FAQ Sheet: Screw Compressors

January 25, 2021

In the following guide, we want to try to answer some of the questions you may have been asking yourself in regards to screw air compressors. If you do not find the answer you are looking for, please get in touch with us.

Keep reading as we answer some of the most commonly questions asked about screw air compressors. We are your partners in compressed air and strive to provide the total solution. 

What is the difference in running costs between a belt driven screw compressor and a direct driven?

There is a difference in running cost between the 2 technologies. The belt driven unit is a less expensive investment but consumes in average 3% extra energy. Also, a little bit more time has to be out to maintenance by adjusting, for example, the belt. The most suitable technology depends on the customer needs.

Does the screw compressors with an integrated dryer require a separate voltage supply?

Depending on the models. For the active range from 30 to 110 kW the transformer can be ordered as an option. When this option is fitted, there is no need for a separate power supply.

What is the recommended period for screw element overhaul?

We strongly recommend every 24,000 running hour. To exceed this limit you are facing a major risk of machine break down, which will cause in an increased service cost or investing in a new compressor.

I just bought a direct driven screw compressor with integrated energy recovery, how do I decide what flow I should have over the energy recovery water circuit?

It depends on the site conditions, and what temperature the customer wishes to have. Below you see a chart that you can use as a reference, the chart below is for a 30 kW and 37 kW compressor.

Softened water for 30 kW  
T. inlet T. outlet Flow (l/min) ΔP Bar
0 60.0 7.2 0.005
5 58.0 8.0 0.006
10 56.0 9.4 0.007
15 54.0 11.0 0.010
20 52.0 13.5 0.015
25 50.0 17.4 0.025
30 46.5 26.0 0.055
35 44.0 48.0 0.170
40 45.0 90.0 0.566
Softened water for 37 kW 
T. inlet T. outlet Flow (l/min) ΔP Bar
0 59.0 9.0 0.007
5 57.5 10.0 0.009
10 55.0 12.0 0.012
15 53.0 14.0 0.017
20 50.0 17.7 0.026
25 47.0 24.0 0.045
30 44.0 39.0 0.117
35 41.0 87.0 0.540

What Certifications do you deliver with the compressor from production site?

The included documentation at delivery is the local certificates. If you are missing your certificate at delivery, you can get in touch with us.

Do you have oil-free choices in the screw compressor range?

There are oil-free screw compressors available. Contact your local customer center for more information.

What Ampere fuse (A) is needed?

"Rules of Thumb" At 575 volts, a 3-phase motor draws 1 amp per horsepower. At 460 volts, a 3-phase motor draws 1.27 amps per horsepower. At 400 volts, a 3-phase motor draws 1.5 amps per horsepower. At 230 volts, a 3-phase motor draws 2.5 amps per horsepower. At 230 volts, a single-phase motor draws 5 amps per horsepower. At 115 volts, a single-phase motor draws 10 amps per horsepower. These numbers above have no safety margin. For example a 10hp compressor=15A, the standard fuse size is 16(A). To be safe for peaks we recommend the 20(A) fuse. Please check with a certified electrician.

What is normally the pay back for an inverter compressor?

The normal pay back is between 1-2 years during normal conditions and 4000 running hours per year. Not seldom we see pay backs within a year.

What is the operating principle of an inverter compressor?

It has almost the same components as a conventional compressor but there is, of course, a few main differences. A frequency driven compressor has an integrated inverter and often a more advanced control system. The inverter adjusts the motor speed to the actual air demand. This is controlled by a sensor that measures the system pressure this is signaled to the controller of the compressor. The controller registers the pressure and sends a signal to the inverter which regulates how much air the compressor needs to produce in order to keep the set pressure.

Why is an inverter compressor saving energy?

Because an inverter/frequency driven compressor is not producing more air than needed. A conventional compressor works within a pressure band. When reaching the higher pressure the machine goes into unload mode (the motor is running but no air is produced). When reaching the lower pressure again until it reaches its unload pressure again. An inverter/frequency driven compressor has less unload time and works towards a set pressure value, this makes the inverter compressors in general 30% more energy efficient than a conventional load/unload compressor.

Why is an internal water separator drain necessary?

First, an internal Water Separator Drain (inside of the compressor) is NOT necessary. But it can have some benefits in two cases: 1) Screw without an integrated dryer: Using a water separator drain, we remove some water from the compressed air, before its delivered to the final area of use with less water content. 2) Screw with an integrated dryer: Using it before the dryer, some of the water gets removed before the dryer, this gives the opportunity to choose a smaller sized air dryer.

Is there any recommendations regarding ventilation of the compressor room?

All compressor rooms require ventilation. Minimum room ventilation can be calculated from the formula: Qv = 1.06 N/ T for Pack unit Qv= (1.06 + 1.3) / T for Full-Feature unit Qv = required cooling air flow (m³/s) N = shaft input of compressor (kW) T = temperature increase in compressor room (usually 7°C) If the compressor is conducted, the required ventilation is the same as the fan capacity of the compressor. This is mentioned in the instruction manual.

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