(574f) Passive Heat Exchanger Temperature Control Using Variable Conductance Heat Pipes

Heat exchangers are important unit operations for a variety of industrial processes. They are often necessary to recuperate waste heat and improve process efficiencies. The heat exchanger's outlet streams often feed processes that are very sensitive to temperature (chemical reactors, separation membranes, fuel cells, etc.) where robust temperature control is a must. Most heat exchangers maintain outlet temperatures by using active means such as temperature controllers with actuated valves and inefficient by-pass streams to maintain a set point. These active control schemes have several drawbacks including the low reliability of moving parts, susceptibility to operator error, and the complicated systems level modeling required to validate control schemes.

Presented in this paper is a passive temperature control technology that utilizes variable conductance heat pipes (VCHP) to control outlet temperature from a counter current heat exchanger. VCHP's are heat pipes filled with a prescribed amount of non-condensable gas. The non-condensable gas expands and contracts in the condenser of the heat pipe as the saturated working fluid's vapor pressure and temperature changes. As a result, the evaporator temperature of the VCHP is nearly constant, regardless of operating conditions (flow rates, inlet temperatures, etc.) A heat exchanger using a tube bank of VCHP's has the unique property that the hot side fluid exits at nearly constant temperature regardless of heat exchanger operating conditions (hot and cold side inlet temperatures and flow rates.) A transient model for a VCHP heat exchanger is presented along with experimental data from a VCHP heat exchanger designed to control the outlet temperature of a hydrogen stream for a fuel cell application. The passive temperature control feature of VCHP heat exchangers is ideal for applications where reliability and safety are of paramount concern.