Space is not the final frontier

Background

There were two over-riding criteria in Oxford’s work to develop linear oil-free compressors for small cryocoolers for space applications; the compressor must be:

• oil free, and
• have a working life of at least 10 years.

The elimination of any contacting surface that can wear is this technology’s key feature, and this has been so successful that it is now the basis of almost all cryocoolers used in space; this technology has now been licensed to major aerospace companies.

During operation a linear motor drives a piston/cylinder system, and with precise assembly the piston moves within the cylinder without contact. Typically the clearance between the piston and cylinder is held at less than ten millionths of a metre allowing compressor pressure differences of approximately 10 bar without power loss. Provided cleanliness in the system is maintained, and suitable materials for construction have been selected then there are no wear or failure mechanisms; this type of compressor can have a life exceeding the 10 year criterion.

Marketing Opportunity

These early designs were too costly for most

commercial applications, but recent developments at Oxford have changed this situation. We can now design oil-free compressors that have major advantages over conventional designs, and yet have the potential for low cost manufacture.

Applications for this new generation of linear oil-free compressors include those involving hazardous gases or those that require high levels of cleanliness. A comparison of the existing compressor technology and the new linear technology will illustrate this.

In a traditional diaphragm compressor a piston generates a fluctuating pressure in a volume of hydraulic fluid. A diaphragm separates the fluid from the gas to be compressed, and changes in hydraulic pressure deflect the diaphragm which in turn compresses or expands the gas volume.

Although diaphragm compressors can be used to compress virtually any gas to very high pressures there are some serious disadvantages:

• Diaphragms have a limited life, and typically have to be replaced at intervals of ~ 5000 hours depending on the service, and as a consequence frequent maintenance is essential.
• Diaphragms can fail prematurely, and a leak detection system is essential.
• The diaphragm compressor is limited to small strokes with low speed operation. This tends to make diaphragm compressors large and heavy for the duty performed.
• They are relatively expensive machines to manufacture and operate.

In a linear compressor the linear movement of the piston within the cylinder compresses and expands the gas in the compression volume to produce a pressure variation. The motor stator complete with windings of the motor is entirely outside the pressure vessel, and the moving magnet inside the pressure vessel is readily contained in an hermetically sealed unit. No oil is used anywhere in the operation of the compressor. A module controls the dynamics of the assembly. There are three advantages offered by this Linear Compressor Technology:

• The significant reduction or even total elimination of maintenance requirements.
• The elimination of any dynamic seals; no failure mode corresponds to diaphragm failure.
• The complete elimination of oil from the compressor environment.

Patent Status

This work is the subject of granted patents and patent applications, and Isis would like to talk to companies interested in developing this opportunity. Please contact the Isis Project Manager to discuss further.