How to properly select and use a compressor?

Read about how compressors work and what parameters to consider when choosing a compressor for your workshop or operation.

Article content:

• How do compressors work?
• How to proceed when choosing a compressor?
• Frequency of compressor use
• Calculation of the required air volume
• Compressed air quality
• Tips for compressors for workshops and operations

A compressor is a device that increases air pressure by compressing it, allowing for its further use. Choosing the right compressor has a fundamental impact on work efficiency and productivity. It is therefore necessary to choose a device with appropriate power, size, noise level, and energy efficiency.

For example, a quiet and compact compressor is more suitable for fine craft work, while a more powerful and robust model is a better choice for industrial use.

In the following lines, we will go through the individual aspects of compressor selection. But first, let's look at the principle on which these pneumatic devices work. At the end of the article, you will find recommendations for specific devices according to different uses.

How do compressors work and what do they look like?

In practice, we most often use two types of compressors that work on different principles: piston and screw.

Piston compressors

Piston compressors are mostly mobile devices, equipped with a handle and mounted on wheels for easy transport.

Compared to screw compressors, they are cheaper, but also noisier and achieve higher vibrations. However, there are also devices in soundproofed versions. Lower noise levels can also be achieved by using multi-cylinder compressors, which can operate at lower speeds thanks to up to three pistons.

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Screw compressors

Screw compressors have fewer moving parts, so their operation is quieter and vibration-free. Compared to piston compressors, they are larger, more durable, and have a longer service life. Their place is rather in industrial operations than in workshops. At the same time, however, they are more expensive.

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How to proceed when choosing a compressor for a workshop or operation?

When selecting a device, first consider a few key questions:

• How often will the compressor be in operation?
• What volume of compressed air will you need?
• What quality of compressed air do you need?

1) Frequency of compressor use

It holds true that for sporadic use in a home workshop or small business, a piston compressor is the ideal choice. You can turn it on and off as needed, but remember that these models wear out with each start. With more frequent use, or longer running times, piston compressors overheat and get damaged. The ideal operating frequency is:

• 4 starts per hour, max run interval 10 minutes (compressors for small workshops and garages),
• 6 starts per hour, max start interval 20 minutes (compressors for professional use).

For continuous and intensive operation, on the other hand, screw compressors are more suitable, designed for continuous operation and higher performance. With screw compressors, you can also expect better power regulation and more efficient processing of compressed air. For both types of devices, ensure timely oil changes, filter element replacement, and generally pay attention to maintenance.

Rules for using a piston compressor

To ensure your piston compressor serves you long and reliably, it's important to follow a few principles:

• Alternate the on and off time of the device in a 3:1 ratio – this means that if the compressor runs for e.g., 9 minutes, let it rest for 3 minutes afterwards.
• Limit the number of starts – maximum 10 starts per hour. Frequent starting leads to excessive wear on the motor and the unit itself.

Whether to choose a piston compressor also depends, among other things, on the total daily volume of compressed air (considering necessary operating breaks – calculation found below).

Rules for using a screw compressor

It is important to use screw devices every day. The compressor must reach an operating temperature of 80 °C at least once every 24 hours to remove excess moisture (using an oil separator), which forms through condensation. Otherwise, the screw block corrodes quickly.

You can regulate the power of a screw compressor by connecting a frequency converter, which reduces and increases the compressor's output, i.e., the supplied amount of compressed air, by changing the drive speed as needed. In cheaper devices, a suction valve plays a similar role.

Details can be found in the article about efficient operation of screw compressors.

2) Required air volume of the compressor

When selecting a compressor model, you need to know how much air you consume in your workshop or operation. This can be done through a thorough analysis of the use of individual pneumatic devices and operations.

This table will serve for a rough estimate of consumption:

The choice of compressor based on air consumption is then guided by the size of the air tank that can be attached to the given device.

At the same time, keep in mind that for large intermittent demands (typically when blowing, paint spraying, sandblasting), a large pressure vessel is more suitable. To power lines and other applications with constant air consumption, you will need a screw compressor with a smaller air tank.

Imagine you are considering buying a piston compressor for your workshop, where your air consumption is 100 l/min, with the following parameters:

• air tank volume: 100 l,
• intake (air suction speed): 250 l/min
• maximum machine pressure: 10 bar

At 1 bar, the compressor could pressurize 1 × 100 liters of air into the tank (i.e., the tank volume), at a pressure of 10 bar, it's 10 × 100 l = 1,000 liters of air.

How long does full pressurization take? We now know that the air tank holds 1,000 liters of air – and at the same time, you know the output is 250 liters per minute. The calculation is then simple: 1,000 l / 250 l/min = 4 min

However, this does not mean that you could immediately consume 1,000 liters of air with the compressor. In order to use the device effectively, the compressor must maintain some pressure in the tank. If its value drops below a certain level, the compressor switches on again and starts drawing in more air.

In practice, this cut-in pressure is usually 6.5 bar. Most pneumatic tools require a pressure force of 6 bar for their operation.

The difference between the maximum (cut-out) pressure of the machine (10 bar) and the cut-in pressure (6.5 bar) is called hysteresis. In our case, the hysteresis is therefore 10 bar - 6.5 bar = 3.5 bar. With 1,000 liters, 650 liters of air always remain in the tank, and only 350 liters are available for immediate consumption.

So, if you consume all 350 liters, the compressor turns on and the device replenishes the consumed air in 1 minute and 24 seconds (350 l / 250 l/min = 1.4 min).

In practice, however, you usually don't wait until the device replenishes all the consumed air:

• First, you turn on the device, wait until it pressurizes, and then in 3.5 minutes, you consume the prepared 350 liters (your working consumption is, as written above, 100 liters per minute, thus: 350 l / 100 l/min = 3.5 min).
• Then you continue working with the compressor while the device replenishes the consumed air. This means that out of the 250 liters the device replenishes per minute, you consume 100 liters in the same time (the effective delivery is therefore 250 l/min - 100 l/min = 150 l/min). The consumed 350 liters are thus replenished in 2 minutes and 20 seconds (350 l / 150 l/min = 2.33 min).

With continued air consumption, the compressor again has a 3.5-minute break (does not need to pressurize air and is not running) and then switches on again and pressurizes for 2.33 minutes (is running). The entire compressor cycle is therefore 5 minutes and 50 seconds (3.5 min + 2.33 min = 5.88 min).

And since we know from the previous lines that a maximum of 10 starts per hour is recommended for piston compressors, we can purchase the device with an intake of 250 l/min for a workshop with a consumption of 100 l/min.

3) Compressed air quality

In standard compressors, due to the presence of lubricants, oil residues get into the compressed air. While this is not a major obstacle in workshop and engineering operations, in certain sectors (beer dispensing, food and drug production, healthcare, etc.), there must be no trace of lubricant in the air. The specific concentration is determined by the compressed air quality standard ISO 8573-1.

In screw compressors, oil is separated from the compressed air, which then passes through a filter (see article on selecting an air filter). There are also screw compressors that do not require oil and are cooled by liquid injection.

Navigating the wide range of compressors is not easy. Therefore, in the following lines, we present the parameters of models for various applications.

Piston compressors for craftsmen and small and medium-sized businesses

Piston compressors suitable for home workshops or businesses are equipped with:

• operation with an automated pressure switch,
• two manometers for monitoring operating pressure and pressure in the air tank,
• direct connection of the compressor to the drive,
• aftercooler for reducing the humidity of compressed air.

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What to take away from the article?

• A piston compressor is ideal for workshop use, while a screw compressor is suitable for continuous industrial operation.
• Piston compressors are mobile, cheaper, but noisier, while screw compressors are quieter, more durable, and suitable for more intensive use.
• Choose the compressor (or air tank) based on, among other things, your air consumption.
• For use in the food industry and healthcare, choose oil-free piston compressors, or screw devices with suitable food-grade oil.