Oil/Water Separators for Compressed Air Systems: Complete Guidelines

Jul 09, 2021 by Brad Taylor

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Do you want to buy oil-water separators for your compressed air system? The answer is probably yes. An external oil-water separator is essential for proper maintenance of your compressed air system and compliance with environmental regulations for disposal of compressed air condensate.  

What Is an Oil/Water Separator? 

An oil-water separator (sometimes called a condensate separator) is exactly what it sounds like: a device that separates oils and lubricants from liquid water. The oil-water separator collects condensate from your air compressor, tank, filters, and dryer, and removes the oil from the water prior to putting it down the drain. It also eliminates the need to have the oily condensate hauled away. 

Why Do I Need an Oil/Water Separator for My Compressed Air System? 

When air is compressed, it produces condensate as a byproduct. That’s because pressurized air becomes saturated when it is compressed. For example, to achieve 100 PSI, the air is compressed 7.89 times atmospheric pressure. As compressed air cools, the cooler air cannot hold the moisture and falls out of the air as condensates. This condensate includes both liquid water (from the moisture in the intake air) and oil from within the compressor. 

The oil comes from the lubricants that protect your air compressor as it runs. The heat generated by air compression causes a small amount of oil to vaporize each time the compressor is used. This oil vapor mixes with water vapor during compression and then falls out of the air along with water condensate as it cools. Because of this contamination, condensate from compressed air systems cannot simply be dumped down a drain or into the environment (e.g., out the wall and onto the ground). 

Air Compressor Condensate Disposal Regulations

The EPA requires condensate to be either collected for proper disposal or treated to remove oil and other contaminants prior to disposal of the water. Air compressor condensate is governed under the 1970 Clean Water Act, which aims to keep harmful contaminants out of groundwater, surface water, and drinking water supplies. Across the country, there are also a variety of state and local statutes regulating the treatment and disposal of wastewater, including air compressor condensate. Read more: 

These regulations set standards for allowable concentrations of various contaminants in wastewater. The federal standard requires that wastewater that is drained into a municipal system contains less than 40 PPM (parts per million) of oil. Water with higher levels of contaminants must be treated or collected and stored in such a way that it will not leach into the environment or into the municipal water supply. Local regulations may be even more stringent; your local waste treatment or water authority will have information specific to your area. 

Aire Tip: Liquid condensate from a compressed air system typically contains around 300 PPM of oil to water, 7.5x higher than the allowable concentrations under Federal law. 

Air Compressor Condensate Disposal Options 

How do you dispose of compressed air condensate? Environmental regulations mean that condensate from a compressed air system cannot simply be discharged into a sanitary drain, dumped somewhere on the plant grounds, or poured into a nearby storm drain or waterway. Manufacturers with compressed air systems have two options when it comes to disposing of oily wastewater. 

  • Collection and storage: Condensate from the air compressor and other system components can be collected from the drain valves into 55-gallon drums or tanks. The water is then hauled away to an approved storage facility. This option tends to be both expensive and inconvenient, involving costs for collection, storage containers, transportation, and storage.  
  • Filtration and treatment: A better option is to filter and treat the wastewater to remove oily contaminants and particulates. After the condensate is treated, the filtered water can simply be discharged down the drain into the sewage system. Over the long run, an air compressor oil-water separator will more than pay for itself in reduced wastewater disposal costs. 

Aire Tip: An air compressor oil-water separator can help you reduce wastewater disposal costs and comply with environmental regulations.

How Does an Oil/Water Separator Work?

The oil-water separator is connected to a collection manifold installed by a contractor or your in-house staff to provide a single connection point to the condensate separator. Condensate sources including the aftercooler on the compressor, the air dryer, any air receiver tank, filters, and any other condensate traps throughout the system. All this untreated liquid is collected and drained into the separator device.

Oil/Water Separation Methods

There are several ways that you can separate oil and water. All of them involve taking advantage of the different chemical and physical properties of oil and water. Some of these include: 

  • Gravity separation: The simplest method involves simply letting the liquids settle naturally. Oils are lighter than water and will gravitate to the top of the column, where they can be skimmed off. (You’ll observe similar separation if you shake vegetable oil and water together and then let it sit for a while.) However, some of the vaporized oil will become emulsified in the water during compression and cannot be separated by gravity alone. For this reason, gravity separation is not considered sufficient to meet the PPM requirements for the disposal of compressed air condensate. 
  • Chemical absorption: This method uses a chemical media that attracts and bonds with oil and repels water. Oils react chemically with the media, trapping them within the filter. 
  • Non-chemical absorption: In non-chemical absorption, the oil is pulled into and held by the media without a chemical bonding process. The material must be oleophilic (oil-attracting) and hydrophobic (water-repelling), so that oil is retained by the media and water passes through. Polypropylene is an example of an oil-absorbent, water-repelling material. 
  • Adsorption: Adsorption is similar to absorption, but instead of being drawn into the core of the media, oil is held on the surface. Oil is attracted to the surface of the adsorbent material electrostatically, while water is repelled. Adsorbent materials may have many, many tiny pores in them to maximize the available surfaces where oil can adsorb.

Air Compressor Oil/Water Separators

Most air compressor oil-water separators work using a combination of absorption and adsorption. These systems typically have multiple stages of filtration. Here’s how it works: 

  • Condensate from drain valves enters the cyclonic depressurization chamber, along with a certain amount of waste compressed air that is released when the drain valves open and close. Air is allowed to depressurize in this chamber and then vented back to the atmosphere so it does not interfere with the filtration process.  
  • Liquids drain down into an oil filter cartridge made of polypropylene fibers, an oil-absorbent material. Most of the oil is collected and stabilized in polypropylene fibers. Absorbing the bulk of the oil here maximizes the life of the activated carbon cartridge. 
  • Activated carbon is the final stage of filtration. Activated carbon is an oil-adsorbent material, containing a large number of extremely small pores that the liquid is filtered through. As liquid drains through the micro-pores, oil is attracted to and held by the pore walls, while water passes through. Activated charcoal is the only filter material that can reduce oily contaminants down to Federally mandated limits, typically reducing concentrations to ~10 PPM. 
  • The remaining water flows out of a drain at the bottom, where it can be safely discharged into a sanitary drain. 

Aire Tip: Zero-Loss Drain Valves will minimize the loss of compressed air from drains and are recommended for optimal performance of the oil-water separator.

Oil/Water Separator Installation and Maintenance

The air compressor oil-water separator is a fairly simple device, but it does require proper installation and regular maintenance. Here are a few things you should know. 

Installing the Oil/Water Separator for Your Compressed Air System

The oil/water separator is typically installed in the compressor room. Be sure to locate the oil-water separator in an area that is convenient to reach for filter cartridge changes. 

The oil/water separator is fed by a manifold that collects condensate piped from each air compressor, dryer, filter, and tank in your compressed air system. Every component that produces condensate must have a condensate drain and be connected to the oil-water separator. 

Sizing the Oil/Water Separator for Your Compressed Air System

The oil/water separator must be properly sized for your compressed air system. They are typically sized according to the maximum airflow (CFM) of your system. The higher the CFM of your air compressor, the more condensate will be created, requiring more filtration. The PneuTech CSO Series of Oil-Water Separators has models for 40, 80, 150, 300, 725, and 1250 CFM systems.

 It is also important to know that the maximum oil absorption, condensate flow, and free air delivery (FAD) for the oil/water separator will vary depending on the environment in which the filtration system is working. In general, the filters can hold more oil in colder environments and can handle a greater condensate flow rate in warmer environments. If the filtration system will be operating in a very cool (<60°F) or warm (>100°F) environment, this may impact system sizing or the frequency of filter changes.  (It should be noted that most ratings are based on a single shift. So your operation runs 24/7, you may need to change the elements more frequently.)

Maintaining the Oil/Water Separator for Your Compressed Air System 

Maintaining the oil/water separator is easy. There are three basic levels of maintenance:

  • Weekly to monthly: Perform a sample test of the water coming out of the separator to check for excess oil. This can be done by collecting liquids coming out of the filtration system in a clear jar. The liquid in the jar is compared to a foggy piece of glass provided with the separator. When the water is foggier than the glass, that indicates that the filter cartridge is fully loaded and there is excess oil in the discharge water. When this happens, it’s time to change the filter cartridge. 
  • Monthly to quarterly: Periodically, open up separation tanks and such off any excess oil that has accumulated on the top with an oil change suction device used in the automotive industry. When oil is separated from water, sometimes it will develop a jelly-like substance that can build up outside of the filter media. This sludge should be cleaned up at this time as well.  
  • Quarterly to annually: Replace the filter cartridges at least annually or more often (quarterly or semi-annually), depending on your usage patterns and the size of your filtration system. When the filter cartridge is fully saturated, it can simply be pulled out of the chamber and replaced with a new one. All of the oil condensate remains safely stabilized in the polypropylene and activated carbon cartridges. The old cartridge should be sealed in a plastic cover and disposed of according to local directives and laws.

The frequency of filter changes will depend on the size and usage patterns for your compressed air system as well as the environment. The total amount of condensate produced by your compressed air system is tied to the temperature and humidity of the incoming ambient air; if you are operating in a warm, humid environment, your system will produce more condensate than if you are operating in a cooler, drier environment. As we explained above, the oil absorption/adsorption capacity of your filter cartridge is also impacted by temperature. The warmer it is, the less oil the filter will be able to hold. If your climate is variable, be sure to check the filter more frequently in warmer months. 

Selecting an Oil/Water Separator for Your Compressed Air System

An oil/water separator is a good investment for your compressed air system. Not only is it a more environmentally friendly solution for handling compressed air condensate, but it can also help you save money on wastewater management. Draining treated water into your regular sanitary drains is much easier and cheaper than collecting, transporting, and storing gallons of contaminated water. 

 If you’re not sure which oil/water separator is right for you, we can help. Our system designers can help you choose the right style and size it properly for your compressed air system and environment. 

Need help selecting an oil/water separator? Call us and let one of our compressed air specialists answer all your questions.

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