Humidification processes Humidity

Since 1904, maintaining the climatic status quo in the tomb of Nefertari has proved difficult, as has been described in previous sections. In addition to the humidification processes occurring in the tomb, well-meaning plaster repairs are a major source of additional water. Threatened murals have been fortified periodically with new plasters, the most recent repairs having been made in 1983. By March 1984, the humidity had increased considerably to an estimated 50 10% RH compared with 31 1% RH in December 1977 (28) and 16 1% RH in March 1981. 

The relative humidity of air at 27"C and 1 atm is increased from 30 percent to 90 percent during a humidification process at constant lemperntuie and pressure. Determine the percent enor involved In assuming the density of air to have remained constant Answer 2.1 percent... 

The difference between these two figures, that is, 0.0557 — 0.0392 = 0.0165kg s is an estimate of the rate at which water must enter the cell. Some of this water would have been in the air anyway, while the rest is added by the humidification process. If the humidity of the entry air is about 70%, then it can be shown that approximately two-thirds of the water is added via humidification. This example illustrates the beneficial effect of... 

Tray or shelf dryers are commonly used for granular materials and for individual articles. The material is placed on a series of trays which may be heated from below by steam coils and drying is carried out by the circulation of air over the material. In some cases, the air is heated and then passed once through the oven, although, in the majority of dryers, some recirculation of air takes place, and the air is reheated before it is passed over each shelf. As air is passed over the wet material, both its temperature and its humidity change. This process of air humidification is discussed in Volume 1, Chapter 13. 

Choose an inlet air temperature that is high enough to negate weather effects (outside air humidity or inside room conditions). The air temperature should not be detrimental to the product being granulated. (To achieve consistent process year round, a dehumidification/humidification system is necessary, which provides the process air with constant dew point and, hence, constant drying capacity.)... 

The use of these relationships in constructing and applying humidity charts is best illustrated by examining a simplified case, that of adiabatic cooling or humidification. Figure 5.4 illustrates this process between air and water that is recycled through the cooling tower. In this operation air is both cooled and... 

This author does not favor the use of static charge meters because their use is subject to operator diligence. It is a safer procedure to avoid the conditions which promote static charge accumulation as a matter of routine. Low ambient humidity is the chief contributing factor in the accumulation of hazardous static charges. In many desert areas (which seem to be favorites for ordnance work), the daytime ambient humidity is below 20% and in temperate regions, winter-time air, and indoor air, has likewise a low humidity. For this reason, also, air conditioned air, in the absence of artificial humidification, should be strictly controlled. In order to maintain a relatively safe humidity above 50% RH, the use of evaporative coolers is helpful in a desert environment. The matter of humidity control in pyrot processing plants was recently considered by Avrami et al (Refs 55 56) Miscellaneous Laboratory Hazards... 

Define the dry-bulb temperature, wet-bulb temperature, and humid volume of humid air. Given values of any two of the variables plotted on the psychrometric chart (dry-buib and wet-bulb temperatures, absolute and relative humidity, dew point, humid volume), determine the remaining variable values and the specific enthalpy of the humid air. Use the psychrometric chart to carry out material and energy balance calculations on a heating, cooling, humidification, or dehumidification process involving air and water at 1 atm. 

On a psychrometric chart (or humidity chart) several properties of a gas-vapor mixture are cross-plotted, providing a concise compilation of a large quantity of physical property data. The most common of these charts—that for the air-water system at 1 atm—is used extensively in the analysis of humidification, drying, and air-conditioning processes. 

Psychrometric charts are plots of humidity, temperature, enthalpy, and other useful parameters of a gas-vapor mixture. They are helpful for rapid estimates of conditions and for visuahzation of process operations such as humidification and drying. They apply to a given system at a given pressure, the most common of course being air-water at atmospheric pressure. There are four types, of which the Grosvenor and Mollier types are most widely used ... 

To illustrate the concept of simultaneous mass and heat transfer, consider the following air humidification problem. Water flows down the inside wall of a 25.4-mm-diameter wetted-wall tower of the design of Figure 2.4, while air flows upward through the core. At a point in the tower, humid air flows at a mass velocity of 10.0 kg/m2-s. The temperature of the gaseous mixture is 308 K, the pressure is 1 atm, and the partial pressure of water vapor in the mixture is 1.95 kPa. Assuming that the process is virtually adiabatic, estimate the temperature of the liquid water at that point, and the rate of water evaporation. 

Air supply subsystem, including blower or compressor, air humidification, heat exchanger and pressure control devices to deliver process air at the required mass flow, pressure temperature and humidity to the fuel cell stack. Separation of water from the used air stream and, in some cases, an expander add more complexity to the air supply subsystem ... 

Humidification [4], Humidification is an operation directed at increasing the amount of vapor present in a gas stream. The vapor may be increased by passing the gas through a liquid that evaporates into the gas. The transfer process through the gas stream takes place by diffusion at the interface. In a certain process, it is necessary to increase the humidity of a gas stream, (a) How much water is required to increase the humidity of 100 kg of a gas having an original humidity of 1 % (w/w, wet basis) until it reaches a humidity of 2 % (w/w, wet basis) (b) What is the mass of the final gas stream with 2 % humidity after the process ... 

Humidification [6], In a certain process, it is necessary to increase the humidity of a gas stream, (a) How much water is required to increase the humidity of A kg of a gas stream having an... 

Benicewicz s group also prepared a modified PPA process for PA-doped PFCB-PBI membrane (Figure 5.7) preparation. In the modified PPA process, polymerization of PFCB-PBI takes place at 140 C. To control the concentration of final polymer, a fixed amount of PA was added at the end of the reaction, then the temperature was increased to 200 C for 24 h. Hydrolysis of PPA to PA took place under controlled conditions (25 C and relative humidity of 55% for 24 h), and at the same time, membranes were formed [37]. At room temperature, proton conductivity of the PFCB-PBI membrane was 0.01 S/cm, and at 140 C, it was 0.12 S/cm without humidification. Although PA doping levels ( 50 PA/RU) were higher, its proton conductivity was lower compared to the other PBI membranes prepared by the PPA process (para-PBl, -0.20 S/cm, -40 PA/RU at 140 C) [13]. The maximum power densities of fuel cells with PA-doped PFCB-PBI membranes were 0.21 W/cm (H2/O2) and 0.15W/cm (H2/air) at 140 C. It was lower compared to para-PBl membranes prepared by the PPA process (maximum power density reached 0.9W/cm at approximately 2.0A/cm, 160 C, atmospheric pressure, H2/O2) [37]. 

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. 

The occupier of a humid factory is required to inform the factory inspector in writing when artificial humidity is first produced (s. 68). Every room in which artificial humidity is produced should have two hygrometers and a humidity table. The readings of the hygrometers shall be taken at certain times of the day and entered on the prescribed form, F48. Artificial humidification is not permitted in a room in which the wet bulb thermometer exceeds 22.5 C (72.5 F) or for certain processes 26.5 C (80 F) or when the difference between the readings of the dry and wet bulb thermometers is less than that indicated in the humidity table. No water which is liable to cause injury to health shall be used for artificial humidification. 

The different water movements are shown in Figure 4.10. Fortunately, all these water movements are predictable and controllable. Starting from the top of Figure 4.10, the water production and the water drag are both directly proportional to the current. The water evaporation can be predicted with care, using the theory outlined below in Section 4.4.2. The back diffusion of water from cathode to anode depends on the thickness of the electrolyte membrane and the relative humidity of each side. Finally, if external humidification of the reactant gases is used prior to entry into the fuel cell, this is a process that can be controlled. 

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