Heat transfer coefficient condensers

To calculate the condensing heat transfer coefficient requires the length of the condensing zone L to be specified. Thus, a value of L must be estimated before the calculation can be made. For the value of L to be correct, it must comply with ... 

Table 9-4 Approximate Values of Condensation Heat-transfer Coefficients tor Vapors at 1 atm. According to Refs. 3 and 45.

Related Calculations. For low values of Reynolds number (4T//x), the Nusselt equation can be used to predict condensing heat-transfer coefficients for vertical tubes ... 

It can be seen from Fig. 7.17 that the condensing heat-transfer coefficient for a fluid with a Prandtl number of approximately 2 (for instance, steam) is not strongly dependent on flow rate or Reynolds number. For this reason, heat-transfer coefficients for steam condensing on vertical tubes are frequently not calculated, but are assigned a value of 1500 to 2000Btu/(h)(ft2)(°F) [8500 to 11,340 W/(m2)(K)]. 

Related Calculations. For most cases, vapor-shear condensation is not important, and the condensing heat-transfer coefficient can be calculated simply as the laminar-film coefficient. 

Heat-transfer coefficient in condensation Mean condensation heat-transfer coefficient for a single tube Heat-transfer coefficient for condensation on a horizontal tube bundle Mean condensation heat-transfer coefficient for a tube in a row of tubes Heat-transfer coefficient for condensation on a vertical tube Condensation coefficient from Boko-Kruzhilin correlation Condensation heat transfer coefficient for stratified flow in tubes Local condensing film coefficient, partial condenser Convective boiling-heat transfer coefficient... 

Local effective cooling-condensing heat-transfer coefficient, partial condenser... 

These data must be carefully interpreted to obtain overall heat-transfer coefficients for condenser tube bundles, e.g., horizontal condensers. Based on for a single tube, the condensing heat-transfer coefficient for a bundle, can be found from Eq. 3.4.6-22 of Ref. [2] ... 

This relation is convenient to use to determine the Reynolds number when the condensation heat transfer coefficient or the rate of heat transfer is known. 

The results obtained from the theoretical relations above are in excellent agreement with the experimental results. It can be shown easily that using property values in Eqs. 10-22 and 10-24 in the specified units gives the condensation heat transfer coefficient in W/m C, thus saving one fromhaving... 

Equation 10 22 gives the condensation heat transfer coefficient ill W/m C when the quantities are expressed in the units specified in their descriptions. 

Equation l,p-22 was developed for vertical plates, but it can also be used for laminar fififi eondensation on the upper surfaces of plates that are inclined by an angle S.ifrom the vertical, by replacing g in that equation by g cos (Fig, 10-27). This approximatiot) gives satisfactory results especially for d 60 . Note that the condensation heat transfer coefficients on vertical and inclined plates are related to each other by... 

Most condensers used in steam power plants operate at pressures well below the atmospheric pressure (usually under 0.1 atm) to maximize cycle thermal efficiency, and operation at such low pressures raises the possibility of air (a noncondensable gas) leaking into the condensers. Experimental studies show that the presence of noncondensable gases in the vapor has a detrimental effect on condensation heat transfer. Even small amounts of a noncondensable gas in the vapor cause significant drops in heat transfer coefficient during condensation. Eor example, the presence of less than 1 percent (by mass) of air in steam can reduce the condensation heat transfer coefficient by more than half. Therefore, it is common practice to periodically vent out the noncondensable gases that accumulate in the condensers to ensure proper operation. 

The drastic reduction in the condensation heat transfer coefficient in the presence of a iioncondensable gas can be explained as follows When the vapor mixed with a noncondensable gas condenses, only the noncondeiisable gas remains in the vicinity of the surface (Fig. 10-29). This gas layer acts as a barrier between the vapor and the surface, and makes it difficult for the vapor to reach the surface. The vapor now must diffuse through the noncondensable gas first before, reaching the surface, and this reduces the effectiveness of the condensation process. 

C Consider film condensation on the outer surfaces of four long tubes. For which orieiitalion of the tubes will ihc condensation heat transfer coefficient be the highest (a) vertical, (b) horizontal side by side, (c) horizontal but in a vertical tier (directly on lop of each other), or ([Pg.619]

You have probably noticed that water vapor that condenses on a canned drink slides down, clearing the surface for further condensation. Therefore, condensation in this case can be considered to be dropwisc. Determine the condensation heat transfer coefficient on a cold canned drink at 2°C that is placed in a large container filled with saturated steam at 95°C. 

Steam condenses at 50 C on the onter surface of a horizontal lube with an outer diameter of 6 cm. The outer surface of the tube is maintained at 30°C. The condensation heat transfer coefficient is... 

Garimella, S., and Bandhauer, T. M. (2001) Measurement of Condensation Heat Transfer Coefficients in MicroChannel Tubes, 2001ASME International Mechanical Engineering Congress and Exposition, New York, NY, United States, Vol. 369, American Society of Mechanical Engineers, pp. 243-249. 

Heat transfer coefficient in condensation Mean condensation heat transfer coefficient for a single tube... 

Convective boiling-heat transfer coefficient Local effective cooling-condensing heat transfer coefficient, partial condenser Fouling coefficient based on fin area Heat transfer coefficient based on fin area Film boiling heat transfer coefficient Forced-convection coefficient in equation 12.67 Local sensible-heat transfer coefficient, partial condenser... 

Gy is the mass velocity (mass flnx) of the entering fluid, defined as the mass flow rate in kg/s (lb j/h) divided by the cross-sectional area of the tube or the flow channel. The snbscript i refers to the physical properties of the condensate and v to the vapor. Equation (6.86a) may be nsed to calcnlate the local condensing heat-transfer coefficient at any quality x by replacing the bracketed term by... 

Because of the opposing effects (which are individnally poorly understood) of the phenomena described in the previons two sections, any calcnlation of condensing heat-transfer coefficient on... 

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