Screw Compressors – Air Compressors 101

Learn about the different screw air compressor types and what is best for your needs

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Screw Compressors – Air Compressors 101

Screw Air Compressors – Air Compressors 101

Air Compressors WA has a large range of Rotary Screw Air Compressors, whether you’re looking for Variable Speed (VSD) Rotary Screw Air Compressors or Fixed Speed Rotary Screw Air Compressors.

Not sure which one is the right one for your needs? Read on to learn about the different types to help you decide.

How do screw compressors actually work?

There are two rotary screw elements (pictured) that spin together with a larger opening between the two elements at one end and a smaller opening at the other end. Air travels into the end of the chamber and as the screw elements spin, the air moves along toward the thinner end and is compressed. As they constantly spin in one direction, and do not go up and down like a piston air compressor, they have the ability to spin at a higher RPM and are more efficient.

The rotary screw elements need to be lubricated and so when the air is being compressed, oil is present in the process. This compressed air and oil mixture is then run through an oil separator, which removes all of the oil from the air.

Variable speed drive, fixed speed – what does it all mean?

Rotary screw air compressors have either a variable speed drive or are fixed speed. Fixed speed air compressors are where the electric motor is fixed in some manner to the air end, or the screw, and when the air compressor is turned on the electric motor will run at 100% of its capacity to turn the screw to make compressed air. So the machine is either turned off and consuming zero power, or it is turned on and running at maximum speed for the unit e.g. 15 kilowatt / 20 horsepower. Variable speed drive air compressors are similar in their design, except for that the power supplied to the electric motor (via an inverter) varies based on some parameters. This means that a variable speed drive air compressor can operate the electric motor at for example 20% power for when air demand is low, and can increase the speed of the electric motor by supplying more power, when the air demand increases.

This is important because two machines of the same power consume different levels of electricity to deliver the same amount of air, when the air demand is not the full capacity of the machine. Consider a scenario, whereby two machines can produce 100m3/hour of air at 100% but the workshop only requires 50m3 of air in that hour period. The variable speed drive will operate at 50% capacity for the whole hour, and the fixed speed will operate at 100% of its capacity for some time then turn off. Once the pressure drops, it will start and then stop again.

The fixed speed is only running half the time, but when it is running, it’s running at 100% of the power of the machine. The electricity draw of this machine is notably higher than the fixed speed, and the result is that the variable speed drive machine saves notable amounts of power over time. After a few years, the power savings alone of the variable speed drive machine may well be in the thousands.

So which one is for me?

Does the air demand of your workshop vary throughout the day? Do you sometimes have 5 machines operating or 10 staff working with air tools, and sometime less machines operating? Then a variable speed drive machine would not be running at full capacity throughout the day and you would be experiencing power savings as the machine is not starting and stopping all day. Variable speed drive air compressors do have a high upfront capital cost when compared to their fixed speed equivalents, but this up-front expense is generally minimal compared to the savings in the three phase electricity bills.

Browse our large range of Variable Speed Drive (VSD) Air Compressors

When to choose a fixed speed?

If your air demand is constantly high, and the air compressor is correctly sized to your air demand, then the machine will be operating at 80-100% all the time and so there is no need to go down the route of a variable speed. For example if you’re in the abrasive blasting industry, your air demand for blasting is generally high and constant. If you had a blasting cabinet, but also sometimes did painting and didn’t use the cabinet, then a VSD may be well suited.

Fixed speed rotary screw air compressors are also generally cheaper than the VSD equivalents, and so if you’re on a budget, they might be good for you.

Browse our large range of Fixed Speed Air Compressors

Floor mounts, tanks, dryers and full featured – what is the best for me?

Floor (or base) mounted screw compressors are ones that have no dryer or tank and simply sit on the floor. This is necessary for larger compressors (e.g. greater than 20-30kw) as the physical size of the unit means it is too big to sit on a tank (known as an air receiver).

Tank mounted (without dryer) screw compressors is when the screw compressor is mounted on top of a horizontal air receiver (tank). This saves floor space compared to having separate units and is a common theme with compressors under 30kw.

Full featured screw compressors is where the screw compressor is mounted on to a tank but also with a refrigerated dryer, as one unit. Some premium brands like Airquip Australia’s Rotary Screw Compressor range and Chicago Pneumatic also have a dryer pre-filter and a dryer post-filter attached, meaning that the whole unit is ready to produce clean air once an electrical connection is made.

Other types

Most screw compressors are powered by a three phase electrical connection, but there are some available in single phase (240v, 15amp). These single phase units are particularly helpful where three phase is not available, such as in the country, or where someone has a requirement of constant air demand of 10-12 cfm FAD and a standard piston cycle with its duty cycle of 50% isn’t going to provide enough air flow.

Portable diesel powered air compressors also have rotary screws.

How do I choose a good one?

Not all screw compressors are equal. Look for high build quality, from a reputable manufacturer with a good track record. For example the Airquip range, where the manufacturer has been around for over 150 years – you really can’t top that.

Find a reputable company who has units and service kits in stock. Find out about the cost of servicing, and how often they should be serviced. Generally speaking a compressor running a workshop 5 days a week 8 hours day will clock 2,000 hours per year, and so servicing is recommended every 2000 hours or 12 months, whichever is sooner. Servicing costs should not be in the thousands of dollars. Ask for a cost for the servicing kit only.

Some might tempt you with a cheap price, but what will you do when it is out of warranty, it needs repairs, and parts cannot be obtained? A screw compressor is an investment, at Air Compressors WA we are a firm believer of that you should be able to focus on your business, and not need to think about your air compressor, as it will just work flawlessly and when it is come for a scheduled service you will be contacted by a friendly representative and a service quotation that won’t break the bank.

Talk to the experts about your requirements

For local, professional products and service, get in touch with us today. Call us at the Canning Vale Workshop on 1300 209 018

Now you know about the different types of screw compressor, use our Find an Air Compressor tool to refine, filter and view the right air compressor for your needs – you can filter by Variable Speed Drive (VSD) and Fixed Speed for example. Speak to an Air Compressors WA Expert today on 1300 209 018 to get the right advice the first time.

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Compressed Air Solutions
and Services

Air Compressors WA provides high quality compressed air solutions from highly recognized brands to an extremely diverse customer base.

Specialising in a wide range of air tools (from diesel air compressors to rotary screw compressors), as well as offering a huge selection of air compressor parts and accessories (including air fitting, air piping, air hose reels, etc.)

We also offer delivery, installation and repair services and is truly your one-stop-shop for air compressors in Perth!