Pure steam heat transfer coefficient

There are two types of correlations for estimating the heat transfer coefficient for condensation inside vertical tube. In the first type of correlations, the local heat transfer coefficient is expressed in the form of a degradation factor defined as the ratio of the experimental heat transfer coefficient (when noncondensable gas is present) and pure steam heat transfer coefficient (Kuhn et al. [1997]). The correlations in general are the functions of local noncondensable gas mass fraction and mixture Reynolds number (or condensate Reynolds number). In the other type of correlations, the local heat transfer coefficient is expressed in the form of dimensionless numbers. In these correlations, local Nusselt number is expressed as a function of mixed Re5molds number, Jakob number, noncondensable gas mass fraction, condensate Reynolds number, and so on. 

In water-cooled tube-and-shell condensers with shell side condensation, overall heat transfer coefficients for essentially pure steam range from 200 to 800 Btu per hour per square foot per °F. 

If a mixture of condensable and noncondensable gases is cooled below its dew point at a surface, the former condenses, leaving the adjacent layers richer in the latter, thus creating an added thermal resistance. The condensable fraction must diffuse through this layer to reach the film of condensate and heat-transfer coefficients are normally very much lower than the corresponding value for the pure vapor. For example, the presence of 0.5% of air has been found to reduce the heat transfer by condensation of steam by as much as 50%. 

Fujii [55] recommends the empirical relationship of Fujii and Oda [79] to calculate the ratio of the heat transfer coefficient with air to the coefficient for pure steam in small tube bundles ... 

The heat transfer coefficients of vacuum condensers depend on many different factors, particularly on the inert gas content and the flow rate in the condensation area. Relatively high heat transfer coefficients exist, for example, in turbine condensers, where sometimes enormous quantities of turbine exhaust steam are condensed under vacuum. The heat transfer coefficients depend on the condenser design and the inert gas content. Values from 2500 to 5000 W (m K) are common. In this case, pure water vapour has to be condensed, so that on the vapour side no fouling is to be taken into consideration. On the water side, the fouling depends on the cooling water quality, and therefore appropriate fouling factors have to be considered. 

When the pure steam experiments are considered as the reference for comparison, a first indicator of the effect of air is a remarkable decrease in centreline and inner wall temperatures. Comparisons show that difference between saturation temperature, corresponding to the pure gas case, and measured centreline temperatures varies between 10 K and 50 K, depending on inlet air mass fraction. In other words, the temperature difference increases considerably as air mass fraction increases. It is found that there is a drastic decrease in the performance of the heat exchanger as the inlet air mass fraction increases. The inhibiting effect of air on condensation manifests itself as reduction in heat transfer coefficient. However, the inhibiting effect of air diminishes as system pressure and gas flow rate increase. The heat transfer coefficient can be based on either the measured centreline... 

Example 15.4 A reboiler is required to supply 0.1 krnol-s 1 of vapor to a distillation column. The column bottom product is almost pure butane. The column operates with a pressure at the bottom of the column of 19.25 bar. At this pressure, the butane vaporizes at a temperature of 112°C. The vaporization can be assumed to be essentially isothermal and is to be carried out using steam with a condensing temperature of 140°C. The heat of vaporization for butane is 233,000 Jkg, its critical pressure 38 bar, critical temperature 425.2 K and molar mass 58 kg krnol Steel tubes with 30 mm outside diameter, 2 mm wall thickness and length 3.95 m are to be used. The thermal conductivity of the tube wall can be taken to be 45 W-m 1-K 1. The film coefficient (including fouling) for the condensing steam can be assumed to be 5700 W m 2-K 1. Estimate the heat transfer area for... 

Water is heated from 15 to 65°C in a steam-heated horizontal 50-mm-ID tube. The steam temperature is 120 C. The average Reynolds number of the water is 450, The individual coefficient of the water is controlling. By what percentage would natural convection increase the total rate of heat transfer over that predicted for purely laminar flow Compare your answer with the increase indicated in Example 12.4. 

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