Book - School of Science and Technology

500 Fans and air treatment equipmentChilled wateroutletFrameChilled waterinletVertical finsFigure 17.27 Cooling coil (six-row arrangement)In a chilled water system, water is circulated through the coil, and this may be useddown to about 3.3 C. Below this an anti-freezing mixture such as calcium chloride brinebecomes necessary.In cooling by refrigeration, the higher the evaporator temperature the less power isconsumed for a given duty, hence it is desirable to keep the cooling coil temperature ashigh as possible consistent with the final air temperature required to meet design conditions.This applies whether the coil is used for direct expansion or with chilled water orbrine.A cooling coil surface is therefore usually designed for small temperature differencesbetween water and air; for instance, for air cooled from 24 Cto13 C, water may be at10 C inlet and 14.5 C outlet (i.e. leaving above the air-outlet temperature). An air washercannot give a performance comparable to this. Coil face velocities are limited to 2.5 m/s;above 2.25 m/s eliminator plates are fitted to reduce moisture carryover on coils performinga dehumidification function.Coolers are generally of finned or blocktype (as in Figure 17.27), comprising banks ofsmall bore tubes threaded through plates between which the air passes, the whole being ofcopper which may be tinned after fabrication or, more normally in present day commercialpractice, made up with copper tubes and aluminium fins, although the life of this formwill be less than the all copper alternative. For applications where coils are to be watersprayed, as discussed later, they must of course be all copper and preferably be tinned.Coils are frequently arranged horizontally, with fins vertical, so as to facilitate drainage ofcondensation when dehumidifying. A drain pan should be provided and be gradedtowards a bottom outlet to prevent stagnant water. The connecting drain pipe shouldhave a `U' trap to maintain a water seal to prevent loss or ingress of air (dependingwhether the coil is on the suction or discharge side of the fan), see Figure 17.28. The pipeshould run to drain via an air gap. It is also a requirement that in certain applications, forexample operating theatres, the drain pan is removable for cleaning purposes. Air handlingunit access for drain pan removal must also be provided.Sprayed cooling coilsIn this application a cooling coil, arranged as normal for a water circulation through thetube internals, is mounted over a water tankand has headers and spray nozzles fitted,facing the upstream face. The nozzles are supplied at low pressure with water drawn by a