The compressed air receiver: the endless question

Identify when and how air receivers enhance compressed air system efficiency

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By Henry Van Ormer, Air Power USA, Pickerington, Ohio

PlantServices.com

Has there ever been a topic in compressed air more contradictory and confusing than what to do with the air receiver? Continue to ask questions of different people and you get contradictory answers:

• "It goes before the system after the air supply" 
• "It does better when put at the end of a system" 
• "It goes after the dryer" 
• "Every air system needs one"
• "Rotary compressors don't need one if there is enough piping capacity"

Let's put these statements in a clearer light. Let's identify when and how to apply compressed air receivers to enhance the efficiency of the compressed air systems. The compressed air receiver is an integral part of all air systems. Understanding their use and operating characteristics gives the plant systems engineer an effective tool.

Pressure stabilization:  A receiver delivers or stores short-term demand that either exceeds or is less than compressor capacity. A pressure change of one atmosphere (less than 15 psi) accommodates a free air volume equal to that of the receiver.

Compressor control:  Receivers with ample volume reduce and slow pressure changes in response to intermittent compressed air use. Compressor controls, normally responding to pressure, smoothly regulate output without frequent ranging through their full control span.

Pulsation dampening:  An ample receiver reduces the probability of excess compressor power or shortened service life resulting from resonant response to the frequency of compressor delivery. Providing a receiver near the compressor discharge dampens pressure pulses from positive displacement compressor (rotary or reciprocating) to a small fraction of their original value

Separation:  A receiver, with its reduction in flow velocity, encourages finely divided particles of liquid lubricant or condensate to drop out of the air stream. Separated liquids drain from the receiver rather than traveling with the compressed air or gas to yield adverse downstream effects.

The proper selection of one or more receiver tanks is based not only on the capacity of the compressor but also on the shop load cycle. The following formula specifies the correct receiver size.

Remember the "old days" when reciprocating compressors powered almost all plant air systems. Every system had a power source (motor) followed by a compressor followed by an air receiver.

With the growth of rotary screw vane centrifugal compressors during the late 1960s and 1970s things changed. Some of the sales features of the rotaries were no damaging pulsations in the discharge air and the oil carryover was "unburned" aerosols instead of burned oil and carbon flaber. Subsequently, as the theory went, if you installed a rotary compressor you did not need an air receiver--as long as you had a minimum storage volume in the pipes of "at least one gallon per cfm of air compressor capacity."

Early rotaries and centrifugals, mostly installed with "modulating" and "blow-off" unloading controls respectively, certainly ran well under these conditions. Further refinement of rotary and centrifugal compressor unloading controls during the early to mid 1980s strove to minimize the inherent inefficiencies of rotary and centrifugal unloading controls. Although these controls sometimes worked well enough with large diameter air systems, the myth began to crumble. More and more installations that relied on pipe volumes for control storage found the promised power savings of their new controls either non-existent or certainly much less than originally promised. In many cases, short-cycling became a problem — usually when the pipe diameter was marginal.

During the late 1980s and early 1990s, technology moved forward with electronic controls and industry management began to focus on "energy costs" as a cornerstone in reducing production costs.

Energy managers soon realized the compressed air was the most expensive utility — it takes 8 hp of electricity to produce 1 hp of compressed air — it was no longer free. Its cost should, and could, be managed.

Early efforts in compressed cost management first focused on the obvious — leak control — usage control. But often after an effective reduction in air usage the energy audit revealed little or no reduction in actual electric power consumed.

Often the major culprit was found to be in the lack of a "control air receiver" between the compressor and the system. Without this air receiver, most unloading controls (with a few exceptions in the upper range) could not establish and hold enough idle time as a percent of running time and most important could not optimize the auto/start/stop control and "shut off."

There are some exceptions to the above scenario but, in general, their condition became more and more a problem. Electronic controls not only allowed more finite control of unloading but also showed even more clearly when the lack of a control air receiver was a problem.

Today, most compressed air system consultants, compressor manufacturers, and design engineers put aside the myth. Today they recommend  a "compressor air control receiver" on every installation with a minimum size of 1 gallon per cubic foot of capacity of the compressed air supply.

Generally this receiver should be located right after the air compressor's aftercooler/separator but before the dryer. Today we often call this the "wet air receiver".