Sizing a rotary screw compressor comes down to three numbers and one pattern: the pressure your equipment actually needs, the capacity (CFM or m³/hr) your demand actually adds up to, and the demand profile (how that demand shows up over the day, week, and year). Get any of these wrong and you either pay too much upfront, pay too much in energy for the next 15 years, or break the machine because you used it for a job it wasn't designed for.

The single most common buying mistake is oversizing. Plants buy a bigger compressor "just to be safe" and then pay for it in energy and maintenance for the life of the machine. The second most common mistake is using a screw compressor for intermittent duty, which kills the machine slowly. Both are covered below.

Pressure: how much do you really need?

Most industrial pneumatic equipment runs on 6-8 bar (87-115 psi). A smaller subset needs 10 bar (145 psi). Specialty applications go higher, but you should know who those are because they are usually obvious (high-pressure cleaning, certain PET bottle blowing, some packaging machines, breathing air at 200+ bar which is a different machine entirely).

The cost of pressure is real: every 1 bar above what you actually need costs roughly 6-7% more energy for the same delivered CFM. Sizing your whole system to the highest-pressure tool on site, when that tool only runs occasionally, can waste 20-30% of your annual energy bill.

Better approach: size the central compressor to the demand of the 90%+ of your equipment, and handle the high-pressure outliers separately. A small high-pressure booster or a dedicated small compressor for the one machine that needs 12 bar is usually cheaper to buy AND cheaper to run than oversizing the whole system.

What to do:

  • Walk the plant and check the actual operating pressure of each piece of equipment (often lower than the nameplate maximum)
  • Identify the highest-pressure user and ask if it really needs that pressure all the time, or just at startup, or never (people inherit pressure assumptions from the previous owner all the time)
  • If one or two outliers drive the rest, isolate them rather than sizing the whole compressor to them

Capacity: CFM / m³/hr (sizing it right)

Capacity is the volume of compressed air your equipment consumes, expressed in CFM (cubic feet per minute) in the US, or m³/hr or liters/second in Europe.

The number to compare across vendors is Free Air Delivery (FAD), which is the actual air the compressor delivers at the operating pressure, measured to ISO 1217. Watch out for sales sheets that quote piston-displacement, nameplate, or "peak" numbers, all of which overstate what the machine actually puts out under real conditions.

How to size correctly:

  1. List every air-consuming piece of equipment on the plant and its nominal consumption at operating pressure. Tool manufacturers publish this.
  2. Apply a utilization factor to each. A pneumatic cylinder firing 4 times per minute doesn't consume air continuously; it consumes the rated CFM for fractions of a second per cycle. Typical utilization factors range from 10-30% for cyclic tools to 60-90% for continuous-process equipment.
  3. Add up the realistic simultaneous consumption. This is your design CFM.
  4. Add 10-15% margin for system leaks (which every system has) and growth.

If the vendor quotes you a sizing without doing this audit, or without measuring your actual demand with a flow logger, treat the quote with suspicion. A serious vendor will measure for at least a week before recommending a size.

A note on units: 1 m³/hr ≈ 0.59 CFM. 1 l/s ≈ 2.12 CFM. The compressor industry mixes these constantly, sometimes within the same datasheet. Always confirm what units you are comparing.

Demand profile

After pressure and capacity, the third sizing question is HOW your demand actually shows up across the day, week, and year. Two systems with identical peak demand can need very different compressor setups depending on the demand pattern.

There are roughly three patterns, with different right answers.

Constant 24/7 demand

A continuous production line, a large bottling plant, a sandblasting operation, a CNC shop running multiple shifts. The compressor needs to deliver close to its rated CFM almost continuously.

This is the rotary screw's natural fit. A right-sized fixed-speed screw running at 70-90% load 24/7 is the most energy-efficient option you can buy. VSD usually doesn't pay back on this profile because there is no modulation to capture savings from.

Variable / peaky demand

Demand swings significantly across the day or shift. A typical machining shop, a mixed packaging operation, a multi-process plant where some equipment runs and some doesn't depending on which orders are running.

Two reasonable approaches here:

  • A VSD compressor sized to handle the peak, modulating down during quieter periods. Works well if the average load genuinely sits in the 30-80% range. See VSD vs fixed speed for the economics.
  • Two fixed-speed compressors + a sequencing controller + a bigger receiver. Compressor A handles base load, compressor B kicks in for peaks. Often the cheaper buy and easier to maintain, with similar energy efficiency.

Which is better depends on your space, your service relationship, and the actual measured demand profile. Don't let a vendor default you to VSD without comparing both options.

Intermittent (1 day a week): why a screw is the wrong choice here

This is the contrarian warning that vendors don't lead with: a rotary screw compressor is the wrong machine for intermittent duty.

A screw compressor needs to reach and hold operating temperature (around 80°C oil temperature) to:

  • Properly seal the rotors (the oil seal between the rotors depends on heat-driven oil viscosity)
  • Boil off condensation in the oil (otherwise the oil emulsifies into a milky mess that destroys bearings)
  • Reach the design lubrication regime that the airend was engineered for

A screw that runs for 2 hours twice a week, then sits cold for days, spends most of its life cold-starting and shutting down before ever reaching operating temperature. The oil emulsifies, the bearings start to fail, the airend dies years before it should, and the warranty claim gets rejected because "the duty profile was outside specification".

The right answer for genuinely intermittent demand is a piston (reciprocating) compressor with a large receiver. Pistons start and stop gracefully, don't care about reaching operating temperature, and last a long time on this kind of profile. The piston is also cheaper to buy.

See the piston buying guide for the right answer on intermittent duty. Don't let a vendor sell you a screw for this profile just because screws are their higher-margin product.

Variable Speed Drive (VSD)

VSD is its own decision and deserves its own page. The short version: VSD is sometimes the right answer for variable demand and sometimes oversold by a vendor who hasn't measured your actual load profile.

The full economics, including when the payback genuinely happens and when it doesn't, is in VSD vs fixed speed. Read it before signing any quote with a VSD premium.

When in doubt: bigger receiver beats bigger compressor

This is the single most underrated investment you can make in a compressed air system, and the cheapest fix for a lot of sizing problems.

The receiver (air tank) smooths out short-term demand peaks so the compressor sees a flatter load. Without a properly-sized receiver, every short demand peak forces the compressor to either run at full speed or short-cycle (start, run briefly, stop, start again), both of which are wasteful and hard on the machine.

With a generous receiver, a smaller right-sized compressor can handle peaks that would otherwise force you to buy a bigger machine "just to be safe".

Sizing rule of thumb: at least 1 gallon of receiver per CFM of compressor output (US units), or roughly 6 liters per l/s of output (metric). More is better, especially for variable-demand systems. A 75 kW screw delivering 400 CFM should have at least a 400-gallon (1,500 liter) receiver. Many plants run with half that and pay for it.

What a bigger receiver actually buys you:

  • Smooths short-term demand peaks so the compressor cycles less
  • Lets a smaller, cheaper compressor cover demand peaks without a "safety oversize"
  • Acts as a coalescing reservoir where water and oil aerosols can settle out (one of the cheapest forms of pre-treatment)
  • Reduces motor start-stop wear on the compressor itself
  • Buffers the system during a brief vendor delay if you need to swap out a major component

The cost: roughly €1,000-3,000 for a quality 500-2,000 liter industrial receiver. The alternative cost: €15,000+ in compressor upcharge for the "just to be safe" oversize, plus higher energy bills for the next 15 years.

I have seen many plants that bought a 75 kW VSD because they thought peaks needed it, when a 55 kW fixed-speed plus a properly sized receiver would have done the same job at much lower total cost of ownership.

When in doubt: more receiver, smaller compressor. Almost always the right call.

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