The figures a distributor needs before they can quote
A distributor can only price a compressor accurately once they know what the job actually demands. Ringing round with "I run a couple of sanders and a spray gun" gets you a guess; sending five figures gets you a number you can compare against the next quote. Each one answers a different question the distributor has to ask before they can spec a machine.
Free air delivery at your working pressure
Free air delivery (FAD) is the volume of air the compressor actually delivers per minute, measured at atmospheric pressure, at the pressure your tools run at. It is not the same figure as displacement, which is the volume swept by the piston or screw element before any losses are taken out, and which is always the larger number. A listing that only states displacement will overstate what reaches the tool, so send the distributor the FAD figure in litres per minute (or cubic feet per minute) at your working pressure in bar. The difference between the two, and how to read a listing that only gives one of them, is covered in Displacement and Free Air Delivery.
Receiver size
The receiver is the tank that stores compressed air between compressor cycles, and its size in litres determines how well the system copes with a sudden draw, such as a spray gun opening up while a sander is already running. Give the distributor the receiver size you have worked out you need, along with your cut-in and cut-out pressures if you know them, so they can quote a tank that matches the demand.
Electrical supply
State the voltage and phase available at the point the compressor will sit, single-phase or three-phase, and any limit on the circuit it will run from. A compressor sized correctly on air can still be the wrong machine if the supply cannot carry its motor, and that is a detail a distributor needs before they quote.
Duty
Duty cycle is the proportion of time the compressor is expected to run under load, whether that is short bursts through the day or near-continuous running on a production line. A machine sized for occasional use and one sized for continuous duty are different specifications even at the same free air delivery, so tell the distributor how the compressor will actually be worked.
Air quality class
Different jobs need different levels of cleanliness in the air itself, from a basic workshop line to painting or instrument work where particles, oil and moisture have to be kept below stated limits. Naming the air quality class you need, referenced to ISO 8573-1, tells the distributor whether the quote needs to include a dryer and filtration or a compressor alone. Where the air feeds an air-fed hood for spraying, the requirements are set separately and are covered in Breathing Air for Spray Booths.
Sending all five figures in one message is what turns a string of back-and-forth emails into a single comparable quote.
Questions to ask before you accept a quote
The figures get you a price. These questions get you a fair comparison between distributors and a clear idea of what happens after delivery.
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Who does the installation and what is left to you
Pipework, electrical connection and siting are sometimes included and sometimes charged separately, so ask what the quoted price actually covers on-site.
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Who carries out the written scheme of examination, and when
If the receiver crosses the bar-litre threshold, ask who the competent person is and how the first examination date is set.
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What the servicing schedule is and who performs it
Ask whether servicing is done by the distributor's own engineers or subcontracted, and what a routine visit covers.
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What the warranty covers and for how long
Ask whether the warranty covers parts, labour or both, and whether it depends on servicing being carried out by that distributor.
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Whether the quoted price includes delivery, commissioning and instruction on use
A lower headline price sometimes excludes one of these and adds it back as a separate charge.
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What happens if the compressor cannot meet the demand once running
Ask what recourse exists if the free air delivery on site falls short of what the figures you sent were based on.
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Whether replacement parts and consumables are readily available
Ask about lead times for filters, oil and wear parts, since this affects how long a fault takes to fix once the compressor is in service.
A written scheme is a legal requirement above the bar-litre threshold. If your receiver is close to that line, check it first.
Comparing the quotes once they land
Quotes for the same job rarely arrive in the same shape, and a lower headline price often turns out to be quoting a smaller job. The fix is to line the quotes up against the same figures you sent.
Same demand, same terms
Check that each quote is pricing to the free air delivery (FAD) figure you sent, stated at your working pressure. A distributor quoting a compressor with a bigger number on the nameplate is not necessarily quoting more usable air; the gap between displacement and free air delivery is exactly the kind of thing that makes two quotes hard to compare on price alone, and it is covered in more depth on displacement and free air delivery.
Set the quotes next to each other on a short list of points:
- Free air delivery at the working pressure you specified
- Receiver size in litres, and whether it changes the bar-litre position you already worked out
- Motor power and the electrical supply it assumes (single-phase or three-phase, and the fuse rating)
- What air quality class the quote assumes, if the air feeds tools, painting equipment or an instrument line
- What is included in the price: installation, pipework, commissioning, and the first service
Where the price difference actually sits
Once free air delivery, receiver size and supply match, a price difference usually comes down to what is bundled in: installation labour, a warranty period, or the written scheme of examination arranged through the distributor. A quote that looks cheaper because it excludes installation is a smaller job, so ask each distributor to itemise.
If a distributor's figures do not match what you sent, ask why before you compare price. It may be a legitimate adjustment for a duty cycle or altitude you did not mention, or it may be a substitution that changes what you are actually buying.
Keep up with what changes after you've bought
Regulation and price changes reach compressed air buyers here as they're published, each tied to what it means for someone running a system.