Compressor Buying Guide

A Simple Honest Guide to Buying a compressor

Before you Buy, let’s start with a few facts we need to consider

CFM (cubic feet per minute) this is the volume of air you require to run a CNC or air tools, this is the amount of air they will consume at a stated working pressure (psi or bar)

Working Pressure, the amount of pressure required in psi or bar for machinery to operate correctly

Example, A CNC requires 14cfm @7bar pressure, you would need a compressor large enough to produce this volume and maintain this pressure, simple so far!

All reciprocating (piston) compressor manufactures state cfm in Displacement, this is a theoretical Volume figure being drawn into the intake, basically making the compressor appear more efficient then it truly is!

you lose a certain amount during compression

What you really need to know is the Free air Delivered (F.A.D) @ a set Pressure

 

Example, A 3Horse Power Motor Compressor States 14-15cfm Displacement, in reality, output it is about a third smaller 8-10cfm Free Air Delivered (F.A.D)

Duty Factor The amount of time the compressor will work for

D.I.Y = 30-50%, Automotive = 50-65%, Industrial 90-100% duty load

Noise Level stated normally in Dba, so be aware when siting the compressor in relation to Staff or Environment

Water, all compressors produce water condensate due to moisture within the air being drawn in from the atmosphere for compression, you need to determine how you want to remove this based on your application i.e. car spraying

DIY Compressors are generally Direct Drive and are manufactured in China, cfm stated in displacement, these are cheap and made to a price and even available from supermarkets

Pros = Cheap, Small, available oil free. 

Cons = Very Noisy, 30% duty, availability of spare parts for Chinese models

Automotive Compressors, Belt Drive ideal for the Garage and automotive trade, cfm stated in displacement unfortunately companies have been introducing Chinese products, stating over the top performance figures, so be aware!

All automotive compressors on our site are Italian and backed up with spare parts

 

Pros = Value for money, belt drive, large selection including sound proofing, 50% duty, 60-65% can be obtained for 3 Phase 415volt versions, cheap to service

Cons = Still noisy without soundproofing, Chinese products cheap, but unreliable

All 1 Phase 240volt compressors have a 50% duty cycle. This means 3 minutes on, 3 minutes off. Up to a maximum of 10 motor starts per hour, a higher rate of duty 60-65% can be obtained for 3 Phase 415volt motors
None are designed for continuous heavy-duty use

Industrial compressors, Rotary Screw, Belt or Direct Drive, High Industrial Compressors 90-100% duty.

This is the choice when production is crucial, or an industrial compressor would be your ideal choice, can be supplied with a dryer

Below two examples ABAC Spinn without Dryer and ABAC Genesis with internal Dryer + Filter

Pros = High industrial unit, 90-100% duty, cfm stated in F.A.D, very quiet, small footprint, with or without dryer, more efficient than reciprocating compressors per kw, intelligent control, multiple options including variable speed available

Cons = high initial purchase price

Industrial Variable Speed = High industrial unit, 100% duty, Direct Drive 1.1 ratio

This is the choice when production is crucial, and you are looking for energy savings, most people will take into account, purchase price, installation and maybe service costs, however what is mostly ignored is the running cost, almost 70% of energy is wasted

Pros = High industrial unit, 100% duty, cfm stated in F.A.D, very quiet, small footprint, with or without dryer, more efficient than reciprocating or fixed speed rotary screw compressors per kw, intelligent control.

Cons = higher initial purchase price compared to a fixed speed model

Water Contamination

Due to there being moisture in the air, there will be moisture in your air system. Heat is generated during compression and to make things simple we can say that the moisture is vapour (steam) when in this state the vapour will bypass to a large degree any filters you put in your system

What we require is condensation to take place by cooling, to some degree, due to tanks being a large surface area, this can happen there, and a simple filter installed, but if the air flow is constant, then another way is required that is more efficient, this is when you require a refrigerant dryer, especially when paint spraying, or running CNCs, this is just like your home fridge, but instead of keeping your food cold, it reduces the air temperature to 3-5 degrees ambient, once the vapour becomes a solid matter i.e. water droplets, it can be removed by a filter

Pros = High industrial unit, 100% duty, very quiet, small footprint, water removal

Cons = Additional cost

Water Contamination Removal Continued

For some industrial application a higher level of water removal is required, i.e. Air assisted Laser cutting, medical or pharmaceutical use

This is when you would install a Desiccant Compressed Air Dryer which can achieve -40 to -70 degrees

Filtration

Filter selection is down to what degree of contamination you need to remove, don’t forget they will only reduce water vapour when the air is cooled, but used mostly for removing very small particles of oil

 

Most Manufactures will now supply an Iso Class they require for the compressed air, which they will require you to supply to conform to their warranty conditions, see below chart

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