What are compressed air aftercoolers and why are they required? Find out everything you need to know about aftercoolers in this article, including the two most common types, why they’re important, and their role within an air compressor system.
What Is An Aftercooler?
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.
A compressed air aftercooler has three primary functions:
- Cool air discharged from air compressor via a heat exchanger
- Reduce compressed air moisture level
- Protect downstream equipment from excessive heat and moisture
Why Are Air Aftercoolers Required?
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 Screw 200°F
Oil-Free Rotary Screw 350°F
2-Stage Reciprocating 300°F
High-temperature air is typically not usable in air-operated equipment, as heat has a detrimental impact on equipment lubrication and sealing materials. 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 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 release excess moisture as a liquid. A general rule is that for every 20°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.
Types Of Aftercoolers
The two most common types of aftercoolers are air-cooled and water-cooled.
Most 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. Similar to the air-cooled version, the typical water-cooled approach temperature is between 10°F and 15°F.
In stationary compressor installations, where cooling water is available, water-cooled aftercoolers are often used. There are a few advantages to using water as the cooling media:
- Water has little seasonal fluctuation in temperature
- Large volumes of cold water can be used
- Cold water can efficiently approach the ambient air temperature, which eliminates condensation downstream
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 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 aftercoolers come 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.
How To Size An Aftercooler
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. The lower that temperature needs to be, the larger the aftercooler needs to be.
The required compressed air temperature is determined by other components downstream in the system. It is critical that each individual component’s maximum temperature is determined and considered to discover the true temperature requirements for the aftercooler.
Disclaimer: Individual air compressor components may vary from the chart above; always check with the component manufacturer for specs and tolerances, including typical and maximum operating temperatures.
Compressor manufacturers may include integral aftercoolers as a component of the compressor system. Otherwise, a stand-alone or freestanding aftercooler is a separate unit installed downstream of the compressor.
The aftercooler should be located as close as possible to the discharge of the compressor.
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