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Before you buy compressed air receiver tank, take some time to learn about the device itself. Our 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.
An air receiver tank (sometimes called an air compressor tank or compressed air storage tank) is 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.
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:
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 as a battery for your compressed air system, except it stores 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.
Adding an air receiver tank significantly improves the efficiency of your compressed air system. They do this by:
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.
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.
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.
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.
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.
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.
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.
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.
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.
The volume of compressed air storage capacity needed by a facility depends on several factors:
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.
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.
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.
Compressed air receiver tanks can be bulky, so many compressed air system owners would prefer to store them outside. Outdoor storage saves precious floor space 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.
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.
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 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.
There are three main options when it comes to the internal lining of your tank.
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.
Some air receiver tanks have treated interior linings to reduce corrosion and maintain air quality. These liners fall into two categories.
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 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 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:
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 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.
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.
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.
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.
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.
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.
All air receiver tanks must also be inspected periodically once they are installed. OSHA does not mandate a specific testing interval, but it is recommended that all air receiver tanks be inspected at least annually. Your insurance company or local governing board may have different requirements. OSHA requires that formal inspections be performed by an inspector holding a valid National Board Commission and in accordance with the applicable chapters of the National Board Inspection Code. Manufacturers are required to keep records of formal inspections and make them available to OSHA representatives upon request.
In between formal board inspections, manufacturers should conduct frequent visual inspections of the air receiver tank to look for signs of corrosion, damage or weld failure. Check drains daily and pressure relief valves quarterly to make sure they are operating correctly. Contact your manufacturer or compressed air system installer immediately if you see any signs of problems with your air receiver tank.
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.
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:
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.
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:
Consult the OSHA guidelines for pressure vessel safety for more information.
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.
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 the air cools in the air receiver tank, the 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.
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.
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FeaturesCompressed air storage is an important part of any compressed air system. Air receivers help ensure better air quality, can lower your air compressor power consumption, and can help keep the system pressure constant. The installation of properly sized storage tanks can help your compressed air-powered machines work better, making plant production operations more reliable. Five valuable uses of compressed air storage are discussed in this article.
Cooling and Water Separation
The air produced by air compressors is often hot and completely saturated with water. This air and water mixture can overwhelm your air dryers, allowing wet air to contaminate downstream components. First passing the flow through a storage receiver allows the air to cool and the entrained water to separate out within the low-velocity “quiet zone” of the vessel. As a rule of thumb, every 20°F of cooling drops the water content in half. The water that condenses and falls to the bottom of the tank can then be removed by automatic drains.
Stabilizing Pressure
The larger the system storage capacity, the more stable the system pressure during periods of high demand. Consider a system with only 120 gallons of storage. A compressed air demand that exceeds the capacity of the installed compressor by 30 cfm will pull the system pressure down by 27 psi in one minute. If the storage receiver were five times bigger, the pressure would only drop by about 5 psi for the same flow event. Having a more stable pressure allows the system pressure to be set lower because you don’t need to compensate for low-pressure events, saving about 1% compressor power for every 2-psi pressure reduction.
Compressor Control
Large system storage can make the compressors run more efficiently. If you have lubricated screw compressors running in load/unload mode, the addition of storage capacity reduces the number of times per minute the compressor will cycle. Each time the compressor cycles, it must release air in its oil separator only to charge it back up when it reloads—not an efficient operation. A 30-hp compressor rated at 120 cfm with a 120-gallon receiver will cycle inefficiently over 160 times per minute. Installing a 660-gallon tank or larger for this size of compressor will reduce the cycles to less than 30 per minute and reduce the power consumption by 10% or more (Fig. 1).
Protect Critical Uses
Sometimes individual compressed air use is particularly sensitive to pressure fluctuations. It could be something like a clamping application or perhaps a clutch for a drive mechanism. If the system pressure varies excessively, there may be problems with these applications because a constant, reliable pressure is needed. In these cases, a properly sized local storage receiver with a check valve can be installed to protect sensitive applications. The check valve traps pressure in the storage receiver for the exclusive use of the local application. This allows main system pressure to vary for short periods of time, but protects sensitive applications.
Reduce High-Volume Uses
Once in a while, a system will have a compressed air demand that uses a large volume of compressed air in a short period of time. This demand might hit the system and overwhelm the system compressors, causing low pressure that might negatively affect other pressure-sensitive compressed air loads. This almost always causes system pressures to be set much higher than needed, causing the compressors to consume more energy (Fig. 2).
System storage receiver capacity can be applied to average out the effect of large, high-volume system demands so the compressors can operate at normal pressure, not affecting sensitive loads. For example, if the previous 30-hp, 120-cfm system with 120 gallons was hit with a 30-second – 240-cfm compressed air demand every five minutes, a 120-cubic foot event, the system pressure might drop by 80 psi during each load cycle. If 660 gallons of compressed air storage were added for the exclusive use of the event, the pressure would only drop by about 20 psi at the large user. The compressed air could be replenished between cycles in 4.5 minutes at a flow of only 27 cfm through a needle valve so that the receiver is full of air just before the next event. If this were done, it is possible other compressed air users would not see any pressure decay during the large event.
Compressed air storage can benefit compressed air systems in many ways. It is a rare case indeed where there is too much compressed air storage capacity. Consider adding storage to your system for better efficiency to save operating costs and increase system reliability.
By Ron Marshall for the Compressed Air Challenge
Learn more about system storage at one of the Compressed Air Challenge seminars or download a fact sheet from the website library. For a schedule of events, see www.compressedairchallenge.org.