Air compressor selection: screw vs reciprocating, CFM, pressure and air quality

Compressed air powers tools, actuators, packaging lines, paint booths and blow moulding machines in factories everywhere. It is also one of the most expensive forms of energy in a plant: over a compressor's life, electricity usually costs far more than the machine itself. Choosing the right type and size pays back every month.

Screw vs reciprocating compressors

Reciprocating (piston) compressors

A piston compresses air in a cylinder, much like an engine in reverse.

• Lower purchase price, simple to maintain.
• Suited to intermittent use: workshops, garages, small factories with variable demand.
• Noisier and less efficient for continuous duty.
• Typically designed to run with rest periods, not 24 hours a day.

Rotary screw compressors

Two interlocking rotors compress air continuously.

• Designed for continuous duty: run all shift, every shift.
• Quieter, smoother air supply, lower maintenance per running hour.
• More efficient at higher demand.
• Higher purchase price.

Rule of thumb: if the compressor will run more than about half the time, a screw compressor usually costs less over its life. For occasional use, a reciprocating compressor is often enough.

Larger plants may also consider centrifugal compressors for very high, steady demand.

Sizing: flow (CFM) and pressure

Flow

Flow is measured in cubic feet per minute (CFM) or cubic metres per minute. To estimate the flow you need:

1. List every tool and machine that uses air and its consumption at its working pressure (from the manufacturer's data).
2. Estimate how much of the time each runs (its duty cycle) and how many run at once.
3. Add the simultaneous demand.
4. Add an allowance for leaks, which in many plants waste a significant share of compressed air, and for future growth.

Make sure you compare quotes on the same flow basis, often called FAD (free air delivery), measured to a recognised standard.

Pressure

Pressure is measured in bar or psi. Most tools run at around 6 to 7 bar. Choose a compressor rated slightly above the highest pressure any user needs, plus the pressure drop through dryers, filters and pipes.

Don't over-pressurise: generating air at a higher pressure than needed wastes energy. Each extra bar typically adds several percent to power consumption.

Oil-lubricated vs oil-free

• Oil-lubricated compressors use oil in the compression chamber. They are cheaper and robust; filters remove most oil from the air.
• Oil-free compressors keep oil out of the compression chamber altogether. They are needed where even traces of oil are unacceptable: food and beverage, pharmaceuticals, electronics, hospitals and some textile processes.

Air quality is often specified using ISO 8573-1 classes for particles, water and oil.

Air treatment: dryers and filters

Compressed air leaves the compressor hot and full of water vapour. As it cools in pipes, water condenses, causing rust, damaged tools and spoiled products.

• Refrigerated dryers cool the air to remove most moisture. Enough for general use.
• Desiccant dryers remove moisture to very low dew points for sensitive processes and outdoor lines in cold weather.
• Filters remove particles and oil aerosols. Use the grade your process needs.
• Auto drain valves remove condensate from receivers and filters.

Air receiver tank

A receiver tank stores compressed air, smooths out demand peaks and reduces how often the compressor cycles. It also lets condensate settle. Size it to your demand pattern and compressor type.

Fixed speed vs variable speed (VSD)

• Fixed speed compressors run at full speed and unload when demand falls. Efficient when demand is steady and close to full capacity.
• Variable speed drive (VSD) compressors adjust motor speed to match demand. They can save substantial energy when demand varies through the day.

Energy efficiency checklist

• Fix leaks. A leak survey is one of the cheapest energy savings in a plant.
• Lower system pressure to the minimum that works.
• Turn compressors off when not needed, or use controls that do it automatically.
• Recover heat from the compressor for water or space heating.
• Use correctly sized pipes to minimise pressure drop.

What to ask suppliers

1. Compressor type and model, rated FAD at your working pressure.
2. Motor power and specific power (kW per unit of air delivered).
3. Fixed speed or VSD.
4. Oil-free or oil-lubricated; air quality class.
5. Dryer, filters and receiver included or quoted separately.
6. Noise level and cooling method (air or water cooled).
7. Service interval, spare parts cost and local service support.
8. Warranty terms.

Frequently asked questions

How much compressed air do leaks waste?

In many plants, leaks waste a significant share of the air produced. Regular leak surveys and repairs are one of the cheapest ways to cut compressor energy costs.

What is FAD?

Free air delivery is the actual volume of air a compressor delivers, measured at intake conditions. Compare compressors on FAD at your working pressure, measured to a recognised standard.

Do I need an oil-free compressor?

If any trace of oil could contaminate your product or process, as in food, pharma, electronics or medical uses, yes. For general industrial tools, an oil-lubricated compressor with proper filtration is usually enough.

Is a VSD compressor always better?

A VSD saves energy when air demand varies. If demand is steady and close to full capacity, a fixed-speed compressor can be just as efficient and costs less.

Find air compressors on GlobalMart

Compare manufacturers and dealers in air compressors, plus related electric motors and pumps. Then post your requirement with your air demand, pressure, air quality needs and operating hours.