How to choose a compressor

The operation of any pneumatic device is based on the principle of compressed gas supply. Most of these mechanisms use ordinary air, which is forced into special containers called receivers. And it is pumped there with the help of a compressor. In this guide, we will analyze in detail the device of compressors, what they are and what to look for when choosing such a unit.

Compressor type
Compressors are structurally divided into three types:The design of screw compressors is based on two rotors (screws) installed in parallel. The injection begins with the drive rotor, which, in turn, supplies air to the driven rotor and then, through its channels, the gas enters the receiver through the entire structure.

They are the most efficient type of compressor, but the pressure required for the operation of the pneumatic tool is created only when working with large rotors driven by powerful three-phase motors. Therefore, screw compressors are often used only in large industrial plants.

In reciprocating oil compressors, air is compressed, as the name suggests, by pistons driven by a crank mechanism. The efficiency of such compressors is lower, due to the fact that most of the energy is spent on overcoming the friction between the piston and the cylinder. These compressors use oil to reduce friction. Oil units serve several times longer than oil-free units, but they have one feature, which often turns into a disadvantage: oil particles are present in the compressed air at the outlet.

The basic design of any reciprocating compressor

Special oil separators and filters partially cope with this problem, but not completely, and if clean air is required, an oil compressor will not work. For example, with fine work with a spray gun (when a spray gun is used, for example, for airbrushing), when blowing parts from dust, when using a plasma cutter – the presence of oil in the compressed air is unacceptable.

But when working with other pneumatic tools – drills, impact wrenches, engravers, etc. – the presence of oil in the air is even useful: it provides lubrication of the rotating parts of the tool.

Manufacturers implement pistons and cylinders of oil-free piston models in different ways in the race for a longer service life: they put garfito-Teflon rings, cover the cylinder walls with anodized mixtures, etc.

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Piston oil-free compressor with receiver

The piston group is manufactured using special materials that reduce friction between the cylinder, pistons and valves. Such compressors are cheaper and easier to operate, but less efficient than those described above.

Piston oil-free compressor without receiver

Another significant plus of this type of compressors is that they, for the most part, are more compact and lighter than all their other counterparts, but only if the design does not imply a receiver – household oil-free reciprocating compressors often come without a receiver.

Receiver volume
The receiver is a metal tank used to store the supply of compressed air.

The larger the volume of the receiver, the less often the compressor will turn on, i.e. increasing the volume of the receiver reduces the load on the compressor and engine.

The downside of a large receiver is that it takes a while for the compressor to build up operating pressure. If the compressor power is not high, and moreover, pressure is taken by some working tool, the pressure in the large receiver (and therefore at the compressor outlet) can take a very long time. Therefore, a large receiver volume must be accompanied by a correspondingly higher performance and power.

The exception is those cases when a short but high performance is periodically required from the compressor. For example, when using an impact wrench, it requires a capacity of 300 l / m, but if you connect it to a compressor, a capacity of 200 l / m with a large (50 l) receiver will be enough for the wrench to start and work for a while.

type of drive
On coaxial models, the compressor crankshaft is rigidly connected to the engine shaft. The simplicity of this design ensures its low cost, which cannot be said about reliability: this option is fraught with frequent peak loads on the engine, which has a bad effect on its durability.

Coaxial compressors can also be implemented in two types:

With crankcase (oil):

Pros: performance, ease of management and maintenance Cons: Requires regular topping up of oil, not designed for frequent use Without crankcase (oil-free):

Pros: produce clean compressed air at the outlet, inexpensive

Cons: not designed for long-term work under load, noisy, small resource

In belt compressors, the engine spins the flywheel using a belt drive, on the axis of which the crankshaft is fixed. This design provides a smoother engine running, extending engine life.

Pros: increased resource, high efficiency

Cons: high noise level, oil level needs to be monitored.

In a two-stage compressor, the intake air is compressed twice: first, to a certain pressure in the cylinder of the first stage, then, after passing through the cooling system, it enters the cylinder of the second stage under pressure, where it is compressed to the final pressure.

From a performance point of view, two-stage gas compression is certainly always more efficient. But, there are some nuances here. For example, in order for a two-stage compression compressor to deliver the capacity for which it is designed, an engine with higher power ratings is also required.

How do you find the power? According to the required performance and maximum pressure. The same parameters are selected based on what tools and in what quantity will be used simultaneously with the compressor.

The higher the motor power, the higher the performance, and, accordingly, the more consumers can be connected to the compressor.

Pneumatic tool and the required compressor parameters:Performance
Compressor capacity is the volume of air delivered per minute. To determine the required value of this parameter, it is necessary to summarize the required performance of all simultaneously operating tools (consumers). When choosing a compressor, it must be borne in mind that the passport usually indicates the volume of intake air under ideal (20 ° C) conditions.

At the outlet, due to air losses in the compressor and due to different operating conditions, the capacity may be 20-30% lower than stated. Therefore, the capacity of the selected compressor must be 30% higher than that required for the operation of the existing tools.

Operating pressure
The working pressure of the compressor is also selected based on the requirements of the tool used. It doesn’t matter how many tools are used at the same time – select the maximum required pressure. This will be the working pressure of the compressor.

Overpressure is contraindicated for some tools (eg spray guns, air blow guns, tire inflation). If such a tool is intended to be used, a pressure adjustment function will be required .

Overheating protection on the compressor is also highly desirable. The forced air cooling system using a special guide allows you to reduce the temperature of the compressor head, avoid overheating and, as a result, increased wear.

When compressed, the air heats up very much, therefore, in many models, the high-pressure pipes are equipped with special cooling fins, and the cylinder body, in a principle similar to motorcycle cylinders, is equipped with additional fins.

All this increases the resource of the compressors and makes it possible to use them at high loads.

No reciprocating compressor can run continuously for a long time. The presence of a receiver of appropriate volume gives the compressor the opportunity to periodically “rest” for cooling, but if the air intake is close to the compressor capacity, then the breaks between starts become too short and the compressor does not have time to cool down. If the air intake exceeds the compressor capacity, the engine will not shut down at all. This can lead to overheating of the compressor and damage to it. That is why the compressor must have a capacity margin and protection against overheating.

Pressure gauge. The presence of a pressure gauge will be useful in any case – it will make sure that the compressor is really working and creates the required pressure.

Noise level. Compressors are some of the noisiest types of electrical equipment. Even if your compressor will be used in production, remember that prolonged exposure of a person to noise levels above 80 dB is harmful. If the compressor is supposed to be used near housing, all the more care should be taken to select a model with a lower noise level.

Mains voltage. Household appliances are often supplied with 12 V, which is sufficient to connect to a car cigarette lighter. More powerful models are equipped with 220 V or 280 V.

Wheels. Compressor weight can be quite significant – up to 100 kilograms! If the compressor is installed permanently and connected to the mains (for example, in a garage), then there is nothing to worry about. But if the compressor is supposed to be used in different places, and its mobility is necessary, it is worth making a choice in the direction of a model that is necessarily equipped with wheels.

Selection Tips
Who needs a compressor? Anyone who needs this or that pneumatic tool in their work:If you have a garage and you yourself carry out some work with the car, then either you already have a compressor and pneumatic tools, or you just do not know how much their presence will facilitate your work.

When you need a cheap, simple compressor for intermittent short-term spray gun and air blow gun operation, choose among the oil-free coaxial drive models .

If you need a compressor for work using wrenches, drills, rivet and nail guns, it is recommended to pay attention to medium power belt oil compressors with a good receiver volume.

If you have to deal with decoration work and it is supposed to use a sandblasting gun, then you need a compressor with a capacity corresponding to the tool (see table) and working pressure.

If you intend to work in a car workshop or the simultaneous use of 2-3 impact wrenches, you will need an oil compressor with a large capacity and a rather large receiver volume .

Screw compressors are recommended for purchase by large enterprises and industrial organizations that use pneumatic equipment requiring a large flow rate of cubic meters of air per minute.