Adiabatic humidification

Figure 28 shows the key features of the humidity chart. The chart consists of the following four parameters plotted as ordinates against temperature on the abscissas (1) Humidity H, as pounds of water per pound of dry air, for air of various relative humidities (2) Specific volume, as cubic feet of dry air per pound of dry air (3) Saturated volume in units of cubic feet of saturated mixture per pound of dry air and (4) latent heat of vaporization (r) in units of Btu per pound of water vaporized. The chart also shows plotted hiunid heat (s) as abscissa versus the humidity (H) as ordinates, and adiabatic humidification curves (i.e., humidity versus temperature). Figure 28 represents mixtures of dry air and water vapor, whereby the total pressure of the mixture is taken as normal barometric. Defining the actual pressure of the water vapor in the mixture as p (in units of mm of mercury), the pressure of the dry air is simply 760 - p. The molal ratio of water vapor to air is p/(760-p), and hence the mass ratio is ... 

FIGURE 4.12 The state change of air in adiabatic humidification. A is the initial state and B is the saturated final I state. 

Eable 4.7)Draw in the Mollier diagram at the 14 °C point of the saturation curve (a) the state change line of the adiabatic humidification and (b) an auxiliary line, associated with the wet bulb temperature measurement, by means of which the state can be defined. The pressure of air is p = 1 bar. 

Comparing this value with the a) and b) point results ot Example 7, we discover that the line of constant enthalpy lies between the determination line of wet bulb temperature and the adiabatic humidification line. The nearer the Lewis number is to 1, the nearer the wet bulb temperature is to the adiabatic humidification temperature. 

The application of open sorption systems can provide dehumidification by the adsorption of water vapor and sensible cooling by adiabatic humidification (after a cold recovery for the dried air) at temperatures between 16 °C and 18 °C. Conventional systems have to reach temperature as low as 6 °C or lower in order to start dehumidification by condensation. For comfort reasons this cold air has to be heated up to about 18 °C before released into the building. This shows that open sorption systems can provide in general an energetically preferable solution. 

This result (which is far from obvious) allows us to perform adiabatic cooling calculations with relative ease using the psychrometric chart. First locate the initial state of the air on the chart then locate the final state on the constant wet-bulb temperature line that passes through the initial state (or on the 100% humidity curve if cooling below the adiabatic saturation temperature takes place) and finally perform whatever material and energy balance calculations are required. Example 8.4-7 illustrates such a calculation for an adiabatic humidification operation. 

The outlet temperature and humidity determined in this manner are represented by a point on the psychrometric chart. If a lower value of Ts is assumed, a higher outlet humidity would be calculated, yielding another point on the chart. The set of ail such points for a specified and m i th defines a curve on the chart, known as the adiabatic saturation curve. If the three stated assumptions are valid, the final state of air undergoing an adiabatic humidification must lie on the adiabatic saturation curve that passes through the inlet state on the psychrometric chart. 

Air at 26°C with a relative humidity of 10% undergoes an adiabatic humidification. Use... 

Figure 4.25 Adiabatic humidification with recycle of water.

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... 

Application of HTU method. The HTU method is applicable to adiabatic humidification. Thus, by definition,... 

MASS TRANSFER IN DRYERS. In all dryers in which a gas is passed over or through the solids, mass must be transferred from the surface of the solid to the gas and sometimes through interior channels of the solid. The resistance to mass transfer, not heat transfer, may control the drying rate. This is most often true in cross-circulation drying of slabs, sheets, or beds of solids. From the standpoint of the gas, this kind of drying is much like adiabatic humidification from that of the solid it is like evaporation when the solid is very wet and like solvent desorption from an adsorbent when the solid is nearly dry. 

Equation (9.3-11) is the equation of an adiabatic humidification curve when plotted... 

In Chapter 9 the fundamentals of humidity and adiabatic humidification were discussed. In this section the performance and design of continuous air-water contactors is considered. The emphasis is on cooling of water, since this is the most important type of process in the process industries. There are many cases in industry in which warm water is discharged from heat exchangers and condensers when it would be more economical to cool and reuse it than to discard it. 

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 ... 

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