If you have a compressed air system, chances are you also have at least one air receiver tank. But do you know why you have an air receiver tank, and what it is doing for your system?
Air receiver tanks don’t always get a lot of attention, but they are an essential component of a 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.
Why an Air Receiver Tank?
An air receiver tank (sometimes called an air compressor tank or compressed air storage tank) is a type of pressure vessel that receives air from the air compressor and holds it 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 compressed air system run more efficiently. The air receiver tank has three main functions:
- It stores compressed air that can be used for short, high-demand events.
- It provides a steady air signal to air compressor controls.
- When used as a “wet tank,” it acts as a secondary heat exchanger, increasing the efficiency of your air dryer.
Average Peaks with 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 air 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 air compressor for larger jobs.
Gain Air Compressor Control
The air receiver tank provides a steady stream of air-to-air control of an air compressor, eliminating short-cycling and over-pressurization. Uneven compressed air utilization causes uneven demand on the air compressor, resulting in rapid cycling of the air compressor controls as the air compressor turns on and off to meet moment-by-moment demand. Each time the system turns on and off (or loads/unloads) it is called a “cycle.” It is better for the air compressor motor to keep these cycles as long as possible.
Over time, frequent short cycling will lead to premature failure of switches and other air 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.
A Secondary Heat Exchanger
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 used, the air discharged from the air compressor may be as hot as 250°F to 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°F to 10°F is not uncommon.
How an Air Receiver Tank Boosts Efficiency
Adding an air receiver tank significantly improves the efficiency of your compressed air system and can even lower your energy and maintenance costs. 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 Air Compressor Operating Pressure
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. 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.
Relying on your air receiver tank for high-demand events 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. On average, every two psi decrease in your system decreases the energy demand by one percent. This can lead to hundreds or thousands of dollars in savings on your energy bills annually.
Increase Dryer Efficiency
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.
Other Air Receiver Tank Benefits
Air receiver tanks improve the efficiency and performance of your system in other ways, as well. Additional benefits include:
- Cycle count reduction:As explained, the air receiver tank reduces cycle 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.
- Pulsation dampening: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.
- Dirt removal: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.
Types of Air Receiver Tanks
Air receiver tanks come in many different sizes and configurations. It’s also important to know there are two types of compressed air storage: wet and dry. The tanks are the same; the difference is in how they are installed.
“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 air compressor and exiting out the top to the dryer.
- 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 air compressor does not experience backpressure because the air does not go through filtration before entering the tank. This results in a steadier pressure signal to the air compressor controller.
“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.
- Dry compressed air is ready to use right out of the tank, so it is immediately available in the case of a high-demand event.
- Dry storage reduces the burden on your air dryer during high-demand events. 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.
Most applications require a combination of wet and dry storage for optimal efficiency and performance.
Optimizing Compressed Air System Performance
The air receiver tank is an unsung hero in the world of compressed air equipment. Ensuring adequate air storage capacity will go a long way toward improving the efficiency and performance of your compressed air system. A compressed air system designer can help you find the right storage volume and wet/dry ratio for your application and air use patterns.
The best news? An air receiver tank is a relatively small investment when compared to the total cost of your compressed air system and associated energy bills — and once installed, your tank will last for many years if cared for properly. Adding more storage is a cost-effective system upgrade with a high ROI. Once you understand all of the ways that your air receiver tank enhances your system, it’s an easy decision to make.
By Derrick Taylor, PneuTech USA