Screw compressors: FAQs

Got a question about screw compressors? We have collected the questions that we are asked most frequently, with the answers, on this convenient FAQ page.

Estimated time to read: 7 minutes

March 14, 2023

What are screw compressors?

Air compressors generally compress air in one of two different ways. The first uses a piston to compress air. Shock and surprise: it’s called a piston compressor. The other method? It uses two screws, or rotors that turn, with the thread on one screw pressing into the gap between the threads of the other screw. This compresses the air in the gap between the threads. The gap is designed to be smaller as the air moves along, allowing it to be compressed even more. You guessed it: this type of compressor is what we call a screw compressor.

What is the difference between a belt-driven screw compressor and a direct-driven screw compressor

A belt-driven air compressor has a V-shaped belt attached to pulleys on the motor and the compressor pump. Hit the power button, the motor runs, the belt turns, the screws of the compressor turn and air gets compressed.

On a direct-drive compressor, the crankshaft is directly connected to the pump. No belt. No pulleys. The compressor pump is driven directly by the motor.

The running costs on belt-driven compressors tend to be higher than for the equivalent direct-driven compressors. This is partly because belt compressors consume an average of 3% more energy. They also demand more maintenance which means more costs and more down-time. However, when you purchase it, a belt-driven compressor usually has a smaller price tag than the equivalent direct-drive compressor. Which is best? It depends on what you’re going to be using it for. And how much you’ll be using it. If you’re unsure, contact ALUP. We will take a look at your needs, put our insight and experience at your service and advise you on which model is best for your needs. 

Do screw compressors with integrated dryers require a separate voltage transformer?

This will depend on the model. If your screw compressor has an active range between 30 kW and 110 kW, you’ll want to have a transformer that reduces mains voltage to a safe and accessible level. In some cases, you’ll have an integrated power supply that does this. In others, you’ll need to order it separately. Unsure whether you need one? Give us a buzz at ALUP. 

What fuse do you need for a compressor?

There’s a kind of rule of thumb here:

·         At 575 volts, a 3-phase motor draws 1.00 ampere per unit of output horsepower.

·         At 460 volts, a 3-phase motor draws 1.27 amperes per unit of output horsepower.

·         At 400 volts, a 3-phase motor draws 1.50 amperes per unit of output horsepower.

·         At 230 volts, a 3-phase motor draws 2.50 amperes per unit of output horsepower.

·         At 230 volts, a single-phase motor draws 5.00 amperes per unit of output horsepower.

·         At 115 volts, a single-phase motor draws 10.00 amperes per unit of output horsepower.

Just note that there is no safety margin for these figures. In other words, if you’re using a single-phase motor at 230 volts and it’s producing 3.0 horsepower, you’re coming very close to blowing a 16 A fuse. Our recommendation? Always check with a certified electrician. Although we know plenty about air compressors, they know more about electricity than we ever will.

What flow should you have over the energy-recovery circuit on a compressor with integrated energy recovery?

This depends on the conditions at your site and what temperatures are acceptable. The chart below is a useful reference for 30 kW and 37 kW compressors.

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 compressors have when they’re delivered?

This depends on the compressor. When your compressor is delivered, you’ll find the local certificates included. Missing your certificate? Or do you need to know what certification a compressor has before you purchase it? Give us a buzz. 

Do you sell oil-free screw compressors?

We have a range of oil-free screw compressors. Perfect for medical, food and tech industries where you need to produce oil-free compressed air. Either check out the range yourself or get in touch with us for a little extra insight and advice. 

How does an inverter compressor work?

Inverter compressors have almost the exact same components as conventional compressors. However, inverter compressors usually have more advanced control systems. This allows them to adjust the motor speed to produce the amount of compressed air you need.

How? There’s a sensor that measures the pressure being created by the compressor. It lets the compressor controller know this pressure. The controller then sends a signal to the inverter. The inverter then regulates the amount of compressed being produced in order to maintain the set pressure. The advantage? The motor only works as hard as is necessary to produce the air pressure you need. 

Why does an inverter compressor save energy?

Inverter compressors only produce the amount of compressed air you need. But a conventional compressor? It will just run until it reaches its higher pressure limit. And then? It just keeps running, running, running, but without actually compressing any more air. This means the air pressure will drop. When it reaches its lower pressure limit, the compressor will start compressing air again.

In general, inverter compressors use about 30% less energy than conventional compressors. 

How long does it take for an inverter compressor to pay for itself?

This will depend on how often and how hard you run your compressors. Typically, if you run your compressor for 4000 hours a year, your compressor will start paying you back after one to two years. We rarely see anyone reach pay-back status within a year. 

Do you need an internal water separator drain?

You don’t need to have an internal water separator drain inside your compressor. But there are benefits to having one.

·         In a screw compressor that does not have an integrated dryer, a water separator drain allows you to remove water from the compressed air before its delivered.

·         For a screw compressor that does have an integrated dryer, the separator drain removes water before your compressed air hits the dryer. As there’s less water in the air, you’ll get by with a smaller dryer. 

Are there any recommendations about ventilation in compressor rooms?

All compressor rooms require ventilation. Exactly how much? It depends on whether your air compressor has a fan or not.

With a fan? The amount of ventilation you require is the same as the fan capacity. You’ll find this in the instruction manual.

What if your air compressor doesn’t have a fan? Prepare yourself ... It’s maths time! There’s a formula to calculate the amount of ventilation you need:

Qv = 1.06 N/ T for Pack unit Qv= (1.06 + 1.3) / T for full-feature units

Qv = required cooling air flow (m³/s)

N = shaft input of compressor (kW)

T = temperature increase in the compressor room (usually 7°C)

Unsure what this means? Give us a buzz at ALUP. We have folks working here who will be thrilled to do the calculations for you. 

Would you like to know more about screw compressors? Do you have a question that wasn’t covered here? Get in touch today. It would be a delight to share the ridiculous amount of knowledge we have! 

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Classification of Compressed Air Quality >


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