Editing Heat pipe (section)

=== Variable conductance ===

Standard heat pipes are constant conductance devices, where the heat operating temperature is set by the source and sink temperatures, and the thermal resistance from the source to the sink. The temperature drops linearly as the power or condenser temperature is reduced. For some applications, such as satellite or research balloon thermal control, the electronics is overcooled at low powers, or at the low sink temperatures. Variable conductance heat pipes (VCHPs) are used to passively maintain the temperature of the electronics being cooled as power and sink conditions change.<ref>{{cite web|url=http://www.1-act.com/vchps-for-passively-controlling-temperature/|title=VCHPs for Passively Controlling Temperature|publisher=Advanced Cooling Technologies}}</ref>

Variable conductance heat pipes add two elements:

* a reservoir
* a non-condensable gas (NCG)

The non-condensable gas is typically [[argon]], except that helium is used for thermosyphons. When the heat pipe is not operating, the non-condensable gas and working fluid vapor are mixed. When the pipe is operating, the non-condensable gas is swept toward the condenser by the flow of the working fluid vapor. Most of the non-condensable gas is located in the reservoir, while the remainder blocks a portion of the condenser. The VCHP works by varying the active length of the condenser. When the power or heat sink temperature is increased, the heat pipe vapor temperature and pressure increase. The increased vapor pressure forces more of the non-condensable gas into the reservoir, increasing the active condenser length and the conductance. Conversely, when the power or heat sink temperature is decreased, the heat pipe vapor temperature and pressure decrease, and the non-condensable gas expands, reducing the active condenser length and conductance.

The addition of a small heater on the reservoir, with the power controlled by the evaporator temperature, allows thermal control of roughly ±1-2&nbsp;°C. In one example, the evaporator temperature was maintained in a ±1.65&nbsp;°C control band, as power varied from 72 to 150 W, and heat sink temperature varied from +15&nbsp;°C to &minus;65&nbsp;°C.

VCHPs can be used when tighter temperature control is required.<ref>{{cite web|url=http://www.1-act.com/pchps-for-precise-temperature-control/|title=PCHPs for Precise Temperature Control|publisher=Advanced Cooling Technologies}}</ref> The evaporator temperature is used to either vary the reservoir volume, or the amount of non-condensable gas. VCHPs have demonstrated milli-Kelvin temperature control.<ref>{{cite web|url=http://www.1-act.com/pressure-controlled-heat-pipe-applications/|title=Pressure Controlled Heat Pipe Applications|publisher=Advanced Cooling Technologies}}</ref>