Bearings get an airing

A Brunel University spin-out has developed a compressor technology that is lubricated by thin air instead of oil and could squeeze the last few drops from dwindling natural gas resources. Jon Excell reports.

UK company Corac has developed an oil-free, high-speed compressor technology that could not only change the face of industrial compression but also squeeze the last few drops from dwindling natural gas natural resource.

Corac’s technology is a refinement of the traditional centrifugal compressor, which works on the principle of an extremely high impeller velocity accelerating a gas to create a rise in pressure. This pressure is then further increased as the flow of air is slowed down by an arrangement of diffusers around the impeller. In most centrifugalcompressors, there are several of these impeller/diffuser combinations.

Corac’s device, which the company prefers to call a turbo compressor, works on the same basic principle, but is designed in such a way that its core element – the spinning armature – is supported solely by a cushion of air.

Air enters the centre of the impeller and is centrifugally spun out to the rims where it is channelled through a diffuser. Corac chairman Gerry Musgrave explained that one of the main differences with his company’s design is that the impeller blade tips are capable of moving at near the speed of sound – 70,000rpm – giving the compressor a somewhat enhanced performance.

Corac has dispensed with the traditional approach of using a gearbox and all of the associated mechanical linkages to drive the impeller. Instead, it is mounted on a single shaft driven by a high-speed permanent magnet. As it spins, this shaft is effectively held in place by Corac’s own air bearings, which, fed from a receiver pressurised by the compressor, prevent the shaft from moving up and down. Air bearings are also used to counter the extreme thrusts generated by the impeller, preventing the shaft from moving backwards and forwards.

A further difference is that the impeller is made of titanium. While this material is frequently found in turbofans and aerospace applications, Musgrave said he believes this is the first time it has been used in an industrial compressor. The advantage of using titanium is its combination of thinness, lightness and robustness.

This makes it possible to have very thin blades, which further improve the impeller’s performance.

An oil-free design has many advantages. These include: no chance of oil getting into the air; negligible friction losses in the air bearings; energy efficiency; less maintenance than a similar system using conventional bearings; and it can run faster.

Corac said that the compressor will be ideal for a range of industries, such as brewing and food processing and packaging, where the absence of oil is desirable – if not essential.

In addition, the size and weight of the compressor and the fact that it has no oil, means that it could be installed right next to a food processing unit. Musgrave claimed that compressors are usually housed in a separate room.

The technology has allegedly sparked quite a bit of interest and Musgrave said that Corac has been asked by a number ‘interested’ parties to demonstrate a family of fully-packaged compressors. Despite Corac’s status as a technology licensing company rather than a manufacturer, it has nevertheless pushed ahead with the development of these demonstration models and Musgrave said he expects to be signing licence deals within six months.

But Corac is not alone with its oil-free claims. Compair and Atlas Copco have recently announced the launch of similar compressors, but based on the alternative rotary screw mechanism.

Rotary screw compressors work on the principle of air filling the void between two helical mated screws and their housing. As the two helical screws are turned, the volume is reduced resulting in an increase of air pressure.

Traditionally, oil is injected into the compression area to enable cooling and lubrication. After the compression cycle, however, the oil and air must be separated.

Compair’s range, the DH-Series, is based on a single screw design that is cooled and lubricated not by oil, but by injected water. While also using a water injection system, Atlas Copco’s AQ range uses a more traditional twin screw design. This, claimed Philip Nichol, Compair’s concept design leader, makes the design less efficient.

‘With a twin screw the bearings run faster,’ he said. ‘there are twice as many of them and they’re heavily loaded because of gas forces trying to push the rotors apart.’ He said that with a single screw design these radial and axial forces are balanced and would contribute towards a longer life.

But Musgrave was unimpressed. He said that in some cases, having water mixed in with the air can create as many problems as oil. He also said that these systems aren’t truly oil-free as the shafts are still driven by gears that are full of oil. ‘If the filter goes, you get oil in the air,’ he said.

In response, Nichol said that while there is oil in the motor, a gap between the motor and compressor make the ingress of oil impossible. And as for the charge of water entering the air, he claimed that the air coming from his compressors contains no more water than from any other kind of compressor.

In a further development of its own technology, Corac has drawn on the principles of its industrial compressor to develop a device that could soon be used to extend the life of natural gas reservoirs by up to five years.

The company’s Downhole compressor, with a diameter of less than 6in, is lowered down an existing gas well tubing to suck the remaining natural gas from the underground reservoir. Musgrave said that the limitations of existing surface-based pumping equipment frequently lead to such reservoirs being abandoned when they still contain around 25 per cent of their original volume of gas.

The compressors, which can be deployed either individually or strung together in series, are lowered down the well and connected to the power electronics – which remains on the surface – by cables.

Norman Liley, who heads the project for Corac, explained that while the reservoir may already be pressurised at anything between 50 and 100 bar, the downhole compressor will increase this to around 120 bar.

It is envisaged that the compressors will remain in place for a number of years. However, over time a well’s pressure differentials will change and Musgrave explained that it will be possible to retract the compressors and alter the pitch of the impeller blades to optimise the efficiency of the device.

He added that feasibility studies into different wells all over the world have indicated that the method gains a far better output from the well than traditional methods.

Corac now has agreement from a number of oil companies for a three-year joint industry programme in which oil companies will pay to take the technology forward. Musgrave added that he expects to soon be able to make a formal announcement concerning a licensing arrangement for the downhole compressor.