By introducing a small mass of NCG (shown schematically below), most heat pipes can be created as a Variable Conductance Heat Pipe. Because NCG is swept to the end of the condenser by the condensing working fluid vapor, it blocks a portion of the condenser, effectively reducing its conductance. If the ambient temperature increases, decreasing the available temperature difference between the condenser and the ambient, the operating temperature of the heat pipe will increase. This causes the operating pressure or saturation pressure of the working fluid at the heat pipe operating temperature to increase and compresses the NCG into a smaller volume. The result is that more of the condenser area is available to condensing working fluid. This limits the increase in the operating temperature of the heat pipe and the component mounted to it, much as in the case of a Constant Conductance Heat Pipe (CCHP). Ideally, the increased conductance of the condenser offsets the increase in the ambient temperature and the heat pipe operates at a constant temperature.
The degree of control depends on the working fluid saturation curve, the desired operating temperature set point, the ranges of ambient temperature and heat load and the volume of gas relative to the volume of the vapor space in the condenser.
