Effect of Noncondensables on Heat Transfer

Noncondensabie gases reduce heat transfer rates in two methods  

The reduction of heat transfer coefficient can be treated as an empirical fouling factor. Data published indicates that the apparent fouling factor can be correlated for condensing steam in the presence of noncondensables as below  

C = concentration of air, weight percent The fouiing factor would be 1/hf. 

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Effect of Noncondensables on Heat Transfer Most of the heat transfer in evaporators does not occur from pure steam but from vapor evolved in a preceding effect. This vapor usually contains inert gases— from air leakage if the preceding effect was under vacuum, from air entrained or dissolved in the feed, or from gases liberated by decom-... 

The following values of overall heat-transfer coefficients are based primarily on results obtained in ordinary engineering practice. The values are approximate because variation in fluid velocities, amount of noncondensable gases, viscosities, cleanliness of heat-transfer surfaces, type of baffles, operating pressure, and similar factors can have a significant effect on the overall heat-transfer coefficients. The values are useful for preliminary design estimates or for rough checks on heat-transfer calculations. 

In some cases, as with pulp-mill liquors, the evaporator vapors contain constituents more volatile than water, such as methanol and sulfur compounds. Special precautions may be necessary to minimize the effects of these compounds on heat transfer, corrosion, and condensate quality. They can include removing most of the condensate countercurrent to the vapor entering an evaporator-heating element, channeling vapor and condensate flow to concentrate most of the foul constituents into the last fraction of vapor condensed (and keeping this condensate separate from the rest of the condensate), and flashing the warm evaporator feed to a lower pressure to remove much of the foul constituents in only a small amount of flash vapor. In all such cases, special care is needed to properly channel vapor flow past the heating surfaces so there is a positive flow from steam inlet to vent outlet with no pockets, where foul constituents or noncondensibles can accumulate. 

C-Y Wang and C-J Tu, Effects of Noncondensable Gas on Laminar Film Condensation in a Vertical Tube, Int. J. Heat Mass Transfer, 31, pp. 2339-2345,1988. 

The analysis was later modified to include some of the factors neglected by Nusselt1213. One of these is the effect of buoyancy forces acting on the liquid film. This results in the pl term in Equation 15.80 being replaced by Pl(pl — Pv ) Such buoyancy forces are usually only important close to the critical point. In most cases, the two most important factors that cause a significant deviation from Equation 15.80 are the presence of vapor shear forces and noncondensable gases in the vapor. Vapor shear forces act to increase the heat transfer coefficient, whereas noncondensable gases act to decrease it. 

For horizontal shell-side condensers, the condensate falls to the bottom of the shell, and vapor and liquid do not coexist, as assumed by the preceding method. The effect this has on the heat transfer must be considered. It is recommended that shell-side condensers with noncondensable gases present be somewhat overdesigned perhaps 20 percent excess surface should be provided. 

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