A step-by-step method to size a compressor for a Gulf plant or workshop: demand audit, cfm and FAD, pressure losses, leak tests, receiver tank formulas, fixed speed or VSD, with a worked example.
Size a compressor in this order: measure or estimate the air your plant uses (cfm, l/s or m3/min) over a real working week, add measured leakage and planned growth, set the pressure from the highest point-of-use need plus losses, then choose the machine, drive and receiver to cover that profile. Compare industrial air compressor suppliers in the UAE on free air delivery at your pressure, not on motor kW.
This guide is for plant engineers, maintenance managers and consultants writing a compressor specification. It goes deeper on sizing than our guide to industrial air compressors in the UAE and Saudi Arabia, which explains FAD, the ISO 1217 test basis, compressor types and brands. Here we cover the demand audit, flow units, leak testing, pressure losses, receiver tank formulas, fixed speed versus VSD and a full worked example. Buyers in the Kingdom can compare air compressor suppliers in Saudi Arabia.
For an existing plant, measure. Guessing from nameplates is how most compressors end up oversized or undersized.
Ask compressor bidders whether they offer a logged survey; measured data beats any estimate.
Without a system to measure, list every consumer and work out average and peak demand:
Atlas Copco's sizing guide sums up the trade-off: an undersized compressor causes pressure drops and incomplete tasks, and an oversized one can suffer mechanical problems. Intermittent demand can be met by a smaller compressor and a receiver that refills between uses; continuous demand needs continuous flow.
Quotes in the Gulf arrive in all of these units, so convert everything to one before comparing:
| From | To | Multiply by |
|---|---|---|
| m3/min | cfm | 35.31 |
| l/s | cfm | 2.119 |
| m3/min | l/s | 16.67 |
| m3/h | m3/min | 1/60 |
| bar | psi | 14.5 |
Watch the basis as well as the unit. Free air delivery is quoted at the compressor inlet and tested to ISO 1217, as the main guide explains. A figure in Nm3 (normal cubic metres) is referred to standard conditions instead; Kaeser's calculator uses 0 °C, 1.01325 bar and 0% humidity under DIN 1343, or 15 °C under ISO 2533. Process data may be given in Nm3/h while compressors are sold on FAD, and the two are not the same number. Ask every bidder to state flow on one agreed basis.
Do not size from motor power. Atlas Copco's rule of thumb is that a typical stationary compressor gives about 4 to 6 cfm per kW, which is a wide band; it also states that rotary screw compressors generally deliver more flow per kW than piston compressors. Compare FAD at your pressure and the specific power (kW per m3/min) for each bid.
Kaeser states that even new, well-maintained networks can lose around 15 to 20% of compressed air to leaks, rising to 30 to 40% on older networks; the main guide gives Atlas Copco's target of below 5%. Fix leaks first, then size for the demand that is left plus a realistic leak allowance.
Kaeser's leakage calculator uses two simple tests, both run with all air consumers switched off:
Kaeser also uses ultrasonic detectors to find individual leaks during normal production.
Start from the highest pressure any consumer needs at its connection, then add every loss between the compressor and that point: dryer, filters, piping, hoses, regulators and quick couplers. Atlas Copco's sizing guide lists losses through dryers, filters and drains; its air receiver guide puts the cost of extra pressure at roughly 1% more energy for every 2 psig, and the main guide gives about 7% per extra bar. So every unnecessary bar is a permanent energy cost.
A receiver stores air to cover peaks and stops a fixed-speed compressor from switching too often. Atlas Copco describes it as a buffer between the compressor and the pressure swings caused by changing demand, and gives a starting rule of 10 to 15 litres of receiver per l/s of compressor flow (3 to 4 US gallons per cfm), depending on compressor type and application. For a proper figure, use two calculations and take the larger result. Both are the formulas behind Kaeser's online receiver calculator.
V = Q × (A − A²) ÷ ((Z ÷ 60) × Δp)
V = q × t ÷ (p1 − p2)
Two practical points. First, rate the receiver for the compressor: Atlas Copco suggests a receiver of at least 10 bar for a fixed-speed compressor running to 8 bar, and 14 bar for a VSD compressor that can reach 12 bar. Second, decide where it goes: Atlas Copco describes wet receivers between the compressor and dryer, which help drop out water, and dry receivers after the dryer, which hold treated air close to demand. Receivers are pressure vessels; ask the supplier for the design code, certificate and the conformity documents your country requires. Find tanks under air receivers and tanks.
The demand profile from Step 1 decides this. Atlas Copco recommends a VSD compressor where air demand fluctuates through the day, because motor speed follows demand and avoids the wide regulation band of fixed-speed machines, and advises including at least one VSD unit in multi-compressor stations. Atlas Copco also claims that only around 8% of compressed air systems have truly stable demand; that is a maker's figure, so check it against your own logged profile.
For a larger plant, one arrangement to ask bidders to price is fixed-speed machines running fully loaded on the base load with a VSD machine trimming the variable part, under a sequencing controller. For a single compressor on a steady two-shift load, a correctly sized fixed-speed unit with a well-sized receiver can be the better buy; Kaeser notes that the larger the receiver, the more smoothly a compressor station switches. Whatever the choice, ask for a savings calculation run on your logged data, not on a generic profile. Our guide to screw versus piston compressors covers the compressor type decision.
The figures below are assumptions chosen to show the method, not recommendations for any particular site.
| Consumer | Rated flow | Share of time in use | Average flow |
|---|---|---|---|
| 4 impact wrenches | 5 cfm each, 20 cfm | 30% | 6 cfm |
| 2 spray guns | 8 cfm each, 16 cfm | 50% | 8 cfm |
| Packaging machine | 20 cfm | 100% | 20 cfm |
| Measured leakage after repairs | continuous | 4 cfm | |
| Total | 38 cfm |
Once the sizing is done, post your compressor requirement so suppliers in the UAE and Saudi Arabia can quote against the same figures. See also screw air compressors and compressed air treatment.
Atlas Copco offers compressed air audits with current loggers left on site for 7 to 14 days, publishes sizing and receiver guidance, and sells fixed-speed and VSD screw compressors through its own offices in the UAE and Saudi Arabia.
Kaeser runs a ten-day air demand analysis (ADA) based on load and idle logging, offers ultrasonic leak detection, and publishes the online calculators for receiver size, leakage and pressure drop used in this guide.
More in this series: Oil-Free Air Compressors for Food, Pharma and Electronics Plants.
Add the air consumption of every tool and machine, weighted by how much of the time each one runs, to get average demand. Add measured leakage and growth, then choose a compressor whose free air delivery at your working pressure covers that total. Check the peak when several consumers run together and cover it with a receiver or extra capacity.
There is no standard figure; it depends entirely on your demand. As a cross-check, Atlas Copco's rule of thumb is that a typical stationary compressor delivers about 4 to 6 cfm per kW, so a 37 kW machine gives roughly 150 to 220 cfm. Always compare free air delivery at your pressure from the data sheet.
As a starting point, Atlas Copco suggests 10 to 15 litres per l/s of compressor flow, or 3 to 4 gallons per cfm. For a real design, calculate the volume needed to limit compressor switching and the volume needed to cover your largest peak, using the formulas above, and use the larger figure.
No. Atlas Copco notes that higher pressure does not mean more flow, and that raising pressure reduces flow from a given compressor. Higher pressure also costs about 1% more energy for every 2 psig. Set the pressure from the highest point-of-use need plus measured losses, and no higher.
Switch off all air consumers. Either time how long the compressor runs loaded over a test period (leakage equals FAD times loaded time divided by test time), or stop the compressor and time the pressure drop in the receiver and pipework (leakage equals volume times pressure drop divided by time). Use an ultrasonic detector to find individual leaks.
If your logged demand varies through the day, usually yes, because a VSD machine follows demand instead of loading and unloading. Atlas Copco recommends at least one VSD unit in a multi-compressor station. For steady, full-load demand a fixed-speed machine with a good receiver can be the better buy. Decide from your own data.
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