Book - School of Science and Technology

Choice of refrigeration plant 545Winter circuitSummer circuitsCondenserEvaporatorLoadCoolingtowerFigure 19.12 Free cooling using an evaporative cooling towerA system of dry coil free cooling may be used to advantage, in particular with systemswhich operate continuously, such as those which serve computer suites. A water andglycol mixture is circulated by a pump between a fan-assisted dry cooler, similar in formto an air cooled condenser, and a cooling coil in the air stream. Typically, such a systemwould be designed such that, when the outside air temperature fell to 5 C or below, fullcooling would be achieved by this method. As the outside air temperature rose above 5 C,cooling would be introduced progressively from a conventional refrigeration plant tosupplement the free cooling effect. A development of this principle is to use the glycol/water circuit and fan-assisted cooler in a dual mode, as a source of free cooling in winterand as a means of heat rejection from the refrigeration machine in summer. The principlesof these approaches are shown in Figure 19.13, the equipment being available in packagedform from specialist manufacturers. It is claimed that a pay-backperiod of two to threeyears may be achieved on the additional capital cost of the equipment.The most efficient form of free cooling in this context is by use of the thermosyphon*principle. This may be applied using either dry coolers or evaporative condensers, thelatter being more effective since they cool the refrigerant towards the ambient wet bulbtemperature rather than the dry bulb, as is the case with dry coolers. The refrigerant ispiped in the normal manner, via the compressor, between a purpose designed evaporatorand the condenser, which must be sited at a higher level than the remainder of the system.A by-pass facility is provided around the compressor.* Pearson, S. F., Thermosyphon Cooling. Institute of Refrigeration, Session 189±90.