Measuring FAD (Free Air Delivery) is a challenging task. The FAD‑measurement methods adopted by air‑compressor manufacturers in‑factory are often infeasible or inapplicable for on‑site testing due to field constraints. Nevertheless, this task can be accomplished without difficulty by deploying appropriate flow‑measurement instruments and mathematical calculation methods.
The basic function of an air compressor is to draw in ambient air, compress it, and deliver compressed air to the compressed‑air pipeline network. Compressor performance is characterized by its rated power and the FAD specifications published by manufacturers. However, on‑site measurements often reveal discrepancies from nominal figures.
The actual performance of an air compressor can be evaluated via several straightforward questions: How much electrical power does the compressor consume? What volume of compressed air does it deliver? What is the exact profile of the pressure curve?
To answer these questions, on‑site measurements must be performed. Measurements are generally divided into two categories:
Ad‑hoc performance measurement: Usually carried out after new‑unit commissioning or prior to compressor procurement. It analyzes compressor performance for comparative evaluation.
Long‑term performance monitoring: Continuously tracks compressor performance to facilitate scheduled maintenance or major overhauls of main compressor units.
Flow rate denotes gas volume measured per unit time. Gases are compressible and expandable; volume varies with temperature. In addition, intake air contains moisture (water‑vapour), and density (mass per cubic metre) changes with altitude and temperature. To ensure measurement consistency, a set of international standards such as ISO 1217 has been established. ISO 1217 specifies acceptance‑test criteria for positive‑displacement compressors (including rotary‑screw compressors).
Clause 3.4.1 of ISO 1217 states:
The actual volume flow rate of a compressor is the volumetric flow rate of gas discharged at the standard discharge point after compression, converted to the gas volume at full temperature, full pressure and full‑composition conditions corresponding to the standard intake state.
This converts the actual discharged‑gas volume to free‑air conditions at the compressor inlet. Therefore, FAD represents the total volume of free air drawn into the compressor, quantified at the compressor discharge port. FAD is expressed in volumetric‑flow units such as m³/min or l/s.
Flowmeters compliant with ISO 1217 first calculate mass flow, and convert mass flow to intake‑port volumetric flow based on inlet‑air density. Real ambient conditions are ideal; for practicality, ISO 1217 recommends using the following reference environmental values within acceptable tolerance bands:
Absolute pressure = 1 bar(a)
Temperature = 20 ℃
Relative humidity = 0 %
Flowmeters further correct volumetric‑flow readings for inlet‑air humidity, condensed‑water upstream of the meter, and motor rotational speed.
Note: Manufacturer‑published FAD figures may be obtained under different inlet‑condition set‑points. When evaluating compressor performance, always refer to the full data‑sheet rather than relying solely on nameplate markings!
Compressing air to higher pressure consumes more energy. Internal air losses and working‑air consumption rise with discharge pressure, so as to prevent motor overload at elevated pressures. For instance, rotary‑screw compressors operate at different speeds at 8 bar and 10 bar discharge pressure.
Compressor efficiency is governed by multiple parameters. Table 1 lists key performance‑influencing factors for the two dominant compressor types (screw and centrifugal). Inlet‑temperature impacts differ between screw and centrifugal compressors, which will not be elaborated here.
Manufacturers test compressor performance per international standards (e.g. ISO 5389) and publish results in data‑sheets. These tests are conducted under factory laboratory conditions rather than real‑world site conditions. Moreover, compressor performance may degrade over service life, necessitating major overhauls. On‑site testing is therefore recommended for performance monitoring. Proactive long‑term real‑time monitoring and continuous performance assessment are strongly advised.
Compressor discharge flow contains air, water, oil and particulates. Some measurement techniques fail because they cannot tolerate entrained water and oil. Others generate excessive pressure drop, incurring extra energy cost. Fundamental requirements for discharge‑side compressed‑air flowmeters are:
Resistance to water and oil droplets
Particulate tolerance
Compatibility with high‑velocity, high‑temperature gas (up to 70 ℃)
Minimal or ideally zero pressure drop
Insert‑type design enabling temporary installation for spot measurements
Table 2 compares common flow‑metering principles and their respective performance characteristics.
As illustrated in Table 2, pitot‑tube flowmeters are the optimal solution for compressor discharge‑flow measurement. This method has proven reliable in industrial practice and is also the standard technique for air‑velocity measurement in aeronautical applications.
Recently some manufacturers (predominantly Chinese suppliers) deploy thermal mass flowmeters mounted on the compressor suction side for performance testing. This measurement approach introduces several flaws that artificially inflate apparent compressor performance:
Internal compressor losses are not captured.
Gas used to pressurize internal compressor volume is counted as delivered output. This air is confined inside the compressor and vented to atmosphere during unload cycles. Large measurement errors arise during loading‑unloading transients, especially under low‑average‑load conditions.
Intake filters are sometimes removed during installation, yielding artificially high‑flow readings. Pressure drop across intake filters reduces inlet pressure by 1‑3 %.
Compressor performance‑test standards mandate flow measurement at discharge, not suction.
Complex installation and bulky equipment footprint.
Only fit for short‑term ad‑hoc testing; unsuitable for permanent long‑term monitoring.
To obtain genuine performance evaluation, measurements shall be performed at the discharge port. Performance qualification depends on measuring gas delivered, not gas drawn in!
Compressed‑air discharge‑flow values obtained via pitot‑tube meters require simultaneous working‑temperature and working‑pressure measurements to compute FAD.
Discharge‑side flow under operating conditions consists of two components:
Air flow: the quantity requiring accurate measurement.
Water flow: to be removed in downstream air‑treatment processes.
Accurate water‑content quantification requires in‑situ pipeline‑humidity measurement. This is challenging because high‑temperature gas is often near saturation, where many humidity sensors malfunction. Extensive testing reveals that setting relative‑humidity within the 80 %‑99 % band introduces a deviation no greater than ±0.3 % to flow‑measurement results.
Based on this finding, a constant relative‑humidity value is applied to subtract water‑vapour contribution and compute “dry‑air flow” under standard reference conditions (20 ℃, 1000 hPa).
Intake‑air humidity affects total dry‑gas output, yet its impact is subordinate to other variables. For example, under hot tropical conditions (32 ℃, 75 % RH), dry‑air volume is merely 3.5 % lower than raw intake‑air volume. Atmospheric‑pressure and climatic variations may produce deviations twice as large. A fouled intake filter can introduce an additional 2‑3 % volumetric deviation.
FAD is finally calculated applying gas‑law principles and manufacturer‑specified intake‑reference conditions.
The resulting FAD represents the compressor’s “true” delivered output. (For valid comparison against manufacturer datasheets, ensure identical reference‑intake conditions.)
Many practitioners assume that power consumption (kW) can be derived solely from current readings for calculating full‑load specific power (m³/min/kW) or estimating flow (m³/min). This approach is incorrect.
Measuring only current ignores power factor and three‑phase load imbalance, which may produce errors of tens of percent.
Power estimates obtained this way cannot accurately quantify load‑factor within the compressor’s performance‑control cycle. Such dynamic calculations require high‑frequency short‑interval sampling, as compressor operating states change continuously over time.
The correct procedure is to measure three‑phase current and voltage with a power meter and derive the power factor.
Power‑calculation formula:
\(\boldsymbol{kW=(A\times V\times1.732\times PF)/1000}\)
kW = Input power
A = Motor current
V = Line voltage
PF = Power factor
After compression, air passes through dryers and filters to remove moisture, particulates and residual oil. Comparing compressor‑outlet flow‑pressure readings against post‑treatment measurements reveals two major categories of losses:
Leakage across components, drain lines, filters and pipe joints, caused by ageing or improper installation. Leak‑management programmes can substantially mitigate these losses.
Purge‑gas consumption for regenerative drying (3‑25 %).
These losses explain the reduction of measured flow after downstream air‑treatment (refer to Figure 2).
Pressure is equally critical to overall‑system performance. Pressure drop across each component must be compensated by raising compressor set‑pressure, which increases energy consumption. Pressure‑loss sources include:
Excessive pressure drop from clogged filters.
Mismatched pipe‑sizing versus air‑flow demand (undersized piping for high flow rates).
The importance of compressor‑performance measurement and monitoring keeps growing. Long‑term monitoring delivers tangible benefits: predictive maintenance enabling intervention prior to component failure; energy‑consumption tracking yielding short payback periods for investment. Combined with periodic leak surveys, operators achieve a reliable, high‑efficiency compressed‑air installation.
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