We all know any thermodynamic power cycle depends on some working fluid for the transfer of heat from source to the point where it is converted to some useful work. Picking a working fluid for any vapour power cycle is not a random decision, since in case of any malfunction caused by it can lead to danger to human life and can cause huge operation and maintenance cost. While choosing a working fluid for a vapour power cycle one should keep following points in his mind. These points suggest the properties of working fluids for a trouble free vapour power cycle.
1. The fluid should have high critical temperature so that the saturation pressure at the maximum permissible temperature is relatively low. It should have a large enthalpy of evaporation at that pressure.
2. The saturation pressure at the temperature of heat rejection should be above atmospheric pressure so as to avoid the necessity of maintaining vacuum in the condenser.
3. The specific heat of the liquid should be small so that little heat transfer is required to raise the liquid to the boiling point.
4. The saturated vapour line of the T-s diagram should be steep, very close to turbine expansion process so that excessive moisture does not appear during expansion.
5. The freezing point of the liquid should be below room temperature, so that it does not get solidified while flowing through the pipelines.
6. The fluid should be chemically stable and should not contaminate the material of construction at any temperature.
7. The fluid should be nontoxic, noncorrosive, not excessively viscous, and low in cost.