# The saturation pressure at the maximum temperature of the cycle should not be excessive. Very high pressures lead to mechanical stress problems, and therefore, unnecessarily expensive components may be required.
# The saturation pressure at the minimum temperature of the cycle (i.e. the condensing pressure) should not be so low as to lead to problems of sealing against infiltration of the atmospheric air into the system.
# The triple point should lie below the expected minimum ambient temperature. This ensures that the fluid does not solidify at any point during the cycle nor whilst being handled outside the system.
# The working fluid should possess a low value of the liquid viscosity, a high latent heat of vaporisation, a high liquid thermal conductivity and a good wetting capability. These ensure that the working fluid pressure drops in passing through the heat exchangers and the auxiliary piping are low and that the heat transfer rates in the exchangers are high.
# The working fluid should have low vapour and liquid specific volumes. These properties affect the rates of heat transfer in the heat exchangers. The vapour specific volume relates directly to the size and cost of the cycle components. Moreover, a high vapour specific volume leads to larger volumetric flows requiring a multiplicity of exhaust ends of the expander at heat engines or compressor in refrigeration cycles and resulting in significant pressure losses. The specific volume of the liquid at the condenser pressure should be as small as possible in order to minimise the required feedwater pump work.
# Non-corrosivity and compatibility with common system materials are important selection criteria.
# The fluid should be chemically stable over the whole temperature and pressure range employed. The thermal decomposition resistance of the working fluid in the presence of lubricants and container materials is a highly important criterion. In addition to making the replacement of the working fluid necessary, chemical decomposition of the fluid can produce non-condensable gases which lower the heat transfer rate in the heat exchangers, as well as compounds, which have corrosive effects on the materials of the system.
# Non-toxicity, non-flammability, non-explosiveness, non-radioactiveness and current industrial acceptability are also desirable attributes.
# The fluid should meet the criteria of environmental protection requirements such as a low grade ozone depletion potential (ODP) and global warming potential (GWP).
# The fluid should possess good lubrication properties to reduce friction between surfaces in mutual contact, which reduces the heat generated when the surfaces move and ultimately increases cycle performance.
# The substance should be of low cost and readily available in large quantities.
# Long-term (operational) experience with the working fluid and possible fluid recycling is also beneficial.