Latent-heat transfer

The heat-transfer process involves (1) latent heat transfer owing to vaporization of a small portion of the water and (2) sensible heat transfer owing to the difference in temperature of water and air. Approximately 80 percent of this heat transfer is due to latent heat and 20 percent to sensible heat. 

A temperature profile of vapor condensing in the presence of a noncondensable gas on a tube wall, as shown in Figure 16 indicates the resistance to heat flow. Heat is transferred in two ways from the vapor to the interface. The sensible heat is removed in cooling the vapor from t to t, at the convection gas cooling rate. The latent heat is removed only after the condensable vapor has been able to diffuse through the noncondensable part to reach the tube wall. This means the latent heat transfer is governed by mass transfer laws. 

Figure 2.42 shows boiling curves obtained in an annular channel with length 24 mm and different gap size (Bond numbers). The heat flux q is plotted versus the wall excess temperature AT = 7w — 7s (the natural convection data are not shown). The horizontal arrows indicate the critical heat flux. In these experiments we did not observe any signs of hysteresis. The wall excess temperature was reduced as the Bond number (gap size) decreased. One can see that the bubbles grew in the narrow channel, and the liquid layer between the wall and the base of the bubble was enlarged. It facilitates evaporation and increases latent heat transfer. 

Equation (57c) implies that the latent heat transferred from the surface by vaporization must be transported to the surface by conduction. For condensation, a development completely analogous to this would apply, but the heat must be transferred from the surface to the interior of the liquid. 

Rapid evaporation introduces complications, for the heat and mass transfer processes are then coupled. The heat of vaporization must be supplied by conduction heat transfer from the gas and liquid phases, chiefly from the gas phase. Furthermore, convective flow associated with vapor transport from the surface, Stefan flow, occurs, and thermal diffusion and the thermal energy of the diffusing species must be taken into account. Wagner 1982) reviewed the theory and principles involved, and a higher-order quasisteady-state analysis leads to the following energy balance between the net heat transferred from the gas phase and the latent heat transferred by the diffusing species ... 

The interfacial boundary condition may be written in terms of the convective heat transfer of sensible heat, latent heat transfer due to evaporation, and, if the surface temperature is high enough, radiant heat transfer. Mathematically, the surface boundary condition is... 

Cooling towers work by moving a predetermined flow of ambient air through the tower with large fans. A small amount of the water is evaporated by the air thus, through latent heat transfer, the remainder of the circulated water is cooled. 

This calculation is typical, in that 94% of the heat is liberated at the 320°F condensing temperature of the saturated steam. Another way of stating the same idea is that a steam reboiler depends on latent-heat transfer, and not on sensible-heat transfer. 

Reduce the rate of latent-heat transfer from the steam. 

The vapor density, like the vapor pressure, can be used as a thermodynamic potential whose total change around a closed path is zero. According to this argument, the effect of the above five factors on vapor density can be mathematically expressed and summed to zero. Beginning at the product water outlet, move to salt water by adding M, compress the salt water to pressure p, and subject it to the thermal loss of latent heat transfer, the diffusion loss of mass transfer, and the viscous loss of pressure in cellophane and manifold passages. This returns the path to fresh water and a closed circuit. 

The enthalpy conservation equation of the multiphase medium, obtained from the sum of the appropriate balance equations of the constituents includes the heat effects due to phase changes and hydration (dehydration) process, as well as the convectional and latent heat transfer,... 

Latent heat transfer Transfer of heat required to bring about a phase change (e.g., condensation or vaporization) in a fluid. (Compare to sensible heat transfer.) Many heat exchangers involve both latent and sensible heat transfer. 

The heat transfer through membrane material takes place by conduction and the latent heat transferred by vapor flow through the membrane, which is small and could be neglected. The equation for heat transfer through the membrane can be written as... 

Sensible heat transfer, Btu/hr. Condensation or latent heat transfer, Btu/hr. 

Latent heat Transfer of heat from the ground upward by the vertical gradient of moisture carrying heat of vaporization. 

The heat transfer capacity is larger than that compared to sensible or latent heat transfer (and this can give very high heat transfer coefficients). 

Cooling water has the highest potential due to the latent heat transfer but pressurization in the cooling loop is essential. Therefore heat transfer oil based cooling concepts are most practical. 

The basic equations covering combined mass- and heat-transfer phenomena have been covered in the literature. The analysis combines the sensible and latent heat transfer into an overall process based on enthalpy potential as the driving force. 

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