Estimating compressed air consumption

If it takes 20 sec for the pressure in a 240-gal.



If it takes 20 sec for the pressure in a 240-gal. pressurized air receiver to drop from 125 to 115 psig, what is the volumetric flow rate of air out of the tank? Does this question sound like something from the professional engineer's Principles and Practices qualification exam? Or does the ability to come up with the answer have practical value in the field?


Engineering design and field evaluations often involve the estimation of loads on existing equipment to verify if the installed machinery is below, at, or above capacity. Compressed air systems are widely used for a variety of applications, including pneumatic controls and power, breathing, laboratory, and process air. As a vital support utility, the proper design and operation of air compressors, receivers, and dryer-filter assemblies are critical.


Air compressor equipment is sized for an application using standard load estimating techniques found in any engineering handbook. However, determining the actual load profile of an installed and functioning compressed air system, even if it is a rough estimate, is of considerable value.


It can give an engineer a better idea of how actual compressed air usage relates to the theoretical load estimates used to size equipment. It can provide evidence of how much reserve capacity a system has for possible expansion. And it can also help to troubleshoot a system experiencing problems.


There are several methods for analyzing compressed air system loads. (See accompanying sidebar, "Determining load profiles.")


The first method provides the most accurate long-term load profile for a system, especially if historical data is collected and stored over an extended period of time. Facilities engineers at some manufacturing plants have installed fixed flow elements in central compressed air systems with electronic datalogging features that can download information on demand. Such flow elements include annubars, orifice plates, and turbine meters. However, plants usually don't have expensive flow elements and datalogging software, particularly if the compressed air system is a relatively minor utility.


The second method provides a reasonably accurate measurement of the compressed air load of a system without having to invest in fixed equipment, but it still means paying a specialist and having data available only during the time the survey is conducted. When a quick estimate is needed, the time and expense of hiring a field specialist are usually not warranted.


The third method is a quick estimate of a compressor's capacity and the demand of the connected system. Although less accurate than the previous two methods, timing a few of the compressor load/unload cycles with a stopwatch and noting the receiver volume and load/unload pressures (sometimes called cut-in and cut-out pressures) can yield useful information.


Some key assumptions are made when using the equation for air demand, C, as shown in the accompanying sidebar, "Estimating compressed airflow rate." Compressed air in the receiver undergoes a polytropic thermodynamic change as air is added to and drawn from the receiver. Pressure, mass, and temperature all change with time as air is drawn from or added to the receiver.


But the equation assumes that the temperature in the receiver is nearly constant and at standard atmospheric temperature. In other words, it is assumed that the heat of compression is small and transferred quickly to the environment. Air in the receiver is also assumed to behave as an ideal gas and that the compressibility effects of a real gas are negligible.


Air compressed under very high pressures or at very low temperatures deviates significantly from the ideal gas relationship, but high-pressure systems up to 3000 psig at ambient temperatures can be relied upon to behave as an ideal gas. It is very important to note that the calculated value of C in the equation is in cfm of free air referenced to ambient pressure and temperature, not cfm at system pressure.


As more cycles are timed, a more complete picture can be obtained. Ideally, a timing session of a couple of hours helps determine an overall average load. Compressors should also be timed during a period of the day known to have the highest compressed air demand.


Even without noting system pressures and computing cfm flows, the relative load-to-unload ratio can indicate if the compressor equipment is adequate or not. If the compressors are loaded up for long periods of time and unloaded only for short periods, the system demand is probably too high for the connected compressors. If the compressors are running under automatic start/stop control, the compressor should be running about one-third of the time and each motor should start not more than 6-8 times/hr. Motors in the 50-hp range should not be expected to start more than 3 or 4 times/hr. The compressor manufacturer should be consulted regarding motor start frequency to ensure reliable operation.


A compressor operating in a load/unload mode isn't as restricted in terms of run time or starts/hr. The load/unload mode is best suited in applications where the compressor is supplying most of the air instantaneously rather than relying on receiver storage. Many compressor packages, particularly rotary screws and rotary lobes, are microprocessor controlled and use energy-saving logic features and proportional loading capabilities. When evaluating or selecting a new compressor for a given application, the compressor manufacturer should always be consulted.


The equation for C is useful not only as a way of timing compressor loads, but also for selecting a receiver size when designing a system. By setting the desired pressure differential bandwidth (based on compressor capabilities and system pressure requirements), and the load to unload timing for a given average design load, C , receiver volume, can be calculated. Once a receiver volume is calculated, select the larger of two standard receiver sizes if the volume falls between them. -- Edited by Joseph L. Foszcz, Senior Editor, 630-320-7135,


Determining load profiles


Here are three valid methods for estimating compressed air system loads.


Permanently install inline flow metering devices with electronic datalogging functions.


Hire a field specialist to set up monitoring equipment and record an accurate load profile over a set period of time.


Manually time compressor loading and unloading frequency




Estimating compressed airflow rate


- Determine the volume of the connected receiver in cu ft.


- Determine the volume of the piping between the compressors and receiver in cu ft, if significant.


- Record the load/unload pressure settings of the compressors.


- Wait for the compressor to turn on, pressurize the receiver, and turn off or unload.


- When the compressor turns off or unloads, start timing (it is assumed that the compressor unloads completely).


- Note the receiver pressure.


- Note the time it takes for the receiver pressure to drop and for the compressor to turn on or load up.


- Repeat these steps over a reasonable period of time.


With this information, average system compressed air demand can be calculated with the following equation:


C = V(P(sub 1) - P(sub 2))/tP(sub 0)




C = air demand, cfm of free air


V = receiver volume, cu ft


P = receiver high pressure, psig


P = receiver low pressure, psig


t = time, min


P = standard atmospheric pressure, 14.7 psia


Calculating the load


Determine the compressed air load on a system consisting of two air-cooled, oil-free compressors, each rated for 165 scfm at a discharge pressure of 125 psig. These compressors discharge to a 240-gal. receiver (V = 32 cu ft) with a dial pressure indicator and pressure transmitter that tie into the compressor's duplex sequencing control panel. The compressors operate automatically in a lead/lag, load/unload control mode. The lead compressor runs continuously while the lag compressor is normally in standby to pick up any demand surges. The lead compressor's cut-in and cut-out pressures are set at 115 psig and 125 psig, respectively.


Timed cycles provide the following information:


Cut-out pressure, P(sub 1) = 125 psig


Time for receiver pressure to drop down to cut-in pressure, t = 13 sec (0.217 min)


Cut-in pressure, P(sub 2) = 115 psig


Using the equation, C = V(P(sub 1) - P(sub 2))/tP(sub 0), and solving for C, the system demand during the measurement period is about 100 cfm of free air.More info


The author is available to answer questions about compressed air consumption estimating. He can be reached at 617-423-7423 or jrattenbury@plbg.van.


Key concepts


Who needs expensive field equipment when a stopwatch will do?


Rough estimates of compressor load profiles can be useful for sizing and troubleshooting.


Accuracy of a load profile increases with cost.


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