What a performance data sheet actually tells you
A data sheet describes one identified compressor configuration, tested or calculated under stated conditions. It is not a promise about your plant. The model name, revision or date, motor rating, working pressure and cooling type together define which machine the numbers belong to. Change any of these and the figures change.
Most sheets for rotary screw and piston compressors give four core values: free air delivery (FAD), maximum or rated working pressure, motor nominal power and total package input power. Better sheets add specific power, no-load power, noise, cooling air flow, oil carry-over, and the test standard used. Sheets that follow a recognised template, such as the standard data-sheet format used by many manufacturers, are easier to compare because they put the same values in the same places.
Read the sheet in a fixed order: identity, operating point, flow basis, power boundary, control state, then notes. The notes and footnotes often carry the conditions that make the headline figure true.
Which figures matter most, and what they mean
| Figure on the sheet | What it means | What to check |
|---|---|---|
| Free air delivery (FAD) | Air actually delivered at the discharge, converted back to inlet conditions | Measured at which discharge pressure, and to which standard (typically ISO 1217) |
| Working pressure | The pressure at which the FAD figure was measured | Is it the maximum pressure or the pressure you will run at? |
| Motor nominal power | The nameplate rating of the main motor | Not the same as input power; ignores fan, efficiency and service factor |
| Total package input power | Electrical power drawn by the whole package at full load | Does it include the fan, dryer, drives and controls? |
| Specific power | Package input power divided by FAD, often kW per m³/min or kW per 100 cfm | Same pressure, same flow basis and same boundary as the other offers |
| No-load or unload power | Power drawn while running unloaded | Important for fixed-speed machines on variable demand |
| Turndown range (VSD) | Minimum and maximum flow under speed control | Specific power at several flow points, not only at full speed |
The most common error is to compare the FAD of one machine at 7 bar (101.5 psig) with another at 8 bar (116 psig), or to compare motor nameplate kW instead of measured package input. Both can make a weaker machine look stronger.
Is the flow figure FAD, displacement or something else?
Identical units do not guarantee identical meaning. A figure in cfm or m³/min may be free air delivery, swept volume (piston displacement), standard flow at defined reference conditions, or normal flow at 0 °C. These can differ widely for the same machine.
- FAD is the useful figure: compressed air actually delivered, restated at the inlet pressure, temperature and humidity.
- Displacement is common on piston compressor sheets and ignores volumetric efficiency, so it is always higher than FAD.
- Standard or normal flow (scfm, Nm³/h) is stated at fixed reference conditions and must be converted before comparing with FAD.
When the basis is not written on the sheet, ask. For conversion between bases, use the FAD and reference condition conversion tool, and for the background read our page on airflow definitions and reference conditions.
Where does the power boundary sit?
Two sheets can both state specific power and still not be comparable, because one includes the cooling fan and the other does not. Before you compare, draw the boundary of each offer:
- Main drive motor and its efficiency class
- Cooling fan motor (air-cooled packages) or pump load (water-cooled)
- Variable speed drive losses, where fitted
- Integrated refrigerated dryer, if the package includes one
- Controller, heaters, drains and other small loads
The right figure for comparison is total package input power measured at the terminals, at the stated working pressure. If one supplier gives only shaft power or motor nameplate rating, the comparison is incomplete and must be marked as such.
Reference conditions and Indian site conditions
Data sheets are produced at reference inlet conditions, typically around 20 °C, 1 bar absolute and a stated humidity. Many plants in Telangana and Andhra Pradesh see compressor room temperatures well above that through the summer: 40–45 °C, and above 45 °C in poorly ventilated rooms. Higher inlet temperature lowers air density and therefore mass flow, raises discharge temperature and can push a package towards its high-temperature limit.
FAD restated at inlet conditions partly accounts for this, but the package still has to reject more heat. Check the maximum ambient rating on the sheet, the cooling air requirement and the room design. Our compressor room ventilation worksheet helps with that part of the decision.
How to read a sheet: a step-by-step method
- Confirm identity: make, model, pressure variant, cooling type, motor rating, sheet revision and date.
- Write down the exact working pressure at which FAD was stated.
- Confirm the flow basis (FAD to ISO 1217 or another stated basis) and the reference conditions.
- Record total package input power at the same point and list what is inside the boundary.
- Calculate specific power yourself: package kW divided by FAD in m³/min. Label it as your calculation.
- For VSD machines, record specific power at several flow points across the turndown range.
- For fixed-speed machines, record unload power and any blow-down behaviour.
- Read every footnote and note any tolerance statement.
- List what is missing as written questions to the supplier.
- Compare only offers that now sit on the same pressure, flow basis and power boundary.
A worked example
Two oil-injected screw offers arrive for a plant that needs about 10 m³/min at 7 bar(g) (about 100 psig) at the point of use.
- Offer A: 10.4 m³/min FAD at 7.5 bar(g), package input 63 kW including fan. Specific power about 6.1 kW per m³/min.
- Offer B: 10.9 m³/min stated at 7 bar(g), motor nominal power 55 kW, no package input stated.
Offer B looks larger and lighter on power, but the comparison is not valid yet. Its flow is at a lower pressure, so it will deliver less at 7.5 bar(g). Its power figure is the motor nameplate, not package input, so fan load and motor efficiency are missing. The correct response is to ask for B’s FAD at 7.5 bar(g) to ISO 1217 and its total package input at that point. If B is to run at 7 bar(g) and A at 7.5 bar(g), remember that each extra bar of discharge pressure typically costs about 7% more power, so the pressure difference alone is worth roughly 3–4% of energy. The specific power comparison calculator puts both offers on one basis.
Tolerances, test standards and acceptance
ISO 1217 is the standard most manufacturers use to measure displacement compressor performance. It defines how flow and power are measured and allows tolerances that depend on machine size. A data sheet value is therefore a nominal figure inside a tolerance band, not an exact guarantee.
If performance matters to the contract, the sheet alone is not enough. Agree in writing the test method, the measurement points, the conditions and the acceptance criteria. Our compressor performance acceptance worksheet sets this out, and the supplier bid normalisation worksheet puts scope differences next to the numbers.
Common mistakes when reading data sheets
- Choosing by the largest cfm figure without checking the pressure behind it.
- Treating motor kW as the power the machine will draw.
- Comparing a VSD machine at full speed with a fixed-speed machine, then ignoring part-load behaviour.
- Mixing displacement, standard flow and FAD in one comparison table.
- Ignoring the dryer, filters and pressure drop between the compressor and the point of use.
- Using an old revision of the sheet after the maker has changed the airend or motor.
- Assuming a used machine still performs to its original sheet without testing it.
How Reddy Compressors helps
We are an independent multi-brand specialist based at Tarnaka, Secunderabad, serving industry since 2002. We sell new and used or refurbished compressors, rent from our own fleet, and service machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar and others. We are not an authorised dealer of any brand, so we can read competing sheets side by side without favouring one maker. We have tie-ups with all major and reputed brands.
When you send us offers, we place them on a common pressure, flow basis and power boundary, list what each sheet does not state, and tell you which questions to put to each supplier. Where you already run machines, our service engineers can log power and pressure through an air audit so the selection is based on your real demand rather than nameplates. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply compressors, rentals and spares across India.
What to send us
- The data sheets or offers you are comparing, with revision dates
- Required pressure at the point of use and the pressure you plan to run
- Expected demand profile: running hours, shifts and load pattern
- Air quality need (dryer, filtration, oil-free or not)
- Site conditions: room size, ventilation and summer temperature
- Nameplate photos of existing compressors, if replacing or adding
Frequently asked questions
What is the difference between FAD and displacement on a compressor data sheet?
FAD, or free air delivery, is the air the compressor actually delivers at its discharge, restated at inlet conditions. Displacement is the swept volume of the compression element, often quoted on piston compressors, and ignores leakage and volumetric efficiency. Displacement is always higher than FAD for the same machine. Compare FAD with FAD, and convert any standard or normal flow figures to the same basis before ranking offers.
How do I calculate specific power from a data sheet?
Divide total package input power in kW by FAD in m³/min at the same working pressure. For example, 63 kW for 10.4 m³/min gives about 6.1 kW per m³/min. Use package input, not motor nameplate rating, and make sure the fan, dryer and drive losses are treated the same way in every offer. Label the result as your own calculation and keep the original figures on record.
Why does the cfm on the data sheet differ from what my plant receives?
The data sheet figure is measured at the compressor discharge, under reference conditions, at a stated pressure. Your plant receives that air after the dryer, filters and pipework, each of which loses pressure, and after leaks take their share. High room temperatures and worn components also reduce real output. An air audit with flow and power logging shows what actually reaches the point of use.
Is a lower specific power always better?
Only if the compared figures share the same pressure, flow basis, power boundary and operating state. A lower figure measured at a lower pressure, or without the fan load, is not better. For variable demand, part-load specific power and unload power matter more than the full-load value, because many compressors spend much of their life below full load. Compare at the operating points your plant really uses.
What does ISO 1217 mean on a compressor data sheet?
ISO 1217 is the international standard for testing the performance of displacement compressors, including screw and piston machines. It sets how flow and power are measured and allows tolerances that vary with machine size. A sheet stating ISO 1217 gives a defined measurement basis, but the value is still nominal within tolerance. For contractual performance, agree a separate acceptance test with method and conditions in writing.
Can I trust the original data sheet for a used compressor?
Treat it as a starting point only. Airend wear, inlet valve condition, separator restriction, cooler fouling and control settings all change real output and power over time. Before buying a used machine, check its running hours and service history and, where possible, run it under load while measuring pressure and power. We test-run the used machines we sell under load before purchase.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Specific power comparison
Compare two compressors on what matters over life: the electrical power needed per unit of air delivered, and what that costs a year.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Machine A: input power (kW): 75
- Machine A: delivered flow (m³/min): 11.5
- Machine B: input power (kW): 80
- Machine B: delivered flow (m³/min): 12.6
- Running hours per year: 6000
- Air actually needed (m³/min): 11
Result
- Machine A specific power: 6.52 kW per m³/min
- Machine B specific power: 6.35 kW per m³/min
- Energy to deliver the air needed: A: 4,30,435 kWh per year
- Energy to deliver the air needed: B: 4,19,048 kWh per year
- Energy difference: 11,387 kWh per year in favour of B
- Cost difference: Enter your electricity tariff (cost per kWh) above
Method. Specific power = input kW ÷ delivered flow (m³/min) at the same pressure and reference conditions. Annual cost = kW × hours × tariff. Use the full-load input power on the same test basis for both machines.
What to take from it. A small gap in specific power is multiplied by thousands of running hours. Compare on the same pressure and reference conditions.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.
Bid normalisation: Net flow and comparable cost
Bring up to three compressor bids to one flow unit, check net flow against the requirement, then compare specific power, energy and installed cost on a like-for-like basis.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Net flow required (m³/min): 20
- Running hours per year: 6000
- Bid A: offered flow: 21.5
- Bid A: flow unit: m³/min
- Bid A: flow stated at the required pressure?: Yes
- Bid A: internal consumption (m³/min): 1.8
- Bid A: total package input power (kW; 0 if not stated): 124
- Bid B: offered flow: 750
- Bid B: flow unit: cfm (ft³/min)
- Bid B: flow stated at the required pressure?: Yes
- Bid B: internal consumption (m³/min): 1
- Bid B: total package input power (kW; 0 if not stated): 126
- Bid C: offered flow: 22
- Bid C: flow unit: m³/min
- Bid C: flow stated at the required pressure?: No / unclear
- Bid C: internal consumption (m³/min): 0
- Bid C: total package input power (kW; 0 if not stated): 0
Result
- Bid A: offered flow: 21.5 m³/min
- Bid A: net flow: 19.7 m³/min – SHORT by 0.3 m³/min
- Bid A: specific power on net flow: 6.29 kW per m³/min (5.77 on offered flow)
- Bid A: energy per year at the required net flow: 7,55,330 for information only: this bid cannot supply the required net flow. kWh = 6.29 × 20 m³/min × 6,000 h (enter a tariff for the cost). Assumes specific power holds at this flow; use the bidder’s part-load data for a VSD or a part-load profile
- Bid A: comparable installed cost: Enter your own prices above
- Bid B: offered flow: 21.24 m³/min
- Bid B: net flow: 20.24 m³/min – meets, margin 0.24 m³/min
- Bid B: specific power on net flow: 6.23 kW per m³/min (5.93 on offered flow)
- Bid B: energy per year at the required net flow: 7,47,124 kWh = 6.23 × 20 m³/min × 6,000 h (enter a tariff for the cost). Assumes specific power holds at this flow; use the bidder’s part-load data for a VSD or a part-load profile
- Bid B: comparable installed cost: Enter your own prices above
- Bid C: offered flow: 22 m³/min (not at the required pressure)
- Bid C: net flow: Not established clarification needed: ask for data at the required pressure
- Bid C: package input power: Not stated ask the bidder for total package kW at the duty point
- Bid C: comparable installed cost: Enter your own prices above
- Eligible bids by energy at the required net flow: B (7,47,124 kWh) only one eligible bid with stated power
- Bids that pass the net-flow check: B compare cost only between these
Method. Flow is converted to m³/min (1 cfm = 0.0283168 m³/min; 1 L/s = 0.06 m³/min); this is valid only when the bids use the same reference conditions. Net flow = offered flow − internal consumption (dryer purge, drains). A bid whose data is not at the required pressure is not credited: it needs clarification, not arithmetic. Specific power = total package input kW ÷ net flow. Energy per year is compared like-for-like at the required net flow: specific power (on net flow) × required net flow × hours, so a larger machine is not charged for air the plant does not use. This assumes the specific power holds at that flow; for a VSD or a plant that runs mostly at part load, use each bidder’s supported part-load power for each hour band instead. Bids are ranked on energy only if they pass the net-flow check. Comparable installed cost = quoted price + required additions − confirmed discounts. Default: the page’s worked example B (20.0 m³/min net required at 8 bar g).
What to take from it. A low price or a larger gross flow cannot offset a failed mandatory requirement. Compare cost only between bids that pass the net-flow check.