Humidification examples

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. 

In direct contact heal exchange, there is no wall to separate hot and cold streams, and high rales of heal transfer are achieved. Applications include reactor off-gas quenching, vacuum condensers, desuperheating, and humidification. Water-cooling lowers are a particular example of a direct contact heal e.xchanger. In direct contact cooler-condensers, the condensed liquid is frequently used as the coolant. 

Since complete humidification is not always attained, an allowance must be made when designing air humidification cycles. For example, if only 95 per cent saturation is attained the adiabatic cooling line should be followed only to the point corresponding to that degree of saturation, and therefore the gas must be heated to a slightly higher temperature before adiabatic cooling is commenced. 

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.)... 

Figure 5.42 shows an example of the effect of humidity on the EIS spectra. It can be seen that the cut-off for anode humidification does not affect the spectra too much compared with the cut-off for cathode humidification. As we know, the fuel cell EIS primarily represents the cathode behaviour. Therefore, cathode humidification can greatly affect the whole impedance spectra. The humidification cut-off at the cathode causes a large difference in both the membrane resistance and the kinetic resistance. Dehydration of the anode also brings about a substantial increase in cathode impedance because a dry anode pulls water away from the cathode and across the membrane, which makes it hard to keep the cathode well hydrated [18],... 

There are historic reports of floods in the area (32 J. Rutherford, personal communication) and of an earthquake in 18 B.C. (33). The movement of the expansive Esna shale bed formation that underlies the whole area is also a source of disruption (32). The Esna shale bed formation, which contains montmorillonite clay, swells upon hydration by about 12.5% (32). Therefore, if an adequate amount of water is available, for example from increased irrigation, humidification, or flooding, the movement of this shale formation accelerates. Collectively, these dramatic natural events are the most probable causes of the loss of plaster and painted murals in the lower chamber of the tomb prior to its discovery, and they increasingly (32) threaten tombs in the Thebes area. 

Dehumidification is the removal of a solute liquid vapor from a gas phase by the solute condensing into its liquid phase. The removal of water vapor in air by condensation on a cold surface is dehumidification. The reverse of dehumidification is humidification. In this unit operation, the flow of the solute is from the liquid phase evaporating into the gas phase. The end result of this movement is saturation of the gas. For example, during heavy rains, the atmosphere may become saturated with water vapor, the degree of this saturation being measured by the relative humidity. Liquid extraction is the removal of a solute component from a liquid mixture called the raffinate using a liquid solvent. In this operation, the solvent preferentially dissolves the solute molecule to be extracted. 

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. 

Practical modes of water management include the supply of water at the anode side by humidification of the anodic feed gas, or the application of a pressure across the membrane that creates the backflow from cathode to anode (anode water removal). Different approaches of effective fuel cell water management are discussed, for example, in Ref. 45. 

Water management can be most simply achieved by providing the gas feed humidification level required to maintain the conductivity of the fuel-cell membrane and of the ionomers in the catalyst layers. Gas feed humidification has been achieved by a variety of methods including, for example, enthalpy exchangers [6] and porous bipolar plates [62]. The two latter approaches rely on utilization of stack-produced water, thereby eliminating the need of frequent water refill . The system in Fig. 29a, can use a condenser to... 

A possibility is to saturate at different temperatures the reactants before they enter into the stack [33]. This approach can be accomplished by several procedures based on external dewpoint, external evaporation, steam injection with downstream condensers, or flash evaporation. High temperature values allow to absorb significant water amount in gas streams and then transport it inside the stack compensating the water losses due to internal fast evaporation. However, the main problem with external humidification is that the gas cools after the humidifier device, the excess of water could condense and enter the fuel cell in droplet form, which floods the electrodes near the inlet, thereby preventing the flow of reactants. On the other hand, internal liquid injection method appears preferable for example with respect to the steam injection approach because of the need of large energy requirement to generate the steam. 

When essentially all of the resistance is on the gas side of the interface, kx ky, and ky or K x kyKi. A well-known example of pure gas-phase resistance is the humidification of dry air with pure water. Because there is no concentration gradient in the liquid, the interface concentration equals the vapor pressure of water at the temperature under consideration. Thus the only driving force is where = partial pressure of water in the air, equivalent to the humidity. Many... 

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