Thus refrigeration is the moving of heat from a place where it is not wanted to a place where it is unobjectionable. The evaporator that was discussed in the previous chapter absorbs the heat, the compressor moves it and the condenser is the door through which the unwanted heat flows out of the refrigeration system.

Hence, like the evaporator, the condenser is a heat exchanger and the same heat transfer laws apply.

Condenser load Condensers receive superheated refrigerant vapour from the compressor, remove the superheat and then condense or liquefy it.

It is the point where the heat and work input into the system are removed. In other words, all the heat absorbed in the evaporator, plus the heat equivalent of the work required for compression, is removed from the refrigerant by means of the condenser.

Condensing media The two basic media to which condensers reject their heat are air and water, depending on the type of condenser. These two media are used because they are available in sufficient quantities and are normally cheap, easy to handle, not dangerous and their normal temperature range is satisfactory to condense refrigerant vapours.

The method of heat exchange is the basis of equipment classification which is as follows:

• Air-cooled condensers

• Water-cooled condensers

Air-cooled condensers Requirements In air-cooled condensers the heat is rejected directly to the air by sensible heat transfer. This system is becoming more and more popular because of its simplicity, its low maintenance requirements and the fact that it does not need water. The disadvantage of the air-cooled condenser is that it usually necessitates higher condensing temperatures and if the condenser is far from the compressor, long refrigerant lines are required.

All the heat given off by the condensing refrigerant increases the air temperature. Hence the temperature increase of the air can be calculated from the basic heat equation.

The temperature of the air entering the condenser is fixed by the selected maximum ambient dry bulb temperature in

the area where the condenser is installed.

It is important that the condenser is rated for this temperature, especially for process cooling application.

Entering air temperatures that are higher than expected will quickly cause higher than design head pressures and possible compressor cut-outs, usually when it can least be tolerated.

Congested or unusual locations may, due to air recirculation or extra heat sources, result in entering air temperatures higher than general ambient.

The condensing temperature must be higher than the entering air temperature for heat transfer to take place. This temperature difference for normal application ranges from 8-20K. The air quantity passing over the air-cooled condenser varies between 75 and 150l per second per kW. This will produce an air temperature difference between 13K and 15K. Condensing temperatures for commercial applications of 40-50 ° C will normally result in an economical selection.( Condensing temperature for ammonia should not be above approximately 35 ° C.)

The final selection of the condenser will be based on price, operating cost, size and noise considerations and is usually done by the manufacturer or the sales organisation.

Description and construction Many different types of air-cooled condensers exist. Coils are constructed of copper, aluminium or steel tubes ranging from about 6-20mm in diameter. Fins are again used to improve the air side heat transfer. Most fins are made of aluminium. Copper fins are sometimes used in corrosive atmospheres but these are more expensive. Most common fin spacing is 300-600 fins per metre( fin spacing 3mm to 1.6mm). The coil design with regard to fin spacing, number of rows and face area has to be considered in relation to refrigerant pressure loss, air resistance and sound requirements. Coil circuitry is also of utmost importance for cost and reliability. Modern trend is towards discharging the superheated refrigerant vapour into several circuits, usually at the top.( See Figure below). As condensation takes place circuits converge into fewer circuits. Refrigerant velocities are maintained in this manner to produce maximum heat transfer.

Air-cooled condenser coil.

Extra circuits are often provided for sub-cooling the liquid refrigerant by approximately 5 to 8K, which will improve the operation of the refrigeration cycle. Coils are mounted in a frame.

The fans used on air condensers can be either axial flow( propeller fans) or centrifugal fans.

Propeller fans are the most common. Very often multiple fans are used in order to control the condenser capacity by reducing the number of fans running during cold ambient conditions. The fan is often mounted on the electric motor driving the compressor on smaller air-cooled condensing units.

Air-cooled condensers are fitted with centrifugal fans where the resistance to air flow is too high for a propeller fan and / or where the noise level of an axial flow fan running at high speeds is objectionable. These fans are usually belt-driven which offers more flexibility but also requires more maintenance.

Arrangement of the coil-fan combination can take almost any form from the horizontal coil with up-flow air to the vertical coil with either horizontal or vertical air flow.

The figure below shows the most common types.

The advantage of the vertical coil condensers is the much smaller space requirements.

The disadvantage of the vertical coil, vertical air-flow condenser is that it is very sensitive to wind direction. When installing this type of condenser, it is important that it be orientated so that the prevailing winds in summer will aid rather than retard the action of the fan.

Wind deflectors must be installed on the discharge side if such orientation is not possible.

The wind direction is immaterial in the other condensers. Space requirements, however, are very much higher, especially for the horizontal coil type. A lower silhouette is very often desirable, especially for roof-top applications.

Water-cooled condensers Requirements In a water-cooled condenser the heat is transferred to water. This water can then either be run to waste or cooled in a cooling tower and recirculated.

In the waste water system the water is usually taken from the municipal water supply and discharged into the drain after it has absorbed the heat from the refrigeration plant. The advantage of this system is its simplicity. The high cost of water, together with the restrictions placed on the usage of municipal water for this purpose, has limited the use of this system to the very small plants.

In the recirculated condenser water system the water is cooled in a cooling tower and returned to the condenser. The water-cooled condenser and the cooling tower can be regarded as one unit because the condenser cannot operate without the cooling tower.

Three types of air-cooled condensers.

In a cooling tower the condenser water can be cooled to a temperature approaching the wet bulb temperature of the air. The advantages of the water-cooled condenser system are: i. Lower condensing temperatures resulting in more economical compressor operation. ii. The cooling water can be transported over long distances, making it simpler to reject heat far more remote from the condenser location.

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