Water evaporation rate

Calculating the heat transfer and water evaporation rates are illustrated by the following example. A cooling tower eools 900 gpm of water from 95 to 85 F. The problem is to determine what the heat rejeetion is, and also what is the evaporation rate. The heat rejeetion is ealeulated as follows ... 

Table 20.1 Various Empirical Relationships Between Wind Velocity Measured at Heigth z Above the Water, u7, and Air-Phase Transfer Velocity of Water , va, Deduced from Observations of Water Evaporation Rates b...

The simulations of volatilization were conducted using the complete model described by Jury et al. (33) where each chemical is present in the soil at a uniform concentration of 1 kg/ha to a depth, L, in the soil and is allowed to volatilize through a stagnant air boundary layer for a specified time period in the presence or absence of water evaporation. The standard conditions or common properties assumed in the simulations are the same as those indicated in Jury et al. (35, 36), i.e., air diffusion coefficient, 0.43 m /d water diffusion coefficient, 4.3 X 10-3 m /d atmospheric relative humidity, 50% temperature, 25°C soil porosity, 50% bulk density, 1.35 g/cm3 soil water content, 0.30 organic carbon fraction, 0.0125 amount of pesticide in soil, 1 kg/ha depth in soil, 1 or 10 cm water evaporation rate, 0, 0.25, or 0.50 cm/d. 

Table II shows the calculated cumulative volatilization after 10 days as affected by depth of pesticides in soil (L) and water evaporation rate (E) expressed as a percent of the 1 kg/ha initially present in the soil at t = 0. Soil water content (0) was assumed to be equal to 0.30, and the organic carbon content of the soil (foe) equal to 0.0125. The volatilization rates shown in Table II are for the ideal conditions and high water evaporation rates assumed in the simulations. They are undoubtedly the upper limits of volatilization to be expected from forest soils. Volatilization was increased greatly by evaporating water, particularly for the compounds with low Kjj values and increasing soil depth decreased volatilization.

Figure 4. Calculated volatilization flux versus time for selected forest pesticides as affected by depth (L) within the soil at water evaporation rate E = 0.25 cm/day.

The mechanism of deviation of the ratio from a constant value for the more volatile substances has already been indicated. For these substances the heat demand slows evaporation. This effect is greater when the high wind blows on the evaporating surface because the heat exchange through the base of the dish is relatively less important. The water evaporation rate has been corrected, in forming the product in the last column, for the ambient air being at 30% relative humidity (RH). 

In this problem, a standard pan is used as shown in Fig. 11-6. The mean wind movement is measured 6 in above the pan rim, and water-evaporation rates are measured with the pan placed on the ground (land pan) or in a body of water (floating pan). For the land pan and with a convectively stable atmosphere, the evaporation rate has been correlated experimentally [13] as... 

Rocklin and Bonner (65) developed a computer method that predicts solvent balance and evaporation times of water-solvent blends at any humidity with any number of water-soluble organic solvents. The method also can be used for regular water-free solvent blends but Ignores humidity. Key considerations of the method are the following it uses the UNIFAC method for calculating activity coefficients it computes the actual evaporation temperature on the filter paper substrate it calculates evaporation rates at the calculated temperature by using the activity coefficients at that temperature humidity is accommodated by applying a correction factor to the water evaporation rate. Experimental data on several systems verified the computer calculations. 

Figure 18.5 Rate-limiting processes in film formation (a) separation of film formation into water ev oration and particle deformation and compaction components, (b) effect of drying conditions on film formation. Bold dashed lines reference rates of processes in (a). Bold solid lines boundary of the limiting film-formation processes at low and high temperature. Light dashed lines possible shifts of film-forming process rates from the reference rates by, for example, plasticization of the latex particles and/or slowing of the water evaporation rate. Points A, B, C and D represent the MFTs resulting from these variations in conditions. (Reproduced with permission from Sperry et al. [53].)...

During drying, the temperature of the drops remains below the wet bulb temperature of the drying gas. However, even if the drying time is short (some seconds) and the drop temperature low because of high water evaporation rate, some molecules of volatile aroma may migrate together with water molecules to the drop surface, where they may either disappear and/or stay unprotected at the surface. 

Water evaporation rate 3,300 g.p.m. pensate for this fall in the reproduction ratio these rods... 

Air/water Evaporation rate Solubility Vapor pressure Vapor density Dissociation constant Specific gravity Particle size... 

Radiation drying is expensive because of the high cost of electrical power or combustion gas to heat the radiators. It is mainly used for short drying times of thin product layers (e.g., lacquer layer on metal sheets [5.33]), ceramic products with thin walls, and thin paper or textile sheets. In the latter case, it is also used in combination with contact drying, if necessary. (Dark toctiles with a relatively high absorption capacity can reach water evaporation rates of ca. 30 kg/(m h) and higher. 

Dryer type Product condition Specific water evaporation rate (kg/(m h)) Specific heat requirement, water evaporation (kJ/kg) Dryer type — design data Operating mode... 

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