates) to a nearly dry state at point 2 by absorbing heat from a controlled refrigerated space thereby producing the refrigerating effect.

The vaporised refrigerant then enters the compressor in which it is compressed, by a work input, ideally to a dry saturated state at a higher pressure and temperature to point 3. The refrigerant next passes through a condenser at constant pressure and temperature until it is completely liquid at point 4 by transferring heat to the surroundings.

© A.Henderson, UHI

The cycle is completed when the refrigerant is expanded through a throttle valve back to its original low pressure, low temperature, wet state at point 1. The enthalpy at point 4 being equal to the enthalpy at point 1.

This cycle of operations is repeated on a continuous basis in order to maintain a pre-determined sub-zero temperature within the controlled space.

Refrigerant

The working fluid that circulates in a refrigeration system is called a refrigerant and may be defined as a substance that, by undergoing a change in phase (liquid to gas, gas to liquid), absorbs or releases a large quantity of heat in relation to its volume, thereby producing a considerable cooling effect.

A refrigerant is a fluid that absorbs heat during evaporation at a low temperature and pressure, and rejects heat by condensing at a higher temperature and pressure. Examples of refrigerants are ammonia, sulphur dioxide, and methyl chloride, although these are no longer widely used, having been largely replaced by fluorocarbons such as Freon (refrigerants R12 and R22).

The Freon refrigerants R12 and R22 are general purpose fluids specially manufactured for refrigeration and these are non-toxic and non-flammable.

Apart from the ability to boil (evaporate) at a low temperature, refrigerants should possess other desirable characteristics such as:

• low cost and commercially available in quantity
• chemical stability
• non-explosive
• suitable working pressures and temperatures
• low specific volume in order to keep pipe sizes small
• the liquid enthalpy should be low and evaporation enthalpy high in order to achieve a high refrigeration effect per kilogram of refrigerant

There is no refrigerant with all these properties, so the choice of a suitable fluid for any particular application must represent some form of compromise. The R family of refrigerants is the safest group and most widely used. All new refrigerants in the R family should have zero ozone depletion potential and be user friendly. Property tables and charts are produced for the various refrigerants similar to those produced for water and steam.

The behaviour of refrigerants is akin to that of water when subjected to heat. Water boils at 100°C when heat energy is supplied at atmospheric pressure. Evaporation then takes place at constant temperature until the vapour is completely dry and in the gaseous state. Further heating raises the temperature and the fluid is in the superheated condition.

When the temperature and pressure of a refrigerant bear a `natural’ stable relationship to each other, the refrigerant is regarded as being in its saturated state.

A refrigerant liquid in its saturated state can be further cooled at the same pressure. It will then become subcooled or undercooled.

A refrigerant vapour in its saturated state can be heated further at the same pressure. It will then become superheated.

© A.Henderson, UHI

Use of thermodynamic tables for refrigerants

In this outcome we have already dealt with the use of property tables for water and steam. With the exception of entropy (s values), our studies now extend into the interpretation and extraction of information covering the ammonia and fluorocarbon refrigerants R717 and R12 as listed on pages 12 and 13 of the Rogers and Mayhew tables.

Refrigerant quantities together with appropriate symbols and units are identified in the table below:

As previously stated, a refrigerant is regarded as being in its saturated state when its saturation temperature, T, and its corresponding pressure, ps bear a `natural’ stable relationship to one another.

The specific volume, vg, of a saturated refrigerant vapour, (i.e. completely dry) can be read directly from the tables against any given pressure.

For a wet vapour, the total volume of the mixture is given by the volume of liquid present plus the volume of dry vapour present. The volume of liquid is usually negligibly small compared to the volume of dry saturated vapour, hence for most practical problems, vx = xvg.

e.g. Spec. Volume of Refrigerant R12 at 1.004 bar and .96 dry

$v_x=0.96\times 0.1594=0.1530m^3{\text{kg}}^{-1}$ 

The heat energy required to change 1 kg of saturated liquid refrigerant to a completely dry saturated vapour (gas) is called the enthalpy of evaporation.

i.e. Enthalpy of evaporation hfg = hg - hf

e.g. Enthalpy of evaporation of refrigerant R717 at 2.680 bar

$h_{fg}=1430.5-126.2=1304.3kJ{\text{kg}}^{-1}$ 

The specific enthalpy, h, of a refrigerant in the superheat condition can be extracted directly from the tables from either of two column headings (50 K and 100 K) dependent on the degree of superheat which is obtained by the difference between the superheat temperature and the saturation temperature at the specified pressure.

Example

Determine the specific enthalpy of refrigerant R717 at a pressure of 2.908 bar and temperature of 20°C.

From tables, the saturation temperature at 2.908 bar is – 10°C.

Hence (T – Ts) = 30°C or 30 K.

Specific Enthalpy, h = 1551.7 kJ kg^-1 (From 50 K column).

If the degree of superheat had been, say, 84K at the same pressure, then

Specific Enthalpy, h = 1665.3 kJ kg^-1 (from 100 K column).