Freeze dryer with improved temperature control

Abstract

The circulation of heat transfer fluid through the low temperature heat exchanger in the temperature control system of a freeze dryer is selectively increased when the temperature of heat being applied to the product shelf assembly in the freeze dryer exceeds the desired temperature. Preferably, a servo-controlled mixing valve selectively bypasses the low temperature heat exchanger so that the rate of cooling is very precisely adjusted without requiring the heat exchanger refrigeration unit to be cycled on and off. Since the refrigeration unit operates continuously, the reliability and operating lifetime of the refrigeration unit is increased. Moreover, the heat exchanger temperature is maintained at a minimum value. Preferably, the servo-control for the mixing valve includes a digitally controlled stepping motor and the adjustment of the mixing valve is sensed by a linear voltage displacement transducer. To prevent short cycling and oscillation, a dead band is introduced in the servo-control loop. To reduce temperature overshoot yet achieve a fast response time, a rate function is included in the temperature control loop as well as nonlinear characterization of the temperature control loop gain to provide a quick response to large control steps. Preferably the same rate function, representative of the thermal inertia or heat capacity of the dryer system, is included in the control loop for the heater as well as the control loop for the low temperature heat exchanger.