Heat exchanger for cooling the air or gas discharge from compressor. It is designed to reduce the temperature and liquefy condensate vapours.
High temperatures are produced when air is compressed, and it needs to be cooled prior to the inlet to air dryer and for further use in the pneumatic applications.
After-cooler is a heat exchanger intended to reduce the temperature of the hot air discharged from the compressor to approximately 15 to 25°C above that of the ambient air (also known as approach temperature). Smaller the degree of approach temperature larger is the size of the heat exchanger.
Two basic types of after-coolers are (1) air-cooled after-cooler and (2) water-cooled after-cooler.
Air-cooled after-cooler uses the ambient air to cool the hot air discharged from the compressor.
In the water-cooled after-cooler, the hot compressed air is passed through the after-cooler tubes and cooling water is passed in the opposite direction through the after-cooler shell.
The counter-current flow provides an effective method for reducing the temperature of the compressed air by reducing the temperature, most of the suspended water vapour and some oil vapour will condense to the liquid state.
Typically, in the aftercooler of lubricated screw compressors, generally 2/3rd section cools the oil and 1/3rd section cools the compressed air.
This cooling process is accompanied by the release of moisture condensate; therefore, condensate separators should be used after the aftercooler. The liquid is then drained away from the system by using condensate separators and condensate drain valves.
(See also rated vs. measured ‘Approach temperature’; ‘Heat recovery’)