Humidifier, adiabatic

If an unsaturated gas is brought into contact with a liquid which is at the adiabatic saturation temperature of the gas, a simultaneous transfer of heat and mass takes place. The temperature of the gas falls and its humidity increases (Figure 13.2). The temperature of the liquid at any instant tends to change and approach the wet-bulb temperature corresponding to the particular condition of the gas at that moment. For a liquid other than water, the adiabatic saturation temperature is less than the wet-bulb temperature and therefore in the initial stages, the temperature of the liquid rises. As the gas becomes humidified, however, its wet-bulb temperature falls and consequently the temperature to... 

In a humidifier in which the make-up liquid is only a small proportion of the total liquid circulating, its temperature approaches the adiabatic saturation temperature 0S, and remains constant, so that there is no temperature gradient in the liquid. The gas in contact with the liquid surface is approximately saturated and has a humidity Jf... 

Further, the relation between the temperature and the humidity of the gas at any stage in the adiabatic humidifier is given by ... 

Calculations involving to systems where the Lewis relation is not applicable are very much more complicated because the adiabatic saturation temperature and the wet-bulb temperature do not coincide. Thus the significance of the adiabatic cooling lines on the psychrometric chart is very much restricted. They no longer represent the changes which take place in a gas as it is humidified by contact with liquid initially at the adiabatic saturation temperature of the gas, but simply give the compositions of all gases with the same adiabatic saturation temperature. 

Another type of process of some importance occurs when adiabatic cooling or humidification takes place between air and water that is recycled as illustrated in Fig. 4.25. In this process the air is both cooled and humidified (its water content rises) while a little bit of the recirculated water is evaporated. At equilibrium, in the steady state, the temperature of the air is the same as the temperature of the water, and the exit air is saturated at this temperature. By making an overall energy balance around the process Q = 0), we can obtain the equation for the adiabatic cooling of the air. 

ADIABATIC SATURATOR. Water is often sprayed into a stream of gas in a pipe or spray chamber to bring the gas to saturation. The pipe or chamber is insulated so that the process is adiabatic. The gas, with an initial humidity and temperature T, is cooled and humidified. If not all the water evaporates and there is sufficient time for the gas to come to equilibrium with the water, the exit... 

HUMIDIFIERS AND DEHUMIDIFIERS. Gas-liquid contacts are used not only for liquid cooling but also for humidifying or dehumidifying the gas. In a humidifier liquid is sprayed into warm unsaturated gas, and sensible-heat and mass transfer take place in the manner described in the discussion of the adiabatic-saturation temperature. The gas is htunidified and cooled adiabatically. It is not necessary that final equilibrium be reached, and the gas may leave the spray chamber at less than full saturation. 

The interaction between unsaturated gas and liquid at the wet-bulb temperature of the gas has been discussed under the description of wet and dry-bulb thermometry. The process has been shown to be controlled by the flow of heat and the diffusion of vapor through the gas at the interface between the gas and the liquid. Although these factors are sufficient for the discussion of the adiabatic humidifier, where the liquid is at constant temperature, in the case of dehumidifiers and liquid coolers, where the liquid is changing temperature, it is necessary to consider heat flow in the liquid phase also. 

In an adiabatic humidifier, where the liquid remains at a constant adiabatic-saturation temperature, there is no temperature gradient through the liquid. In dehumidification and in liquid cooling, however, where the temperature of the liquid is changing, sensible heat flows into or from the liquid, and a temperature gradient is thereby set up. This introduces a liquid-phase resistance to the flow of heat. On the other hand, there can be no liquid-phase resistance to mass transfer in any case, since there can be no concentration gradient in a pure liquid. 

The simplest case, that of adiabatic humidification with the liquid at constant temperature, is shown diagrammatically in Fig. 23.6tz. The latent-heat flow from liquid to gas just balances the sensible-heat flow from gas to liquid, and there is no temperature gradient in the liquid. The gas temperature Ty must be higher than the interface temperature 7) in order that sensible heat may flow to the interface and 2 must be greater than X in order that the gas be humidified. 

ADIABATIC HUMIDIFICATION. Adiabatic humidification is similar to adiabatic saturation except that the air leaving the humidifier is not necessarily saturated and, for design, rate equations must be used to calculate the size of the contact zone. The inlet and outlet water temperatures are equal. It is assumed in the following that the makeup water enters at adiabatic-saturation temperature and that the volumetric-area factors and are identical. The wet-bulb and adiabatic-saturation temperatures are equal and constant. Then... 

An air stream at 87.8 C having a humidity H = 0.030 kgH20/kg dry air is contacted in an adiabatic saturator with water. It is cooled and humidified to 90% saturation. 

This is the equation., of a curve on the psychtomelric hart, the adiabatic-saturation curve t which passes through the points (>X, t ) on the 100 percent saturation curve and (T,, i i)- Since the humid heat contains the term T,, the curve is not straight but slightly concave upward. For any vapor-gas mixture there is an adiabatic-saturation temperature t such that if contacted with liquid at tg, the gas will become humidified and cooled. If sufficient contact time is available, the gas will become saturated at (y, / ) but otherwise will leave unsatuxated at (Y y /g2)> point on the adiabatic-saturation curve for the initial mixture. Eventually, as Eq. (7.20) indicates, the sensible heat given up by the gas in cooling equals the latent heat required to evaporate the added vapor. 

IliustradoD 7.13 A horizonlal spray chamber (Fig. 7.19) with recirculated water is used for adiabatic humidification and cooling of air. The active part of the chamber is 1 m long and has cross section of 2 m. With an air rate 3.5 mV at dry-bulb temperature 65.0 C, Y 0.0170 kg water/kg dry air, the air is cooled and humidified to a dry-bulb temperature 42.0 C. If a duplicate spray chamber operated in the same manner were to be added in series with the existing chamlUr, what outlet conditions could be expected for the air ... 

Before addressing the properties and construction of the humidity charts, we consider a small variation in the simple contact of water with flowing air, which led to the wet-bulb conditions. In this modified arrangement, shown in Figure 9.2, a stream of air is humidified in contact with constantly recirculated water. Both the water and the exiting gas stream attain adiabatic saturation temperature, which is lower than the dry-bulb temperature of the entering air because of evaporative cooling. 

Without close approach is defined as operation at a humidifier outlet temperature sufficient to maintain ESP performance, typically 250° to 27S°F. Close approach " is defined as a 20°F approach to adiabatic saturation (approximately J25°F) measured at the humidifier outlet for the coals used or 14S°F. The humidifier is located between the air heater and the ESP. 

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