AIR RECEIVERS

There are two different applications air receivers in your compressed air system. They differentiate themselves by their location. The first I will refer to as the PRIMARY receiver and is located near the compressor after the aftercooler but before any filtration and/or drying equipment. Every positive displacement (reciprocating, rotary screw or vane) air compressor should be piped to a PRIMARY air receiver after the aftercooler and separator. This provides the following for your air compressor and system:

a. Dampens fluctuations in pressure so that compressor control functions are smoother and slower.

b. Acts as a secondary separator to help remove condensed oil and water.

c. Provides a vent point for performing certain service and troubleshooting functions.

d. Allows you to perform certain time vs. volume tests with regard to compressor capacity or increased air requirements.

A rule of thumb for sizing primary air receivers for applications from 90-125PSIG is as follows:

SECONDARY receivers would be located very close to the point of air use at a piece of equipment that uses a large volume of air on an intermittent basis. Intermittent is the key word here. If you had a piece of equipment using a large volume of air on a constant basis, a secondary receiver won't do anything to help your system. Let's say that you buy a new piece of equipment for your plant that uses 240 CFM for 3 minutes, then it uses 0 CFM for 1 minute and the cycle repeats. You have a compressor that delivers 300 CFM at 125 PSIG. The new equipment and the other equipment in your plant requires 100 PSIG minimum to operate correctly. The other equipment uses a constant 100 CFM. Your system started out at 125 PSIG but when you turned on the new unit the pressure dropped below 100 PSIG and never recovered. A secondary receiver will definitely help in this case, but what size should it be. Use the following formula:

Where:

V	=	receiver volume in cubic feet
T	=	time in minutes
P0	=	atmospheric pressure (psia)
P	=	difference between initial pressure and final pressure
C	=	intermittent air requirement in CFM
S	=	Constant air supply available in CFM
V	=	3 (240-200) 14.7
		25
V	=	1764
		25
V	=	70 cubic feet

The closest larger standard size I could find was 89 cubic feet. Since this is considerably larger, what is the lowest point the pressure will drop to? Use the same formula, substituting X for P

89 = 1764

X

X=19.8

initial pressure 125

P -20

PSIG min. receiver pressure 105

Now we need to make sure that the system has time to recover between cycles. Use the following formula:

Pressure Rating - There is a danger in the use of air receivers of unsound or questionable construction. The American Society of Mechanical Engineers has established a code, now incorporated into the laws of most states and municipalities, governing the construction of unfired pressure vessels. Receivers should satisfy this code, as well as any other state or municipal codes which apply locally. the code receiver will have a stamp stating the maximum working pressure of that vessel.

Safety Relief Valves - You must have a relief valve capable of relieving the capacity of the connected compressor/s at the pressure you intend to operate. The relief valve can not be set higher than the maximum rated working pressure shown on the ASME stamp affixed to the tank. In addition, you can not operate your system at or right below the pressure relief setting. If you did, the relief valve will tend to chatter or whistle and wasted energy will escape to atmosphere. Another "rule of thumb" is to have your relief valve set 10% above the highest working pressure of the system. Example:

Condensate Drains - the various types of condensate drains - manual valve, float drains, and electric drains were covered in TECH TIP 1.

Pressure Gauges - A 4" pressure gauge with large graduations makes it easy to read. If you buy a gauge with a range double that of your anticipated working pressure, then the needle will point straight up when you are in normal operation. A combination snubber/shutoff valve allows you to change out the gauge without bleeding down the system and prevents pressure spikes from damaging the gauge.

Installation Suggestions - Mount the receiver on a 4" high concrete house keeping pad. Pipe inlet air in the lower opening and out an upper opening that is on a perpendicular axis to the inlet. This will assist in preventing condensate carryover.

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