Book - School of Science and Technology

554 Refrigeration: water chillers and heat pumpsthe higher the efficiency the closer will be the water outlet to the wet bulb temperature. Agood efficiency will give a difference of about 3 K, so that, if the external design wet bulbis taken at 20 C (refer to Table 3.11), the cooling water will be brought down to 23 C,and, with a 5 K temperature rise through the condenser, the outlet will be at 28 C. Thistemperature then, in turn, forms the basis for design of the compressor and condenser(e.g. condensing at about 37 C).Condenser water treatmentAs evaporation takes place, there is a continuous concentration of scale-forming solidswhich may build up to such a degree as to foul the condenser tubes. Similarly, theevaporative surfaces of the cooler suffer a build-up of deposit.In order to overcome this problem:. A constant bleed-off of the water is required, the rate of which may be calculated fromthe known analysis of the water, the evaporation rate and the maximum concentrationadmissible.. As is common practice, the water may be treated by a regular chemical dosage whichalso generally contains biocides to prevent micro-biological growth.In the case of the closed circuit water cooler, the condenser tubes and the tower tubebundles do not suffer internal fouling but the external surfaces of the latter will, in time,become coated with solids. The need for water treatment of the spray water thus remains.The total water loss from an evaporative cooler is the sum of evaporation loss, bleedrate and windage loss (water droplets carried from the tower by natural or by induced airmovement) which would normally be between 3% and 5% of the condenser water flowrate. It is normal for water storage equivalent to 24 hour usage to be held on site as areserve against mains failure.Packaged cooling towersWhereas past practice was to use structural containment to form the tower proper and touse purpose built internals, water distribution arrangements, etc., cooling towers are nowcommonly factory fabricated for delivery to site in a minimum number of parts. Whilstthis development reduces site works, there is a tendency to design to minimum size alsowith the result that the depth and capacity of the base tankis inadequate. This conditionmay lead to problems of overflow when the condenser pumps are stopped or, moreseriously, to the formation of a vortex at tankoutlet due to an insufficient water depth.To reduce the riskof this problem, only the minimum length of condenser cooling waterpipeworkshould be installed above the water level in the cooling tower tank, therebyreducing the quantity of water that will drain into the tankwhen the pump circulation isstopped; excessive drain-backwill cause water loss from the system via the tankoverflow.In all evaporative coolers, water loss by whatever cause is made up by `fresh' waterthrough a float valve.Heat recoveryAir-conditioned buildings offer an inherent opportunity for some form of heat recoverysystem, utilising the heat to be rejected from the condenser of a refrigeration plant. The