Evaporative Humidifier

The research and development outlined in this monograph was carried out in close collaboration with industry and a wide range of products were tested, for feedback visit the Web site http //www.simsoftks.com. The case studies which follow demonstrate the industrial potential of this methodology the first one deals with the assembly planning of an evaporative humidifier as shown in Fig. 7.1 and the second one deals with the costing of a moulding tool. 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

Acid cleaners based on sulfamic acid are used in a large variety of appHcations, eg, air-conditioning systems marine equipment, including salt water stills wells (water, oil, and gas) household equipment, eg, copper-ware, steam irons, humidifiers, dishwashers, toilet bowls, and brick and other masonry tartar removal of false teeth (50) dairy equipment, eg, pasteurizers, evaporators, preheaters, and storage tanks industrial boilers, condensers, heat exchangers, and preheaters food-processing equipment brewery equipment (see Beer) sugar evaporators and paper-mill equipment (see also Evaporation Metal surface treati nts Pulp). 

Evaporation of medium provides a little cooling. The inlet air to particulate filters must not be near saturation because condensation of moisture on the filter medium mobihzes contaminating microorganisms so that their chances of penetration are greatly increased. Sometimes humidified air is used, and the filter unit is heated to prevent condensation. However, this is common only for small equipment where the extra operations are relatively easy to install and maintain. 

The analysis of the different cycles examined here, which range from the simplest cycle such as evaporative cooling to the more complex cycles such as the humidified and heated compressed air cycle, are rated to their effectiveness and to their cost is shown in Table 2-1. The cycles examined here have been used in actual operation of major power plants, thus there are no cycles evaluated that are only conceptual in nature. The results show addition from 3-21% in power and the increase in efficiency from 0.4-24%... 

Humidification efficiency The ratio of the actual mass of water evaporated by a humidifier to the theoretical mass of water needed to achieve saturation at a given temperature. 

Water is injected into the air stream in a fine mist by pumped jets or spinning disc. For practical purposes, the psychrometric plot follows a wet bulb line. The air provides the latent heat of evaporation, resulting in a fall in dry bulb temperature. If water were to be supplied at up to 100°C the humidified condition would be at a correspondingly higher total heat of 420 kJ per kg water supplied. 

Where dust control is important the system should only be used with a supply of demineralized water to avoid solids being passed into the conditioned space. The temperature of the air must be sufficient to hold the quantity of moisture being supplied, any excess being deposited in the duct. Unless drained away, this can give rise to corrosion and to incorrect control by re-evaporation when the humidifier is switched off under control. 

Where water is evaporated from a circuit, such as a cooling tower, evaporative condenser, or humidifier, it must be remembered that there is no way of avoiding a steady bleed-off or frequent flushing, to restrict the concentration of dissolved solids. Much trouble arises from the efforts of well-wishing but misguided persons who stop the flow of bleed-off to save water . 

On the large scale, air and water are brought into countercurrent contact in a cooling tower which may employ either natural draught or mechanical draught. The water flows down over a series of wooden slats which give a large interfacial area and promote turbulence in the liquid. The air is humidified and heated as i( rises, while the water is cooled mainly by evaporation. 

Some workers have attempted to base the design of humidifiers on the overall heat transfer coefficient between the liquid and gas phases. This treatment is not satisfactory since the quantities of heat transferred through the liquid and through the gas are not the same, as some of the heat is utilised in effecting evaporation at the interface. In fact, at the bottom of a tall tower, the transfer of heat in both the liquid and the gas phases may be towards the interface, as already indicated. A further objection to the use of overall coefficients is that the Lewis relation may be applied only to the heat and mass transfer coefficients in the gas phase. 

Behavioral and environmental modifications may significantly improve dry eye, especially in mild cases. Evaluate the patient s environment for air drafts. Consider adding a humidifier in low-humidity areas. Schedule regular breaks from computer work or reading. Lower the computer screen to below eye level to decrease lid aperture. Evaluate medication use and make therapeutic substitutions to medications that do not exacerbate dry eye. Spectacle sideshields or goggles may reduce tear evaporation.30... 

Humidification can be done at the system level by having humidifier section at the air-handling unit supplying fresh air. The humidifier can be electric or gas type. Another method is to use evaporative pads such as GLASdek from Munters. This eliminates the use of gas or electricity for humidification. 

Looking at open sorption system as an indirect heat storage, the system would look like Figure 250. The charging process is driven by the district heat input. While the district heat return flow, the condensate, is providing the low temperature heat input to the humidifier for the evaporation. 

The cold recovery device consists of an exhaust air humidifier with an integrated heat exchanger and the supply air heat exchanger, which are connected by a fluid circuit. The first can be described as an indirect evaporative cooler. The cold recovery device is able to transport 83% of the maximum possible enthalpy difference from the exhaust air to the supply air. 

Figure 254. Heat exchanger and humidifier of the supply air and the indirect evaporative cooler of the exhaust air...

EDTA titrations are routinely used to determine water hardness in a laboratory. Raw well water samples can have a significant quantity of dissolved minerals that contribute to a variety of problems associated with the use of such water. These minerals consist chiefly of calcium and magnesium carbonates, sulfates, etc. The problems that arise are mostly a result of heating or boiling the water over a period of time such that the water is evaporated, and the calcium and magnesium salts become concentrated and precipitate in the form of a scale on the walls of the container, hence the term hardness. This kind of problem is evident in boilers, domestic and commercial water heaters, humidifiers, tea kettles, and the like. 

It was apparent from the very earliest tests that control of thin moisture films on the surface of reactive particles was the key to success. The main three competing arrangements, as compared by Statnick et al. [4th Annual Pitt. Coal Conf. 1987)] involved slurry spray dryers, where lime and water were injected together, versus systems where the gas was humidified by water injection before or after injection of limey dry powder reagents. It turns out that there are tradeoffs among the costs of hardware, reagent, and water dispersion and reagent purchase and disposal. Systems where water evaporates in the presence of active particles are usually less expensive overall. 

A Simple example will clarify the point. Consider wintertime operation, having all inside air and heavy evaporation in order to control conditions. Contrast this with summertime operation having a large fraction of outside air with outside environmental conditions such that little humidification is required. (Furthermore, solids levels in the humidifier water tend to be higher in winter than summer, in some cases.)... 

Batra, Fornes eind Hersh (23) recently demonstrated that a significant amount of the material captured by vertical elutria-tors in the model card room at North Carolina State University arises from solids generated by the evaporation of humidifier water. Table IX shows variation in dust concentrations in the card room with and without the contribution of humidifier particulates (23). 

Humidifiers can be a significant source of airborne particles if tap water is used because as the water evaporates from the aerosol, the solids that were in water are left as particles. For example, Highsmith et al. (1988, 1992) showed that the airborne particle concentration increased linearly with the total dissolved... 

Incubate the plate overnight at 4°C m a humidified chamber to minimize evaporation. 

For the case in which the exiting gas-vapor mixture is saturated, we have the term T, Jfas, Has. The liquid enters at T and the gas is humidified by evaporation of liquid and cooled. By expansion of the humid heat terms in the equation derived for problem 3.9,... 

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