The Lewis factor and its influence on the performance prediction of wet-cooling towers

The effect of the Lewis factor, or Lewis relation, on the performance prediction of natural draft and mechanical draft wet-cooling towers is investigated. The Lewis factor relates the relative rates of heat and mass transfer in wet-cooling towers. The history and development of the Lewis factor and its application in wet-cooling tower heat and mass transfer analyses are discussed. The relation of the Lewis factor to the Lewis number is also investigated. The influence of the Lewis factor on the prediction of wet-cooling tower performance is subsequently investigated. The Poppe heat and mass transfer analysis of evaporative cooling are considered as the Lewis factor can be explicitly specified. It is found that if the same definition or value of the Lewis factor is employed in the fill test analysis and in the subsequent cooling tower performance analysis, the water outlet temperature will be accurately predicted. The amount of water that evaporates, however, is a function of the actual value of the Lewis factor. If the inlet ambient air temperature is relatively high, the influence of the Lewis factor, on tower performance diminishes. It is very important, in the view of the Lewis factor that any cooling tower fill test be conducted under conditions that are as close as possible to the conditions specified for cooling tower operating conditions.