Condensers

As the name suggests, condensers are used for condensing of refrigerant vapours.In the refrigerant line it is fitted after the compressor. That means after compression in compressor, refrigerant vapours, which are at high pressure and high temperature,are passed through condenser. Condenser is actually a nest/bundle of tubes/pipes.Hot refrigerant vapours pass through this bundle of pipes and transfer their latent heat to the pipes. So, the pipes become hot and are simultaneously cooled from outside either by air or water. In this way, the heat energy of refrigerant vapours is ultimately transferred to cooling air or water through the wall of pipes. As the refrigerant vapours release their latent heat, the condensing commences i.e. vapours converts to liquid. It is exactly same as when steam is cooled and converted to water. Thus condenser converts high pressure and high temperature vapours into high-pressure liquid continuously. The refrigerant flow through condenser pipes due to pumping force of compressor. After coming out of the condenser the high pressure liquid refrigerant is either stored in the receiver or made to flow through pipe to reach to the expansion valve. Thus the working of condenser is very simple.

It can be easily understood by looking at this example. We know that hot milk put in a glass will cool slowly as compared to if it is put in an open pan. The reason is that in case of pan the area of contact with air increases thus rate of heat released is more. In condenser also we do the same thing i.e. we increase area by flowing the vapours through large bundle of pipes.

Depending on the construction and cooling medium used, condensers are mainly classified in three types as mentioned below.

•  Air-cooled condenser
•  Water-cooled condenser
•  Evaporative condenser

i. Air Cooled Condenser

In an air-cooled condenser atmospheric air takes away the latent heat of refrigerant flowing through bundle of condenser tubes and condenses the refrigerant. The tubes made of copper or steel are bend in the shape of 2 to 8 rows. From one side refrigerant enters and on the other side it exits. Some fins of aluminum in the shape of wire or plate are also welded to the bundle of pipes to increase the heat transfer area. These fins help in cooling of refrigerant and tubes. The heat of refrigerant spreads in the tube and fins and easily transferred to surrounding air because of more area of contact with air. Sometimes a fan is also used which blows air over the condenser tube and fins. It increases the heat transfer rate in the same way as we try to cool the hot tea by blowing air by mouth over tea. When no fan is used, the condensers are called natural convection condenser and when fan is used these are called forced convection condensers. A natural convection air cooled condenser with wire shape fins is generally used in a domestic refrigerator. And a forced convection air-cooled condenser with plate shaped/ flat fins is generally used in a room air conditioner. Generally air-cooled condensers are not used in large plants due to their less heat transferring capacity.

ii. Water Cooled Condenser

In a water-cooled condenser water acts as a cooling medium i.e. it absorbs the latent heat of refrigerant passing through bundle of condenser tubes. The heat carrying capacity of water is more than that of air. So, water-cooled condensers are more effective. But these are costly because of additional water flowing systems. So, their use is restricted to large plants only. These condensers are
commonly used in commercial and industrial refrigerating units.

In this type of condenser, the tube rows are arranged in an enclosed shell/cylinder. These are also called shell and tube condensers. Here the cooling water enters from one end of tube and flow inside the tube bundle/rows arranged in shell and exit from the other end. In the enclosed shell, there is one entrance for refrigerant vapours and exit for refrigerant liquid. So, when refrigerant enters the shell and flow outside the cooling water tubes in the shell, the latent heat of refrigerant vapours is transferred to the cooling water through the wall of tubes and it condenses i.e. convert into liquid and exit from the shell. The cooling water also becomes hot after taking heat of refrigerant and exit from the other end of tube bundle. For continuous working of condenser, the hot water coming out of condenser is required to be cooled again for reuse in condenser. Spraying or flowing through showers in the cooling tower or a spray pond separately cools thishot water. While spraying some of the water evaporates by absorbing latent heat from the rest of water and thus cooling it. That cooled water is again re-circulated to the condenser.

iii. Evaporative Condenser

In an evaporative condenser the bundle of condenser tubes is wetted from outside by water spray . This water when evaporates in the flow of atmospheric air, absorbs heat from the refrigerated vapor inside the tubes and condense it. You already know that in case of water-cooled condenser, water takes away the heat of refrigerant and becomes hot. Then spraying in air in a cooling tower cools this hot water. In cooling tower, water is cooled by evaporation. Now here in case of an evaporative condenser, the water is pumped from the sump to a spray header and sprayed through nozzles over the condenser tubes in which refrigerant vapours are flowing. The heat of refrigerant crosses the wall of tubes and transfers to the water that is wetting the outside surface of tubes. At the same time, a fan draws air from the bottom side of the condenser and discharge out of the top of condenser.

Due to flow of air the water over the surface of tubes evaporates and cools the rest of water. The cooling by evaporation in air can be easily understood by an example. If you dip your hands in a bucket of water and shake in air, you will feel cooling effect on your hands. That is only because the water in your hands evaporate in air and absorb its latent heat from your hand thus making you feel coolness.When shaking of hand is more, more is the air flow over hands, more is evaporation and more is cooling effect.