A cooling system is an essential component of an industrial air compressor. There are two basic types of cooling methods to choose from: air-cooled or water-cooled. Which type of cooling system is better? It depends on your compressor type, the amount of air you need to cool, and the ambient conditions. Here are some things to consider when choosing between an air-cooled and a water-cooled air compressor.
Why Do You Need a Cooling System for Your Air Compressor?
Air compressors generate heat—and a lot of it. This is known as the heat of compression. It all comes down to the laws of physics: as the volume of a gas is reduced, both the temperature and pressure go up in accordance with the combined gas law:
The more air that is compressed, the more heat is created. Additional heat is also produced in the motor and air-end of the compressor. In an oil-injected rotary screw air compressor, a majority of this heat is removed internally by circulating oil. However, the air coming out of the compressor is still very hot; typical outlet temperatures for an oil-injected rotary screw compressor may be more than 160°F. An oil-free air compressor or two-stage reciprocating compressor may have discharge air temperatures of 300-350°F. These high temperatures can cause problems downline in your compressed air system–particularly for your air dryers if not reduced.
Most air dryers are rated for inlet temperatures of no more than 100°F. When temperatures are higher, the dryer can’t work as efficiently or at all. Hotter air carries more moisture, putting strain on the drying system. If the dryers aren’t able to remove enough of the moisture, it will result in condensation in other parts of the distribution system as the air cools. Excess heat can result in other issues as well:
- Excess heat can degrade seals and wash away lubricants in downstream tools and equipment, leading to early failure of system components.
- Failing to control the heat created by air compression will heat up the compressor room and surrounding environment, potentially leading to overheating of the air compressor itself.
- If the compressor is operating indoors, excess heat generated by the compressor will put a strain on air conditioning systems in the summer.
An aftercooler is designed to dissipate excess heat and mitigate these heat-related issues.
What Is an Air Compressor Aftercooler?
Industrial air compressors need a cooling system to remove excess heat generated by compression. An aftercooler is a type of heat exchange system that absorbs heat from the compressed air and carries it away where it can be dissipated. As the compressed air cools, it reaches its point of saturation. In this way, the aftercooler acts both as a cooling system and the first stage in the drying system for compressed air. In fact, about 70% of moisture in compressed air falls out in the aftercooler, leaving just 30% to be handled by the dryers. Without an effective aftercooler system, the dryer will need to be oversized to handle the additional moisture.
Rotary type air compressors are almost always sold with an integrated after-cooling system. There are two main types of industrial air compressors: air-cooled and liquid-cooled, also known as water cooled. Both types of aftercoolers can provide significant benefits. An aftercooler will:
- Remove about 70% of the excess moisture from the air, reducing strain on the air dryer.
- Ensure that air going into the dryer does not exceed the temperature specifications for the dryer.
- Protect downstream equipment from excessive heat and moisture.
- Improve safety and reduce fire risk by cooling discharge air before it comes in contact with compressed air hoses.
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Air-Cooled Compressors vs. Liquid-Cooled Compressors
Both air-cooled and liquid-cooled aftercoolers act as heat exchangers, removing excess heat from outlet air. The main difference is the medium used to transfer the heat. As the names suggest, an air-cooled compressor uses air, and a liquid-cooled compressor uses a liquid, usually water or glycol/water mix.
An air-cooled aftercooler uses ambient air to bring down the temperature of compressed air. This is by far the most common form of aftercooler for rotary screw or rotary vane air compressors. Compressed air moves through a series of coils inside the aftercooler. A fan blows cool ambient air over the coils, carrying away excess heat. Cooling fins provide additional surface area for air to move across, increasing the heat transfer capacity. As an alternative to mounting a fan with its own motor, a belt guard air aftercooler (used on some piston-style compressors) uses airflow generated by the compressor’s belt system, which can save some space.
An air aftercooler reduces the temperature of compressed air to within roughly 15-20°F of ambient temperature, also known as the approach temperature. If the ambient temperature is 85°F, you can expect the air cooler to reduce outlet temperatures to about 100°F. At the same time, some of the excess water vapor drops out of the air as it cools. Condensation must be removed via a drain valve.
Air aftercoolers provide effective cooling for most industrial compressed air applications. They are also simple to maintain. However, an air-cooled air compressor requires plenty of space and ventilation to ensure adequate airflow for cooling. Ambient air must also be relatively cool; the aftercoolers will not work well if the compressor room is allowed to get too hot, which could lead to air compressor overheating. The compressor must be located in an area near an outside wall or with access to roof ventilation to vent the excess heat outdoors. In some systems, it is possible to capture the excess heat to provide heating for the building or perform other work.
Aire Tip: The optimal operating temperature range for an air compressor is between 50-85°F. Read more: Optimal Temperature Range for Compressed Air Equipment.
Water-cooled aftercoolers use cool circulating water to bring down the temperature of compressed air. While other liquids can be used in some cases, water is by far the most common substance used for liquid cooling of compressed air. As compressed air moves through the coils of the aftercooler, water flows around the tubes to carry away the heat. A water-cooled system can be more costly to install and maintain, but liquids are much more efficient in carrying away heat. That means they can dissipate more heat using less energy. Water cooling typically can get compressed air to within 10-15°F of the water temperature. There are two main types of water-cooled aftercoolers.
- A closed-loop water cooling system recirculates the same water through the system. The water heats up as it comes in contact with the hot pipes carrying discharge compressed air. The warmer water passes through a heat exchanger that cools it back down before it returns to the beginning of the path. Alternatively, water may pass through a refrigeration system to be re-chilled. (Process industries using refrigerated water for other purposes may be able to draw on excess capacity for cooling.) The heat exchanger may be similar to an air-cooled aftercooler, with fans and cooling fins. A closed-loop system uses less water overall, though they will need to be inspected and topped off regularly. If the pipes are exposed to the elements in a colder climate, glycol will be added to the system to prevent the pipes from freezing and cracking. However, this will reduce the heat transfer properties of the water.
- In an open (or evaporative) water-cooling system, a continual supply of fresh water is used to cool the coils containing compressed air. Water is sprayed across the coils and drains off or evaporates. In some facilities, heated water is reclaimed for use in boilers, industrial processes or other “gray water” applications. These systems use a lot of water, which can be a cost concern if water must be purchased.
An open water-cooling system often relies on nearby bodies of water (pond, river, lake, well, or even ocean) rather than relying on a municipal water supply. However, water used for cooling must be high-quality. A water tower and filtration system will typically be needed when using environmental water. These costs must be considered when contemplating an open-water cooling system. Because building and maintaining a water treatment system is cost-prohibitive, these systems are more commonly used in process industries that already have this infrastructure in place for other purposes.
Should You Buy an Air-Cooled or Water-Cooled Air Compressor?
Which type of cooling system is best for your air compressor? That depends in part on the air compressor size, your environment, and if you have an existing cooling system.
- Air-cooled aftercoolers are less expensive and require less maintenance, making them the best choice for a majority of air-using applications. More than 95% of rotary screw air compressors are air-cooled. An air-cooled aftercooler can handle discharge temperatures of up to 350°F and pressures up to 250 PSI.
- Water-cooled air compressors are a better choice for high-temperature applications, high dust environments, very large compressors, or where the installation in the plant is opened and air conditioned. An open water-cooled system can handle discharge temperatures of up to 450°F and pressures up to 435 PSI.
A water-cooled system may also be a better choice if:
- You do not have adequate space and airflow for effective air cooling.
- Ambient air temperatures get very high (90°F and above) on a regular basis.
It is also possible to supplement an air-cooled compressor with a separate trim aftercooler if air cooling is not able to get outlet temperatures below 100°F. This can be especially helpful if additional cooling is needed only when temperatures rise into the 90s and above in the summer months. The ancillary aftercooler can be turned on only when it is needed.
Sizing Your Air Compressor Aftercooler
Aftercoolers must be sized appropriately to get the anticipated amount of cooling. Sizing for an air compressor aftercooler depends on four variables:
- The CFM of the air compressor
- The PSI of the compressed air system
- The discharge temperature of air from the compressor
- The ambient temperature
The more air you are moving (higher CFM), the higher its pressure, and the greater the difference between discharge temperature and required temperature, the larger the aftercooler will need to be. Most air compressor aftercoolers are sized to bring compressed air temperatures down to within 10-20°F of the ambient air temperature or cooling liquid temperature, known as the approach temperature.
Aire tip: When sizing an air-cooled aftercooler, always size it for the highest ambient temperatures expected in your environment at 100% humidity.
Caring for Your Air Compressor Aftercooler
Your aftercooler required regular maintenance for proper operation. That includes:
- Regularly drain condensate from the system. A zero-loss drain valve will drain excess liquid automatically while conserving compressed air.
- For air-cooled systems, inspect and clean the fan system on a regular basis. Keep the coils and cooling fins free of dust and debris, which will reduce cooling efficiency.
- For water-cooled systems, monitor water quality on a regular basis and inspect and clean the system.
Need Help Keeping Your Compressor Cool?
If your air compressor is overheating, or you’re having trouble getting compressed air down to acceptable operating temperatures for downstream equipment, it may be time to take a look at your aftercooler. We can help you determine whether you just need some maintenance or if you need a larger aftercooler system.Talk to an air expert today.