Dehumidification

Abstract

Humidification/dehumidification (H/D) is in fact a small to medium capacity desalination method driven by solar energy or waste heat. In this chapter, initially a short historical introduction is given on how this method is derived from solar distillation procedure by using separate devices (champers or towers) for evaporation and condensation processes in contrary to solar distillation where the two operations take place in only one chamber. The basic operating principles of the two configurations, such as closed water–open air and closed air–open water of the single H/D, are presented. The majority of all H/D concepts are driven by solar energy and/or by waste heat.

As these H/D systems are based on H/D principles in separate chambers, a short description of Mollier (h–x diagrams) and psychrometric charts, used in drying, with chart examples is presented to facilitate calculations of H/D in the corresponding chambers. The main relationships of mass and heat transfer and mathematical models of the single effect or multiple effect configurations for the humidifier (evaporator) and the condenser (dehumidifier) are given. The corresponding temperature–enthalpy diagrams for the two chambers are presented as well as examples for calculations. The dewvaporation procedure having the two chambers in contact, leaving the vapor to be transferred through a separating wall, is included.

Multiple effect H/D offers the possibility for air to be loaded by reheating with as much as possible vapor in the humidification chamber. Mathematical models of this H/D concept including Mollier charts of humidity calculations are also presented.

Various other concepts of operation that exist, such as the desiccant absorption/desorption H/D process using zeolite or lithium bromide as vapor absorbent, the membrane drying dehumidification using selective membranes, varied H/D cycle, and the thermocompression method, are shortly described. At the end of this chapter, the thermoeconomics of this method is described.

Greenhouses are important in dry regions which have intensive solar radiation and no usable water resources. The combination of solar-driven H/D method and solar stills with greenhouses to supply the necessary humidity to the greenhouse plants gives a good solution to produce simultaneously fresh water and crops in dessert lands. A short description of this combination and figures of installations is presented.