Preventing freezing in a heat exchanger can include passing first and second cooling fluids through the heat exchanger, monitoring a temperature and/or pressure of the second cooling fluid entering the heat exchanger, heating the second cooling fluid entering the heat exchanger by opening a valve when the monitored temperature and/or monitored pressure is below a trigger temperature and/or trigger pressure, and closing the valve when a condition is met. The first cooling fluid can have a first freezing temperature and the second cooling fluid can have a second, lower freezing temperature. Opening the valve can cause the second cooling fluid, in gaseous phase, to mix with the second cooling fluid, in liquid phase, upstream of the heat exchanger.