A Compressed Guide to Air Compressors

We cornered Mark Mlyniec – Global Industrial's Lead Merchant in charge of pneumatics, hydraulics, and generators – and we riddled him with questions. What happens to an air tool if you increase the PSI in the compressor? Does it become more responsive? What does CFM stand for? When should I replace my air compressor? Should you use an impact wrench to change a tire? Mark has the answer to all these questions, and probably anything else you might like to know about pneumatics.

In this guide, however, we are going to focus on one main question, and it's the question Mark hears most regularly: What type of air compressor should I buy?

So if you're puzzled by the CFM rating1 on your composite twin-pawl ratchet, here's a short guide to help you:

Comparing Compressors; A short guide

There are two main types of compressor: reciprocating and rotary screw; and they're both as exciting as their names suggest. 'Reciprocating' in this instance refers to the reciprocating pistons which are used to pressurize air. The second type uses a rotating screw – or two interlocking screws one rotating against the other – usually rotating continuously, to pressurize air.

Mark says: “Rotary screws pressurize air faster and more quietly than comparable reciprocating models with a smaller footprint. They are more expensive in up-front costs and service, but are less expensive to operate.”

Within reciprocating compressors, the category can be further sub-divided into single-stage and two-stage compressors. Like small-venue and large-venue air guitars, you ask? Almost, but not quite. As the name suggests, single-stage compressors perform the pressurization process only once – compared to rotary screw which operate continuously – which means that the maximum PSI the single-stage system produces is limited. Single-stage compressors vary in size with tank volumes from 60 to 120 gallons.

Two-stage compressors complete the pressurization process twice, such that the effect is compounded, leading to a maximum PSI output that is higher in a shorter cycle period. Typically they have a larger capacity, with tank volumes from 60 to 2000 gallons.

Mark says: “Two-stage compressors are found in large garages, paint shops, manufacturing facilities, mills, assembly plants; and are ideal for applications with runs of hoses or piping that use multiple tools simultaneously or tools with higher demand.”

Rotary screw compressors are typically much larger installations – we're talking fridge-sized – and tend to be used to drive tools which require much larger volumes of air. Rotary screw compressors can also be sub-divided into two further categories: oil-free and oil-injected. With oil-free compressors, there is no oil seal. An oil-seal minimizes the loss of compressed-air from the pressurization stage. As such, in oil-free compressors, the maximum PSI that can be sent to the air-tool tends to be lower than oil-injected compressors. However, oil-free compressors provide many advantages to industries and premises where it is necessary that the air is uncontaminated by oil and its associated by-products, e.g. healthcare.

Is it time to replace my air compressor?

If you are considering upgrading your compressor: your old single-stage no longer cutting it? You need more power to stay ahead? Or maybe you've got more ratchets on your rack – more wrenches in your kit – more rivets for your bucking bar? Ask the following questions:

Marks says:

How old is your air compressor, or compressors?
How often do you service the compressor?
Is the tank drained regularly?
Do you see oil leakage, or hear air leakage?

And the answers:

1. How old is your air compressor?

Air compressors used in applications of industrial manufacturing last 5 to 15 years, more or less depending on specific use, and how they are maintained. If your compressor is in the double digits, it’s in the tail end of its life.

2. How often do you service the compressor?

Individual needs vary, but this is one of the critical factors that will determine the state of the compressor. Compressors are loud, and there are various methods for reducing the noise level, one of which is to house the compressor in a casing using materials known for dampening properties. In such installations, however, the compressors are often forgotten and serviced less regularly as a result.

3. Is the tank drained regularly?

As a component in air, water vapor ranges from 0.01% to just under 5%. This might not seem like very much but when you compress the air in your tank, this vapor becomes moisture, which causes metal to oxidize quicker than it would usually. Draining it regularly extends its life.

3. Do you see oil leakage, or hear air leakage?

If the answer is "Yes", and then you should replace your compressor. If the compressor is not sealed then it will not be able to pressurize to the PSI that your tools require, meaning that the tool may be operating without sufficient power, meaning longer use to complete the same task and more wear for your tools, which will then need to be replaced sooner – or if the leak is substantial, your tools may not run at all; and this is not the type of leak you can fix with a bucket.

If you're new to air compressors, you'll need to match your air compressor to the air tool. Mark says: “The rating tells you how much air the tool will use, and for multiple tools, you'll need to add the ratings together.”

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