Industrial Utility Efficiency

A Case for Using Compressed Air Flow Meters

To accurately assess compressed air systems, it is best to gather as much information as possible, for as long as practical so a clear picture of the operation of the system can be formed. The basic equipment needed includes pressure and amp/power data loggers, but in the past decade some excellent advances in instrumentation design has made very affordable flow meters available. These devices make compressed air auditing much more accurate, especially for systems using air compressors running in inlet modulation, or variable displacement, where it becomes quite difficult to calculate flow from the power input. This article discusses using flow meters for an example compressed air assessment.

Creating a baseline of a compressed air system, basically capturing some “as found” readings as in Figure 1, gives an auditor a reference from which he can work to improve the system being audited.

A typical pressure/amp/flow profile of a small two compressor system. This profile could not be used for a valid baseline because there was a Monday holiday, this is something that should be considered when auditing. The profile showed a breathing air system that was consuming 80% of the total compressed air production. Click to enlarge.

We can see from simple visual analysis of Figure 1 the pressure is adequate, but is the system efficient and can something be done to improve it? To find the answer to this question we must measure the system power and energy, and then calculate or measure how much air is produced for a given amount of electrical input. Very often the calculations and measurements will yield a parameter called Specific Power, stated in kW per unit output (we commonly use kW per 100 cfm in North America, but we can use any applicable units, or even cfm per horsepower). 

This is where flow meters come in, it is quite easy to measure amps, then calculate power and energy, but it takes some complex calculations, and detailed understanding of compressor operations to convert this same power measurement into flow. Figure 2 shows the theoretical flow versus power characteristics of 5 different air compressor control modes. We can see that the modulation mode with no blowdown and VSD modes are the only curves that are close to linear. The load/unload curve is shown as a straight line, however this only applies to lubricant free compressors. The lubricated compressor load/unload curve is shown in Figure 3, the shape of the curve is not linear and varies with storage size, pressure band width, and sump blowdown time. What usually happens in the field is the auditor assumes one certain curve, but the compressor is really operating on another, due to mis-adjustment or some other issue, resulting in big errors in flow calculation. Simply measuring the flow with a properly installed flow meter is much simpler and more accurate.

These typical flow versus power curves are often used by compressed air auditors to calculate flow output from power.  Some curves are non-linear requiring complex equations that may not match real life performance. (Source: Compressed Air Challenge). Click to enlarge.

Load/unload operation for lubricated screw compressors depends on storage size, pressure band width and blowdown time.  Incorrect assumptions about these curves can lead to large errors in the flow estimates. (Source: Compressed Air Challenge). Click to enlarge.

When improving a compressed air system, with goals to increase efficiency and reduce electrical costs, the baseline flow parameter is very important. It is the starting point to find higher efficiencies and to use as an input to calculate potential energy savings. An auditor will thoroughly examine a system to find waste and misuse on the demand side of a system, then assess the air compressors, dryers, and filters on the supply side for potential control or efficiency improvement base on the reduction of waste flow.

All waste that can practically be removed will be subtracted off the baseline flow profile, then the resulting new flow is run through a simulation using proposed more efficient supply side equipment. The result is a simulated new power profile. When the new energy profile is subtracted from the original energy baseline, it yields an estimate of the possible energy savings.

Flow Measurements

There are a great number of types and styles of flow meters available, it is not the purpose of this article to compare them, but by far the most affordable, and most commonly used in compressed air auditing work is the thermal mass style, based on the principle of hot wire anemometer measurement. This type of meter needs be installed on a dry system located downstream of the system air dryers because any free water will artificially cool the hot probe, causing the meter to read high. Should a system need to be measured at a point where wet compressed air flows, there differential pressure style flow meters available that are designed for this service  Some companies also offer ultrasonic meters that do not need probes inserted into the pipe.

Calculating Specific Power

The profile in Figure 4 shows the challenge in measuring some abnormal systems. The compressors are running in modulation mode with no blowdown, however the input power remains very flat for all levels of flow.  We can see that for a flow increase of 340 cfm (70%) the power increases only 16 kW (15%).  If we had no flow meter installed and were calculating flow from the typical power curve, assuming 3 percent power change for every 10 percent flow change, we would have had a large error in our calculations. But simply installing a flow meter allows us to quickly assess the true flow/power characteristics.

This example pressure/flow/power curve shows abnormal performance of the compressors as the flow changes.  Attempting to calculate flow from power in this case would lead to large errors. Not captured by the flow meter is the purge flow of a desiccant dryer in this system, this needs to be accounted for in the system calculations (Source: Click to enlarge.

The chart also shows some automatic calculations of the system specific power, for this example we use kW per 100 cfm, we can see that during light loading the SP increases to a high of 135 and to a low of 25 when a single compressor is running fully loaded, this being the most efficient point for a modulating compressor. Specific power is calculated simply by dividing the average power consumption by the average flow within the measurement period, something that can very easily done using a spreadsheet or a viewing program with mathematical functions.

The potential savings in optimizing the compressor control can be estimated by having a look at the rated specific power of potential new replacement compressors, data that can be taken from Compressed Air & Gas data sheets, or calculating the performance of the existing compressors running in a more efficient control mode, such as load/unload with large storage capacity.  In this case we know that new compressors should be able to supply this load at about 18 kW per 100 cfm if they operated in VSD mode. The actual average specific power for this example system is 48.5 kW per 100 cfm.

But before we jump to conclusions there is one more added piece, a fixed cycle desiccant dryer is operating in this system. The flow meter is installed downstream of the dryer, meaning the dryer purge is not measured by the flow meter. In this case special testing is needed, we need to determine how much flow the dryer purge is consuming to create a proper baseline. This purge was measured at 120 cfm. This means the actual flow produced by the compressors is about 520 cfm. The real specific power of the compressors in this system is 30 kW per 100 cfm if we take the dryer purge into account. Of course, the dryer purge is also one source of extra savings if we could optimize using dew point control or a different dryer style.

The flow meter makes it easy to assess baseline low flow periods as well, the 120 cfm in consumption we see during weekend periods is mostly leakage, something that can be easily addressed by an aggressive repair effort. Once done the results can be seen by tracking the flow meter readings.


The use of flow meters when assessing compressed air systems is recommended to increase the accuracy of the measurement. The measurement and resulting key performance indicator calculations are a lot simpler using flow meter data. Flow meters can be used to track changes in the system to ensure savings are sustained after efficiency measures are applied.

For more information about this article, contact Ron Marshall at Marshall Compressed Air Consulting, tel: 204-806-2085.

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