Cooling system employing a heat exchanger with phase change material, and method of operation thereof

A heat exchanger for use, e.g., in a rear door of a server rack in a computer room, made up of pipes clad with a layer of a phase change material, such as a paraffin, so that latent heat is absorbed in the conversion from the solid to the liquid state. Preferably, each pipe in turn is opened by a control valve and chilled coolant flows through the pipe until the phase change material reverts back to the solid state. Then, the control valve is again closed, and the phase change material is further heated by waste heat so that it is melted once more. Preferably, the control valve for only one pipe at a time is opened, so that latent heat is being absorbed by the phase change material around all pipes but one.