The rate equation for heat transfer is: Q = U × A × MTD where:…
This section discusses the principles of process condensation and gives rules-ofthumb for steam condensation.
Condensing surfaces are below the dewpoint temperature of the condensing fluid and are covered by a film of condensate. The resistance to heat transfer between the vapor and the condensate is negligible for single component condensation. Essentially all of the resistance to heat transfer is convection and conduction across the condensate film. Major factors affecting condensate film thickness and heat transfer are whether the condensing fluid flows vertically downward or horizontally, whether the film is laminar or turbulent, whether the film is vapor shear controlled (high vapor velocity) or gravity controlled (low vapor velocity), and whether the condensing fluid is flowing in tubes or outside tube bundles.
The resistance to heat transfer between the vapor and the condensate film is significant for multicomponent condensation. Least volatile components condense first, concentrating more volatile components near the vapor-liquid interface. This tends to inhibit condensation. This vapor phase resistance is governed by vapor shear and turbulent mixing in the vapor.
Total condensers usually operate in the vapor shear controlled turbulent film regime near the inlet, in gravity controlled regimes in the middle, and may have a liquid flooded zone near the outlet. The heat transfer coefficients may vary from about a thousand at the inlet to about one Btu/hr × °F × ft2 at the outlet. This type of condenser is usually analyzed by solving for the limits of the various flow regimes and applying the correlations appropriate to each regime. This is an iterative process and involves a very large number of correlations. Computer simulation with the HTRI CST program is recommended for these cases.
Some partial condensers operate with vapor shear controlled turbulent film condensation throughout, which is very similar to two-phase heat transfer discussed earlier. The two-phase methods discussed earlier may be applied to condensers in this case.
Condensing steam is a common and efficient heat medium. Steam condensing coefficients are usually between 2000 and 3000 Btu/hr × °F × ft2 in the vapor shear controlled regime and a few hundred in the gravity controlled regimes. Most steam heated exchangers involve total steam condensation and have an outlet condensate pot to avoid a condensate flooded zone in the exchanger. For these cases, a design steam side heat transfer coefficient of 1000 Btu/hr × °F × ft2 is recommended.
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