Air Compressor Rating

All air compressors and air compressor tools (pneumatic tools) are rated after operating pressure and air flow/air consumption. This way of rating equipment is much like system for rating electrical components.

In an electrical system, e.g. the one you have in your house, the voltage has the same role as pressure has in an air system. So, if your intake cable delivers 110V to you electrical consumers, your consumers must be rated for the same voltage, in the same way pneumatic tools operated by the air compressor must have the same pressure rating as the compressors output pressure. So if the compressor delivers 120 psi the consumer must have an 120 psi rating. If the air compressor tool rating is lower than the air compressors, which is often the case, we simply install a reduction valve upstream of the consumer, much like we would install a transformer to reduce the voltage in an electrical system.

So we now know that the air pressure of a compressed air system, is the same as the voltage in an electrical system. So what is the effect?

Well, the effect of a compressed air system is the air flow. If we think about it for a second, it becomes clear. To create air pressure, you don't have to put a lot of sweat into it. By pressing down on a bicycle pump you generate air pressure and it doesn't take much effort from you either. By if you where to generate an air flow with that same bicycle pump, you would have to start pumping and do a lot more work. It's the same for an air compressor, even small compressors can, and do, generate high air pressures, but if we where to generate high air flow combined with high pressures, you would need a bigger and bigger air compressor as both pressure and air flow increases.

There is at least one crucial difference between air systems and electrical systems. In electrical systems you have a fuse, cutting the voltage to the consumer if the effect demand gets above the rating for the electrical circuit. We do not have this in air compressors, but if the air consumption (effect) becomes greater than the capacity of the compressor, 1 of 3 things will happen:

1. The compressor will run until the consumption falls below the limit of the compressor, at which point it will stop.
2. The compressors motor overcurrent/temperature protection will cut the power to the compressor due to overheating of the motor, stopping the compressor. (some compressors have timers, stopping them if they run for to long).
3. The compressor will break down and stop.

The Three Air Compressor Types

Even though there are many different models of air compressors, there are only three different types. Each type has its advantages and drawbacks. Picking the right air compressor when setting up a compressed air system is crucial. Not only can you face the risk of getting a substandard system with to low air flow, excessive noise and high power consumption, but you also run the risk of buying a compressor that will wear out early and be to expensive to both operate and purchase.

This article will cast light on which air compressor types that will be suitable for different compressed air systems.

We can, as said earlier, divide air compressors into three different types:

1. Reciprocating air compressors
2. Rotary air compressors
3. Rotodynamic air compressors

Each of the three compressors have very different characteristics making them ideal within their own field.

Reciprocating compressors:

These compressors are commonly known as piston- or membrane compressors. They are called that because they have either a piston, or a membrane, that is being pushed up and down inside a compression chamber to compress air. They are known to be very robust and can operate for many years with minimum maintenance. Their low price and work pressure (normally 8 bar) makes them ideal for carpenters, DIY home improvement, small garages and workshops. You can even find bigger models in industry and production plants. 

In the picture to the right: Campbell Hausfeld Piston Air Compressor

Rotary air compressors:

Rotary air compressor consist of for different compressor principles: scroll, vane, lobe and screw compressors. The most common air compresor for industry is the screw- and scroll compressor, while vane and lobe compressors are more common in laboratories and medical industry. The screw compressor compresses air by rotating a screw, which is interlaced with another screw, inside a compression chamber. This rotating motion creates air pockets in-between the two screws and the compression chamber. As the screw is turned by the compressor motor, the air pockets volume gradually decreases, compressing the air trapped in the air pocket. At the end of the compression chamber the trapped compressed air is released into the compressed air system. Unlike most other compressors, screw compressors use air compressor oil to cool the compressor, and to lubricate and seal gaps between the screw and compression chamber.
The scroll compressor works by having to mirrored interlaced spirals, mounted on each its plate, rotating in opposite direction with one spiral slightly off center. Air is trapped between the interlaced spirals and because of the direction of the spirals rotation, the trapped air is guided in towards the center of the two spirals. As the air is lead inwards the air pockets volume gradually decreases, resulting in an increase in the air pressure. At the center of the two spirals the air is "vented" into the compressed air system. The screw compressor is ideal for industry and for compressed air systems with high consumption, such as in a spray booth or when using air tools like jackhammers. The scroll compressor has a lower air flow and can be more or less compered to the piston compressor. Its advantage to the piston compressor is that it runs much quieter.

Rotodynamic air compressors:

These compressor are quite common because of their small size and incredible air flow. These compressors are the ones you would find mounted on a combustion engine and in the working end of a turbine. This compressor type is basically an air pump, but unlike a pump it's capable of creating air pressure high enough for fuel to self ignite. We divide this compressor type into two: 1. centrifugal compressor and 2. axial compressor. The centrifugal compressor is what you would typically find on a car engine. The centrifugal air compressor  sucks in air at the center of it housing, accelerating it inside the compressor house with its impeller (fan wheel) before the air is jettisoned out of the side of the compressor, through a diffusor disc. The rapid speed decrease that happens over the diffusor disc is what causes the pressure increase. An axial compressor works much after the same principle, but instead of the air being ejected out on the side of the compressors, it's being sent strait back and ejected in the other end, where a diffusor disc slows down the air speed and causes an increase in the air pressure. The centrifugal compressor is common in tight spaces that requires extremely high air flow. For this reason it is often the choice in combustion engines on cars etc. The axial compressor is is used in turbines and heavy industry like petroleum refineries and production plants.

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Compressed Air Dryer

When compressing air you not only increase the pressure and density of the air, but you also decrease the airs ability to contain water. So, if we assume that the temperature is constant, the higher the air pressure the less water mass it can contain.

This is because we are lowering the condensation threshold when pressurising air. The side effect of this is of course condensation of water, everywhere the air temperature fall below the dew point. inside the air system. This creates lots of unwanted effects like malfunctioning of  equipment and tools, corrosion inside system, loss of air volume because it is replaced by water, application failure especially regarding to spray painting and chemical application using compressed air.

There are several ways to dry compressed air. The most common method, used in small systems, is just to let the air cool down inside an air tank. This will give the air time to condensate any moisture that is above the pressurised airs limit. This is also the reason why it is so important to drain air tanks. Failing to drain an air tank minimum once daily can result in an increase in the compressors start/stop intervals, longer running time du to reduced air tank capacity, and even, in some extreme cases, condensed water in the compressed air system downstream of the air tank.

Other alternatives to remove moisture from compressed air is to use moisture filter, or moisture traps. These are basic filters design to trap moisture particles. These filters do not remove evaporated water from the air, they just remove all water condensation droplets contained in the air. The filters come with a bowl where the water is collected. The bowls can be manually drained with a drain valve, or automatically drained via a float mechanism.

For industry, paint shops, and medical clinics, an air dryer might be in it place. These are machines that work on the same principle as an air condition unit, refrigerator, or freezer. The hot, humid, high-pressure air enters the air dryer. Inside the air dryer cooling coils, cooled by refrigerant, cools the air to several degrees below the anticipated lowest temperature of the whole compressed air system. The result is that the humidity of the air is reduced to a point where no condensation can occur anywhere in the system, because the humidity is to low for the air to reach sits condensation point.