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Receivers, or air tanks / reservoirs, are found either horizontally or vertically under the compressor head assembly in reciprocating, rotary vane, rotary screw and other styles of compressors.
Usually, the higher the capacity of the compressor assembly (the higher the horsepower), the larger the compressor tank should be.
The capacity of reservoir tanks (Please note that we're using all of the synonyms for receivers that we can think of in this text to ensure that you understand that all of the terms mean the same thing) is normally measured in U.S. gallons.
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Almost all compressors operate on a “demand” basis.
There is a pressure switch on the downstream side of the compressor that checks the pressure in the compressor receiver, and by extension the pressure in your plant air lines.
As demand downstream makes the pressure in the mains air line drop, so too will the pressure in your compressor air tank, and the pressure switch will signal the compressor to “kick” in and generate higher pressure. When the pressure in the downstream line reaches the high pressure set point, the compressor stops.
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The difference between the two pressures, and the frequency that the air pressure oscillates between the two points, dictates how often the compressor fires up.
Most electro-mechanical equipment “doesn’t like” to be cycled on and off. The frequency of the on/off cycle may lead to premature component failure, draw higher energy resources, etc. Your compressor is an electro-mechanical device.
One way industry deals with this is to provide continuous run compressors. When the downstream air pressure drops, these compressors begin compressing air until the high pressure set point is reached. At that point, they stop compressing air, but the motor still runs.
In non-continuous run compressors, to help eliminate too-frequent on/off cycling, compressors are coupled with receivers.
Depending on your compressor size, and the volume of air required for your applications downstream, the receiver may be measured in a few cubic inches to many hundreds of gallons.
A rule of thumb for receivers is that they should have 1 gallon of capacity for every CFM of compressor output. Therefore, since a 25 HP compressor can theoretically generate about 100 CFM at 90 PSI, the receiver for that compressor should be 100 gallons in size.
The receiver’s size relative to the downstream demand, will allow the compressor to build up a reservoir of compressed air at the desired pressure in the receiver.
Usually this volume will exceed all downstream demand for a period of time. Therefore, once the receiver is filled, your air-driven equipment downstream from the compressor can - perhaps - cycle on and off a number of times without the compressor having to “kick in” every few seconds to maintain the downstream pressure set point.
It’s important then to have a receiver size large enough that the compressor has “rest” time, not just to help prolong compressor life, but due to some ancillary benefits to having a large receiver as noted below.
If it’s of sufficient capacity, the air in the receiver can cool once the compressor has been shut off. It’s the process of compressing the air (friction between the air molecules) that generates the heat.
As the air in the receiver cools, it reduces the capacity for holding moisture, and eventually the temperature in the receiver will reach the dew point where more water is dropped from the compressed air than would be the case if the air stayed hot.
Less water in the compressed air means fewer problems to deal with downstream as noted on this page, if you haven't been there yet: Water problems.
Sometimes the size of the main air lines and the secondary air lines in a plant is so huge that the mains become themselves a secondary receiver and the air moving through those lines cools further, and more water condenses.
If it were us, we'd have - depending on what the CFM requirements or types of applications we're using compressed air for - the largest practical air receiver possible for our compressor system.
Here's more info on compressors in general receivers.
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