Air Compressor FAQ – Technical & Buying Guide (UK)

The correct size depends on the airflow required (measured in CFM, l/min, m3/hr, M3/min, l/sec) and the air pressure needed (measured in bar, psi or pa). The receiver size (measured in liters or gallons) should be of a volume that is appropriate to the airflow from the compressor and on smaller model’s users might consider space and portability as being more of a factor as a larger air receiver will take up smore pace and be heavier and less portable.

Many air compressors used for pneumatic tools and devices will offer a pressure range between 102 psi (7 bar) and 145 psi (10 bar). In these situations what differentiates a larger compressor from a smaller unit is the amount of air flow provided and NOT so much air pressure delivered.

The Power Supply available will often determine the size of air compressor that can be used. For instance, a 230v single phase supply (13/16 amps) will normally limit the size of compressor that can be used to 2.0 -2.2 kW. Larger units up to 5.5 kW can be run from a 230v single phase power supply, however these shall normally require a dedicated power supply (16amps to 40 amps ) that is connected independently and directly to the incoming power distribution board* For applications where a 380/400/415V three phase power supply is available then the power range of air compressor that can be used is only limited by the amount of power available to the site and the validity of the connection available. Air compressors rated at 400kW or larger are used in large scale manufacturing and industrial processes, however smaller compressors at 1.5 kW or smaller are often used with a three-phase power supply when available. Where electrical power is not available the applications may consider petrol or diesel engine powered compressors or compressors connected to an alternative power source from an excavator, commercial vehicle or other source that may use an auxiliary power output or belt drive arrangement.

The size (power input) of the compressor will determine the capacity (air flow) available to the user. Normally for every 1 kW (1.4HP) of motor power, a compressor will deliver between 100l/min and 170 l/min (3.5 cfm to 6 cfm) Free Air Delivered at a pressure of 7 bar (102 psi).

The size/type of air compressor shall be determined by the air flow required by the process or application and in some cases the air pressure where this value is higher than the normal range of 7- 10 bar (102 psi to 145 psi). Consider the tool(s) or equipment that requires the maximum air flow and then consider how often this is used and by how many people at one time. Normally the air flow requirement is stated by the manufacturer of the equipment, however, consider that in the case of pneumatic tools, the more powerful that they are and the more that they are used equates to a higher air flow requirement. Once you have ascertained a maximum value for the air flow required you should then add an additional 20% as a safety margin to take care of wear and tear, air losses and other factors.

Home workshop users typically requires 6–14 cfm, whilst small workshops and car maintenance shops 14–30 cfm, and industrial applications 20 cfm to 500 cfm or even much more. Always consider the FAD (Free Air Delivery) of an air compressor for accurate comparison.

For a compressor being used by one or two people the formula above is quite simple, however for more complicated applications and production environments things become a little bit more complicated, but not too much so! The same factors apply

Add up all of the pneumatic tools and air cleaning guns being used at one time and by any one person, and if multiple pneumatic tools are being used by an individual user, consider those tools with maximum air flow requirement. You will then have a list of the people using pneumatic devises and the maximum air flow for each person/application

Make a list of all the machinery and devices that require compressed air and the flow needed for each, such as machining centers, automation, media blasting, pumps, testing apparatus, autoclaves, plastic molding equipment and so on.

Once you have a complete list, you can then calculate a theoretical value for the maximum amount of air flow needed in total of all of the equipment in use that requires compressed air. We then consider a few points.

Displacement (in cfm or l/min etc.) often refers to theoretical suction volume displaced by an air compressor ie in piston compressors this will be the volume or displacement of the cylinder(s) multiplied by the rpm of the compressor at atmospheric pressure. The FAD (Free Air Delivery) measurement relates to the actual (Measured) usable airflow at a given operating pressure. This value takes account of losses in the compression process and represents the real value for the compressed air flow that is produced and available to the user. The FAD should always be used when comparing compressors or when sizing for a suitable compressor. As a general guide when referring to piston compressors the FAD value will be approximately 60% of the displacement value, so for example a piston compressor with an air displacement or suction volume of 10 cfm will provide an FAD output at around 6 cfm.

Oil-free piston compressors do not use oil as a lubricant and instead use sealed bearings and self-lubricated pistons and liners. These models provide clean air and are suitable for dental, medical and food environments. Rotary scroll compressors likewise do not use oil within the compression process. Oil-lubricated compressors are better suited for standard and heavy-duty industrial applications. The lubricants that can be used are varied and include mineral, synthetic and even food grade types.

A direct drive compressor is characterized by the compressor pump unit being directly connected to the electric motor, resulting in a compact design, fewer moving parts and lower maintenance requirements. In smaller piston units the motor shaft and compressor shaft is manufactured as a single item. On larger compressors including screw compressors the direct drive arrangement will include a flexible coupling.

A belt driven compressor uses a pulley and belt system between the motor and the pump/compressor block. This allows the pump to run at a slower speed, reducing wear, heat build-up and noise. Belt driven piston compressors are commonly used in workshops and light industrial environments where durability and longer run times are required. Many smaller to medium size rotary screw compressor (typically 2kW to 75kW) will also be fitted with a belt drive arrangement.

A piston compressor (also known as a reciprocating compressor) uses one or more pistons/cylinders that are driven by a crankshaft and connecting rods. These compressors are commonly used in garages and workshops for standard duty applications. Piston compressors are not suitable for continuous use. Piston compressors normally comprise a single or two stage design, however three and four stage piston compressors can be used for pressure up to 400 bar or greater

A portable air compressor is a compact unit designed for mobility. They are typically used for DIY tasks, tyre inflation and light trade work and are often designed to work with a 230v or 110 v power supply. Portable piston compressors may also be fitted with a petrol or diesel engine. The large ‘towable’ compressors used on highways and construction sites are also referred to as portable

A rotary screw compressor uses two parallel non contacting screws or rotors (male and female profile) suspended on bearings. The rotors are assembled into a block or stator with end covers and a drive shaft. The complete compressor assembly is often referred to as an ‘air- end’. To compress air, atmospheric air is introduced into the air-end through a regulator. The air is trapped within the lobes of the rotors and pushed along the stator. The space between the screws or rotor is reduced during rotation resulting in a single stage compression process. Rotary screw compressors are ideal for manufacturing facilities and higher flow applications. A Rotary screw compressor from a reputable manufacturer shall provide 100% duty cycle rating and are often used around the clock. These models also feature reduced noise levels and simple installation. Rotary screw compressors are often fitted with variable speed drive.

24–50 litres is suitable for DIY use, 90–150 litres for workshops, and 200+ litres for industrial systems. Larger tanks reduce motor cycling and improve pressure stability. Larger compressors shall require an air receiver that has sufficient volume to allow a constant air pressure to the network and to allow the compressors regulation devises to operate efficiently. The larger the air compressor, the larger the air tank or more correctly named air receiver, shall be. A 5.5 kW (7.5HP) compressor will normally require a receiver at around 200 liter. A 22 kW (30HP) compressor shall require a 500 Liter receiver, and a 75 kW (100 HP) compressor shall need a 1500 Liter receiver and so on. Using a larger receiver is not an issue technically speaking, however using a smaller receiver will cause problems for the compressor operation and may result in erratic air pressure, increased energy consumption and may mean more wear to components.

Duty cycle refers to how long a compressor can run within a work period. A 50% duty cycle means 10 minutes running per 20 minute period and this is typical for piston compressors. A 100% duty cycle indicates suitability for continuous operation typically applied to rotary screw compressors and some piston reciprocating compressors.

A standard on/off regulated rotary screw fixed speed compressor has a fixed output so therefore the output of the compressor can be either 100% or zero. Fixed speed screw compressors are either off/on or in idle mode during use.

A Variable Speed Drive compressor utilizes an inverter that is able to adjust the drive motor speed according to air demand. By adjusting the motor speed and thereby the compressor rotor speed the output of the compressor can be reduced or increased in real time according to the demand, avoiding costly idle operation and reducing energy consumption, noise and mechanical wear.

An air dryer removes moisture from compressed air. It is essential for many processes including spray painting, pneumatic devises, food and drug production, power generation, CNC machinery, dental equipment and any application where moisture could cause damage and reduced productivity.

Condensation forms naturally during compression as a product of the ambient air. The amount of moisture in the air will be determined by physical factors including humidity. The saturated air is introduced in to the compressor during compression and then exits the air compressor at relatively high temperature. As the compressed air cools condensation occurs. Some of this condensation is often collected in the tank or air receiver where it can be collected and drained, however a large percentage of the moisture shall enter the system or pipes where it becomes a major problem in terms of corrosion, damage to equipment and spoilt products. The problem of condensation in compressed air can be dealt with using a compressed air dryer of which there are several types.

A refrigerated air dryer uses refrigeration technology that initially cools compressed air to allow condensation to occur, allowing its removal before it enters the compressed air system. Refrigeration dryers are relatively low cost and require little maintenance. Refrigerated dryers are commonly used throughout industrial, automotive and other applications where industrial grade (Class 4 pressure dewpoint 3’C to 7’C) dry air is specified.

Membrane dryers use an arrangement or stack of tubes or capillaries that separate the constituent components that form water. Used for smaller more stringent applications for dental air and similar applications. Membrane dryers are low maintenance though the membrane tubes do not tolerate the ingress of oil, so pre filtration is essential. They also consume a proportion of the compressed air generated for the ‘purging ’process’ and this value (apx 15 % should be allowed for. Air quality (Class 3/2 pressure dewpoint -10C to -20C typically).

Adsorption dryers are used universally for more critical applications in food production, power generation, pharmaceutical and many other critical fields. These are normally of a heatless or heat regenerative type technology. Adsorption or pressure swing adsorption dryers are available in all sizes from very small 2 cfm or less through to 1000 cfm +. Adsorption dryers require specialised maintenance and during operation shall require regeneration of the drying columns using a proportion of the dried compressed air from the system (up to 15% ). Heatless dryers can often be fitted with dewpoint sensing in order to reduce this value to a more manageable less costly 4%. Regenerative dryers are often used for larger applications using various regeneration technologies such as heaters, blowers or vacuum. These technologies allow a much more economical operation. Adsorption dryers contain various types of adsorption media including activated alumina, molecular sieve as well as conventional desiccant material. Air quality (Class 3/2/1 pressure dewpoint -20C to -70C typically).

Small single-phase compressors up to 2.2kW 3 HP may often operate on 230V supply. Smaller compressors up to 1.5kW (2HP) may utilize a 110 V single phase supply. Larger industrial systems may require 380/400/415V three-phase power. Compressors from 2kW up to 400kW+ may well utilise a suitable three phase power supply. Very large centrifugal compressors may operate at high voltage ie 3000V.

Oil-lubricated piston compressors typically require oil changes every 500–1000 hours or six months. Air filters and valves should be replaced annually, the receiver tank should be drained daily. A Rotary screw air compressor is serviced annually or every 2000 hours typically.

With proper maintenance, piston compressors typically last 5–10 years, while rotary screw compressors can last 10–15+ years.