What is a compressed air receiver tank—and how do you know if you need one? Our 2020 guide to compressed air receiver tanks explains how they work, what they do, and how you can use them to maximize the efficiency of your compressed air system.
What Is a Compressed Air Receiver Tank?
An air receiver tank (sometimes called an air compressor tank or compressed air storage tank) is exactly what it sounds like: a tank that receives and stores compressed air after it exits the air compressor. This gives you a reserve of compressed air that you can draw on without running your air compressor.
An air receiver is a type of pressure vessel; it holds compressed air under pressure for future use. The tanks come in a range of sizes and in both vertical and horizontal configurations.
The Purpose of an Air Receiver Tank
An air receiver tank provides temporary storage for compressed air. It also helps your air compression system run more efficiently. The air receiver tank has three main functions in your compressed air system:
- It stores compressed air that can be used for short, high-demand events.
- It provides a steady air signal to compressor controls.
- When used as a “wet tank”, it acts as a secondary heat exchanger, increasing the efficiency of your air dryer.
Compressed Air Storage
The primary role of an air receiver tank is to provide temporary storage for compressed air. Storing compressed air allows the system to average the peaks in compressed air demand over the course of a shift. You can think of your air receiver tank like a battery for your compressed air system, except it is storing air instead of chemical energy. This air can be used to power short, high-demand events (up to 30 seconds) such as a quick burst of a sandblaster, dust collector pulse, or someone using a blowgun to dust themselves off. The air in the tank is available even when the compressor is not running. Storing compressed air reduces sudden demands on your air compressor, prolonging the life of your system. Using an air receiver tank may also allow you to use a smaller horsepower compressor for larger jobs.
The air receiver tank provides a steady stream of air to compressor controls, eliminating short-cycling and over-pressurization. Uneven compressed air utilization causes uneven demand on the air compressor, resulting in rapid cycling of the compressor controls as the compressor turns on and off to meet moment-by-moment demand. Each time the system turns on and off (or loads/unloads) is called a “cycle”; it is better for the compressor motor to keep these cycles as long as possible. Over time, frequent short cycling will lead to premature failure of switches and other compressor components. Rapid cycling can result in excessive wear of the motor contactor or even a direct motor short because of winding insulation. The air receiver tank eliminates short cycling and provides more consistent system pressure to controls.
As air is compressed under pressure, its temperature increases; this is a simple law of physics known as the Pressure-Temperature Law. Depending on the type of air compressor you are using, the air discharged from the compressor may be as hot as 250 – 350°F. This is too hot for most air-operated equipment to use directly. Hotter air also contains more moisture, which will result in excess water vapor that will condense in control lines and tools if it is not removed. The condensed air must be cooled and dried before it is utilized. A heat exchanger is used to remove excess heat caused by compression. The air receiver tank acts as a secondary heat exchanger; as air sits in the tank or slowly flows through it, it naturally cools over time. The air receiver tank supports the work of a primary heat exchanger; lowering the temperature of the air an additional 5 – 10°F is not uncommon.
Efficiency Benefits of Air Receiver Tanks
Adding an air receiver tank significantly improves the efficiency of your compressed air system. They do this by:
- Reducing waste of compressed air from excessive sump blowdowns
- Lowing the pressure requirements for the air compressor and air network
- Increasing the efficiency of the air dryer by reducing moisture
Reduce Waste of Compressed Air
As the air compressor cycles on and off, compressed air can be wasted. Every time a rotary screw air compressor unloads, the sump tank (oil tank) is vented. Compressed air is released during the venting. Over time, this adds up to the loss of thousands of cubic feet of compressed air that could otherwise have been used to power processes in your facility. A properly sized air storage tank reduces frequent cycling and venting.
Reduce Operating Pressure for the Air Compressor
Compressed air storage also allows you to reduce the pressure at which your air compressor operates. Without a store of compressed air to draw on, the system will have to operate at higher pressures so it is always ready to meet peak demands. In essence, you are asking your system to operate as if your facility is always running at maximum demand. This leads to increased energy use and wear and tear on the system. On average, for every 2 PSI that you increase the pressure of your system increases the energy demand by 1%. This can lead to hundreds or thousands of dollars added to your energy bills annually. As explained above, adding an air receiver tank to your compressed air system will even out these peaks in demand, allowing you to meet intermittent periods of high demand without increasing the overall pressure of your system.
Increase the Efficiency of the Dryer
The heat exchanger function of the air receiver tank helps to improve the efficiency of your air dryer. As air passes slowly through the receiver tank, it cools. Cooler air can’t hold as much moisture as warm air, so excess moisture condenses and falls out of the air as a liquid. The water drains out of a valve at the bottom of the tank. By removing some moisture in advance, the air receiver tank reduces the amount of work the air dryer needs to do. This improved efficiency translates to additional energy savings for your system.
Wet vs. Dry Compressed Air Storage: What’s the Difference?
When shopping for an air receiver tank, you may be asked whether you want “wet” or “dry” compressed air storage. The difference is in the location of the air storage tank in your compressed air system; there is no difference in tank construction or design.
- “Wet” storage tanks are located before the air drying system. Air flows through the tank in this configuration, entering through the bottom port from the compressor and exiting out the top to the dryer.
- “Dry” storage tanks are located after the air dryers to store compressed air that has already been dried and filtered. It is not necessary to flow the compressed air through the tank for dry storage.
Advantages of Wet Compressed Air Storage
With wet air storage, the receiver tank is positioned in between the air compressor and the air dryer. Wet air enters the receiver tank from the air compressor through the lower port in the tank and exits through the upper port to enter the air drying system. A wet air receiver tank has several benefits.
- As explained above, wet storage increases the efficiency of your air dryer by allowing excess water and lubricant to condense out of the air before it hits the dryer.
- A wet air storage tank also prolongs the life of the pre-filter element, which is located in between the wet storage tank and the dryer. Since the air going through the filter is cleaner and dryer than it would be directly out of the air compressor, slugging of the filter with liquids is minimized, along with resulting pressure drop on the air dryer side of the system.
- The compressor does not experience back pressure because the air does not go through filtration before entering the tank. This results in a steadier pressure signal to the compressor controller.
Advantages of Dry Compressed Air Storage
On the other hand, a dry air storage tank has advantages as well. Dry compressed air is ready to use right out of the tank.
Without a dry air tank, air from the wet tank will have to go through the air dryer before it is used. During periods of high demand, the dryer is at risk of becoming over-capacitated as the system tries to pull air through at higher volumes than the dryer is rated for. If the dryer cannot keep up with the demand, drying efficiency is reduced, potentially leading to unwanted water in the air lines.
Finding the Right Ratio of Wet to Dry Compressed Air Storage
For most applications, it makes sense to have a combination of wet and dry storage.
The ideal ratio of compressed air storage is 1/3 wet to 2/3 dry capacity. For example, if you have a total of 1,200 gallons of compressed air storage, 800 gallons should be dry storage and 400 gallons should be wet. Dry air is ready to use on-demand. The wet air tank increases the efficiency of the dryer and acts as a secondary reserve when dry air is exhausted. Dry air storage needs to be greater than wet storage to minimize the risk of over-capacitating the air dryer during periods of high demand.
An exception to this rule is for applications that have steady airflow without sharp peaks in demand. In this case, there is no need for a dry storage tank because air will simply flow through it without being stored up. This is often the case in robotic manufacturing facilities where airflow is consistent and predictable.
How Much Air Storage Capacity Do You Need?
The volume of compressed air storage capacity needed by a facility depends on several factors:
- The air compressor capacity in cubic feet per minute (CFM)
- Peak CFM requirements at moments of maximum demand
- The consistency of airflow
- The diameter of the piping
Calculating Compressed Air Storage Requirements
A good rule of thumb for most applications is to have three to five gallons of air storage capacity per air compressor CFM output. So if your air compressor is rated for 100 CFM, you would want 300 to 500 gallons of compressed air storage. As explained above, 1/3 of the total storage capacity should be wet storage and 2/3 should be dry storage.
Flow Consistency and Compressed Air Storage Requirements
While the standard rule works well for many applications, you will also want to consider other variables in determining your compressed air storage needs. Flow consistency has a large impact on storage requirements.
- Facilities with very steady airflow, such as robotic facilities, typically don’t need as much stored air. That’s because they don’t have frequent high bursts of demand that rely on stored air. In this case, air storage can be reduced to 2 gallons per CFM of air compressor capacity. All storage should be wet storage in this case, as explained above.
- Facilities with high variability in airflow and large peaks in demand may require larger volumes of stored air. This extra capacity will ensure that the system will be able to keep up with periods of high demand. Testing to determine CFM at peak demand will be needed to calculate air storage requirements.
Pipe Diameter and Compressed Air Storage Requirements
The final consideration in determining compressed air storage requirements is the size of the pipework in the system. The pipes also store air for your compressed air system, and the larger the pipes, the more storage they provide. For systems with pipework of 2” or greater diameter, it may be worthwhile to consider that volume into the calculation.
Can the Air Receiver Be Stored Outdoors?
Compressed air receiver tanks can be bulky, so many compressed air system owners would prefer to store them outside. Outdoor storage saves precious floorspace in the facility.
It also helps to reduce strain on your HVAC system in warm weather. The compressed air storage tank radiates heat as hot air from the compressor cools within the tank, raising temperatures in the compressor room. Storing your tank outside avoids excess heat buildup in the compressor room and also helps the storage tank perform its secondary job as a heat exchanger more efficiently.
However, outdoor storage only works in milder, non-freezing climates. Make sure your climate is suitable for outdoor placement of your compressed air tank.
Climate Considerations for Air Receiver Tank Storage
Outdoor storage of the air receiver tank is only appropriate for environments that stay above freezing year-round. In freezing temperatures, outdoor tanks can ice up and even rupture—a costly and potentially dangerous outcome. If your area experiences freezing temperatures during part of the year, it is safest to keep your tank indoors.
Tips for Outdoor Storage of Air Receiver Tanks
If you are storing your air receiver tank outdoors, be sure to conduct frequent inspections to monitor for corrosion. Any signs of corrosion should be addressed immediately to maintain the integrity of the tank.
If your area is subject to cooler temperatures with an occasional risk of icing, take special care of your tank in cooler weather. The tank will generate some heat on its own. However, if temperatures drop too far, the tank is still at risk of freezing. Insulating your tank and providing auxiliary heating during cold weather may be necessary to prevent damage.
Air Receiver Tank Internal Lining Options
There are three main options when it comes to the internal lining of your tank.
- Bare steel interior with primer exterior (typical)
- Epoxy coated or galvanized interiors
- Stainless steel
Steel Air Receiver Tanks
The majority of air receiver tanks are bare steel on the inside with a primer coating on the outside to reduce corrosion. The exterior paint is commonly matched to the compressor equipment. A basic steel tank works well for most applications and is the least expensive option. However, they may be prone to corrosion if too much liquid is allowed to build up inside the tank.
Epoxy Coated and Galvanized Air Receiver Tanks
Some air receiver tanks have treated interior linings to reduce corrosion and maintain air quality. These liners fall into two categories.
- Epoxy coatings are sprayed onto the interior as a liquid and then cured into a tough, anti-corrosive coating. Epoxies work by creating a moisture-proof barrier between the air and the base metal of the tank.
- Galvanized tanks are treated with a protective zinc coating that halts the formation of rust. Zinc protects the base metal by reacting chemically with corrosive agents before they can reach the base.
Both methods provide long-lasting protection for the interior of the tank, but they do add to the cost and lead time. Coated or galvanized tanks are better at maintaining air purity because they reduce the risk of particulates caused by corrosion entering the airstream. Applications needing higher purity air, or users concerned about the longevity of their air tanks, may want to consider one of these options.
Stainless Steel Air Receiver Tanks
Stainless steel air receiver tanks are primarily used for specialty applications where very high-purity air is required. They are the most expensive option, but they are highly durable and corrosion resistant and maintain exceptional air purity. Hospitals, labs, electronics manufacturers and other applications requiring high-purity air should consider a stainless steel tank.
Air Receiver Tank Accessories
Air receiver tank accessories are essential for tank safety and operation. While the tank itself is just a large sealed metal tube, all tanks must have at a minimum:
- A drain to release excess liquid building up inside the tank
- A gauge to monitor interior pressure
- A safety relief valve
Electronic Auto Condensate Drain
Automatic drain valves eliminate the need for daily manual draining of liquid inside the air receiver tank. An electric automatic drain valve is programmed to open at set intervals to let accumulated liquid drain out.
Zero Air-Loss Condensate Drains
Zero air-loss condensate drains also provide automatic drainage of the tank. Instead of draining at set intervals, they use a float mechanism to control drainage. The drain will only open when needed, saving energy and reducing air loss from the tank.
The pressure gauge provides a visual indicator for the interior pressure of the air in the tank. You need the gauge to monitor pressures and ensure that the tank is not under stress from over-pressurization.
Pressure Relief Valves
A pressure relief valve is required for all air receiver tanks per OSHA and ASME guidelines. The pressure relief valve opens automatically to release some air if pressures in the tank are too high. This safety mechanism is essential to minimize the risk of a dangerous rupture due to over-pressurization. The relief valve is typically set to 10% higher than the working pressure of the compressed air system but never more than the rated pressure of the tank’s ASME certification.
Vibration pads are not required for all applications, but they are recommended if the air compressor is mounted on top of the tank. Vibration pads absorb vibrations from the compressor motor and reduce fatigue on the tank.
ASME Certification for Air Receiver Tanks
Many buyers wonder if ASME certification is important for air receiver tanks—and the answer is yes. All air receiver tanks used in industrial applications must be certified by ASME for safety and performance.
What are the ASME Standards for Air Receiver Tanks?
The American Society of Mechanical Engineers, or ASME, is an organization that sets engineering codes and manufacturing standards for a variety of machines, parts and system components. ASME acts as an independent quality assurance organization to ensure the safety and quality of manufactured items. An ASME certification stamp means that the manufacturer has met all safety and engineering standards for their product.
ASME has developed a set of codes and standards for pressure vessels, including air receiver tanks. The ASME Boiler and Pressure Vessel Certification Program sets rules governing the design, fabrication, assembly, and inspection of pressure vessel components during construction. These rules include engineering standards for the thickness of the tank body, welds and joints, connections, and other components of the tank. Tank manufacturers must conform to all of the rules to obtain ASME certification.
Can I Buy an Air Receiver Tank Without ASME Certification?
Non-code air receiver tanks should never be used, especially for industrial applications.
Some big box stores carry non-code air receiver tanks. While these may be cheaper, they have not undergone the rigorous manufacturing processes and quality testing needed to ensure that they are safe and reliable. Using a non-code air receiver tank could put your life and the lives of your coworkers at risk.
Inspecting Your Air Receiver for Code Violations
If you are not sure whether or not your air receiver tank meets code requirements, you should have it inspected. Your local Fire Marshall may provide this service. They will stop in and test your tank with ultrasonic metal thickness testing technology. If your air receiver tank does not pass the inspection, it should be decommissioned and replaced immediately.
Air Receiver Tank Safety
Air receiver tanks hold air under immense pressure. This creates safety hazards if the tank is not up to code or is not maintained properly.
Causes of Air Receiver Tank Failure
Pressure vessels must be built to withstand high internal pressures over a long period of time. Over time, corrosion, stress and fatigue can make tank failure more likely. The most common causes of air receiver failure are:
- Faulty design/use of non-code tanks
- Operation above maximum allowable working pressure (over-pressurization)
- Improper installation
- Weld failure
- Improper repair of cracks/leaks
- Exposure to extreme environmental conditions (freezing or overheating)
- Safety valve failure
Air Receiver Tank Occupational Hazards
The high internal pressures within an air receiver tank make failure extremely hazardous. Cracking or weld failure can cause the tank to burst with explosive force, projecting large pieces of metal or fragments of shrapnel at high speed. Air receiver tank failure may result in extensive damage to the facility and nearby equipment and severe injury or death for nearby workers.
Maintaining Air Receiver Tank Safety
It is essential to follow all safety guidelines listed in the owner’s manual for your air receiver tank. To improve tank safety, be sure to:
- Only use ASME-certified air receiver tanks.
- Never over-pressurize the tank; follow operating guidelines for maximum pressure.
- Make sure that the tank has a pressure gauge and it is functioning correctly.
- Periodically inspect the tank for corrosion, signs of weld seam stress, cracks, thinning of the vessel walls, and other defects.
- Make sure that the tank has an ASME-certified safety relief valve and the valve is working correctly.
- Drain the tank frequently to prevent liquids from accumulating inside the tank.
- Have all alterations or repairs completed by certified professionals to ensure that the repair meets quality standards.
- Provide safety training for air receiver tank operators.
Consult the OSHA guidelines for pressure vessel safety for more information.
Can Your Air Receiver Tank Help You Save Money?
An appropriately-sized air receiver tank will improve the efficiency of your system—and can even reduce your operating costs for your compressed air system. Your air receiver tank reduces energy consumption and saves wear and tear on your system.
Your compressed air receiver tank is like a battery for your facility, providing an extra reservoir of compressed air you can draw on during periods of high demand. This lets you reduce the overall operating pressures for your system, resulting in lower energy costs. You may also be able to purchase a smaller air compressor with lower CFM capacity by relying on your air receiver tank for high demand events.
Cycle Count Reduction
As explained above, the air receiver tank reduces cycles counts for your air compressor by evening out peaks in compressed air demands. Lower cycle counts add up to lower energy use and less wear and tear on other system components, extending the life of your air compressor.
The air receiver tank functions as a pulsation dampening device, absorbing vibrations from the air compressor motor and pulsations in the air stream. This reduces fatigue on piping and other system components.
As air cools in the air receiver tank, excess liquid condenses and falls out of the air. This results in less work for the air dryer and less energy consumption.
Particulates can enter the airstream due to corrosion within the system, motor exhaust from the air compressor, or particulates in facility air. Many of these particulates will fall out of the air along with condensate within the air receiver tank. The excess dirt is then simply drained away with the liquids. As a result, the air entering the air dryer is both cleaner and drier than air directly from the air compressor.
The Importance of Air Receiver Tanks
Your air receiver tank is an essential component of your compressed air system. Having a properly sized air receiver tank ensures the safe and efficient operation of your system and provides a reservoir of extra power for use during periods of peak demand.
If you’re not sure how much air storage capacity you need, or if you have questions about maintaining your tank for safe operation, the experts at Fluid-Aire Dynamics can help. We will perform an assessment of your compressed air usage patterns and recommend an air receiver tank that will fit your needs. We can also help you inspect, repair or upgrade your current storage system.
Let your compressed air tank do its job for you! Call us today and ask for pricing.