22 May 2002
The use of natural refrigerants has attracted considerable interest in the past decade because of environmental concerns attached with CFC, HCFC and HFCs. This paper discusses potential cooling systems working with natural refrigerants. Single, two stage and cascade systems operating at different temperature levels were studied for both direct and indirect expansion. It was concluded that a single stage ammonia system offers the best COP for evaporating temperatures up -10°C. For evaporating temperatures up to -35°C a two stage ammonia system offers the best performance. A propylene two stage system is also applicable. For evaporating temperatures up to -45°C, propylene offers the best COP. Also in this temperature range an ammonia-CO2 cascade system is a promising candidate. CO2 operating as a secondly refrigerant can have significant cost advantages over glycol/brine systems. For evaporating temperatures below -70°C an ammonia-ethane cascade system is proposed. Various glycols and brines were analyzed for the indirect systems. It was concluded that natural refrigerants offer interesting refrigerant alternatives for energy efficient and environmentally friendly refrigerating equipments. However, safety precautions due to flammability must seriously be taken into account.