Effect of noncondensible gas

Mori, Y, K. Hiyikata, and K Utsunomiya, The Effect of Noncondensable Gas on Film Condensation Along a Vertical Plate in an Enclosed Chamber, ASME Journal Heat Trans., V. 99, May (1977) p. 257. 

Effect of Noncondensable Gas. As shown in Fig. 14.3, the presence of a noncondensable gas creates an additional thermal resistance to condensation heat transfer due to the required diffusion of the vapor molecules through a gas-rich layer near the surface of the condensate. This additional resistance can reduce the condensation heat transfer rate substantially. 

Effect of Noncondensable Gas. As described earlier, Rose [47, 48] has suggested an approximate method to calculate condensation heat transfer in the presence of a noncondensable gas. For forced convection over a single horizontal tube, he recommends the following relationship (similar to Eq. 14.52 for a flat plate) that relates the mean condensation rate to the free stream conditions ... 

Ullman and Letan [189] studied the effect of noncondensable gas on condensation of bubbles in a subcooled liquid. Bergles and Bar-Cohen [190] provide information on the use of direct contact condensation of bubbles in a so-called submerged condenser. Zangrando and Bharathan [191] established computer models to characterize direct contact condensation of low-density steam (with high noncondensable gas concentrations) on seawater. 

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. 

Upon computing the bubble point of the overhead product, we find that the measured reflux temperature is well below the estimated boiling point. Thus, we choose the subcooled condenser model. The steady-state concept of the subcooled condenser often does not exist in practice. Instead, the condenser is in vapor-liquid equilibrium with the vapor augmented by a blanket of noncondensable gas (that has the effect of lowering the dew point of the overhead vapor). The subcooled condenser is a convenient work-around for steady-state models (as is needed here), but not for dynamic models. We assume a partial reboiler. 

EFFECT OF NONCONDENSABLE GASES ON RATE OF CONDENSATION. The presence of even small amounts of noncondensing gas in a condensing vapor seriously reduces the rate of condensation. The partial pressure of the condensing vapor at the surface of the condensate must be less than it is in the bulk gas phase... 

Presence of even a small amount of noncondensable gas in the condensing vapor leads to a significant reduction in heat transfer during condensation. The buildup of noncondensable gases near the film vapor interface inhibits the diffusion of vapor from the bulk mixture to the liquid film. The net effect is to reduce the effective driving force for heat and mass transfer. Figure 22.25 shows the temperature and noncondensable gas boundary layer near the film and vapor interface. 

Figure 16. Temperature profile showing effect of vapor condensation on a tube wall in the presence of a noncondensable gas.

The effect of a noncondensable gas in the system with a condensable vapor is to significandy reduce the condensing side film coefficient. Henderson and Marcello present data to illustrate the effect. Figures 10-85, 10-86, and 10-86A present the effect of AT with a steam-air system and toluene-... 

The potential for corrosion as a result of the reactions of noncondensable gases present in steam-water circuits is a major area of risk. The dissolved oxygen (DO) content of MU water is recognized as a primary source of gas entering a boiler system, and effective deaeration of MU and FW is therefore critical. 

Interference from Noncondensable Gases. Foreign gas in the still comprises air from mechanical leaks and traces of dissolved gas that have survived the degasser. The effectiveness of the degasser was tested by measuring dissolved oxygen, according to the ASTM procedure which yielded the data of Table III. 

In experiments with mixtures of condensable and noncondensable gases, adsorption of the condensable gas component can restrict or even completely block permeation of the noncondensable gas [61,62], This effect was first noticed by Barrer and Pope in experiments with sulfur dioxide/hydrogen mixtures [63] some of the data are shown in Figure 2.40. Sorption of sulfur dioxide on the pore walls... 

Toepler pumps are used for the collection and transfer of noncondensable gases. They are not capable of creating great vacuums, but they can be very effective pumps regardless. A 500-cm3-size Toepler pump can remove 99.9% of a two-liter gas bulb in 10 minutes. 

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