Air Compressor Buying Guide

What is an Air compressor?

An air compressor is primarily a device that converts power into potential energy stored in the form of pressurized air or compressed air. Unlike other sources of power, it does not involve conversion from another form of energy such as heat at the point of application.

Where is Compressed Air Used?

1. Compressed air can be used in piston displacement. It can also be used to rotate a shaft or it can be used to produce a high-speed jet, like in a paint sprayer.
2. Compressed air also provides torque and rotation power for pneumatic tools, such as drills, brushes, nut runners, riveting guns and screwdrivers.

Classification of Air Compressor

Air compressors can be classified as low-pressure compressors (With discharge pressure of 150 psi or less), medium pressure(151-1000 psi) and high-pressure compressors (more than 1000 psi).

Moreover, we can also classify air compressors as:
• Positive displacement
• Dynamic positive displacement

In this, a specified amount of air is trapped in a chamber and the volume occupied by it is reduced, to increase the pressure prior to the discharge. The most common types of air positive displacement compressors are found in small and medium-sized industries are rotary screw, vane, and reciprocating compressors.

Dynamic air compressors comprise of centrifugal and axial machines and are used for large manufacturing plants.

How to Choose the Right Air Compressor?

#1) Try to analyze the requirements of the air tools that will be used.

Consider the pressure and volume requirements of any tool. For e.g, Heavy-duty tools will require more pressure and thus more volume. If one fails to choose a compressor that is large enough for the intended use, the work efficiency will decrease to a considerable extent.

#2) Choose between piston-type and portable compressors:

Piston-type compressors rely on a motor which builds up air pressure when it gets depleted. They store compressed air in tanks.
Portable compressors do not offer tanks and thus run continuously in order to deliver air.

#3) Look at the horsepower (HP) of the air compressor.

Range for horsepower on an air compressor is between 1.5 and 6.5 HP which can be used for small and medium-sized applications. Air compressors with larger HP are usually reserved for industrial uses.

#4) Try to look at Cubic Feet per Minute or CFM:

CFM measures volumetric flow. The CFM changes in accordance with the psi of the compressor, it implies that the two tools with different psi will not necessarily have CFMs that we can just add together.
If the psi of the tools are different we can then look for Standard CFM (SCFM) at the time of evaluating compressors. Standard CFM is measured as 14.5 PSIA, at 68 °F (20 °C), with 0% relative humidity.

#5) Consider your power source:

Depending on the availability of electricity we can opt for a drive system with an electric motor or drive the compressor via gasoline engine.

#6) Size of the tank:

If we are using a piston-type compressor we should determine how large our tank should be. If we are operating it for low duty applications, i.e. when using a nail gun we can use one with a small tank. But in case we want to work for extended periods of time a compressor with a large tank would be preferred.

Air Compressor Installation Location

The air compressor should ideally be located as close to the area where the compressed air has to be used. Moreover, it should be located in a dry, clean, cool and well-ventilated area. It should be kept away from dirt, vapor and volatile fumes that may clog the filter and valves.

Ambient temperature conditions can vary depending on the manufacturer and should be first reviewed before purchasing a compressor. Typical ambient conditions generally range from 32˚F to 115˚F.

Compressor Controls and System Performance

Air systems seldom operate at full load all of the time and hence the ability to efficiently control flow is important.

1. Start/Stop –

This is the easiest and most efficient control strategy. It can be applied to either reciprocating or rotary screw compressors. Essentially, the motor driving is turned on or off in response to the discharge pressure of the machine. For generating start/stop signals a pressure switch can be used. This is generally applicable for compressors having a size smaller than 30 horsepower.

2. Load/Unload –

This control mode is sometimes called online/offline control. It keeps the motor running but unloads the compressor when the discharge pressure becomes adequate. Unloaded rotary screw compressors do not produce any compressed air output while it typically consumes 15-35% of their full load power demand.

Variable Displacement:

Some lubricated rotary screw compressors vary their output capacity using special capacity control valves, also called spiral or poppet valves. The output pressure and power consumption of the compressor can be controlled without starting/stopping or loading/unloading the compressor. At points above 60% loading, this control method has good efficiency.
At flows below 40% capacity, one can use pressure switch activated unloading controls to reduce power consumption at lower flows.

Variable Speed Drive (VSD):

As per the air demand, this control method changes the speed of the compressor.
The variable speed drive controls can be equipped with both lubricated and oil-free screw compressors to adjust the drive motor speed as per the demand requirements to maintain constant pressure.

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