Rotary screw air compressors

The main benefit of rotary screw compressors is the 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 2.2-900 kW range of workplace and industrial rotary screw compressors. Recent introductions offer advanced designs such as a vertical, close-coupled configuration. Another distinguishing feature are interior permanent magnet motor drive and inverter systems. Such systems are capable of achieving energy savings of up to 50 per cent over conventional fixed speed designs.

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

Using a 2kW screw compressor is usually the mark of an entry professional who needs to work beyond the limitations of the supermarket variety compressor - typically a piston machine. We're looking at the serious business that wants a compact, durable machine, to be able to use their compressor all the time, and prompt manufacturer support among others. The main factors that should drive the purchase of a screw compressor in this size range are free air delivery (FAD), working pressure, durability, availability from suppliers, delivery times and finance options. The last is perhaps one of the most important, because cash flow and the availability of tax breaks can make or break a business that is trying to become established. As we progress in size, the compressor becomes more and more ingrained in the business processes. It's gradually moving from being an element that supports some of the machinery in the workshop into an indispensable supply in the factory. The typical buyer will start to consider energy efficiency more and more, and it's not uncommon to see a variable-speed drive type compressor in the compressor room.

Whereas up until the 30 kW mark, compressors would normally be a standalone item somewhere out of sight, things change as we enter this higher power range. Because the compressor is a bigger part of the factory, we’re looking at businesses that typically have duty and standby compressors. This is to ensure that a breakdown does not grind production to a halt. For the typical buyer, the additional factors that drive the choice of compressors in this range are the energy efficiency and the availability of spare parts and service. As such, in this size range, the variable speed driven compressor is the preferred choice of buyers. The knowledgeable buyer should be hearing terms like low footprint, modular design, service by factory-trained technician and genuine parts. They should also be used to receiving copies of energy usage reports based on their existing factory and the proposed solution. As we progress in size, compressor lifecycle cost grows to be a more significant part of the selection process as well.

Once we reach the 90kW mark, compressor breakdowns and failure have an eye-watering price tag. And so does their energy consumption. The large industrial user no longer looks at the compressor price on its own, because they know that focusing on price alone could bring more headaches than it is worth down the line. The key factors in buying criteria are now the lifecycle cost and uptime guarantees from service. The buyer should also consider how well their purchase fits in with any other existing compressors and factory maintenance systems, remote monitoring capabilities and running optimisation solutions such as central controllers. Energy recovery should also play a significant part in the selection choice, to lessen the impact of an energy bill that is the better part of the lifecycle cost a compressor this size. At this point, the buyer’s focus should be to deliver a solution that connects the compressor room and can improve the efficiency of the existing system. This offers not only an immediate benefit from a more efficient compressor, but also optimises the usage of any existing purchases.