A cooling apparatus includes: a first fluid flowpath including the following elements, in downstream flow sequence: a subcooler having a first side in fluid communication with the first fluid flowpath and a second side configured to be disposed in thermal communication with a source of cooling fluid; a flow control valve; a primary evaporator assembly including at least one primary evaporator configured to be disposed in thermal communication with a primary heat load; and a pressure regulator operable to maintain a refrigerant saturation pressure within the primary evaporator at a predetermined set point.