Air Compressor Buying Checklist: What to Confirm Before You Sign the Order

Who this page is for: plant engineers, maintenance managers, purchase teams and owners who have two or three compressor quotations on the table, or are about to ask for them, and want a disciplined way to decide.

Key facts

• Free air delivered (FAD) is the figure that matters, not displacement and not motor kW. It must be stated at a named pressure and at stated reference conditions, or two quotations cannot be compared.
• Every 1 bar of unnecessary pressure costs roughly 6 to 8 % more energy for the same air. A set point of 8 bar(g) (116.0 psig) where 7 bar(g) (101.5 psig) would do is a permanent running cost.
• Specific power, in kW of package input per m³/min of FAD, is the fairest single number for comparing efficiency. Typical modern oil-injected screw packages at about 7 bar(g) sit around 6 to 7.5 kW per m³/min; confirm against each maker’s data sheet.
• Electricity usually outweighs the purchase cost of an industrial compressor within a few years of two-shift running, so a small efficiency difference can outweigh a large difference in scope.
• Scope gaps are where quotations hide: dryer, filters, receiver, drains, starter, cabling, piping, foundation, installation and commissioning are often left out of one quotation and included in another.
• The selection guide covers the ten common selection mistakes; this page is the practical checklist and comparison method that sits alongside it. See the complete compressor selection guide.

Step 1: Confirm the demand before anyone quotes

A quotation can only be as good as the requirement it answers. If the requirement is “a 30 hp compressor”, every supplier will offer a 22 kW machine of their own choosing and none of the offers will be comparable. Write the demand down in the units the machine will be judged on.

• Air needed, in m³/min FAD (with cfm): add up the consumption of the tools and machines that run together, apply a realistic diversity, add leakage, and keep a stated margin. A plant that needs 3.2 m³/min (113 cfm) should say so, not “about 100 to 150 cfm”.
• The demand pattern: one shift or three, steady or swinging, weekend running, and the peak that lasts more than a minute. This decides between fixed speed, variable speed or a base-plus-trim pair.
• Growth that is planned, not imagined: name the machine or line that will be added and when. Oversizing for vague growth leaves a machine running unloaded for years.
• Measured data where it exists: if a compressor is already running, a few days of logged pressure and loaded hours is worth more than any estimate.

Step 2: Fix the pressure at the point of use, then the set point

Ask what pressure the most demanding tool or machine needs at its inlet, then add the losses between the compressor and that point: dryer, filters, piping, hoses and regulators. That sum is the compressor set point. Quote requests should state both numbers, for example “7.0 bar(g) (101.5 psig) needed at the machine; compressor set point 7.5 bar(g) (108.8 psig)”. A supplier who offers a 10 bar(g) (145.0 psig) machine “to be safe” is selling you higher running cost and, for the same motor, less air.

Step 3: Specify the air quality

Name the air quality class the application needs for particles, water (pressure dew point) and oil, or describe the use so the supplier can propose it: general workshop air, instrument air, paint finishing, food contact, pharmaceutical or electronics. The answer decides whether you need an oil-free compressor or an oil-injected machine with treatment, and whether a refrigerated dryer (typically +3 to +7 °C pressure dew point) is enough or a desiccant dryer is required. Oil-free claims should state the standard class and the test basis.

Step 4: Check the site, the room and the power supply

• Room and ventilation: the space, the clearances the maker requires, how hot air leaves the room and where cool air enters. Indian summer room temperatures above 40 °C need checking against the machine’s maximum ambient rating.
• Power supply: voltage and its variation, transformer and cable capacity, the starting method (star-delta, soft starter or drive) and whether the machine must run on a generator during outages.
• Cooling: air-cooled or water-cooled, and if water-cooled, the water quality and temperature.
• Access: how the machine reaches its position, and whether there is room to remove the motor or airend later.

Step 5: Define the package scope, line by line

Scope is where quotations differ most and where comparisons go wrong. List every item and ask each supplier to mark it included, excluded or optional.

Step 6: Confirm service, parts support and documents

• Who will carry out the routine service, from where, and how a breakdown call is handled.
• Which consumables the machine uses (filters, separator, oil, kits) and whether they are available locally for the expected life of the machine.
• The documents you will receive: data sheet with FAD and package input power at the quoted pressure, test certificate, receiver and safety-valve documents, wiring diagram, operation and maintenance manual, and the warranty terms in writing.
• For used and refurbished compressors, the warranty and inspection basis in writing. Ours carry a six-month warranty from the delivery or invoice date, covering the whole compressor and excluding consumables, misuse or wrong oil, service by others, and accident or site damage.

How to compare quotations like-for-like

Put every quotation into the same table before forming a view. Do not compare the headline motor rating; compare what the package delivers and what it draws.

1. Restate FAD at one pressure. If one quotation states FAD at 7 bar(g) and another at 8 bar(g), ask the supplier to restate it at your set point. FAD falls as pressure rises for the same machine.
2. Check the reference conditions. FAD should be stated to the same recognised test basis, with the inlet conditions named. A figure “at inlet” from one and “at standard conditions” from another are not the same thing; see FAD and reference conditions.
3. Use package input power, not motor rating. The package figure includes the main motor, the fan motor and drive losses. A 22 kW motor can mean 24 to 27 kW at the meter.
4. Calculate specific power = package input kW ÷ FAD in m³/min, at the same pressure. Lower is better.
5. Equalise scope. Add the missing dryer, receiver or drains to the quotation that leaves them out, at their own power draw and pressure drop.
6. Compare running energy over the life. Annual kWh ≈ demand in m³/min × specific power × loaded hours. Then use the lifecycle cost comparison calculator with your own tariff and prices.

Acceptance at delivery

The order is not complete when the machine arrives. Check the nameplate against the quotation (model, pressure rating, motor kW, voltage), confirm every scope item is present, and run the acceptance checks before you sign the delivery note: pressure reached and held at the set point, load and unload behaviour, discharge temperature after a warm run, dryer dew point, drains working, no leaks, and protection devices tested. Our commissioning and final acceptance method sets out the full sequence.

New or used?

A used or refurbished compressor can be the right choice for a standby duty, a temporary project, a known model already in your fleet, or when delivery time matters more than the last point of efficiency. Apply the same checklist: confirm FAD at your pressure, the running hours, the airend and motor condition, and what has been replaced. See used and refurbished compressors for sale.

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Common mistakes

• Comparing FAD figures stated at different pressures or different reference conditions.
• Ignoring package input power and judging by the motor nameplate.
• Comparing an offer with “dryer optional” against one that includes the dryer, so the two quotations answer different scopes.
• Leaving commissioning and acceptance out of the order, so there is no agreed test of what was promised.
• Accepting service, parts or warranty terms verbally instead of writing them into the order.

How Reddy Compressors helps

We have supplied, rented, serviced and supported compressors since 2002 as independent, multi-brand suppliers. We can measure your existing demand and pressure, write the requirement in the units above, prepare a like-for-like comparison of quotations, supply new or used and refurbished machines of all major makes, and commission and service what you install. Our own rental fleet, 50 to 2,000 cfm (1.4 to 56.6 m³/min) and 2 to 30 bar, can cover a gap while a decision is made. We support sites across Telangana, Andhra Pradesh and districts within about 300 km of Hyderabad, with supply across India.

What to send us

• The quotations you have received, or your draft requirement.
• The tools and machines the air serves, with their pressure needs.
• Nameplate photos of any existing compressor, dryer and receiver.
• Running hours, shift pattern and any pressure complaints.
• Room photos and the electrical supply details.

Use the equipment selection enquiry, or Call/WhatsApp 99590 60718 or 92468 88812.

Frequently asked questions

What should a compressor quotation always state?

Free air delivered at your set pressure, the reference conditions and test basis, package input power at that pressure, the full scope with inclusions and exclusions, the air quality offered, documents to be handed over and the warranty terms.

Is motor kW a fair way to compare compressors?

No. Two 22 kW machines can deliver noticeably different amounts of air and draw different power at the meter. Compare free air delivered and package input power at the same pressure, and calculate specific power.

What is a good specific power for a screw compressor?

For a modern oil-injected screw package at about 7 bar(g), typically around 6 to 7.5 kW per m³/min of free air delivered. Lower is better; confirm each figure against the maker’s data sheet at your pressure.

Why does the pressure stated on a quotation matter so much?

The same machine delivers less air and draws more power per unit of air as pressure rises. A quotation stated at 7 bar(g) cannot be compared with one stated at 8 bar(g) until both are restated at your set point.

Should the dryer and receiver be in the same order?

Usually yes. Buying them together makes one supplier responsible for sizing, pressure drop and performance, and lets you compare quotations with identical scope.

What warranty comes with a used or refurbished compressor?

Ours carry a six-month warranty from the delivery or invoice date, covering the whole compressor and excluding consumables, misuse or wrong oil, service by others, and accident or site damage.

Can you check quotations we already have from other suppliers?

Yes. Send them with your requirement and we will put them on the same basis of pressure, free air delivered, input power and scope so the differences are clear.

Related pages

• Complete compressor selection guide
• Technical specification and bid evaluation
• Supplier bid normalisation worksheet
• Specific power comparison calculator
• Air compressor running cost calculator
• Commissioning and final acceptance
• Used and refurbished compressors for sale
• Compressor log book formats

Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 5 November 2026.