Heat exchangers surface coefficients

Figs 5.4-34 to 5.4-37 show results of the measurements and calculations. In Figs 5.4-34 and 5.4-35 the results of temperature and heat flow measurements are shown. Isothermal operation was quite easy to reach due to the relatively low heat of reaction and the high value of the product of the heat-transfer coefficient and the heat-exchange surface area Art/ in relation to the volume of the reaction mixture. Peaks in the heat flow-versus-time diagram correspond to the times at which isothermal operation at the next temperature level started. After each peaks the heat flow decreased because of the decrease in the concentrations of the reactants. 

If an error of 10% due to the simplification is accepted the maximum distance of the phase front to the heat exchanger surface smax is given in Table 22. For a typical heat transfer coefficient if water is taken as heat transfer fluid, two different cases can be observed. For the pure PCM, the maximum thickness allowed before the simplification leads to serious errors in the result is only 0.5 mm. In that case the simplification is of no practical use. If the... 

Loss of agitation or circulation of the reactant mass or other reduction in the heat-transfer coefficient or contact with the heat exchange surface... 

Heat Transfer Heat-exchange surfaces have been used to provide the means of removing or adding heat to fluidized beds. Usually, these surfaces are provided in the form of vertical or horizontal tubes manifolded at the tops and bottom or in a trombone shape manifolded exterior to the vessel. Horizontal tubes are extremely common as heat-transfer tubes. In any such installation, adequate provision must be made for abrasion of the exchanger surface by the bed. The prediction of the heat-transfer coefficient for fluidized beds is covered in Secs. 5 and 11. 

A = heat-exchanger surface area, ft2 U = heat-transfer coefficient, Btu/[(h)(ft2)(°F)]... 

Heat removed by condensation is easy. The heat-transfer coefficient U for condensation of pure, clean, vapors may be 400 to 1000 Btu per hour per ft2 of heat exchanger surface area, per °F of temperature-driving force. The U value for subcooling stagnant liquid may be only 10 to 30. Condensate backup is the major cause of lost heat transfer for heat exchangers, in condensing service. 

According to Newton s law of heat exchange, the heat exchanged by the reactor depends on the overall coefficient of heat exchange, U, on the heat exchange surface... 

In the presence of significant thermal effects, one of the above-mentioned fluid dynamic scale-up criteria must be considered, together with the criterion US/V = constant, which can be made more realistic by considering that heat exchange surface S can be different from SL and by introducing a proper functional relationship for U. If the internal resistance to heat transfer prevails, U may be intended as the internal heat transfer coefficient h, so that the relationship... 

In general, on increasing the reactor volume, the required heat transfer surface increases faster than S3 this effect is enhanced by the decrease of the heat transfer coefficient. This prescription cannot be obeyed by the lateral surface of the reactor (which increases as S2) so that an internal or external additional heat exchange surface, whose dimensions can be fixed independently from the reactor dimensions, must be provided. 

For reactors of a larger diameter (e.g. more than 600 mm) it is more advisable to use a bundle of small tubes as heat exchangers. Such a distribution of heat exchange surfaces virtually does not inhibit fluidising and ensures that the heat is efficiently withdrawn from the whole surface of the apparatus. It should be also kept in mind that tube bundles allow one to select the reaction space and place rotating gas distribution devices between sections this considerably increases the coefficient of heat transfer and ensures a more uniform gas distribution in the reaction zone. 

In addition, the heat transport at the boundary between the fixed bed and the heat exchange surface is also decisive for the heat exchange. The latter heat transport is generally described by a wall heat-transfer coefficient otB.. It lumps the complex interplay between convective flow at the tube wall and conduction transport by contact between the fixed bed and the heat exchange surface. Heat transport in packed tubes has been investigated and discussed in detail [8, 21]. How-... 

Other prevalent issues concern the fact that most of the heat recovery for air preheating is performed on clean flue gases, which allows for higher heat exchange coefficients and lower heat exchange surfaces. 

While the curvature K doesn t exceed some value Kmax it is enough of the capillary ability of porous coating to transport of liquid, when curvature rises to K>Kmax the drainage of heated surface begins. On reaching the certain quantity of heat flux a heat exchange surface above liquid-level doesn t get sufficient amount of liquid, dry spots appear and then spreads to all over this part of surface. The liquid level is lower the maximum heat transfer coefficient is decreasing. 

An exponential expression with the quotient of alkaline contents and slagging coefficient fg is introduced for the extension of the evaluation to fouling of brown coals on heat exchanger surfaces in the temperature range of O < lOOO C after Lautenschlager (1 ) as well as Below and Rundgin (1 ), but predominantly after interpretation of operational experience with salt coals (Kluge (12)). 

Some complex compact heat exchanger surfaces have been studied using mass transfer methods, for example, naphthalene sublimation [109] and chemical reaction between a surface coating and ammonia added to the air stream [110]. These elegant but tedious methods yield local mass transfer coefficients that can be used to infer heat transfer coefficients by the usual analogy. This detailed information, in turn, should aid in the development of more efficient surfaces. Numerical studies have also yielded useful predictions for laminar flows [111, 112]. 

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