A Pressure Controlled Heat Pipe (PCHP) is a variation of a Variable Conductance Heat Pipe (VCHP), where the amount of Non-Condensable Gas (NCG) or the reservoir volume is varied. Like a VCHP, the NCG is swept toward the condenser end of the heat pipe by the flow of the working fluid vapor. The NCG then blocks the working fluid from reaching a portion of the condenser, inactivating a portion of the condenser.
In a VCHP, the fraction of condenser blockage is determined by the reservoir size, the non-condensable gas charge, and the operating pressure, and cannot be adjusted once the VCHP is sealed. In contrast, the condenser blockage in a PCHP is actively controlled. In some designs, an actuator drives a bellows to modulate the reservoir volume. By decreasing the reservoir volume, more of the condenser is blocked. In other designs, non-condensable gas is added and removed to the reservoir, also allowing active control of the condenser length.
The operation of a bellows/piston type of PCHP is shown in Figure 1. Initially, the piston is withdrawn at higher powers, so that most of the condenser is open. When the heat load is reduced, the piston pushes additional gas into the condenser, helping to maintain the heat pipe at a constant temperature. While a VCHP will passively also increase the condenser blockage, PCHPs are able to react faster, and more precisely.
Figure 1. Pressure Controlled Heat Pipe (PCHP) varies the reservoir volume for precise temperature control
The two applications for Pressure Controlled Heat Pipes are: