TABLE OF CONTENTS Large counterflow wet-cooling towers usually have a rain zone beneath the fill in which 10 20% of the total heat rejected by the tower may occur. In view of this contribution to the overall performance, knowledge of the characteristics of the rain zone is important for reliable prediction of the total performance and also to exploit the contribution of this region.
In any detailed analysis of the performance characteristics of a wet counterflow cooling tower, the transfer processes in the spray or rain zone may not be ignored. Earlier studies considered the transfer processes in the rain zone to be relatively unimportant or too complex to analyze. Rish was one of the first to include the rain zone in his comprehensive analysis of counterflow cooling towers. He ignores variations in drop sizes in the spray or rain regions above and below the fill.
Lowe and Christie derive the mass transfer and pressure drop for counterflow conditions, assuming no drop collisions or agglomeration occur. Their data is applicable to small drops only, where the drops fall at their terminal velocity in the major part of the flow field. In most real towers, large drops may not even attain their terminal velocity prior to reaching the pond.
Missimer and Bracket as well as Sedina conducted rain zone model tests. Hollands modeled the operation of a spray cooling tower mathematically. He concluded a uniformly sized drop distribution is desirable and the mean droplet size should be as small as 1 2 mm.
