Rotary screw air compressors explained

Unlike piston compressors, in screw compressors, there are no valves or other mechanical forces that can cause unbalance. This allows a screw compressor to operate at high speeds while combining a large flow rate with small exterior dimensions. The ideal applications of rotary air compressors are continuous, workplace and industrial applications. There are two primary versions, oil-free and oil-injected, with options of fixed speed or variable speed drive operation.

The main benefit of rotary screw compressors is their energy efficiency. But this type of compressor has many other benefits. These include:

• low noise output
• small footprint for point-of-use installations
• no duty cycle
• continuous operation at temperatures of up to 46 degrees C
• low oil-carryover (as little as 3 ppm) in oil-lubricated machines
• zero loss of capacity over time

The installation footprint can be further reduced with "full-feature" variants. Such machines have an integrated refrigerant dryer (dewpoint at compressor output of +4°C).

 

There are a wide variety of options available in the 2.2-500 kW range of workplace and industrial rotary screw compressors. Recent introductions offer advanced designs such as a vertical, close-coupled configuration. Another distinguishing feature is interior permanent magnet motor drive and inverter systems. Such systems are capable of achieving energy savings of up to 50 percent over conventional fixed speed designs.

Here we will look a bit closer at twin screw air compressor technology. What is a screw compressor and what is its basic working principle?

Also referred to as a twin screw compressor, the screw element technology is one of the types of rotating displacement compressor, which developed in the 1930s. The main characteristic is a male and a female rotor element, driven either by the male rotor or by a timing gear:

• In oil-injected screw compressor technology, the male rotor drives the female rotor
• In oil-free compressor technology, a timing gear drives both rotors for harmonic running, with minimum calculated clearance between both elements

The basic principle of a screw compressor is that the male and female rotors are spinning in opposite directions. This draws air in-between them. As air progresses along with the rotors, the decrease of space between the rotors and their housing leads to air compression.

The compressed air is then displaced to the outlet. Although they belong to the same class, rotary screw compressors are more complex than piston compressors. As such the speed of the screw rotors is optimized at a certain level. This is to minimize mechanical losses (due to heat at very high speed) and volumetric losses (air losses due to very low speed) during compression.

A good example of a screw compressor that can produce large volumes of compressed air and with a small footprint is our GA VSD⁺ air compressors. You can learn more about compressed air technologies on our Wiki. Or contact our team today to get the right-sized screw compressor for your business.

This variant is also sometimes called an oil-flooded compressor or an oil-injected compressor. The technical name, however, is oil-lubricated.
Oil-lubricated rotary screw compressors, inject oil into the compression chamber. This oil cools and lubricates the compressor element. It also helps to remove the heat from the compression process, and aids minimising leakage in the compression chamber. As the next step up from piston technology, oil-injected compressors have as many varied uses as there are industries. This type of compressor is usually picked by users needing large volumes of medium pressure air.
One of the main selling points for users is the absence of a duty cycle. An oil-injected rotary screw compressor could run for the entire length of a working day and suffer no ill effects. As a matter of fact, it would be beneficial. This is particularly important for manufacturing, where a stopped compressor will impact production.
It is also applicable where the air demand is difficult to predict or where attempting to control this demand is undesirable. Would you want your compressor to decide when you can work?

Much like the oil-lubricated variant, oil-free screw compressors also have other names. Users will most often refer to them as oilless compressors or no oil compressors. The names, however, can be somewhat misleading. The naming convention for these compressors comes from the presence (or absence, in our case) of oil in the compression process. An oil-free compressor will still use lubricant for its other processes, such as motor cooling or removing heat from the rotors. Some variants will use water as an alternative lubricant. Our AQ water-injected compressors are such an example. Other variants rely on a special coating applied to the rotor elements, which eliminates the need for lubricants. That is the concept behind our most popular ZR oil-free rotary screw compressors. The oilless compressor is generally used by sensitive industry applications. This is because they adhere to the rigorous standards expected in food and beverage or pharmaceuticals. They are also better suited to the stringent requirements in the oil and gas industry.