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How to measure the performance of installed air compressors

Measuring the Free Air Delivery (FAD) of an air compressor can be challenging, a recent blog first published on ‘Compressed Air Best Practices’ looks at how to select the flow meter and summarizes parameters to be considered in the FAD measurement task.   The ultimate job of an air compressor is to produce compressed air…

  • installed air compressors

Measuring the Free Air Delivery (FAD) of an air compressor can be challenging, a recent blog first published on ‘Compressed Air Best Practices’ looks at how to select the flow meter and summarizes parameters to be considered in the FAD measurement task.


The ultimate job of an air compressor is to produce compressed air by sucking in ambient air, pressurising or compressing it and discharging it into the compressed air network. Air compressor power is defined by its power rating and the FAD specifications stated by the manufacturer when new, however, as time goes by, an on-site measurement can be very useful. There are several simple questions, which eventually explain the performance of an air compressor:


  • How much electrical power is my air compressor consuming?
  • How much compressed air is my air compressor delivering?
  • What’s my pressure profile?


To answer these questions, you may want to measure your air compressor on your own.


Performance measurements usually comes into two types:

  • Temporary performance measurement, done as part of system assessments, or before investing in a new air compressor.
  • Permanent performance measurement, which is performed to monitor the performance at all times in order to trigger services or overhauls timely.


How is FAD Defined?

Flow is measured in volume per unit of time. But air can be compressed easily and its volume changes with the temperature. Meanwhile, the air that an air compressor sucks in contains humidity (water vapor), and the air density (air per m3) changes with variation in the altitude, temperature and weather patterns. To ensure everyone is talking about the same thing, industry and international standards have been written. For example, ISO 1217 relates to performance testing of displacement air compressors. This standard includes packaged rotary screw air compressors as well.


Clause 3.4.1 of ISO1217 states:

“Actual volume flow rate of a compressor is the actual volume of gas, compressed and delivered at the standard discharge point, referred to conditions of total temperature, total pressure and composition prevailing at the standard inlet point.”


This is the actual volume of air delivered by the air compressor referred back to the conditions of the free air at the compressor inlet, so, FAD is the amount of free air drawn into the compressor that is actually delivered by the air compressor at its compressed air outlet.


FAD uses volumetric flow units such as m3/min and l/s etc. The types of flow meters allowed by ISO 1217 initially calculate a mass flow of air, which is then converted to an intake volume flow rate based on a value for the density of the air at the intake to the air compressor. Ideally these are the actual conditions, but for convenience, ISO 1217 suggests the following conditions, provided the actual conditions are within an allowable tolerance:

  • Pressure = 1 bar absolute
  • Temperature = 20 °C
  • Relative humidity = 0%


Corrections are then made for the intake humidity and the amount of water condensed upstream of the flow meter and the speed of the motor compared to its rated speed.

Because air compressor manufacturers may state their FADs at different inlet conditions, it’s advised to look into the air compressors data sheet and not just take the nameplate numbers.


It takes more power to compress air to a higher pressure, also, air losses and control-air use increase with pressures within an air compressor so the compressor motor isn’t overloaded at higher pressures. For example, the compression module in an 8-bar rotary screw compressor operates at a different speed than in a 10-bar machine.


What Affects the Efficiency of an Air Compressor?

Air compressor manufacturers measure the performance of air compressors according to international standards (ie, ISO 5389 for centrifugal air compressors) and describe the results in their data sheets. However, these measurements are performed in the factory conditions and not real on-site conditions.

Moreover, the performance of the air compressor may degrade over its life-time and an overhaul might be required. To judge the performance of an air compressor, an on-site measurement is recommended. Further, an active measurement such as real-time monitoring is very important for real-time performance estimation of air compressors.


Methods for Measuring the Discharge Flow

The discharge flow of an air compressor contains air, water, oil and particles. Some measuring methods fail because they cannot handle water and oil in the flow, others are not suitable because they cause a pressure drop, which in turn, leads to a waste of energy and money. The requirements for a flow meter used at the compressor discharge include:

  • Resistant to particles.
  • Resistant to water and oil drops.
  • Ability to handle high velocities and temperatures up to 70 °C.
  • Minimal pressure drop, better no pressure drop.
  • Insertion-type flow meter suitable for temporary measurement.


Measuring Flow at the Intake Side

Recently, some manufacturers have introduced thermal mass flow meters to be installed at the intake side of the air compressor to determine the compressor performance.  In many ways these flow meters beautify/exaggerate the performance because they do not consider:

  • Losses inside the air compressor, which cannot be measured.
  • Air used to “pump up” (pressurize) the air compressors internal volumes is measured as air delivered, but it’s only stored within the air compressor. It is vented (during blow down) to atmosphere when the air compressor unloads.
  • For an air compressor operating load/unload this blow-down air can quickly become a big error, especially at low average loads.
  • Installation sometimes requires removal of intake filter, thus showing flow rates that are too high. The pressure drop across an air filter can reduce the intake air pressure by 1 to 3 percent.
  • Air compressor performance tests standards require the flow measurement at the outlet, and not at the inlet.
  • Complicated installation and bulky equipment.
  • Applicable to short term tests, no permanent installation.


An air compressor user should insist on measurement at the outlet to determine the performance.


How to Calculate the FAD From the Discharge Flow

The discharge flow measured by a pitot tube flow sensor must be computed into FAD with additional measurements on the operating temperature and operating pressure.

It’s important to understand the operating flow at the air compressor discharge is made of two components:

  • Airflow, which is what you want to measure.
  • Water flow, which is removed later in the air-treatment section.


To measure the water contents precisely, it’s required to measure the humidity in the pipe, you must take into account the almost- saturated-humidity conditions at high temperatures.


Many humidity sensors cannot work at such conditions, when the humidity is set to a value between 80 to 99 percent, the measurement error is not more than ± 0.3%.


Based on this finding, you can use a constant setting for the relative humidity, therefore,

you are able to deduct the water contents from the flow and calculate the “dry air flow” at the standard conditions (i. e.,20 °C, 1000 hPA).


The intake humidity affects the amount of dry air delivered, but the error is less than other factors, e.g on a hot tropical day of 32 degC and 75% RH, the dry air volume is 3.5% less than the intake air volume.


The barometric pressure with changing weather can vary by twice this amount. A dirty air intake filter can also vary the intake airflow by 2 to 3 percent.

By using the gas law and following the intake conditions specified by the air compressor manufacturer you eventually get the FAD computed.

The so-calculated FAD is what the air compressor “really” delivers. If you want to compare it with the air compressor data sheet, make sure to use the same intake conditions.


How to Measure Power

Quite often, amperes alone are thought to be an accurate measurement of kilowatts (kW), which is then used to calculate full-load specific power (m3/min/kW) or to estimate flow (m3/min) of an air compressor. This is incorrect!


If you measure only amperes, you cannot know the power factor and the imbalances between the three phases. This will lead to errors of 10 to 30 percent.


It’s also very difficult to accurately use this estimated kW to calculate the percent of full load in the capacity-control cycle. These complex calculations can only be done using short-term data as the condition of the air compressor and its controls vary with time.


A correct power measurement requires amperes and voltages measured at all three phases of the air compressors using a power meter, which can calculate the power factor. The power equation is as follows:

kW = (A x V x 1.732 x PF) / 1000


kW = Input kilowatts

A = Motor current (amperes) V = line voltage

PF = Power factor


For more information on air compressors, why not get in touch with a member of our friendly team HERE.


This blog was initially published on Air Best Practices.