Boiling heat-transfer coefficient estimation

This method uses simple, unsophisticated, methods to estimate the two-phase pressure drop through the exchanger and piping, and the convective boiling heat transfer coefficient. The calculation procedure is set out below and illustrated in Example 12.11... 

Many empirical relations have been developed to estimate the boiling heat-transfer coefficients for water. Some of the simplest relations are those presented by Jakob and Haw kins [15] for water boiling on the outside of submerged surfaces at atmospheric pressure (Table 9-3). These heat transfer coefficients may be modified to take into account the influence of pressure by using the empirical relation... 

Estimate the nucleate-pool-boiling heat-transfer coefficient for a water-26% glycerin mixture at 1 atm in contact with a copper surface and AT, = 15°C. 

Simplified empirical equations to estimate the boiling heat-transfer coefficients for water boiling on the outside of submerged surfaces atT.O atm abs pressure have been developed (J2). 

EXAMPLE 4.8-1. Rate of Heat Transfer in a Jacketed Kettle Water is being boiled at 1 atm abs pressure in a jacketed kettle with steam condensing in the jacket at 115.6°C. The inside diameter of the kettle is 0.656 m and the height is 0.984 m. The bottom is slightly curved but it will be assumed to be flat. Both the bottom and the sides up to a height of 0.656 m are jacketed. The kettle surface for heat transfer is 3.2-mm stainless steel with a /c of 16.27 W/m-K. The condensing steam coefficient fi, inside the jacket has been estimated as 10200 W/m"-K. Predict the boiling heat-transfer coefficient /iq for the bottom surface of the kettle. 

An electrical current passed through a long wire (0.001-m diameter) dissipates 4085 W/m. The wire is contained in water at atmospheric pressure and has a surface temperature of 128°C. Determine the boiling heat transfer coefficient and estimate the liquid-surface interaction coefficient. 

Estimate temperature distribution in the evaporator, taking into account boiling-point elevations. If all heating surfaces are to be equal, the temperature drop across each effect will be approximately inversely proportional to the heat-transfer coefficient in that effect. 

The equation given by Bromley (1950) can be used to estimate the heat-transfer coefficient for film boiling on tubes. Heat transfer in the film-boiling region will be controlled by conduction through the film of vapour, and Bromley s equation is similar to the Nusselt equation for condensation, where conduction is occurring through the film of condensate. 

Estimate the heat-transfer coefficient for the pool boiling of water at 2.1 bar, from a surface at 125°C. Check that the critical flux is not exceeded. 

Single-phase flow region at the inlet the liquid is below its boiling point (sub-cooled) and heat is transferred by forced convection. The equations for forced convection can be used to estimate the heat-transfer coefficient in this region. 

Chen s method was developed from experimental data on forced convective boiling in vertical tubes. It can be applied, with caution, to forced convective boiling in horizontal tubes, and annular conduits (concentric pipes). Butterworth (1977) suggests that, in the absence of more reliable methods, it may be used to estimate the heat-transfer coefficient for forced convective boiling in cross-flow over tube bundles using a suitable cross-flow correlation to predict the forced-convection coefficient. Shah s method was based on data for flow in horizontal and vertical tubes and annuli. 

The equations for estimating nucleate boiling coefficients given in Section 12.11.1 can be used for close boiling mixtures, say less than 5°C, but will overestimate the coefficient if used for mixtures with a wide boiling range. Palen and Small (1964) give an empirical correction factor for mixtures which can be used to estimate the heat-transfer coefficient in the absence of experimental data ... 

The boiling film coefficient for a kettle reboiler can be estimated from the correlation for pool boiling. Equation 15.96 gives one such method due to Palen15. However, the correlation requires the heat flux to be known, and therefore the heat transfer area to be known. Hence the calculation will need to be iterative. An initial estimate of the overall heat transfer coefficient of 2000 W-m 2-K 1 gives ... 

Heat-transfer coefficients for boiling are usually large compared with those for ordinary convection. Estimate the flow velocity which would be necessary to produce a value of h for forced convection through a smooth 6.5-mm-diameter brass tube comparable with that which could be obtained by pool boiling with AT, = I6.7°C, p = 100 lb/in2 abs, and water as the fluid. See Prob. 9-10 for data on properties. 

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