How to Choose the Right Rotary Screw Air Compressor

Start With Real Air Demand

Before comparing compressors, measure what your plant actually consumes. Add up the free air delivery (FAD) of every tool and process that can run at the same time, then convert each figure to the pressure at which it operates. A 7 bar tool fed at 10 bar does not need 10 bar, and quoting demand at the compressor's rated pressure rather than the working pressure is the most common sizing error. Add 15 to 25 percent for leakage and future growth; a well-maintained plant still loses 10 to 15 percent through joints, hoses and fittings. If demand is intermittent, log it over a full production week rather than estimating from nameplates.

Choose the Pressure Rating

Rotary screw compressors are typically offered at 7, 8, 10 and 13 bar. Each step up in pressure costs roughly 6 to 8 percent more energy per cubic metre delivered, and that penalty applies on every running hour. Buy the lowest rating that covers your highest-pressure tool plus the pressure drop in the system. If most of the plant runs at 6.5 bar but one machine needs 8 bar, consider a small dedicated unit for that machine instead of raising the whole system.

Fixed Speed or Variable Speed Drive

Fixed speed: best where demand is steady and the compressor runs at or near full load for long periods. Lower capital cost and simpler maintenance.
Variable speed drive (VSD): best where demand fluctuates through the day or week. The drive matches motor speed to demand, avoiding unload cycles. Payback is typically 12 to 24 months in plants with large swings, but can exceed the machine's life where demand is flat.
Multiple smaller fixed-speed units with one VSD trim compressor: a common approach where demand varies widely and redundancy matters.

Air Treatment Matched to the Application

General plant air usually needs a receiver tank, a refrigerated dryer to a pressure dew point around 3 degrees C, and a coalescing filter. Instrument air needs a drier dew point, typically -20 degrees C or lower, plus finer filtration. Breathing air requires a dedicated purifier and continuous monitoring. Oil-free processes need an oil-free compressor, not just a filter, because filters do not guarantee zero oil carryover. Size the receiver at roughly 1 litre per 1 litre/min of FAD as a starting point, larger where demand pulses.

Installation and Serviceability

Allow at least 1 metre of clearance for ventilation and ensure ambient temperature stays below the manufacturer's limit, typically 40 to 45 degrees C; every 5 degrees above that reduces capacity and shortens oil life. Mount on a level, rigid foundation and fit anti-vibration pads. Size pipework for a pressure drop under 0.3 bar from compressor to the furthest tool, and drain condensate automatically at the tank, dryer and low points. Check filter and separator intervals, spare parts availability and whether local service support exists before you buy, not after.

Mistakes Buyers Make Most Often

Sizing on the tool's rated cfm rather than its actual consumption at working pressure; ignoring pressure drop so the compressor runs at a higher setting than needed; and undersizing the receiver, which forces short cycling and unstable pressure. Each of these adds energy cost every hour the system runs.

Send Kaishan your demand profile and pressure requirement, and our engineers will recommend a matched compressor, treatment package and control strategy for your site.

Frequently Asked Questions

What size rotary screw compressor do I need for my workshop?

Add the free air delivery of every tool that can run at once, converted to your actual working pressure, then add 15 to 25 percent for leakage and future growth. For example, three tools drawing 1.5 m3/min each at 7 bar gives about 4.5 m3/min, so specify roughly 5.5 m3/min at 8 bar. If demand is intermittent, log it over a full production week rather than relying on nameplate figures.

Is a variable speed compressor worth the extra cost?

It depends on how much your demand fluctuates. Where airflow varies widely through the day, a VSD typically pays back in 12 to 24 months by matching motor speed to demand and avoiding unload cycles. Where the compressor runs at or near full load continuously, a fixed-speed unit costs less to buy and is simpler to maintain, and the VSD premium may never be recovered.

What pressure rating should I choose, 8 bar or 10 bar?

Choose the lowest rating that covers your highest-pressure tool plus system pressure drop. Most general plant air runs at 7 or 8 bar. Each step up to 10 or 13 bar costs roughly 6 to 8 percent more energy per cubic metre delivered, and that penalty applies every running hour. If only one machine needs higher pressure, a small dedicated compressor is often cheaper than raising the whole system.

Do I need an air dryer and receiver tank with a rotary screw compressor?

Almost always yes. A receiver tank stabilises pressure and reduces short cycling; size it at roughly 1 litre per 1 litre/min of free air delivery, larger for pulsing demand. A refrigerated dryer to about 3 degrees C pressure dew point suits general plant air, while instrument or breathing air needs a lower dew point and finer filtration. Oil-free processes require an oil-free compressor, not just filters.

How much ventilation does a rotary screw compressor need?

Allow at least 1 metre of clearance around the machine for air intake and cooling, and keep ambient temperature below the manufacturer's limit, typically 40 to 45 degrees C. Every 5 degrees above that reduces capacity and shortens oil life. If the room is small, duct warm exhaust air outside rather than recirculating it, and make sure the foundation is level and rigid.