The selection of the working fluid of a Heat Pipe should be made according to the following criteria:

1. Freezing, boiling and critical points in must be consistent with the operating temperature range.
   
   
2. High liquid and vapor densities and high latent heat of vaporization are desirable in order to transfer large amounts of heat with a minimum fluid flow.
   
   
3. A high value of surface tension is desirable in order to provide satisfactory capillary pumping, to enable the operation against gravity and generate a high capillary driving force.
   
   
4. Low vapor and liquid viscosities mean less friction among the fluid molecules, so that the resistance to fluid flow will be minimized.
   
   
5. High thermal conductivity to aid heat transfer between fluid, wall and wick.
   
   
6. Good wetting characteristics (small contact angle) to provide satisfactory capillary pumping.
   
   
7. Compatibility in relation to corrosion and non condensable gas formation with container wall and wick.
   
   
8. Chemical stability.
   
   
9. Environmental acceptability (need to avoid CFC’s).
   
   
10. Safety: toxicity or health hazard, flammability and reactivity risks.
    
    
11. Available and inexpensive in high-purity (chemical grade) state.
    
    
12. Fulfillment of other special requirements defined by the Heat Pipe application.

It is useful to define a fluid property group that known as the liquid figure of merit (Chi, 1976) as follows:

\[\label{eq1}N_l=\frac{\rho_l\cdot\Delta h_v\cdot\sigma}{\eta_l}\tag{1}\]