Which thermal property is considered undesirable for a refrigerant?

A low critical temperature for a refrigerant can be considered undesirable because it indicates that the refrigerant will easily transition from gas to liquid phases at relatively low temperatures. This characteristic can restrict the refrigerant's efficiency and performance in cooling applications, as it may not be ideal for achieving the necessary temperature ranges in refrigeration cycles.

In refrigeration systems, a higher critical temperature allows for better efficiency and performance, as the refrigerant can operate effectively at higher pressures without transitioning to a liquid state. This is particularly important in situations where the condenser must reject heat—if the refrigerant's critical temperature is too low, it may limit the system's ability to operate effectively at higher ambient temperatures or under varying load conditions.

In contrast, the other parameters mentioned, such as high critical temperature, low viscosity, and high thermal conductivity, tend to contribute positively to the performance of a refrigerant. A high critical temperature is advantageous for system efficiency in maintaining gaseous states under pressure. Low viscosity promotes better flow characteristics within the system, and high thermal conductivity ensures effective heat transfer.