Cooling Tower

When a component is transferred from a gas phase to solution in a liquid phase, the latent heat associated with the condensation is evolved. In the operations where mass transfer proceeds by equimolar counter diffusion, as in distillation or in operations where latent heat effect are small, as in liquid –liquid extraction, gas absorption in dilute solutions, and leaching, heat transfer is of minor importance as a rate controlling mechanism. In others, particularly where there is a net transfer of mass from gas phase to condensed phase or vice versa, the heat transfer rate is important. In these cases, heat transfer significantly limits the rate at which mass can be transferred. Of those operations where both heat transfer and mass transfer affect the rate, humidification and dehumidification involve two components and two phases. The liquid phase, most often water, is single component, and the gas phase consists of a non-condensable gas, usually air, in which some vapor from the liquid phase is present.

HUMIDIFICATION: INDUSTRIAL APPLICATIONS

Humidification process may be carried out to control the humidity of a space or, more usually, to cool and recover water by contacting it with low humidity air. The water that has lost heat to the atmosphere can then be reused in heat exchanger throughout a plant. Alternatively, the water could be cooled in the surface heat exchanger. The choice is one of economics, with the designer balancing the loss of cooling water inherent in the air- water contact cooler against the cost of supplying and handling the cooling source to the surface cooler and the higher cost of surface units.

HUMIDIFICATION EQUIPMENT

The most obvious form of humidification equipment is the spray chamber. Here, the contacting liquid is sprayed as a mist into the gas stream. Gas velocity is kept low so that the contact time is high so that there will be only small amount of liquid physically entrained in the gas stream. These units are usually restricted to the small-scale operations and are frequently used in humidity control of a room or plant where either humidification or dehumidification of the inlet air is required.