Abstract: A control device regulating flow rate of a two-phase mixture of steam and water through a vent conduit in fluid communication with the exit from the fourth pass of a four-pass heat exchanger and a low pressure source. The heat exchanger conducts a first fluid vapor through the first and second tube side passes while condensing a portion of the first fluid vapor therein and separating the resulting mixture into condensate and vapor portions at the exit of the second pass. The separated first fluid vapor is then routed through the third and fourth passes where additional condensation of first fluid vapor takes place. A high ratio of first fluid vapor to first fluid condensate is maintained in all four passes to prevent tube failure due to cyclic tube temperatures resulting from alternate flooding and draining of the tubes' interior. The first fluid exiting from the fourth pass is a two-phase mixture of vapor and condensate which must both be vented to a lower pressure source.