Condensation average heat transfer coefficient

Using Eq. (9-28) as a starting point, develop an expression for the average heat-transfer coefficient in turbulent condensation as a function of only the fluid properties, length of the plate, and temperature difference i.e., eliminate the Reynolds number from Eq. (9-28) to obtain a relation similar to Eq. (9-10) for laminar condensation. 

Select the calculation method to be used. Use the Dukler theory [18], which assumes that three fixed factors must be known to establish the value of the average heat-transfer coefficient for condensing inside vertical tubes. These are the terminal Reynolds number (4T///), the Prandtl number (cfx/k) of the condensed phase, and a dimensionless group designated Ad and defined as follows ... 

Below we discuss relations for the average heat transfer coefficient h for the case of laminar film condensation for various geometries. 

At Reynolds numbers greater than about 30, it is observed that waves form at the liquid-vapor interface although the flow in liquid film remains laminar. I he flow in this case is said to be wavy laminar. The waves at the liquid-vapor interface tend to increase heat transfer. But the waves also complicate the analysis and make it very difficult to obtain analytical solutions. Therefore, we have to rely on experimental studies. The increase in heat transfer due to the wave effect is, on average, about 20 percent, but it can exceed 50 percent. The exact amount of enhancement depends on the Reynolds number. Rased on his experimental studies, Kutateladze (1963) recommended the following relation for the average heat transfer coefficient in wavy laminar condensate flow for p p, and 30 < Re < 1800,... 

Equation 10 22 for vertical plates can also be used to calculate the average heat transfer coefficient for laminar film condensation on the outer surfaces of vertical lubes provided that the tube diameter is large relative to the thickness of the liquid film. 

SOLUTION (a) Condensation heat transfer on a tube is not influenced by the presence of other tubes in its neighborhood unless the condensate from other tubes drips on it. In our case, the horizontal tubes are arranged in four vertical tiers, each tier consisting of 3 tubes. The average heat transfer coefficient for a vertical tier of N horizontal tubes is related to the one for a single horizontal tube by Eq. 10-33 and is determined to be... 

The average heat transfer coefficient for film condensation on the outer surfaces of a horizontal tube is determined to be... 

A large heat exchanger has several columns of tubes, witb 33 tubes in each column. The outer dtaiueler of the tubes is 1.5 cm. Saturated steam at 50"C condenses on the outer surfaces of the tubes, wliich are maintained at 20°C. Determine (n) the average heat transfer coefficient and (i) the rate of condensation of steam per m length of a column. 

Velkoff and Miller [336] investigated the effect of uniform and nonuniform electric fields on laminar film condensation of Freon-113 on a vertical plate. With screen grid electrodes providing a uniform electric field over the entire plate surface, a 150 percent increase in the heat transfer coefficient was obtained with a power expenditure of a fraction of one watt. Choi and Reynolds [337] and Choi [338] recently reported data for condensation of Freon-113 on the outside wall of an annulus in the presence of a radial electric field. With the maximum applied voltage of 30 kV, the average heat transfer coefficients for a 25.4-mm outside diameter by 12.7-mm inside diameter annulus were increased by 100 percent. 

FIGURE 14.5 Average heat transfer coefficients for film condensation on vertical plates. 

Laminar Free Convection. Laminar film condensation of a quiescent vapor on an isothermal, smooth horizontal tube, as depicted in Fig. 14.11, may be treated approximately by a Nusselt-type analysis, yielding the following average heat transfer coefficient ... 

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 ... 

Tube Bundles. The average heat transfer coefficient in a bundle of finned tubes is influenced by both vapor shear and condensate inundation, although the effects are not as large as for smooth tubes [88,102-107]. At low vapor velocities, Webb and Murawski [107] express the local coefficient for the Mh row in terms of the local film Reynolds number ... 

Film-condensation coefficients for vertical surfaces. Film-type condensation on a vertical wall or tube can be analyzed analytically by assuming laminar flow of the condensate film down the wall. The film thickness is zero at the top of the wall or tube and increases in thickness as it flows downward because of condensation. Nusselt (HI, Wl) assumed that the heat transfer from the condensing vapor at 7, K, through this liquid film, and to the wall at 7 K was by conduction. Equating this heat transfer by conduction to that from condensation of the vapor, a final expression can be obtained for the average heat-transfer coefficient over the whole surface. 

EXAMPLE 4.8-2. Condensation on a Vertical Tube Steam saturated at 68.9 kPa (10 psia) is condensing on a vertical tube 0.305 m (1.0 ft) long having an OD of 0.0254 m (1.0 in.) and a surface temperature of 86.11°C (187°F). Calculate the average heat-transfer coefficient using English and SI units. 

C is condensing on a bank of five vertical tubes each 0.305 m high and having an OD of 25.4 mm. The tubes are arranged in a bundle spaced far enough apart so that they do not interfere with each other. The surface temperature of the tubes is 97.78°C. Calculate the average heat-transfer coefficient and the total kg condensate per hour. 

C is condensing on a horizontal tube bank with five layers of tubes (N = 5) placed one below the other. Each layer has four tubes (total tubes = 4 X 5 = 20) and the OD of each tube is 19.1 mm. The tubes are each 0.61 m long and the tube surface temperature is 97.78°C. Calculate the average heat-transfer coefficient and the kg condensate per second for the whole condenser. Make a sketch of the tube bank. 

Condensation in Microchanneis, Figure 11 Effect of sizes of hydraulic diameter on the cross-sectional averaged heat transfer coefficient [13]... 

Steam (at 120°C) condenses on the outside of a horizontal pipe at 30 kg/hr. Water flows through the pipe (0.025-m diameter 0.8 m long) at an average velocity of 1 m/sec. The inlet water temperature is 16°C. Assuming that the only important thermal resistance is the water convection, find the average heat transfer coefficient. Latent heat of the steam is 2202 kJ/kg. 

The average heat transfer coefficient a for film condensation of a pure unmoved vapor [59, 60] is given by the following ... 

Heat-transfer coefficients in this book have the units of Btu/[(h)(ft2)(°F)], where the ft2 term refers to the surface area of the surface condenser. The °F term refers to the condensing steam temperature, minus the average tube-side cooling-water temperature. 

Collecting information and its processing will convert them in data. So, data represent agglomerated information, which are partially or finally processed. Examples of data can be found as a parameter, which describe evaluated information to be used for the specific purpose. In this respect the average inlet temperature of cooling water in the condenser is data obtained by the averaging procedure adapted for this purpose. Also heat transfer coefficient used in the design of condenser is the data obtained by the experimental procedure for the heat transfer evaluation. 

Condensation of mixed vapors of immiscible liquids is not well understood. The conservative approach is to assume that two condensate films are present and all the heat must be transferred through both films in series. Another approach is to use a mass fraction average thermal conductivity and calculate the heat-transfer coefficient using the viscosity of the film-forming component (the organic component for water-organic mixtures). 

Calculate the heat-transfer coefficient using both mechanisms and select the higher value calculated as the effective heat-transfer coefficient hL. The annular-flow assumption results in heat-transfer coefficients that vary along the tube length. The condenser should be broken into increments, with the average vapor and liquid flow rates for each increment used to calculate heat-transfer coefficients. The total is the integrated value of all the increments. 

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