Pressure Controlled Heat Pipes
Pressure Controlled Heat Pipes (PCHPs) are actively controlled Variable Conductance Heat Pipes (VCHPs). In a conventional VCHP, the location of the vapor and non-condensable gas (NCG) interface in the condenser section moves in response to changes in the vapor pressure to vary the thermal conductance of the heat pipe.
A PCHP-control mechanism assembly in test stand
This provides passive control of the evaporator (and heat source) temperature. The precision and accuracy of the control are proportional to the NCG reservoir size. Tight control requires a very large reservoir. The PCHP bypasses this limitation by actively controlling the location of the vapor-NCG interface. The NCG reservoir of a PCHP is typically made of variable volume materials (e.g. bellows). A temperature change at the evaporator causes a change in the position of a stepper motor, which in turn changes the volume of the reservoir. These changes produce very precise control of the location of the NGC interface and result in very precise control of the evaporator temperature.
The figure below shows a diagram sketch of the PCHP operating principle.
PCHP operating principle
PCHPs can be used in applications that require very precise temperature control, including:
- Temperature calibration
- Temperature stabilization of Optics
- Fuel cell reformers
- Variable conductance heat exchangers
- Rapid temperature changes for electronics test sets
In the past several years, ACT has developed the PCHP technology including various control methodologies that is capable of near milli-Kelvin temperature control. The following figures show ACT's PCHP hardware and test data examples.
Comparison of evaporator temperature stability over changes of heat sink temperature
between a VCHP (open loop) and a PCHP (closed loop)
Comparison of evaporator temperature stability over changes of heat load between a VCHP (Open Loop)
and a PCHP (closed loop) for two different set point temperatures