Heat transfer evaporation from

An organic liquid is boiling at 340 K on the inside of a metal surface of thermal conductivity 42 W/m K and thickness 3 mm. The outside of the surface is heated by condensing steam. Assuming that the heat transfer coefficient from steam to the outer metal surface is constant at 11 kW/m2 K, irrespective of the steam temperature, find the value of the steam temperature to give a maximum rate of evaporation. 

If the liquid droplet is initially at a higher temperature than the gas dew point, then the liquid s vapor pressure would be greater at the gas-liquid interface than the partial pressure of the vapor in the gas. Under these conditions, the liquid will evaporate and water vapor molecules will diffuse into the gas stream. The latent heat needed for evaporation will first be derived from the sensible heat of the liquid drop, causing it to cool down. When the liquid temperature has dropped below the dry-bulb temperature of the gas, heat begins to flow from the gas to the liquid. The rate at which this heat transfer occurs increases as the temperature differential becomes greater. After sufficient time, the heat transfer rate from gas to liquid matches the rate of heat requirement for the evaporation. Here, the temperature of the liquid remains at some constant low value known as the wet-bulb temperature. 

On a clear night the effective radiation temperature of the sky may be taken as -70°C. Assuming that there is no wind and the convection heat-transfer coefficient from the air to the dew which has collected on the grass is 28 W/m2 °C, estimate the minimum temperature which the air must have to prevent formation of frost. Neglect evaporation of the dew, and assume that the grass is insulated... 

For the cycle, the total enthalpy change is zero. At the compressor, outside energy Wm is needed, and at the evaporator, heat transfer qm from the matter to be cooled is used to evaporate the refrigerant R-134a. 

Of the many different methods for calculating heat transfer, that from Chen [4.92] will be discussed here, as it was established with a model that is plausible in physical terms. It also has the advantage that it is not only valid for convective evaporation but also for saturated boiling. Similarly to saturated boiling, it is assumed that the heat transfer coefficient is a combination of two parts which are... 

FIGURE 6.2 Forced circulation evaporator such as used for simultaneous brine evaporation-crystallization. Mechanical movement of brine past the heat-exchange surface avoids decreased heat transfer efficiency from crystallization on this surface. Constructed of Monel or Monel-clad steel for parts contacting brine. (Reprinted from Kirk-Othmer [10], with permission.)... 

As with the paddle dryers, these are mostly considered to be indirect dryers since heat transfer is from the jacket. If the product is a sticky, pasty material one may wish to use this design. The advantage of the pan dryer is the availability of several heated agitator designs which improve the overall heat transfer rate appreciably over a simple heated jacket the reason is the same as mentioned in the previous section on paddle dryers. As mentioned earlier, venting of the dryer is necessary to remove the evaporated vapors. 

As the heat-transfer area varies during the evaporation process, the overall heat-transfer coefficient is best defined in relation to the initial drop area. By calculating the overall resistance to heat transfer directly from the temperature driving force, the total evaporation time and the total heat content of the drop, Sideman, Hirsch, and Gat (SI la) obtained a relationship between the average overall heat-transfer coefficient and the initial diameter. For single pentane drops evaporating in sea water,... 

Continuous metal hand (heated by forced convection air, IR, direct steam or direct hot water) Heat transfer coefficient from impinging hot air. U = 0.06-0.09 kW/m K air velocity 15-25 m/s 5-50 kg water evaporated/m drying surface 1.5-2 kg steam/kg water evaporated power required is 20-30 kW with values relatively independent of the size. 

Here we do not consider in detail the modeling of the mass and heat transfer processes from the droplet surface into the surrounding gas. The evaporation or condensation of droplets has been addressed in detail by Kukkonen et al. (1989), Kulmala and Vesala (1991), Vesala and Kukkonen (1992), and Vesala and Kulmala (1993) see also Kulmala et al. (1993). Vesala (1991) has discussed the validation of various mass flux and droplet temperature equations against laboratory-scale experimental data. 

The results of model version J are also depicted (as broken lines) in Fig. 4.25. This model version is similarly simple and almost identical to model version C, apart from the fact the heat transfer coefficient from the heating element is now calculated by assuming an infinitely large specific heat capacity of the particles in Martin s model. This is equivalent to saying that the evaporative sink in the... 

Emulsion polymerization also has the advantages of good heat transfer and low viscosity, which follow from the presence of the aqueous phase. The resulting aqueous dispersion of polymer is called a latex. The polymer can be subsequently separated from the aqueous portion of the latex or the latter can be used directly in eventual appUcations. For example, in coatings applications-such as paints, paper coatings, floor pohshes-soft polymer particles coalesce into a continuous film with the evaporation of water after the latex has been applied to the substrate. 

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