Heat transfer fogging

Steinmeyer, D. E., Special Problems in Process Heat Transfer (Fog Formation in Partial Condensers), AIChE 71 Nat l. Meeting, Dallas, Feb. (1972). 

If condensation requires gas stream cooling of more than 40—50°C, the rate of heat transfer may appreciably exceed the rate of mass transfer and a condensate fog may form. Fog seldom occurs in direct-contact condensers because of the close proximity of the bulk of the gas to the cold-Hquid droplets. When fog formation is unavoidable, it may be removed with a high efficiency mist collector designed for 0.5—5-p.m droplets. Collectors using Brownian diffusion are usually quite economical. If atmospheric condensation and a visible plume are to be avoided, the condenser must cool the gas sufftciendy to preclude further condensation in the atmosphere. 

Ryan et al. [Chem. Eng. Progr, 90(8X 83 (1994)] showthat separate mass and heat transfer-rate modeling of an HCl absorber predicts 2 percent fog in the vapor. The impact is equivalent to lowering the stage efficiency to 20 percent. 

Calculations. To check a design for possible fogging, a procedure is presented that rightly considers mass transfer and heat transfer as two separate processes. 

The calculations are made as follows. The exchanger is divided into small increments to allow numerical integrations. A tube wall temperature is first calculated and then QAV. The gas temperature and composition from an increment can then be calculated. If the gas composition is above saturation for the temperature, any excess condensation can occur as a fog. This allows the degree of fogging tendency to be quantified. Whenever possible, experimental data should be used to determine the ratio of heat transfer to m.ass transfer coefficients. This can be done with a simple wet and dry bulb temperature measurement using the components involved. 

Koestel, A., M. Gutstein, and R. T. Wainwright, 1963, Fog-Flow Mercury Condensing Pressure Drop Correlation, Proc. 3rd Annual High Temperature Liquid Metal Heat Transfer Tech. Meeting, ORNL-3605, Vol. 2, 198, ORNL, Oak Ridge, TN. (3)... 

Topper, L., 1963, A Diffusion Theory Analysis of Boiling Burnout in the Fog Flow Regime, Trans. ASME, J. Heat Transfer 85 284-285. (5)... 

Sometimes the dehumidification process can lead to fog formation. Consider a gas with the initial conditions represented by point A in Fig. 23.7. In this case the dehumidification path from A to B reaches the equilibrium curve and then crosses it into a region of supersaturation. The rate of heat transfer (temperature change) outruns the mass-transfer rate (humidity change) so that the gas becomes supersaturated. If the gas contains dust or other particles that can serve as nuclei for droplet formation, the supersaturation may be relieved by condensation on these particles instead of on the bulk liquid surface. This can lead to a persistent, troublesome fog. Fog formation may be avoided by making sure that the initial gas temperature is well above the equilibrium value, as at point A, or by adding heat to the gas during the dehumidification process. ... 

The term e/(ee — 1), which appears in equations 1 and 2, was first developed to account for the sensible heat transferred by the diffusing vapor (1). The quantity 8 represents the group M4-C 4 / hg, the ratio of total transported energy to convective heat transfer. Thus it may be thought of as the fractional influence of mass transfer on the heat-transfer process. The last term of equation 3 is the latent heat contributed to the gas phase by the fog formation. The vapor loss from the gas phase through both surface and gas-phase condensation can be related to the partial pressure of the condensing vapor by using Dalton s law and a differential material balance. 

During a study of the applicability of "spray" or "fog" evaporation to sea water desalination, it was found that this technique was particularly useful for scale deposition studies. Thus, test conditions are reproducible and heat transfer coefficients are very high, so that the effect of scale formation is readily apparent. Three novel methods for the control of scale deposits on the evaporating surfaces of a spray evaporator were explored. One involves the addition of small quantities of low molecular weight polyacrylic acid to the feed water, which prevents the formation of adherent scale. The methods are applicable under certain conditions to scales formed from sea water containing substantial amounts of calcium sulfate in addition to alkaline scale-forming substances. While spray evaporation appears to be of limited application in water desalination, the scale-control methods developed are probably applicable to other types of evaporator, particularly of the long-tube type. 

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