Thermosyphons are passive heat transfer devices that operate on the simple process of convection through vaporization and condensation. Heat transfer will only occur when the low end of the unit is warmer that the high end.
Ther refrigeration cycle is similar to that of mechanical refrigeration plant in that the refrigerant changes phase from liquid to vapour to liquid as it boils and condenses. The temperature differential across the thermosyphon drives the heat transfer like the compressor in a mechanical plant and the larger the temperture differential the higher the heat transfer rate.
In the context of Thermo Probes and Thermo Piles, the conductance of a thermosyphon is a function of three main resistances:
• Heat flow to the evaporator
• Thermosyphon internal resistance
• Heat flow away from the radiator
Sloping Evaporator Thermoprobes
Sloping Evaporator Thermoprobes are found on the majority of existing passive subgrade cooling system installations beneath slab-on-grade structures. These units utilize a sloped evaporator to insure that the condensate will flow from the condenser to the lowest portion of the evaporator. Typically, evaporator slopes are between 10% and 3% – with the median being 5%.
Structures founded on grade over permafrost utilizing Thermoprobes for subgrade cooling also have two other very important components of design: subgrade insulation and NFS fill. the typical thermal design methodology for this type foundation economically balances the three major components. The insulation reduces the heat load on the subgrade and limits the thawing of the underlying NFS fill during the summer season when the Thermoprobes are dormant. The NFS fill contains the seasonal thaw and provides a heat sink beneath the structure. The Thermoprobes remove heat from beneath the structure primarily during the freezing season and refreeze the NFS fill so that it is completely frozen at the beginning of the thawing season.