How small can a screw compressor be

Anyone who needs a new compressor or would like to expand or optimize the existing compressed air system must, among other things, deal with the two common compressor systems: the piston and screw compressors. It is not possible to make a general statement as to which of the two types of compressor is the “better”. In this first article in our three-part series "Economical Compressed Air Supply", it is therefore a matter of describing both concepts as well as showing their functionality and possible areas of application.

When it comes to investing in a new compressed air station, you should take your time and provide sufficient information. However, it is not uncommon for general statements to be made. Sometimes you can hear, for example, that reciprocating compressors have old, outdated technology and are therefore not suitable for continuous operation. But they are transportable and therefore ideal for use on construction sites. In the same breath it is said that screw compressors are more modern, quieter, etc.

We now want to explain one by one whether and what is actually true of such or similar statements.

Reciprocating compressors

A typical piston compressor system always includes the actual compressor block. This consists of the compressor, which either has a directly flange-mounted motor or is driven by a V-belt and is mounted on a compressed air tank.

The compressed air tank is used to store, calm and cool the compressed air.

• The compressed air must be stored so that sufficient reserves are available in the event of a sudden increase in demand that exceeds the actual delivery volume of the compressor.

• The compressed air must be calmed down, as the piston compressor generates a pulsating volume flow which is not exactly beneficial for the service life and the functional reliability of the downstream tools.

• The compressed air must be cooled so that the condensate does not first accumulate in the compressed air line and cause problems for the consumers. The compressed air tank can never be big enough. A container that is too small does not have sufficient storage volume. It is filled quickly and of course also emptied very quickly. As a result: the compressor switches on and off very frequently and the engine is stressed too much.

Container size: You can calculate the container size with complicated formulas. This is certainly justified in larger companies. Because there it is about very large amounts of compressed air.

On the other hand, there are practical empirical values ​​for the dimensioning of compressed air tanks, which are normally sufficient. As a simple rule of thumb for normal operation:

• The size of the compressed air tank, measured in liters, should be as large as the delivery rate of the compressor in liters / minute (example: a compressor delivers 500 l / min, then a tank with a volume of approx. 500 l is ideal).

As a rule, you can also rely on the information provided by the compressor manufacturers, who have already assigned the appropriate compressed air tank to each compressor in their offers, brochures and price lists.

How it works: The typical piston compressor is driven by an electric motor. A pressure switch, attached to the compressed air tank, switches the motor off when the maximum compressor pressure is reached and on again when the lower switching point of the pressure switch is reached.

The pressure switch must be attached to the compressed air tank, since calm compressed air is available at this point. A pressure decrease directly on the compressor with pulsating pressure would not guarantee proper switching. The pressure difference between the switch-on and switch-off pressure is usually 20% of the maximum pressure. A 10 bar compressor switches between 8 and 10 bar, a 15 bar compressor analogously between 12 and 15 bar. Too small a switching differential below 2 or 3 bar would cause too frequent switching. B. 6 to 10 bar could lower the minimum pressure in the container so far that proper operation of the consumer is no longer guaranteed.

The piston compressor basically works in intermittent operation (start-stop operation). It only runs when compressed air is required. Otherwise it switches off without costly idle operation.

Operating modes: Piston compressors are available on the market for a wide variety of applications. The operator can choose as required.

There are

• Devices with 100% duty cycle are suitable as base load compressors for continuous load operation

• Devices for intermittent operation with approx. 60% duty cycle.

Piston compressors can therefore also be suitable for continuous operation.

Delivery ready for connection: Mobile compressors for operation on construction sites already have an electrical connection cable with plug and a coupling with hose connection as a compressed air connection. They are ready to use straight away.

In the case of stationary systems, which are usually larger than the construction site compressor, the electrical and compressed air connections must be carried out on site by specialists.

The complete, ready-to-connect compressor systems must comply with EC regulations. This is documented on the one hand by a declaration of conformity and on the other by the CE mark attached to the system.

If the compressor becomes too small: If the pressure suddenly drops in the compressor system during operation or if the compressor runs without interruption and no longer reaches its switch-off pressure, then something is wrong. But what? One might think that the compressed air tank is too small and a second tank could help. However, this is not necessarily correct: because a second container could indeed increase the total storage volume and thus provide a somewhat larger buffer for a short time. But this is not a patent solution. If 1100 l / min are constantly required instead of 1000 l / min, then a larger container is of no use in the long run. Then either a compressor with a higher delivery rate or an additional compressor must be installed.

A second compressor feeds the same compressed air tank together with the existing one. In the case of a stationary installation, this is usually possible without any difficulties. If the second compressor is just as powerful as the first, then 2 x 1000 l / min are available with a compressed air requirement of 1100 l / min. So there are enough power reserves for possible expansions or additional peak demand. The control of the second piston compressor is also problem-free. A separate pressure switch with slightly offset switching limits, attached to the compressed air tank, connects the new compressor as peak load to the existing base load compressor. If you still want to ensure that both compressors are evenly utilized, the machines can be controlled time-dependently with a timer using an automatic base load changeover switch.

The advantages of a double system over a larger system are obvious:

• lower current peaks when starting up

• longer availability

• Higher operational reliability, because if one compressor fails, the second is always available.

Equipping the stationary piston compressors with an additional soundproof hood for problem-free operation directly at the workplace is just as unproblematic as adding compressed air dryers and / or filters for dry and oil-free compressed air.

Oil-free or oil-lubricated ?: Finally, there is the question of the type of compression: oil-free or oil-lubricated. It is then compressed oil-free if

• Oil proportions in the compressed air are absolutely not permitted

• There are no options for checking or changing the oil

• oily condensate cannot be processed

• Costs and follow-up costs for additional compressed air treatment are to be saved.

For "normal" operation, conventional oil-lubricated compressors are certainly the ideal compressed air generators. This question should be discussed in detail with the compressor manufacturer's experts in individual cases.

Conclusion: The "prejudices" mentioned at the beginning about reciprocating compressors are not correct.

• Piston compressors do not have any outdated technology: their design has matured over decades, and new materials and technologies are constantly being incorporated into new developments.

• Piston compressors are available with soundproofing.

• Piston compressors can be supplied ready for connection, with the CE mark.

• There are piston compressors that can be operated continuously and are also suitable for highly intermittent operation.

Screw compressors

Screw compressors are offered in a soundproofed or even super-soundproofed version. This makes them ideal for installation directly at the workplace.

A compressed air tank also belongs to a compressor station with a screw compressor. It has the same functions as a reciprocating compressor. Only the pulsation damping is omitted, since a screw compressor already generates low-pulsation compressed air.

Tank size: Compared to the piston compressor, the compressed air tank downstream of a screw compressor is 2/3 smaller. The screw compressor fills the smaller container faster than a reciprocating compressor. The container is, of course, empty again more quickly. This means: a screw compressor switches more frequently than a piston compressor, which puts more stress on the engine. You will now be wondering why this is possible with a screw compressor. Rightly so, because: this is just as impossible with a screw compressor as it is with a piston compressor. It is just controlled differently.

How it works: Screw compressors are also controlled via pressure switches. However, these are not attached to the container, but inside the screw compressor housing, since low-pulsation compressed air is already available at this point.

The pressure switch also does not have this wide spread of 2 bar, as is common with piston compressors. The pressure difference between the switch-on and switch-off pressure is only 0.5 - 1 bar. In addition to the small compressed air tank that increases the switching frequency of the compressor, there is another factor that increases it.

The solution: The screw compressor does not switch off when it reaches its maximum pressure. It continues to work in idle mode to protect the engine. However, the idling operation of screw compressors means around 25 to 30% power requirement without generating even a liter of compressed air. When compressed air is required, the compressor starts from idle to load operation without unnecessarily loading the engine.

So that the compressor does not idle for too long, the compressor manufacturers keep the idle time within limits. Each manufacturer has its own controls. It is important that the idle time and thus also the idle costs are minimized. The idle control takes into account the maximum permissible switching of the electric motor. These in turn depend on the motor size.

The Bielefeld compressor manufacturer Boge, for example, equips all of its screw compressors with an intelligent control and monitoring concept (ARS). This extends the service life of the compressor, reduces running costs and increases the availability of the compressed air system. The following functions make this possible:

Dynamic full load / idle control: A microcontroller dynamically determines the consumption trend from the current operating status and a programmed ideal value.

Automatic selection of the most favorable operating mode. Inexpensive intermittent operation (start-stop operation) is always aimed for.

Automatic optimization of the motor switching cycles. The maximum permissible motor switching cycles determine in which operating mode the compressor works.

Extension of the screw compressor station: The extension of a screw compressor station can be seen in the same way as the piston compressor station. Only one thing should be considered in the case of multiple systems. The pressure switches of the compressors are sensibly adjusted in their switching limits like a cascade. Example:

• Base load compressor 9 to 10 bar

• Medium load compressor 8.5 to 9.5 bar

• Peak load compressor 8 to 9 bar.

This means that with a minimum pressure of 8 bar in the compressed air tank, a compressor has to compress up to 10 bar. This is a waste of energy, because a pressure increase of 1 bar means an increase in energy of approx. 10%.

Multiple system controls: With modern multiple system controls, each compressor is no longer assigned its own pressure switch. A pressure sensor with a bandwidth of approx. 0.5 bar regulates all compressors. To guarantee 8 bar in the compressed air tank, all three compressors work between 8 and 8.5 bar. A pressure increase of 1.5 bar to 10 bar is saved.

Example: A 30 kW peak load compressor saves 1.5 bar pressure increase from 8.5 to 10 bar. That is 15% of 30 kW = 4.5 kW x 2000 operating hours = 9000 kWh x 0.25 DM / kWh = 2250.00 DM.

A 30 kW medium load compressor saves 1 bar higher compression from 8.5 to 9.5 bar. That is a saving of 1500 DM.

A 30 kW base load compressor saves 0.5 bar higher compression from 8.5 to 9 bar. That is a saving of 750 DM.

In total, savings of DM 4,500 will be made.

Another type of economical compressor control are compound systems consisting of reciprocating and screw compressors. The screw compressor works in base load mode. It runs constantly and covers the compressed air requirement with its delivery quantity. There are no idle times. A piston compressor switches on when there is peak demand. It only works when there is a need for compressed air. So you have a healthy mix of screw and piston compressors without unnecessary idle costs.

Final consideration

When choosing a suitable type of compressor, it is not a priority whether a compressor is “modern”, already soundproofed or perhaps already installed in a neighbor's house. In addition to the purchase price, the most economical type of compressor should be selected.

• If the demand for compressed air is constant, a compressor that is also continuously operated under load, ie a screw compressor, is the best choice.

• If the demand for compressed air fluctuates strongly, a compressor that works in intermittent operation without idling costs, i.e. a piston compressor, is recommended.

In addition, a combination of both compressor systems with the screw compressor as the base load and the piston compressor as the peak load should be included in the considerations.

The second part (in the next BM) will deal in detail with the subject of "Oil-free or oil-lubricated compression".

BM online 10 | 1998

TagsCompressed airPiston CompressorScrew Compressor

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