How to verify compressor performance data

Why the same compressor can have several different numbers

Buyers often put two quotations side by side and compare the free air delivery (FAD) and kilowatt figures directly. The trouble is that those figures may not describe the same thing. A brochure value may be a nominal figure for a model family. A datasheet may quote FAD at a specific working pressure under ISO 1217 conditions. A quotation may round the value or quote it at the maximum pressure rather than the pressure you actually need. A site measurement taken in a hot compressor room in May is different again.

None of these numbers is necessarily wrong. Each answers a different question. The risk comes when unlike figures are compared as if they were alike, or when a family-level figure is assumed to apply to the specific serial number being sold, rented or overhauled.

Common reasons the numbers differ:

• Different working pressure: FAD falls as discharge pressure rises, and a 10 bar (145 psig) variant usually delivers less air than the 7.5 bar (108.8 psig) variant of the same frame.
• Different reference conditions: inlet pressure, inlet temperature and humidity change the mass of air drawn in.
• Package versus bare airend: airend data excludes the losses of the inlet filter, inlet valve, cooler and separator.
• Shaft power versus electrical input: motor efficiency, fan motors, dryer load and VFD losses are included in one figure and not the other.
• Production revision: later builds may use a different airend, motor or controller with different performance.
• Condition: a worn airend, a choked separator or a slipping belt reduces delivered air on an older machine.

The key terms you need to read a performance claim

For displacement compressors, ISO 1217 is the recognised test method for acceptance testing, and its annexes set out how FAD and specific energy are measured and what tolerances apply. Industry verification schemes publish data sheets in a common format so that package input power and FAD can be compared at the same pressure. When a figure is quoted without reference to a test method, treat it as a nominal value until proven otherwise.

Rank the source before you use the number

Not all sources carry the same weight. A simple ranking prevents a brochure value from quietly overriding an acceptance test, or a salesperson’s verbal figure from becoming a design basis.

How to check a performance figure step by step

1. Identify the machine: make, model, pressure variant, serial number and year, ideally from a nameplate photograph rather than an invoice.
2. Record the source of the figure: document title, revision or date, page, and who supplied it.
3. Confirm the basis: FAD or displacement, package or airend, shaft or electrical input power.
4. Note the test conditions: working pressure, inlet temperature, inlet pressure, humidity, cooling-air or cooling-water temperature and test standard.
5. Check applicability: does the document cover this serial and revision, this motor and this cooling arrangement?
6. Normalise before comparing: put all candidates at the same working pressure and the same power basis before calculating specific power.
7. Decide what still needs verification: request a data sheet, a test report or a site measurement where the gap matters to the decision.
8. Record the conclusion as confirmed, indicative or unresolved, so that the next engineer knows how far to rely on it.

Comparing quotations fairly

When a plant compares two or three new screw compressors, the most useful single figure is specific power at your actual working pressure, calculated on total package input power and FAD. A machine with slightly higher FAD but higher input power may cost more to run over its life. Because energy is usually the largest part of a compressor’s lifecycle cost, a difference of a fraction of a kW per m³/min matters on a machine that runs two or three shifts.

Points to align before comparing:

• Working pressure: compare at the pressure you will run. As a rule of thumb, each extra bar of discharge pressure costs about 7% more power.
• Control type: fixed-speed load/unload and variable-speed machines behave differently at part load. Full-load specific power alone does not tell you the part-load picture.
• Scope: integrated dryer, filters, receiver, heat recovery and controller features change input power and price.
• Cooling: air-cooled and water-cooled versions differ, and high-ambient variants may have larger coolers and fans.
• Electrical supply: 415 V, 50 Hz performance should be quoted for Indian supply, not converted from another frequency.

For part-load and station-level comparison, see our specific-power assessment method. It explains how to compare machines across a real demand profile rather than a single full-load point.

Used, rented and overhauled machines: rated is not actual

A used or refurbished compressor carries its original rating on the nameplate, but its actual delivery depends on airend clearances, inlet valve condition, separator differential pressure, oil condition, belt or coupling condition and cooling performance. A rental machine that has run hard on a quarry site will not necessarily deliver its catalogue figure without servicing.

Practical ways to check actual output on site:

• Receiver pump-up test: time how long the compressor takes to raise a receiver of known volume between two pressures, then calculate delivered air. It is simple and repeatable when the receiver volume and system isolation are known.
• Flow meter in the discharge line: a properly installed thermal or vortex meter, with adequate straight pipe, gives a direct reading.
• Electrical logging: record kW, load and unload time and pressure over a representative period to see how the machine behaves in real duty.
• Thermal indicators: discharge temperature on an oil-injected screw machine normally runs at about 75–95 °C. Higher values point to cooling or oil problems that also reduce efficiency.

Record ambient temperature with every site test. On a summer afternoon in Telangana or Andhra Pradesh, compressor-room temperatures of 45 °C or more are common, which reduces inlet air density and can bring the machine close to its high-temperature limit.

Red flags in performance claims

• A FAD figure with no stated pressure.
• Specific power calculated on shaft power and compared with another supplier’s package input power.
• Displacement figures for a piston compressor compared with a screw compressor’s FAD.
• A figure copied from a different pressure variant or an earlier production revision.
• “Equivalent to” claims for a replacement machine without a data sheet for both.
• Rounded headline values such as “100 cfm class” used as a design basis for a tool or process.
• A used machine sold on its nameplate rating with no test, no service history and no hour-meter reading.

Keeping a performance record that lasts

For each important compressor, keep a short performance file: the commissioning or acceptance data, the data sheet used at purchase, the serial and revision, and each later site measurement with its date, method, instruments, ambient conditions and the service state of the machine. When a performance complaint arises years later, this file shows whether the machine has degraded, whether demand has grown, or whether the original figure was never comparable in the first place. The same file supports overhaul-versus-replace decisions and audits.

How Reddy Compressors helps

We are an independent multi-brand compressor specialist based at Tarnaka, Secunderabad, serving industry since 2002. We sell new and used/refurbished compressors, rent from our own fleet, and service and supply spares for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar and other brands. We are not an authorised dealer of any brand, which means we can read competing data sheets neutrally. We have tie-ups with all major and reputed brands.

• We put competing quotations on a common basis: the same working pressure, the same power basis and the same scope.
• We identify the machine from its nameplate and serial before accepting any performance claim for a used, rented or overhauled unit.
• We carry out site checks such as pump-up tests, pressure and power logging as part of air audits, and record the method with the result.
• Our 20+ service engineers and technicians cover the whole of Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India.

What to send us

• Nameplate photographs of the compressor, motor and dryer, if fitted.
• The quotation, data sheet or brochure page containing the figure you want checked.
• Your actual working pressure at the point of use and at the compressor.
• Running hours, service history and any recent measurements.
• A short note on the decision: purchase, rental, used machine, overhaul or a performance complaint.

Frequently asked questions

What is the difference between FAD and displacement on a compressor?

Displacement is the theoretical volume swept by the compression element, usually quoted for piston compressors. Free air delivery is the air actually delivered at the discharge, converted back to inlet conditions, after leakage, heating and valve losses. FAD is always lower than displacement, often by a significant margin on small piston machines. Compare machines only on FAD at the same working pressure, never one machine’s displacement against another’s FAD.

How do I compare specific power between two compressor quotations?

Ask both suppliers for total package input power and FAD at the same working pressure, measured to the same test standard, then divide kW by m³/min. Make sure both include or exclude the same items, such as an integrated dryer or cooling fans. If one supplier quotes shaft power and the other quotes electrical input, the comparison is invalid. For variable-speed machines, compare several load points rather than full load only.

Why is my compressor delivering less air than the nameplate rating?

Common causes are a partly closing inlet valve, a choked inlet filter, high separator differential pressure, worn airend or piston rings, slipping belts, high inlet temperature, or a pressure setting higher than the rated variant. Leaks downstream can also make a healthy machine look undersized. A pump-up test or flow measurement, taken with ambient temperature recorded, shows whether the compressor itself has lost output or the demand has grown.

Does ISO 1217 apply to every compressor performance figure?

No. ISO 1217 is the recognised acceptance-test method for displacement compressors, such as screw and piston types, and many manufacturers quote FAD to it. Centrifugal compressors are tested under a different standard. A brochure or quotation may not state any standard at all. Where performance matters to a contract or a decision, ask which standard and which annex the figure refers to, and at what working pressure.

Can I trust the performance of a used compressor from its nameplate?

The nameplate tells you what the machine was designed to do, not what it does today. Delivered air on a used compressor depends on airend or cylinder wear, valve condition, separator and filter state, cooling performance and control settings. Before buying or renting a used machine, ask for running hours, service history and a simple site output test. We check identity and condition before quoting any used or refurbished unit.

How much does high ambient temperature reduce compressor output?

Hot inlet air is less dense, so the mass of air delivered falls as compressor-room temperature rises, even though volumetric FAD at inlet conditions may look similar. High ambient also raises discharge temperature and can push an air-cooled package towards its high-temperature trip. In Indian summers, good room ventilation, ducting hot cooler air outside and drawing intake air from a cool, shaded point matter as much as the rating.

Related pages

• How to read a compressor nameplate
• Serial ranges and production revisions
• Selecting a verified equivalent compressor
• Specific-power assessment
• Used compressor technical due diligence
• Compressed air audits and measurement