Turbo compressors

A new turbo-compressor can use many `zero effect’ substances such as refrigerants. What is more, power consumption has been reduced

The production of fluorocarbons (CFCs) ended on 1 January, 1996. For most users, the phaseout will not cause severe side effects. For air conditioning and refrigeration equipment, however, there was a large inventory of CFC-using equipment and shortages of phased out refrigerants during 1996. This has already caused sharp increases in refrigerant process and problems in servicing equipment that depends on the phased out refrigerants.

In response to these difficulties, Governments have initiated programs designed to foster recovery of CFC refrigerants, reduce refrigerant leakage and stimulate early conversion of CFC-using equipment to first generation substitutes, such as HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons). But these are only interim solutions to the problem because HCFCs and HFCs are themselves due to be phased out by the end of the next decade.

In fact, there are only two real long term solutions to the problem: the development of a new generation of `safe refrigerants’ that can be used in existing equipment, or the development of a new generation of refrigeration and air conditioning equipment that can use as refrigerants existing substances that are known to have a zero-effect on the ozone layer.

There are several zero-effect substances that can be used as refrigerants that do not contain chlorine or bromine that have been declared safe for the ozone layer. But their molecular mass and relatively high boiling temperature have made them unsuitable for use in current refrigeration equipment.

For this reason, research continues into finding yet more `safe’ products that can be used on existing equipment. But it is not just a matter of finding new refrigerants. Existing lubricants are not compatible with the first generation substitute refrigerants, so new lubricants will have to be developed. There are also a number of research projects underway to design new equipment that can use zero-effect coolants but, until now, none of these has been successful.

Now a team of Russian engineers working on the development of cryogenic equipment has developed a new turbo-compressor that can use many of the zero effect substances as refrigerants. Many of these substances are available in commercial quantities and at competitive costs, making them suitable substitutes in refrigeration and air-conditioning equipment.

VIC Dynamic Products, the UK distributor, claims that for the first time, a centrifugal compressor of less than 100kW capacity has been successful in gleaning all the traditional benefits of centrifugal compressors from a turbo-compressor as small as 3kW. And, through the creation of a new thermo-dynamic cycle, power consumption has been reduced significantly compared with traditional cooling cycles without these elements.

The turbo-compressor system, which is patented world wide, is based on a two-stage centrifugal turbo-compressor, driven by a high speed asynchronous electric motor integrated with the compressor body.

Indeed, the breakthrough in miniaturising the compressor was made possible by the development of the electric motor. The variable speed motor is believed to be the first of its kind that can operate up to frequencies of 1200Hz and speeds up to 72,000rpm. The speed is controlled by a standard variable speed inverter.

The two smallest turbo-compressor models operate at speeds of up to 60,000rpm, and use gas dynamic bearings to overcome the problems of short bearing life when operating at those speeds. The rotor shaft `floats’ on a cushion of the circulating coolant which also acts as a lubricant. The larger turbo-compressor models operate at lower speeds and use conventional bearings that are self-lubricating. This means that there is an absence of oil in the entire system.

The traditional thermodynamic cycle has been changed by introducing two additional elements into the cooling cycle, an intermediate vessel for separating vapour and fluid, and a recuperative heat exchanger for intermediate cooling and heat regeneration. Together, they have improved the efficiency of the cooling cycle so that energy consumption is significantly less than for cycles without these elements.

Unlike traditional cooling cycles, the turbo-compressor never switches off. Cooling is maintained by variations in the turbo-compressors’ speed of rotation, which is controlled by the variable-output frequency inverter. This enables more precise temperature regulation and reduces costs.

Figure 1: The VIC system is based on a two-stage centrifugal turbo-compressor, driven by a high velocity asynchronous electric motor integrated with the compressor body