The Purpose and Functions of Compressed Air Storage Tank
We’re often asked if a particular air compressor installation requires the use of an air receiver tank. Most applications will benefit from the use of air storage whether it’s a vertical or horizontal air tank. The choice of what style of tank is generally made by the installation location and the amount and type of space available. Vertical receiver tanks are readily available in sizes from 10 – 2560 Gallons, and horizontal receivers are available from 5 – 2560 Gallon capacities.
The receiver serves many important functions. It damps pulsations from the discharge line of the compressor, resulting in essentially steady pressure in the system. It serves as a reservoir to take care of sudden or unusually heavy demands in excess of the compressor capacity. It prevents too frequent loading and unloading of the compressors. In addition, it serves to precipitate some of the moisture and possible oil carryover that may be present in the compressed air as it comes from the compressor or that may be carried over from the after-cooler.
The minimum receiver capacity for certain applications may be calculated, but experience and judgement are important at this point. It is advisable to consult the compressor manufacturer with regard to the capacity required, particularly for a system receiver. Receivers are also used to meet heavy, short time demands of certain equipment, and the manufacturers of this equipment could supply the information on the air requirements in such cases.
The time interval during which a receiver can supply air without excessive drop in pressure may be found from the equation:
T = V P1 – P2
Where T = time, minutes
P1 = initial receiver pressure, psig
P2 = final pressure, psig
P0 = atmospheric pressure, psia
C = air equipment, cubic feet of air per minute
V = receiver capacity, cubic feet
S = supply from the compressor(s)
Read more about sizing your air receiver tank.
This equation assumes that the temperature of the receiver is constant at standard atmospheric temperature and that P0 is standard atmospheric pressure. It also assumes that no air is supplied to the receiver during the time interval. If 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.
There is a danger in using air receivers 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 unfired pressure vessels. Receivers should satisfy this code, as well as the other local government or insurance regulations that may apply to this type of product.
The use of special tanks of lighter construction that are manufactured for pressures less than 125 psig is not recommended for air systems even if at the time of installation no higher pressures are anticipated. The slight savings, if any, do not compensate for the risk that at some future time the lighter tank may be inadvertently used for higher pressures.
Receivers must be equipped with ASME-approved safety valves, pressure gauge and a drain valve.
Receiver safety valves must not be set at a pressure higher than the working pressure for which the receiver is stamped. These safety valves must be set slightly higher than the operating pressure, so it follows that the operating pressure of a system should be about 5 percent lower than the pressure rating stamped on the receiver.
Adequate space should be allowed for draining condensate from the receiver with an auto drain trap.
If the receiver is located outdoors, the safety valve and the pressure gauge should be indoors or insulated to prevent freezing and the tank should have some method of positive moisture evacuation. Removal of the moisture in cold weather is important as this moisture will accumulate and get carried back into the flow of out-going air. If the temperature should drop below 32° F, the condensate line would freeze and the pipe may become damaged.
The storage of quantities of moisture in receiver tanks leads to the formation of rust and scale on the inside of the tank, which can become loose and get carries down-stream in the outgoing air. This rust and scale can cause problems of blockage in air using components and premature blockage of filters.
The compressed air leaving a vertical receiver tank should always be at a higher level than the inlet port. The reason for this is that the moisture and other contaminants, which condense out of the incoming compressed air, collect in the bottom of the receiver tank. These liquid contaminants are normally drained away, however should the drain line become blocked or the drain trap fail to function, the liquid contaminants level would rise and get 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.
The reason for the large volumes of liquids in the air receiver is that the tank is made of steel, and the wall temperature of the vessel is the same as the ambient temperature.
If the compressor is supplied with an air-cooled aftercooler, there is always a difference between the inlet and discharge air temperature (the approach temperature). As the cooling is not 100 percent efficient, the air discharging from the aftercooler will always be higher than the ambient air being used to cool the hot compressed air.
As soon as air leaving the aftercooler enters the receiver tank it comes in contact with the cooler steel wall of the tank, which is usually at ambient temperature. At this point, moisture starts to condense out of the compressed air as the air chills. If the air is stored 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.
The location of the inlet and discharge ports, as well as the pressure gauge and safety relief valve orifices on a given air receiver varies with the manufacturer.
If the air storage vessel is being used in conjunction with a compressed air drying system, the location and type of receiver tank should follow the recommendations of the dryer supplier.
A “wet” receiver is upstream of the dryer and a “dry” receiver is down stream of the dryer. Each location choice has advantages and disadvantages.
As mentioned, air receiver tanks are important 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 you have any questions about this article or anything mobile compressor related, please contact us.