Abstract: A process liquid circulation and temperature control system including a pump for circulating water or other process liquid through a path including a heating unit, a cooling unit, and a process apparatus that is returned to the pump; the pump and the heating and cooling units are actuated in response to temperature/pressure variations sensed at the return from the process apparatus and at the delivery thereto. One pressure control measures the differential pressure across the pump and inhibits system operation except when the differential exceeds a predetermined magnitude in a given direction; another pressure control inhibits operation if the return pressure falls below a given threshold; a circuit that determines .DELTA.P/T prevents boiling of the process fluid. For both temperature and pressure controls, the delivery and return sensors are calibrated to match each other on system start-up.

Abstract: The invention involves a closed loop control method and apparatus for starting up a steam-heated heat exchanger by controlling the heating-steam pressure. According to the invention a correcting variable is established by a controller from an actual pressure value, measured by an actual value transmitter, and from a reference value, generated by a reference value generating device, the heating-steam pressure being modified by at least one control valve on the basis of said correcting variable. The pressure reference value S is selected from three pressure set values, S.sub.T, S.sub.P1 and S.sub.P2, the lesser value of S.sub.T and S.sub.P1 being established as an intermediate pressure set value S' (that is, S' = Min (S.sub.T, S.sub.P1)), and the pressure reference value S is selected as being the greater value of S' and S.sub.P2 (that is, S = Max (S', S.sub.P2)), where S.sub.T represents that permissible maximum desired pressure value which is a function of the momentary metal temperature T.sub.