Air Compressor Aftercooler Guide to Compressed Air Coolers

Aftercoolers are heat exchangers used in compressed air systems to cool the air discharged from the compressor. This guide will provide you with all the relevant information so that you understand what air compressor aftercoolers are and how they work.

Table of Contents

• What is an Air Compressor Aftercooler?
• Why do Air Compressors Need Aftercoolers?
• Different Types of Aftercoolers
  • Air-Cooled Aftercoolers
  • Water-Cooled Aftercoolers
• How to Size an Aftercooler
• Air Compressor Aftercoolers, Filters & Dryers
• Useful Reading
• FAQs (Frequently Asked Questions)

What is an Air Compressor Aftercooler?

Aftercoolers are mechanical heat exchangers designed to remove the heat of compression and reduce the amount of water vapor in compressed air systems by condensing the vapor into a liquid form.

An air compressor aftercooler has these three important primary functions:

1. Cools air discharged from the air compressor
2. Reduces the moisture level of the compressed air
3. Protects downstream equipment from excessive moisture and heat

It’s crucial to include aftercoolers to ensure the proper functionality of pneumatic tools and devices at the end of the line. Liquid water or excessive heat can cause significant damage to pneumatic equipment and so aftercoolers can help to prevent that.

Why do Air Compressors Need Aftercoolers?

It doesn’t matter what type of air compressor you have, the air coming out of the compression process is always going to be hot. This high-temperature air is not suitable for use in most air-operated equipment as it can have a detrimental impact on the pneumatic equipment’s lubrication and sealing materials.

When air exits the air compressor, it cools down upon entering the tank and tends to dump the moisture as it begins cooling (if the aftercooler is integrated into the compressed air system by the manufacturer, otherwise the aftercooler will be installed downstream from the compressor).

If it does not dump the moisture then it can wind up in the compressor systems’ airlines and then be able to reach your pneumatic tools at the end of the line. The moisture also contributes to rust, scale build-up, and possible freezing issues.

Atmospheric air contains water in a vapor state, and as this air is drawn into an air compressor and pressurized, the moisture becomes concentrated. Due to the high temperatures from compressing air, this moisture remains in a vapor state above the dew point temperature.

Now you must be wondering but what is the dew point temperature? It’s the temperature at which the air becomes saturated at 100% of its capacity to hold water in a vapor state. When it reaches this dew point temperature, excess moisture must be released as a liquid.

Different Types of Aftercoolers

Aftercoolers are typically available as either air-cooled or water-cooled systems. Both boast benefits depending on your requirements.

Both will typically have a moisture separator installed at the aftercoolers discharge. This will help remove the liquid moisture and solids from the compressed air due to centrifugal forces making them collect at the bottom of the moisture separator. The separator should boast an automatic drain to remove the vapor-liquid and solids but in some cases may have a semi-automatic or manual drain.

Let’s take at the two types in more detail.

Air-Cooled Aftercoolers

Air-cooled aftercoolers cool the hot compressed air using ambient air. It does so by forcing the ambient air over the cooler by a motor-driven fan as the compressed air travels through its spiral finned tube coil or plate-fin coil (depending on the design).

This forced cooler ambient air removes the heat from the compressed air and allows the liquid water to form and then be removed in the moisture separator and drain valve.

Air-cooled aftercoolers tend to be sized to cool the hot compressed air to around 15-20 degrees Fahrenheit of the ambient air temperature entering the system. As this compressed air cools down, it’s possible to remove up to 75% of the water vapor present and turn it into a liquid which can then be removed from the compressed air system.

A belt guard air-cooled aftercooler is mounted to the air compressors v-belt guard (depending on the compressor). Ambient air is forced over the compressor and air-cooled aftercooler by the compressor belts pulley fins which facilitate the heat transfer. Not only this, but when the pulley forces the air over the compressor, a safe operating temperature is able to be maintained.

Water-Cooled Aftercoolers

Water-cooled aftercoolers tend to be sized to cool the compressed air to around 10-15 degrees Fahrenheit. In stationary air compressor installations where cooling water is available, water-cooled aftercoolers are the more popular choice. They boast a few advantages over air-cooled aftercoolers and these are:

• Cold water can efficiently approach the ambient air temperature which helps eliminate condensation downstream
• Large amounts of cold water can be used
• Water has little seasonal fluctuation in temperature

Water-cooled aftercoolers come in a number of varying styles with the most common being a shell and tube heat exchanger. Typically, the compressed air flows through a bundle of tubes inside the shell of the aftercooler in one direction. As this air flows, water flows through the shell in the opposite direction allowing the heat from the air to be transferred to the water.

Liquid water forms as the compressed air cool and this moisture is then captured and removed by the moisture separator and drain valve.

How to Size an Aftercooler

Compressed air aftercoolers are sized based on their cold temperature difference, which is either 10, 15, or 20 degrees Fahrenheit. To put this into simple terms, the compressed air temperature at the outlet of the aftercooler is equal to the cooling medium temperature plus the cold temperature difference.

When sizing the aftercooler for the specified inlet air temperature and flow, the lower the temperature you require typically the larger your aftercooler will need to be.

You must also determine the maximum temperatures of the other components in the compressed air system in order to discover the true temperature requirements for the aftercooler you require. These include but are not limited to:

• FRL’s (filter regulator lubricators)
• Drain valves
• Oil Water Separators
• Coalescent Filters
• Desiccant dryer

For further information on any of both please visit our specific guides!

Air Compressor Aftercoolers, Filters & Dryers

Air compressors always generate warm to hot, moisture-laden, compressed air. If this hot, wet, compressed air is passed through an aftercooler, the aftercooler causes the air temperature of the compressed air to lower. A typical aftercooler is shown in the image below.

As a result of the cooling of the compressed air by the aftercooler, the compressed air can carry less moisture and water condenses out of it, and the air stream becomes dryer.

Downstream from the aftercooler, a general-purpose compressed air filter will strip much of the free water (actual liquid water) from the air stream, furthering the work the aftercooler has done.

At this point, and depending on how dry the compressed air has to be for your use (remember, the drier the compressed air has to be the higher the cost is to make it so) sometimes the compressed air is run through an air dryer.

Dryers work better with cooler compressed air, so it makes sense then if your compressor is working hard and pumping really hot air into the mains, this air gets passed through an aftercooler before it gets to the dryer.

Consider your compressed air moistness shown on a line from wettest to driest. At the wettest end, you will have the air as it exits the compressor pump, and the driest end is the application that is using the air.

In order to get the compressed air dry enough for your air-using equipment, you will install equipment that dries compressed air between your application and the wet compressed air source which is the compressor.

Between the compressor and your air-use application, you will commonly see – and again, depending on how dry the air has to be for your use – aftercooler(s), general-purpose filters, refrigerant or desiccant air driers, more general-purpose filters, point of use air dryers, another air filter and so on.

You will need to keep treating your compressed air with additional air-drying equipment until it reaches your point of use as dry as it needs to be for you.

That’s why some plants have tons of “air-treatment” equipment, and others have a lot less.

Useful Reading

Air Compressor Intercooler Guide to Compressed Air Intercoolers

Air Compressor Heat Exchanger Guide

Intercooler vs Aftercooler In Compressed Air System

FAQs (Frequently Asked Questions)

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If you have any questions about air compressor aftercoolers, then please leave a comment below, with a photo if applicable, so that someone can help you!