Book - School of Science and Technology

550 Refrigeration: water chillers and heat pumpswater by an amount which is dependent upon the quantity of the water which is circulatedthrough the condenser.The lower the temperature of the condenser water the less power will be required toproduce a given cooling effect; and it also follows, conversely, that with a given size ofplant the greater will be the amount of cooling possible.Water from a well or borehole or from the main supply will always be the coldest, theformer at 12 C and the latter at about 18 C in summer. In the case of mains supply, thequantity to be wasted, however, generally rules out this method. For instance, a 700 kWplant with power input of about 120 kW with a 10 K rise through the condenser wouldrequire a flow of (700 ‡ 120)/(10 4:2) ˆ 20 litre/s and it is likely that water chargeswould exceed the cost of current for running the compressor many times over. Analternative is river water, which, subject to Water Authority approval, may be useddirectly for condenser cooling purposes. Before such a solution is adopted, however,consideration must be given to the need for filtration, the quality of the water and whetherthe composition is aggressive to the system materials; maintenance will need to be of thehighest quality.Applications exist outside the British Isles, however, where well water is more freelyand cheaply available, which produce economical solutions. In one known instance inEurope, such well water is first passed through a pre-cooling coil integral to the airhandlingplant prior to use in the condenser. Similarly, in some parts of the Caribbean,clear sea water is available via fissures in the coral which may be pumped in quantities ofup to 1000 litre/s through specially designed condensers.However, cooling the water by evaporation is more generally adopted. Evaporativecoolers depend on the ability of water to evaporate freely when in a finely divided state,extracting the latent heat necessary for the process from the main body of water, which isthen returned, cooled, to the condenser. In the case stated above, the consumption ofwater with an evaporative cooler (assuming no loss of spray by windage) would be only820/2258 ˆ 0:36 litre/s.Evaporative coolers divide themselves into two categories, i.e. natural draught and fandraught. The former is represented by:The spray pond. In this type the water to be cooled is discharged through sprays over ashallow pond in which the water is collected and returned to the plant. To preventundue loss by windage the pond is usually surrounded by a louvred screen. Owing tothe large area needed for the spray pond system and, moreover, the consequentprobability of pollution, this type of equipment is seldom possible for air-conditioningapplications.Natural draught cooling tower. In this, water is pumped to the top of a tower whichcontains a specially designed `packing' of plastic or other material which will notsupport microbiological growth, arranged so as to split up the water stream and presentas large an area as possible to the air, which is drawn upwards due to the temperaturedifference, and by wind. The base of the tower is formed into a shallow tankto collectthe water for return to the plant. Again, owing to its size and height, this type of cooleris not frequently used for air-conditioning.Condenser coil type (evaporative condenser). Where the refrigerating machine is near thepoint where an outdoor cooler may be used, the condenser heat exchanger may bedispensed with and the refrigerant delivered to coils outside, over which water isdripped by a pump. The water collects in a tankat the base and is recirculated. Alouvred screen is usually necessary surrounding the coils.