A cooling system is an essential component of an industrial air compressor. There are two types of cooling methods to choose from: air-cooled or water-cooled. When buying a new compressor, choosing the right type of cooling system is a critical consideration to ensure efficient operation.
Table of Contents
- Why Do You Need a Cooling System for Your Air Compressor?
- What Is an Air Compressor Aftercooler?
- Air-Cooled Compressor vs. Water-Cooled Compressor: What’s the Difference?
- Requirements for Air Compressor Cooling Systems
- Costs of Air-Cooled vs. Water-Cooled Air Compressors
- Water-Cooled vs. Air-Cooled Air Compressors: Pros and Cons
- Things to Consider When Choosing an Air Compressor Cooling System
- Should You Buy an Air-Cooled or Water-Cooled Air Compressor?
- Sizing Your Air Compressor Cooling System
- Caring for Your Air Compressor Aftercooler
- Need Help Choosing the Right Air Compressor Cooling System?
Why Do You Need a Cooling System for Your Air Compressor?
Air compressors generate heat — and a lot of it. One of the major considerations in compressed air system design is how to keep the air compressor cool.
In an oil-injected rotary screw air compressor, a majority of this heat is removed internally by circulating oil also known as coolant. The air exiting the compressor is very hot and must be cooled before going downstream; typical outlet temperatures for an oil-injected rotary screw compressor may be more than 160°F. Oil-free air compressors or two-stage reciprocating compressors may have discharge air temperatures of 300-350°F. These high temperatures can cause problems downstream of your compressed air system – particularly for your air dryers.
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 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.
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 the 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-screw air compressors (including single-state oil-flooded rotary screw, double-stage air compressors and oil-free 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. 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.
Air-Cooled Compressor vs. Water-Cooled Compressor: What’s the Difference?
Both air-cooled and water-cooled (or liquid-cooled) aftercoolers act as heat exchangers, removing excess heat from outlet air. The main difference between air-cooled and water-cooled compressors is the medium used to transfer the heat. As the names suggest, an air-cooled air compressor uses air, and a liquid-cooled air compressor uses a liquid, usually water or glycol/water mix.
An air-cooled air compressor 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.
- For self-cooling air compressors with built-in air cooling, 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.
Air aftercoolers provide effective cooling for most industrial compressed air applications. They are also simple to maintain.
- An air aftercooler cools 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.
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.
An air compressor water cooling system uses circulating water to cool air temperatures. While other liquids can be used in some cases, water is by far the most common substance used for air compressor cooling.
- As compressed air moves through the coils of the aftercooler, water will circulate around the tubes to carry away the heat.
- These systems 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.
- Liquid cooling systems typically can get compressed air to within 10-15°F of the water temperature.
- Because they are more efficient for cooling, a water-based cooler is better for high-volume, high-pressure applications.
There are two main types of water-cooled aftercoolers: closed-loop cooling systems and evaporative cooling systems.
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 in a closed-loop cooling system 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 a closed-loop system.
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. However, they provide the most efficient cooling for high-volume production environments and those operating at high pressure (above 250 PSI).
Requirements for Air Compressor Cooling Systems
When installing a new compressor, it’s critical to ensure that installation requirements are met, including ventilation, water availability and other factors. Each of the two types of cooling systems has its own requirements for safe and efficient operation. Failing to meet these could lead to air compressor overheating. Understanding these requirements will help you decide which type of aftercooler is right for you.
Installation Requirements for Air-Cooled Air Compressors
An air-cooled screw compressor or air-cooled reciprocating compressor requires plenty of space and ventilation to ensure adequate airflow for cooling. Other factors include:
- Ventilation: Ensure that the installation site has proper ventilation and an adequate flow of cool air for effective heat dissipation. The compressor must be located in an area near an outside wall or with access to roof ventilation to vent the excess heat outdoors. This may require installing exhaust fans or ductwork to direct hot air away and prevent overheating.
- Ambient temperature: Air-cooled compressors are more sensitive to the ambient temperature. In extreme temperature environments, additional cooling or heating measures might be necessary to maintain optimal operating conditions.
- Space requirements: Air-cooled compressors generally require more space around the unit to allow for sufficient air circulation.
Installation Requirements for Water-Cooled Air Compressors
Air compressors with liquid aftercoolers require access to a reliable water supply, water treatment, and appropriate water disposal solutions. These systems often use a combination of heat exchangers and water-cooling loops connected to an external water source, such as a cooling tower or a water chiller.
Key considerations for water-cooled systems include:
- Water supply: Ensure that the installation site has access to a reliable and consistent water supply that meets the compressor's flow and pressure requirements. 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.
- Water quality: Water used for compressor cooling must be high quality. Water treatment, such as filtration or chemical additives, may be required in liquid cooling systems to prevent scale buildup, corrosion, or biological growth in the cooling system. This is especially important if using environmental water.
- Water disposal: Consider the water disposal requirements, as some local regulations may restrict the discharge of used cooling water. In such cases, a closed-loop or recirculating cooling system may be necessary.
Costs of Air-Cooled vs. Water-Cooled Air Compressors
When comparing the costs of compressors with air- and water-based cooling systems, it is important to evaluate initial cost, energy expenditures, infrastructure requirements, cost recovery potential, and other factors. Assessing these factors in the context of your specific application and facility requirements will help you determine the most cost-effective cooling system for your needs.
- Air-cooled compressors generally have a lower initial cost, as they require simpler cooling systems and fewer components.
- Water-cooled compressors typically have a higher initial cost, as they require additional components, such as heat exchangers, water pumps, and cooling towers or chillers.
Power Demand and Energy Costs
- Air-cooled compressors may use more power in hot environments, as the compressor has to work harder to dissipate heat, potentially increasing operational costs.
- Water-cooled compressors often have lower power demand, as water is a more efficient heat transfer medium. However, the energy cost for operating water pumps, chillers, or cooling towers should also be factored in; these elements may make water-cooled compressors more expensive to operate than air-cooled.
- An air-cooled compressor must be located in an area with adequate ventilation and sufficient space for air circulation, which may necessitate additional infrastructure investments, such as exhaust fans or ductwork.
- A water-cooled compressor needs access to a reliable water supply, water treatment systems, and appropriate water disposal solutions. This may require investments in water supply infrastructure, treatment equipment, and cooling systems, such as cooling towers or chillers. 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. A closed-loop cooling system requires less water and will have lower infrastructure costs.
Cost Recovery Through Heat Capture or Grey-water Reuse
- Air-cooled compressors may offer limited opportunities for heat recovery, as the heat is dissipated into the ambient air. However, some facilities may be able to capture this waste heat for space heating or other purposes.
- Water-cooled compressors provide greater potential for heat recovery, as the heat is transferred to the cooling water. This heated water can be used for process heating, space heating, or even domestic hot water. Grey-water from the cooling system can also be reused for non-potable applications, such as irrigation, reducing water consumption and disposal costs.
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Water-Cooled vs. Air-Cooled Air Compressors: Pros and Cons
Both types of cooling systems have their place in industry, and each has unique pros, cons and typical uses. This table summarizes the differences between air-cooled and water-cooled compressors.
|Air-Cooled Compressors||Water-Cooled Compressors|
- Less expensive
- Lower maintenance
- Simple operation
- Used to Heat Facility
- Higher cooling efficiency
- Quiet operation
- Lower space requirements
- Better cooling ability in high ambient temperatures
- Requires more space
- Hard to keep compressor room cool
Does not work well in high ambient temperatures
- More systems maintenance
- Higher operating costs
- Open systems require large volumes of high-quality water
|Typical Use||- General industrial compressed air applications below 200HP||
- High ambient temperature applications
- Larger HP applications with large volumes of air to cool (e.g., centrifugal compressors)
Things to Consider When Choosing an Air Compressor Cooling System
When choosing between an air-cooled and water-cooled air compressor, several factors need to be considered to determine the most suitable option for your specific application. Here's a breakdown of key considerations.
- Initial cost: Air-cooled compressors generally have a lower initial cost, while water-cooled compressors may require additional investments in components and infrastructure.
- Operating cost: Consider energy expenditures, including the cost of operating cooling equipment and potential savings from heat recovery or grey water reuse.
- Air-cooled compressors may require more frequent maintenance due to their exposure to dust and debris in the ambient air. Additionally, fan or blower components may need servicing or replacement. However, maintenance is generally simpler.
- A water-cooled compressor is more complex and may require more complex and costly maintenance, such as water treatment, pump servicing, and heat exchanger cleaning.
Space and Ventilation Requirements
- Air cooling systems require adequate space for air circulation and proper ventilation to dissipate heat. This may involve investing in exhaust fans or ductwork.
- Water cooling systems typically have a smaller footprint, as they do not need as much space for airflow, but they do require access to water supply and disposal infrastructure.
- Air-cooled compressors are generally suitable for lower CFM requirements, where heat generation is more manageable with ambient air cooling.
- Water-cooled compressors are often more effective for high-volume applications with very high CFM requirements, as they are more efficient in removing heat.
- Industries that already have infrastructure in place for process water may benefit from a water cooling system. Water-cooled compressors may also benefit industries with high CFM demands and those requiring stable temperature control for compressed air, such as food processing or electronics manufacturing.
- Industries that require ruggedness and simplicity, such as construction or automotive repair, may prefer air-cooled compressors for their lower initial cost, ease of maintenance, and adaptability to various environments. Air cooling systems are also a better choice in facilities where water access is a concern.
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 cooling systems 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 pressure up to 250 PSI.
- Water cooling systems are a better choice for high-temperature or high-pressure 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 air 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 cooling system can be turned on only when it is needed.
Sizing Your Air Compressor Cooling System
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 operating pressure (PSI) of the compressed air system
- The discharge temperature of air from the compressor
- Ambient temperatures
The higher the volume of air you are moving (higher CFM), the higher the 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.
Not sure of your CFM requirements? Check out our CFM calculator.
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 liquid cooling systems, monitor water quality on a regular basis and inspect and clean the system.
Need Help Choosing the Right Air Compressor Cooling System?
If you’re in the market for a new air compressor, make sure you choose the right type of cooler. Fluid-Aire Dynamics can help you evaluate your compressor cooling options and choose the best cooling system for your needs. We can also help you size, install and maintain your system.
Talk to an Aire expert today.