SET-POINT ACTIVE CONTROL OF VAPOR-MODULATED LOOP HEAT PIPE

Abstract

A novel advanced active temperature control method, based on a variable- conductance vapor-modulated loop heat pipe (VMLHP) technology, is presented and discussed. The pressure-regulating valve (PRV) is a key element of the VMLHP. The back-pressure reservoir of the PRV has been charged by actuating fluid to provide the system thermal regulation by means of pressure control inside of the VMLHP. Argon and ammonia (single- and two-phase fluids) have been investigated as PRV actuating agents. The active control of the VMLHP is achieved by the PRV actuating fluid pres - sure adjustment provided by the PRV temperature conditioning. A miniature heater and Peltier element have been attached to the PRV body for this purpose. For argon, the results show that the temperature set point can be modified up to 5°C without exceeding the hardware temperature limitations with very little heater/Peltier element power. A ratio of about 1°C set point increase per 10°C argon temperature increase is obtained. Regarding ammonia, the test goal was to keep evaporator temperature constant, what- ever the power applied to VMLHP (up to 170 W), by means of a PID controller. In this case, again very little control power was needed (less than 4 W) to achieve a precise temperature control. A possible application of this active control is to counterbalance the heat fluxes which can reach the PRV back-pressure reservoir from a hot/cold environ - ment or from the superheated vapor of the working fluid. This is very important when high-temperature control precision (less than 0.1°C) is required.