Film condensation superheated vapor

A further, and likewise small, deviation from Nusselt s film condensation theory is found for superheated vapour. In addition to the enthalpy of vaporization, the superheat enthalpy CpQ ( g — < s) has to be removed in order to cool the superheated vapour from a temperature da to the saturation temperature t9s at the phase interface. Instead of the enthalpy difference Ahv according to (4.28), the enthalpy difference... 

A further consequence is that the superheated vapor will fail to directly condense only if the surface temperature is above the saturation temperature, and this will only occur if the local heat flux is higher than would be obtained if the vapor were condensing on the surface. Note that this implies, first, that a desuperheater is not necessary to desuperheat a vapor if the only concern is that the heat-transfer rate of a superheated vapor is too low, and second, that care must be taken with superheated vapors such that the local heat-transfer rate and surface temperature are not too high for the coolant stream, causing excessive fouling or film boiling (see next section on vaporization). 

All modules use the 2-fluid model to describe steam-water flows and four non-condensable gases may be transported. The thermal and mechanical non-equilibrium are described. All kinds of two-phase flow patterns are modelled co-current and counter-current flows are modelled with prediction of the counter-current flow limitation. Heat transfer with wall structures and with fuel rods are calculated taking into account all heat transfer processes ( natural and forced convection with liquid, with gas, sub-cooled and saturated nucleate boiling, critical heat flux, film boiling, film condensation). The interfacial heat and mass transfers describe not only the vaporization due to superheated steam and the direct condensation due to sub-cooled liquid, but also the steam condensation or liquid flashing due to meta-stable subcooled steam or superheated liquid. 

For example, vaporization may occur as a result of heat absorbed, by radiation and convection, at the surface of a pool of hquid or as a result of heat absorbed by natural convect ion from a hot wall beneath the disengaging surface, in which case the vaporization takes place when the superheated liquid reaches the pool surface. Vaporization also occurs from falling films (the reverse or condensation) or from the flashing of hquids superheated by forced convec tion under pressure. 

The effect of superheat of the vapor will next be considered. In the previous analysis it was assumed that the vapor reservoir was at the saturation temperature. If the vapor reservoir is superheated to temperature, 7", the enthalpy of the vapor, hv, condensing onto the film will be ... 

If the degree of superheat is large, it will be necessary to divide the temperature profile into sections and determine the mean temperature difference and heat transfer coefficient separately for each section. If the tube wall temperature is below the dew point of the vapor, liquid will condense directly from the vapor onto the tubes. In these circumstances it has been found that the heat transfer coefficient in the superheating section is close to the value for condensation and can be taken as the same. So, where the amount of superheating is not too excessive, say less than 25% of the latent heat load and the outlet coolant temperature is well below the vapor dew point, the sensible heat load for desuperheating can be lumped with the latent heat load. The total heat transfer area required can then be calculated using a mean temperature difference based on the saturation temperature (not the superheat temperature) and the estimated condensate film heat transfer coefficient. 

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