Finned tubes condensation

The effect of vapor velocity on finned tube condensation is less than that on a smooth tube [98-100], Cavallini et al. [101] proposed the following relationship for the average heat transfer coefficient on a finned tube during forced convection conditions ... 

Surface Condensers Surface condensers (indirect-contact condensers) are used extensively in the chemical-process industiy. They are employed in the air-poUution-equipment industry for recoveiy, control, and/or removal of trace impurities or contaminants. In the surface type, coolant does not contact the vapor condensate. There are various types of surface condensers including the shell-and-tube, fin-fan, finned-hairpin, finned-tube-section, ana tubular. The use of surface condensers has several advantages. Salable condensate can be recovered. If water is used for coolant, it can be reused, or the condenser may be air-cooled when water is not available. Also, surface condensers require less water and produce 10 to 20 times less condensate. Their disadvantage is that they are usually more expensive and require more maintenance than the contac t type. 

Open Tube Sections (Air Cooled) Plain or finned tubes No shell required, only end heaters similar to water units. Condensing, high level heat transfer. Transfer coefficient is low, if natural convection circulation, but is improved with forced air flow across tubes. 0.8-1.8... 

The usual applications for finned tubes are in heat transfer involving gases on the outside of the tube. Other applications also exist, such as condensers, and in fouling service where the finned tube has been shown to be beneficial. The total gross external surface in a finned exchanger is many times that of the same number of plain or bare tubes. 

D,.g = an equivalent diameter of a finned tube for calculation of condensing coefficient, ft. 

W" = tube loading for condensing, Ib/hr (ft of finned tube length). 9 ... 

Condensation Outside Horizontal Tubes. Figure 8.14(d) shows a condenser with two tube passes and a shell side provided with vertically cut baffles that promote side to side flow of vapor. The tubes may be controlled partially flooded to ensure desired subcooling of the condensate or for control of upstream pressure by regulating the rate of condensation. Low-fin tubes often are advantageous, except when the surface tension of the condensates... 

The condenser on a certain automobile air conditioner is designed to remove 60,000 Btu/h from Freon 12 when the automobile is moving at 40 mi/h and the ambient temperature is 95°F. The Freon 12 temperature is 150°F under these conditions, and it may be assumed that the air-temperature rise across the exchanger is 10°F. The overall heat-transfer coefficient for the finned-tube heat exchanger under these conditions is 35 Btu/h ft2 - °F. If the overall heat-transfer coefficient varies as the seven-tenths power of velocity and air-mass flow varies directly as the velocity, plot the percentage reduction in performance of the condenser is a function of velocity between 10 and 40 mi/h. Assume that the Freon temperature remains constant at 150°F. 

For low-fin tubes, the laminar condensing coefficient can be calculated by applying an appropriate correction factor F to the value calculated using the preceding equation for laminar-film condensation. The factor F is defined thus ... 

The procedure outlined in this example can also be used when designing equipment using low-fin tubes. The same approach is used when condensing or boiling on the outside of low-fin tubes. 

Steam heating Condensing Low-to-high pressure gas heating and cooling finned tubes are often used) Reboilers Low-pressure (<50 psig) steam heating Reboilers... 

Now consider a finned-tube unit with the same I.D. and O.D. tubes. Service conditions are kept identical—i.e., fluid velocities, temperatures, etc., are constant. The increases in ho and over plain-tube exchangers are assumed negligible, especially for phase-change applications, such as condensation or evaporation on the fins. Thus ... 

Steam is not to be condensed on conventional low-finned tubes its high surface tension causes bridging and retention of the condensate and a severe reduction of the coefficient below that of the plain tube. 

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