The Purpose & Functions Of Air Receiver Tanks

We’re often asked if a particular air compressor installation requires an air receiver tank. The quick answer is most applications will benefit from air storage, even though it is not required for many rotary screw applications.

Air receiver tanks can be vertical or horizontal; tank choice is typically determined by the installation location, including the amount and shape of available space. Vertical air receiver tanks are readily available in sizes from 10 to 2,560 gallons, and horizontal receivers are available in 5 to 2,560 gallon capacities. 

The air receiver tank serves many important functions:

• Dampens pulsations from the discharge line of the compressor, resulting in steadier system pressure,
• Serves as a reservoir to take care of sudden or unusually heavy demands beyond the compressor capacity,
• Prevents too frequent loading and unloading of the compressor, and
• Captures moisture and possible oil carryover that may be present in the compressed air as it comes from the compressor or aftercooler.

Air receiver tanks are also used to meet heavy, short-time demands of certain equipment. Manufacturers of this equipment can supply information on the air requirements in such cases.

The tank capacity required for certain applications can be calculated, but experience and judgment are essential. It is advisable to consult the compressor manufacturer concerning the capacity required.

Air Receiver Tank Capabilities

The amount of time an air receiver tank can supply compressed air without an excessive drop in pressure can be determined using this equation:

T = V   P1 – P2
                Cp0

Where:

T = time, minutes
P1 = initial receiver pressure, psig
P2 = final pressure, psig
p0 = atmospheric pressure, psia
C = air consumption, cubic feet of air per minute
V = receiver capacity, cubic feet

This equation assumes that the air receiver tank’s temperature is constant at standard atmospheric temperature and that p0 is standard atmospheric pressure. It also assumes that no air is supplied during the interval.

If newly created compressed air is being supplied steadily to the receiver at a rate of S cubic feet of free air per minute, then C in the above equation may be replaced by C minus S:

T = V  P1 – P2
       (C-S)p0

Where:

T = time, minutes
P1 = initial receiver pressure, psig
P2 = final pressure, psig
p0 = atmospheric pressure, psia
C = air consumption, cubic feet of air per minute
V = receiver capacity, cubic feet
S = supply from the compressor(s)

You can read more about sizing your air receiver tank here.

Safety Standards For Air Receiver Tanks

There is a danger in using air receiver tanks of questionable or unsound construction. The American Society of Mechanical Engineers (ASME) has established a code, used extensively throughout North America as law, governing the construction of pressure vessels. Air receiver tanks should satisfy this code, as well as any other local government or insurance regulations that apply to this type of product.

Air receiver tanks must also be equipped with ASME-approved safety valves, a pressure gauge and a drain valve.

The safety valves must not be set at a pressure higher than the working pressure for which the receiver is stamped. These safety valves must also be set slightly higher than the operating pressure, so it follows that the operating pressure of a system should be about 10 percent lower than the pressure rating stamped on the receiver.

Moisture Removal For Air Receiver Tanks

Removal of moisture in cold weather is essential, as water accumulates and gets carried back into the outgoing airflow. When temperatures drop below 32° F, the condensate line can freeze, and the pipe may become damaged. Therefore, if the air receiver tank is outdoors, the safety valve and the pressure gauge should be indoors or insulated to prevent freezing, and the tank should have some method of moisture removal.

Meanwhile, storing moisture in air receiver tanks leads to the formation of rust and scale on the inside of the tank, which can become loose and get carried downstream in the outgoing air. This rust and scale can contaminate the air and cause blockages.

The compressed air leaving a vertical air receiver tank should always be higher than the inlet port because the moisture and other contaminants, which condense out of the incoming compressed air, collect in the bottom of the air receiver tank. These liquid contaminants usually drain away, but if the drain line is blocked or the drain trap fails, the liquid contaminant level rises and gets carried into the outgoing air stream. We talk about air treatment methods in this article.

To prevent this from occurring, it is a good practice to periodically check that the tank is free of moisture by tripping the auto drain trap manually.

Why Is There Water In Air Receiver Tanks?

The reason for the large volumes of liquids in the air receiver is that the tank is made of steel, so the wall temperature of the vessel is the same as the ambient temperature.

If the compressor has an air-cooled aftercooler, there is always a difference between the inlet and discharge air temperature, known as the approach temperature. As cooling is not 100% efficient, the air discharging from the aftercooler will always be higher than the ambient air used to cool the hot compressed air.

As soon as air leaving the aftercooler enters the air receiver tank, it meets the colder steel wall of the tank, which is usually at ambient temperature. At this point, moisture condenses from the compressed air as the air chills. If the air stays in the tank for enough time, the temperature of the air in the tank will be the same as the ambient, and no more moisture will condense out.

Learn more in the article, “Why Is There Water In My Compressed Air.”

Installing An Air Receiver Tank

Air receiver tanks are beneficial for most air compressor installations. Knowing how and where to install them and with what components will help ensure your air system operates as optimally as possible.

If the air receiver tank is used in conjunction with a compressed air drying system, the location and type of receiver tank should follow the recommendations of the dryer supplier.

For the best possible outcome, air compressor systems and air receiver tanks should always be designed and installed by those with experience.