Humidification and Dehumidification

Humidification and dehumidification (drying) of air are required in many commercial and industrial applications for the following reasons  

The process of air humidification is achieved by means of a device called a humidifier. 

Many different types of humidifiers are in common use the humidification process is simply achieved by adding moisture into the air to be conditioned. 

III health and associated Encourages the growth of Lowers the body s resistance to 

Electrostatic shocks Eliminated at high moisture conrenr. Increased at low moisture contciu, causing discomfort to occupants and damage to electronic components. High explosion risk. 

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Humidification and dehumidification refer to the transfer of water between a gas stream and a water stream. 

This chapter deals with the basic air-handling processes filtration of particles and gaseous substances from the supply and recirculated air, air heating and cooling, heat-recovery processes, and humidification and dehumidification. It also describes fans and ductwork. 

Figure 9.14 shows the processes of humidification and dehumidification in a skeleton psychrometric chart, while Table 9.5 explains the cycle. 

A device called an air washer is also used for humidification and dehumidification, It consists of a chamber incorporating a water spray system, a collection tank, and an eliminator section. The eliminator plates are necessary to reduce the incidence of water droplets that are carried out of the plant into the duct run. 

The system is used in humidification and dehumidification. However, grid and slat packings are more commonly used tj pes. 

For the gas cooling case in which Le = 1, we must redefine our basic equations for humidification and dehumidification developed back in Chapter 5. 

Distillation and gas absorption are the prime and most common gas-liquid mass-transfer operations. Other operations that are often performed in similar equipment include stripping (often considered part of distillation), direct-contact heat transfer, flashing, washing, humidification, and dehumidification. 

FIGURE 13 The air handler has several stages of filtration combined with heating, cooling, humidification, and dehumidification capability. (Courtesy of Air Enterprises.)... 

Besides the obvious processes of humidification and dehumidification of air for control of environment, interaction of air and water is a major aspect of the drying of wet solids and the cooling of water for process needs. Heat and mass transfer then occur simultaneously. For equilibrium under adiabatic conditions, the energy balance is... 

Humidification and dehumidification involve the transfer of material between a pure liquid phase and a fixed gas that is insoluble in the liquid. These operations are somewhat simpler than those for absorption and stripping, for when the liquid contains only one component, there are no concentration gradients and no resistance to transfer in the liquid phase. On the other hand, both heat transfer and mass transfer are important and influence one another. In previous chapters they have been treated separately here and in drying of solids (discussed in Chap. 24) they occur together, and concentration and temperature change simultaneously. 

PHASE EQUILIBRIA. In humidification and dehumidification operations the liquid phase is a single pure component. The equilibrium partial pressure of solute in the gas phase is therefore a unique function of temperature when the total pressure on the system is held constant. Also, at moderate pressures the equilibrium partial pressure is almost independent of total pressure and is virtually equal to the vapor pressure of the liquid. By Dalton s law the equilibrium partial pressure may be converted to the equilibrium mole fraction in the gas phase. Since the liquid is pure, is always unity. Equilibrium data are often presented as plots of Pg vs. temperature at a given total pressure, as shown for the system air-water at 1 atm in Fig. 23.1. The equilibrium mole fraction y is related to the saturation humidity by Eq. (23.2) thus... 

The operations which include humidification and dehumidification, gas absorption and desorption, and distillation all have in common the requirement that a gas and a liquid phase be brought into contact for the purpose of diffusional interchange between them. The equipment for gas-liquid contact can be broadly classified according to whether its principal action is to disperse the gas or the liquid, although in many devices both phases become dispersed. In principle, at least, any type of equipment satisfactory for one of these operations is suitable for the others, and the major types are indeed used for all. For this reason, the main emphasis of this chapter is on equipment for gas-liquid operations. 

Humidification involves the transfer of water from the liquid phase into a gaseous mixture of air and water vapor. Dehumidification involves the reverse transfer, whereby water vapor is transferred from the vapor state to the liquid state. Humidification and dehumidification can also refer to vapor mixtures of materials such as benzene, but most practical applications occur with water. To better understand humidity, it is first necessary to discuss the vapor pressure of water. 

In humidification and dehumidification, intimate contact between the gas phase and liquid phase is needed for large rates of mass transfer and heat transfer. The gas-phase resistance controls the rate of transfer. Spray or packed towers are used to give large interfacial areas and to promote turbulence in the gas phase. 

Mass transfer between liquids and gases depends on the vapor pressure of the components as functions of temperature. Thus appropriate selection of operating temperature and pressure allows the reverse (desorption or stripping, and dehumidification) to be performed. The purpose of absorption and stripping operations is to remove and recover the maximum amount of a particular component from a feed stream. It is most efficiently accomplished in multiple stages, as in tray or packed columns. Humidification and dehumidification arc similar in principle, but are directed toward control of an environment short of equilibrium (e.g., <100 percent humidity) for them, a single stage is ordinarily sufficient. 

Humidification and dehumidification also apply to environmental control where a certain moisture content is desired in the air. As pointed out earlier, an operation of this sort is generally conducted in a single stage, so control is actually not difficult. Yet the significance of the terms and principles is sufficiently confusing to deserve a general review and definition ... 

The Air-Water System Humidification and Dehumidification, Evaporative Cooling... 

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