Book - School of Science and Technology

Refrigeration plant components 551Water inletMotorOpen waterdistribution troughEliminator platesTower fillFanAir inletFigure 19.16 Low silhouette cross-flow cooling tower (Heenan Marley)Fan draught systems are more commonly used owing to the compact space into whichthey may be fitted. Where possible they are placed on the roof, but if this is impracticablethey may be used indoors, or in a basement, with ducted connections for suction anddischarge to outside. All fan assisted coolers must be fitted with effective moistureeliminators at the air discharge to minimise the carry-over of moisture which may becontaminated.The following methods are typical:Cross-flow type, Figure 19.16. Low silhouette with the water pumped to the top of thetower and discharged over a fill material. Air is drawn in horizontally across the fill tobe discharged vertically or horizontally by a fan located at the outlet.Induced draught cooling tower, Figure 19.17. In this case the water is delivered to the topby the condenser pump, and cascades over a packing as with the natural draught type.An axial fan is arranged to draw air upwards over the slats at high velocity so that amuch reduced area of contact is required.Forced draught cooling tower, Figure 19.18. This type is similar in function to the induceddraught type except that the fan, normally centrifugal, discharges air into the towerat low level pressurising the shell slightly and forcing the air upwards through thepacking.Film cooling tower. Similar to the induced draught type; in this case the water is notsprayed or broken up in any way, but is allowed to fall from top header troughs downslats arranged in egg-crate form. The water remains as a film on the surface of the slats.Higher air velocities than usual are permissible without riskof carrying over of water,and yet air resistance is low due to the open nature of the surfaces.Closed circuit water coolers, Figure 19.19. In this case the condenser water is notevaporated but circulated through tube bundles which act, as it were, as the packingwithin the tower. In most other respects such coolers resemble induced draught orforced draught units. An independent water supply is required for the local evaporationcircuit which is pump-recirculated over the tube bundles. Such coolers are useful forapplication to heat recovery systems where fouling of condenser water must be avoided;