Cold idea prompts hot prospects

Tom Shelley reports on an energy saving way of cooling freezers, and some of the spin-off benefits

Gas turbo-expanders, rotating at up to 400,000rpm, form the basis of novel commercial and eco-friendly super freezers. Using only air as a working fluid, they are much 'greener' than conventional machines and are particularly attractive economically when they form part of a combined cold/heat/power generating system. The company that makes them, already supplying units to commercial food freezing factories, sees great potential for the technology in the processing of old rubber tyres, turning them into useful products for profit, and in lower-energy-consuming desalination plants. Turbo expanders form the basis of an alternative method of refrigeration based on what is termed, the Reverse Brayton thermodynamic cycle. Working fluids are normally dry air or nitrogen, although others gases can be used. It is normally configured as a closed system to avoid the need for continuous moisture removal from makeup air. If this were not done, moisture would freeze on turbine blades and ice particles could damage the rotating equipment. The high-pressure working fluid is in a gaseous state throughout the system, unlike conventional refrigeration systems where heat is transferred by the evaporation or condensation of a liquid. As with mechanical refrigeration, the compressor raises gas pressure and temperature. The gas is first cooled by external ambient air and then further cooled by passage through a core heat exchanger, giving up additional heat to the cold air stream returning from the load heat exchanger. The expander then drops the gas pressure suddenly by a large amount, cooling it further, the cooled air is used to extract heat from the load. Even after the gas has extracted heat from the load, it is still cold enough to pre-cool the air to the expander. The gas stream applies a force to the expander blades, causing a rotation of the shaft, providing some of the power requirements of the compressor. The remaining power requirement is met by a motor or engine driving the compressor-expander shaft. Until now, such machines have mainly been of interest to those engaged in aerospace and defence work, because of their compactness, their ability to operate on violently moving platforms and their ability to work under conditions of low or zero gravity. They are also favoured for special applications because of their quietness. A small, Moscow-based company, NPO Turboexpanders, has, for some time, been manufacturing such machines for non aerospace and non high technology commercial work. The company consists of 15 "highly qualified specialists", headed by Dr Alexander Kobalashvili, who has, apparently, spent the last several decades mathematically analysing and modelling the high-speed fluid bearings crucial to the technology. These have linear rotation speeds of 180m/s, amounting to 40,000 to 400,000rpm, depending on shaft diameter. The company uses gas bearings on machines with thermal capacities from 0.1 to 20-25kW; cold gas and oil bearings for machines rated from 2-3 to 50kW; and hydrostatic oil bearings for capacities between 50 and 200kW. The goal is to achieve bearing lives of more than 30,000 hours under operating conditions. This requires rotor perturbations to be less than 10 microns. The machines also have to minimise heat gain and maximise the separation between the cryogenic and warmer parts. A specially designed annular channel minimises heat gain from the machine's running gear, and the internal separation of high and low pressure cavities is accomplished by means of a ring seal which, nonetheless, allows a quick changed of the rotor should this be necessary. The impeller is a radial-axial, centripetal, reactive type. It is made in semi-open and closed versions. The nozzle and impeller combination is said to achieve 75 to 78% efficiency in small machines, and 82 to 85% in larger machines. One of the company's configurations of a combined cycle machine comprises: an internal combustion engine operating on gas or diesel oil; an electric generator; an air compressor; a cold generator; and a system of heat exchangers. The company estimates that a basic machine powered by a 110kW gas engine will generate 35kW of electricity, 310kW of low- level heat and 15kW of low-temperature cold air. Such a unit is capable of fulfilling all the energy needs of a plant producing up to 10 tonnes of frozen food products per day. Potentially – and even more significant – project co-ordinator Yaroslav Shkop says: "Our approach also offers an economic way of processing rubber from old tyres and elastomeric polymers using a process based on freeze grinding. Only with the help of low temperature processing is it possible to make the fine powders of rubber and polymers, with sizes less 1mm across, that are in high demand on the world market. Turboexpander technology allows this process to be undertaken at one fifth to one seventh of the cost of processes based on the use of liquid nitrogen. "Furthermore, water desalination by means of the gradual freezing of sweet water from salted water becomes extremely economical if a turboexpander is driven by the reduction of pressure of natural gas in a pipeline prior to its distribution to consumers. "The company owns several old but reliable machines that allow us to carry out the precision machining of the high-accuracy parts for turboexpanders. At present, our company fulfills one-shot orders from Russian companies making equipment for the fast freezing of fish, seafood, vegetables and fruit. We are now looking for a strategic partner-investor to help us set up a modern equipment and production base to allow us to undertake new research and development and to expand our production of units both for sale in Russia and to export to other countries." If the Russian development only results in an economical way of recycling old car tyres, so they are no longer dumped in the woods and lanes of Kent, our local county, we at Eureka would consider it extraordinarily useful. Dr Yaroslav Shkop Freezers using turbo-expanders and the Reverse Brayton thermodynamic cycle are commercially viable Economics become particularly attractive if they are used as part of combined heat/cold/energy generating systems They are also an economically viable method of freezing old car tyres so they can be ground up and recycled and desalinating sea water