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.
Classic purposes for an air receiver
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.
ResultToday, 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".
Receiver before or after the dryer?A receiver is always of some help in cooling and cleaning the air entering the dryer which certainly is beneficial to continued dryer performance. The receiver location "before the dryer" is a better location than after the dryer for control air sensing. However there is one case when the receiver before the dryer may be problem. If it is located closely downstream from the dryer, then there is a continuing surge demand. The stored wet air in the receiver added to normal air supply may "overload" the dryer and deteriorate performance. If this appears to be a potential problem, have a professional look at it closely.
Establishing a demand side control systemDuring the 1990s a significant system modification began to gain popularity demand side control. The idea was to manage your compressed air usage by controlling the system from the demand side rather than the supply side.
To do this at all effectively the plant must first separate the supply side from the demand side. The supply side consists of the air compressor, the aftercooler, the separator, the wet receiver, the dryer, and the filters. This side is set up to produce air as efficiently as possible with no more than one compressor at part load and the compressors off whenever possible
The demand side draws compressed air from the supply side at the lowest effective pressure at a steady pressure. All air consumption in the plant is "regulated air" at "the lowest effective pressure" for optimum production.
Realizing that any higher pressure automatically uses more air, the astute system designer wants no more than a "steady minimum pressure" at the point of use.
Storage air receiver & flow controllerThese are used between the supply side and the demand side to let them each operate independently of each other. The flow controller pressure regulator creates the 10 psig minimum "pressure band" between the stored air and the system entry pressure. A single good quality, correctly applied, pilot-operated pressure regulator delivers this steady pressure with little, if any, noticeable pressure droop. There are more elaborate, commercial controllers available for flow and pressure that are microprocessor driven--but in all cases be sure they have, or are installed with, a bypass.
The receiver should only store clean dry air its sizing should be between 2 and 4 gallons per cubic foot of maximum air demand. In all reality it cannot be too large. However there is an economic point beyond which initial cost and installation outweighs the payback for new installations. Often plants already have large receivers in their systems that are not currently set up correctly.
Do the final sizing of this air receiver after reviewing the current and anticipated load Profile. Consider normal flows and the duration and intensity of event demands. Sometimes plants will find it effective to install a central demand-side control system and a sub-system downstream at a particular process.
It is important that this receiver be sized to handle all events without turning on an extra compressor or running inefficiently whenever possible.
A system running off the pressure switch without a controller for system flow or pressure automatically uses more air as the pressure rises. Look what is really happening! As the system usage goes down the pressure rises automatically increasing the usage in cfm! Is this what we want?
Use of a pilot operated pressure flow regulator or other intermediate controller to allow smooth flow of air to the system at the lowest possible pressure. This avoids "running off the pressure switch" in which the pressure spikes create "continuing flow through artificial demand with no increase in productivity."
Production should find a constant, steady pressure very conducive to stabilizing their productivity. The same steady, fixed pressure increases the quality of most production runs by "fixing" the repeatability standards.
Once the system is stabilized with the flow and pressure controlled, experiment to find the lowest effective pressure that automatically "optimizes" the flow demand.
A 600 cfm demand in an air system with 100 psig inlet pressure feeding an unregulated system will automatically be reduced as shown below:
90 psig 540 cfm
85 psig 510 cfm
80 psig 485 cfm
75 psig 460 cfm
This same concept could be applied to feeding "sub system" areas that use lower pressure. Note that eliminating the artificial demand created by 105 psig in order to hold a minimum 90 psig system pressure reduces the overall demand approximately 13% (20% from 115 to 90 psig). This translates into significant power savings 12 and 17% as long as the proper unloading controls are in place.
Other places for receivers in the system:Feeding a surge demand so it doesn't pull down the whole system. This is a very common usage, again basic pneumatics. This air receiver should also have a flow controller or regulator to optimize and minimize the flow to the surge. Size the receiver so that it stores enough air to feed the surge for the duration without going below the lowest pressure. It must also have time to receive before the next surge demand. The basic formula for this is:
T = V*(P<->2<->-P<->1<->)/(N*P<->0<->)
T = Decay or Pump up (in minutes)
V = Volume of Air Receiver (in gallons)
P2 = initial pressure (in psia)
P1 = final pressure (in psia)
N = net system flow (in acfm)
P<->0<-> = atmospheric pressure (in psia)
This strategy can be used to isolate a particular process or even a sector of a plant. For example, it answers the question "Should the air flow through the air receiver or is it okay to go in and out of the air receiver using the same opening?
This has been an age old question which many people have discussed in depth. The answer depends on many variables. Is there a flow controller regulator? Pipe size versus flow capacity? Flow demand intensity, and so forth.
However a friend of mine, Russ Morine at Compressor Technology, ran a test on two-100 hp rotary screw units several years ago with a recording flow meter that shows what's happening in general. In the case where the air flowed in one opening of the air receiver and out another, the air flow was smooth and normal. In the case where the air entered and exited the same opening, the air flow was continuously surging in "extremely short cycles".
What we believe happens here is this.
- Air doesn't move until there is a pressure differential.
- The constant pressure surging between the entry air and stored air causes extreme turbulence.
- Turbulence causes the required extra pressure (1/2% per psig of pressure) to move into the system.
- Apparently the pressure spikes build up from constant "over shoot" that actually increases system pressure somewhat on leaving the air receiver.
- This could actually cause an increase in air usage with no increase in productivity artificial demand.
ConclusionRoute your compressed air to go through the receiver whenever possible, with one exception. The one exception would be when an air receiver is installed at the end of a dead head line to back feed and help hold the pressure up at the end of the line. When a receiver is used in the corner of a loop system, the air flow should definitely go through the air receiver.
Here are some other important considerations for air receivers. If a receiver is used, it should be complete with safety valve, pressure gauge, hand holes or man holes, drain valve, and base. The safety valve must not be set at a pressure higher than the working pressure for which the receiver is stamped. Since these safety valves must be set slightly above the operating pressure, it follows that the operating pressure of a system must be at least 5% lower than the pressure stamped on the receiver. The receiver should be set on blocks or a small foundation to keep it dry and to prevent rust. Provide adequate space for draining the receiver.