Compressed Air Aftercoolers

What are aftercoolers and why are they required?

An aftercooler is a mechanical heat exchanger designed to remove the heat-of-compression from a compressed air stream and to condition the air so it can be used in air-operated equipment.

Regardless of the type of compressor being used, the compressed air discharged from that air compressor is going to be hot. That temperature will vary according to the type of compressor being used.

Typical Compressor Average Outlet Temperatures:

Oil-Injected Rotary Screw200 Deg. F.
Oil-Free Rotary Screw350 Deg. F.
2-Stage Reciprocating300 Deg. F.
Centrifugal225 Deg. F.

High temperature air is typically not usable in most air-operated equipment as it has a detrimental effect on the equipment lubrication and sealing materials. This hot air also contains large quantities of moisture vapor which, as it condenses, contributes to rust, scale build-up, washing out of lubricant and possible freezing issues.

Water exists in a vapor state in all atmospheric air (relative humidity) and as the air is drawn into a compressor and is pressurized, this moisture is concentrated in each cubic foot. Due to the high temperature of the air, the moisture remains in a vapor state (above the dew point temperature).

The “dewpoint” is the temperature at which this air becomes saturated at 100% of its capacity to hold water in a vapor state and, with any additional cooling, must give up free moisture as a liquid. A general rule is that for every 20 Deg. F. rise in temperature the air can hold twice the moisture load in a vapor state above the dewpoint.

As the compressed air cools this water vapor condenses into a liquid form and is removed from the air stream. As an example, if an aftercooler is not used, a 200 cubic feet per minute (CFM) compressor operating at 100 psig can introduce as much as 45 gallons of water into the compressed air system each day.

The two most common types of aftercoolers are air-cooled and water-cooled.

air-cooled aftercoolersMost air-cooled aftercoolers are sized to cool the compressed air to within 15°F to 20°F of ambient cooling air temperature (approach temperature). As the compressed air cools, up to 75% of the water vapor present condenses to a liquid and can be removed from the system.

A moisture separator, installed at the discharge of the aftercooler, mechanically removes most of the liquid moisture and solids from the compressed air. Utilizing centrifugal force, and in some cases baffle plates, moisture and solids collect at the bottom of the moisture separator. An automatic drain should be used to remove the moisture and solids.

In stationary compressor installations, where cooling water is available, water-cooled aftercoolers are often used. The advantages of using water as the cooling media are a cooling media with little seasonal fluctuation in temperature and the ability to supply quite a cold water supply approaching the ambient air temperature ensuring no further condensation downstream. As with the air-cooled version, the typical water-cooled approach temperature is between 10-15 Deg. F.

The Functions of a compressed air aftercooler

  • Cool air discharged from air compressors via a heat exchanger
  • Reduce compressed air moisture level
  • Protect downstream equipment from excessive heat and moisture

Coolers are usually sized with a CTD (Cold Temperature Difference) of 10°F, 15°F or 20°F. This means that the compressed air temperature at the outlet of the aftercooler will be equal to the cooling medium temperature plus the CTD when sized for the specified inlet air temperature and flow.

Compressor manufacturers may include aftercoolers within the compressor package. In general these compressors are referred to as integral aftercoolers. A stand-alone or freestanding aftercooler is a separate unit installed downstream of the compressor.

Air-Cooled Aftercooler

Air cooled aftercoolers use ambient air to cool the hot compressed air. The compressed air enters the air cooled aftercooler. The compressed air travels through either the spiral finned tube coil or a plate-fin coil design of the aftercooler while ambient air is forced over the cooler by a motor-driven fan. The cooler, ambient air removes heat from the compressed air. Liquid water forms as the compressed air cools. The moisture is removed by the moisture separator and drain valve.

Belt Guard Type Air Cooled Aftercooler

A belt guard air-cooled aftercooler mounts to the compressor’s v-belt guard. The compressor’s belt pulley has fins designed to force ambient air over the compressor and air cooled aftercooler. The air passing over the aftercooler facilitates the heat transfer. The pulley also sends air over the compressor helping maintain proper operating temperature.

Water-Cooled Aftercooler

Water-cooled aftercoolers comes in different styles. The most common style for compressed air service is a Shell and Tube Heat Exchanger/Aftercooler. This aftercooler consists of a shell with a bundle of tubes fitted inside. Typically the compressed air flows through the tubes in one direction as water flows through the shell side in the opposite direction (counter-flow). Heat from the compressed air is transferred to the water. Liquid water forms as the compressed air cools. The moisture is removed by the moisture separator and drain valve.

Recommended location of the aftercooler

The aftercooler should be located as close as possible to the discharge of the compressor.

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