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S & P Coil Products LtdHeat Pipes for Displacement VentilationHeat PipesA heat pipe is an extremely efficient thermal conductor, relying internally on the boiling and condensation of a working fluid to transfer heat along its length. By utilising the latent energy of the working fluid the internal temperature of the heat pipe remains constant while it pumps heat between areas of high and low temperature to which it is exposed externally. The requirements of displacement systems are such that the conditioning of primary supply air is most efficiently undertaken by utilising the unique features of heat pipes outlined above. Displacement VentilationThe principle advantage of this method of space ventilation lies in the reduced air volumes that are required to effectively condition the space. Relatively low volumes of primary air are introduced at low level at a temperature of 3 to 4°C below the design temperature of the space. The stable layer of cool air rises when in contact with people and/or equipment, picking up heat and moisture. This warm air collects at roof level where it is extracted or cooled by a secondary cooling system (chilled panels and beams). In order to prevent any condensation on secondary cooling equipment the primary air must be cooled to a dewpoint of approximately 12°C and supplied at a temperature of 18°C to a space maintained at 21 to 22°C. To achieve this supply condition, during the cooling season, the primary, ventilation air must be overcooled to reduce the dewpoint prior to being reheated to the supply temperature. Based on a design outside condition of 28.0/20.0°C the primary air process is as follows, based on an air delivery of 1kg/s: Outside air (28.0/20.0°C), enthalpy = 57.3kJ/kg Wrapping a heat pipe around the cooling coil in the ventilation AHU acts to precool the air prior to it reaching the cooling coil and then add this energy back into the air stream as reheat energy downstream of the cooling coil. The heat pipe effectively reduces or eliminates the load associated with overcooling while simultaneously reducing or eliminating the reheat load. The loads associated with the introduction of the heat pipe are given below along with the savings that would be accrued based on the 1kg/s of primary air. The savings can simply be multiplied by the mass flow rate for other air quantities. The processes are demonstrated on the accompanying psychrometric chart. Outside air (28.0/20.0°C), enthalpy = 57.3kJ/kg As the temperature of the outside air changes the degree of energy transferred by the heat pipe varies. In order to accurately control the temperature of the primary supply air a trimmer heater battery should be included as the final control element of the outside air AHU. Design FeaturesThe heat pipes are arranged to ‘wrap-around’ the main cooling coil and add, typically, no more than 300mm to the overall length of the AHU. Heat pipes are manufactured from copper tubes expanded into continuous aluminium fins in an identical manner to a conventional cooling coil, as such they are available in a continuous range of sizes to match exactly the dimensions of the main cooling coil. Optional material specifications are available to again match the specification of the other cooling and heating coils in the system.
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S & P Coil Products Ltd
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