R744 carbon dioxide

This early refrigerant is again attracting much interest. High latent heat and a heat transfer coefficient combined with high pressure and density under operat­ing conditions results in the ability to produce large amounts of cooling with very small displacement compressors and small diameter pipelines. Its main disadvantage is its low critical temperature (see Chapter 2). The efficiency of a vapour compression cycle becomes very poor if the condensing tempera­ture approaches the critical temperature. This is also a disadvantage with HFC refrigerants, but carbon dioxide is much worse.

It is being applied in the low stage of cascade systems using the vapour compression cycle. The transcritical cycle is very effective where a heating requirement calls for water to be heated over a large temperature range, as is the case when hot water for services is being generated from a cold supply. In these cases the counter flow gas cooler is a good solution. Much work has also been done on the use of carbon dioxide for vehicle air conditioning. Another aspect of the R744 transcritical cycle is the ability to revert to a conventional vapour compression cycle in low ambient conditions. Good average seasonal efficiency may be achieved in locations where heat can be rejected at tempera­tures well below 30°C for most of the year, where operation in the transcritical mode is only under high ambient temperature conditions. High pressures and high pressure differentials allow the use of small tubing. Condensing tempera­tures can be brought right down under low ambient conditions and there is still adequate pressure difference to drive the refrigerant round the circuit. Use of an economizer can significantly enhance the sub-critical cycle performance.

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