Dependence overall heat transfer

The effects of axial mixing and of overall heat transfer are shown in figure 2, where the curves 1 and 2 show the steady state profiles at zero axial mixing, while curves 0 and 3 are calculated with D=0.5m2/s. Curves 2 and 3 are calculated from the temperature dependent overall heat transfer coefficients of [4] which are higher than the value of 4kJ/s/m2/K used for... 

If solvent recovery is maximized by minimizing the temperature approach, the overall heat-transfer coefficient in the condenser will be reduced. This is due to the fact that a large fraction of the heat transfer area is now utilized for cooling a gas rather than condensing a Hquid. Depending on the desired temperature approach the overall heat-transfer coefficients in vent condensers usually range between 85 and 170 W/m K (ca 15 and 30 Btu/h-ft. °F). 

The rate of heat-transfer q through the jacket or cod heat-transfer areaM is estimated from log mean temperature difference AT by = UAAT The overall heat-transfer coefficient U depends on thermal conductivity of metal, fouling factors, and heat-transfer coefficients on service and process sides. The process side heat-transfer coefficient depends on the mixing system design (17) and can be calculated from the correlations for turbines in Figure 35a. 

The overall heat-transfer rate is almost entirely dependent upon the film coefficient between the inner jacket wall and the solids, which depends to a large extent on the solids characteristics. Overall coefficients may range from 30 to 200 J/(m s K), based upon total area if the diyer walls are kept reasonably clean. Coefficients as low as 5 or 10 may be encountered if caking on the walls occurs. 

The inside eonveetive heat transfer eoeffieient h is the only element of the overall heat transfer eoeffieient U that varies with the agitation speed N. If heat is removed from an agitated reaetor using an internal eoil or external jaeket, the overall heat transfer rate depends on the rotation speed of the agitator N and if the proeess side offers the major resistanee. This is expressed by... 

The radiant heat transfer coefficient becomes important above about 600°C, but is difficult to predict. Baskakov et al. (1973) report that depending on particle size, hr increases from approximately 8% to 12% of the overall heat transfer coefficient at 600°C, to 20 to 33% of h at 800°C. 

Since the overall heat transfer coefficient U depends also on the coolantflow rate, it must be emphasized that loss of coolant flow or fouling of the heat transfer surface on the coolant side has a similar effect as shown for loss of agitation. 

The overall heat transfer coefficient between the gas-liquid dispersion on the tray and the cooling medium in the tubes is dependent upon the gas velocity, as pointed out by Poll and Smith148 , but is usually in the range 500-2000 W/m2 K. 

Overall heat transfer coefficients for any form of evaporator depend on the value of the film coefficients on the heating side and for the liquor, together with allowances for scale deposits and the tube wall. For condensing steam, which is a common heating medium, film coefficients are approximately 6 kW/m2 K. There is no entirely satisfactory... 

For most processing equipment, the low thermal conductivity of polymers strongly influences the overall heat transfer coefficient between the bulk of the polymer and the contacting metal surfaces, creating limitations in heat transfer rates. Heat transfer rates between processing equipment and the polymer depend on many factors, including thermal conductivity, machine clearances, and screw... 

In the convection zone of the heater, some heat also is transferred by direct radiation and reflection. The several contributions to overall heat transfer specifically in the convection zone of fired heaters were correlated by Monrad [Ind. Eng. Chem. 24,505 (1932)]. The combined effects are approximated by item 10 of Table 8.16, which is adequate for estimating purposes. The relation depends on the temperature of the gas film which is taken to be the sum of the average process temperature and one-half of the log mean temperature difference between process and flue gas over the entire tube bank. The temperature of the gas entering the convection zone... 

The overall heat-transfer coefficient U depends upon the properties of the dry product and the method of heal transfer. The heat-transfer rate A is influenced by the mechanical design of the heating elements and the conditioning of the frozen mass. The temperature gradient AT is limited by the maximum allowable temperatures al the sublimation interface and dry-layer surface. In the constant-rate period, the lirst one-half to two-thirds of the drying cycle, about 8fl + of the water is removed. 

The first term depends entirely on the physical properties of the reactor contents and degree of agitation. It represents resistance to heat transfer of the internal film and of eventual deposits at the wall, which may determine the overall heat transfer [3], Therefore, the reactor should be regularly cleaned with a high pressure cleaner. Both last terms depend on the reactor itself and on the heat exchange system, that is, reactor wall, fouling in the jacket, and external liquid film. They are often grouped under one term the equipment heat transfer coefficient (cp) [4, 5],... 

The Q term in Eq. (2.5) depends on the heat removal scheme used. If a circulating jacket water system is used, the jacket is essentially at one temperature Tj, and the heat transfer rate depends on the jacket area, the overall heat transfer coefficient and the differential temperature driving force... 

The overall heat transfer coefficient U (kW m-2 K-1) depends on the velocity through the tubes ... 

In a liquid-liquid exchanger, the total heat transferred (Q) from the hot process fluid to the cooling water is dependent on the overall heat transfer coefficient (U), the heat transfer area (A), and the log mean temperature difference (ATm). Therefore, any of these can be manipulated to control Q. 

In enclosure fires, radiation may be the dominant mode of heat transfer. For flames burning in an open atmosphere, the radiative fraction of overall heat transfer ranges from less than 0.1 to 0.4, depending both on the fuel type and the fire diameter [45], Owing to the important role that radiation plays in fires, all fire CFD models have a radiation model that solves the radiation transport equation (RTE) [46,48] ... 

In some cases it is not possible to consider the modes separately. For example, if a gas, such as water vapor or carbon dioxide, which absorbs and generates thermal radiation, flows over a surface at a higher temperature, heat is transferred from the surface to the gas by both convection and radiation. In this case, the radiant heat exchange influences the temperature distribution in the fluid. Therefore, because the convective heat transfer rate depends on this temperature distribution in the fluid, the radiant and convective modes interact with each other and cannot be considered separately. However, even in cases such as this, the calculation procedures developed for convection by itself form the basis of the calculation of the convective part of the overall heat transfer rate. 

For a hollow cylintfer exposed to a convection environment on its inner and outer surfaces, the electric-resistance analogy would appear as in Fig. 2-6 where, again, 1A and TB are the two fluid temperatures. Note that the area for convection, is not the same for both fluids in this case, these areas depending on the inside tube diameter and wall thickness. In this case the overall heat transfer would be expressed by... 

The overall heat transfer coefficient is a composite number. It depends on the individual heat transfer coefficients on each side of the tube and the thermal conductivity of the tube material. The individual heat transfer coefficient in turn depends on the fluid flow rate, physical properties of the fluid, and dirt factor. The temperature along the tube is not uniform. The hot and the cold fluids may flow in the same (cocurrent) or in opposite (countercurrent) directions. Generally the hot and cold fluids come in contact only once, and such an exchanger is called single pass. In a multipass exchanger, the design of the... 

Figure 4 plots, against suspension density, the heat transfer coefficients measured by Basu (1990) over a wide range of bed temperature for 296 pm sand, by Kobro and Brereton (1986) at a temperature of 850°C for 250 pm sand and by Grace and Lim (1989) at 880°C for 250-300 pm sand. The overall heat transfer coefficient is shown to increase with bed temperature. Before radiation becomes dominant in heat transfer, the observed rise in heat transfer coefficient with bed temperature may be explained as follows. The gas convective component is expected to decrease mainly because of the inverse dependence of gas density on temperature. On the other hand, the particles convective component will increase with temperature, thus leading to an increase in gas conductivity, because the latter is dominant for... 

Here we use a simplified overall heat transfer coefficient, h , to approximate these heat transfer processes. If the heat transfer is primarily by convection, the heat transfer coefficient will depend on the velocity of the gas over the green body [110]. If, however, the heat transfer is primarily by radiation, then the heat transfer coefficient will depend on the temperature of the furnace [110, p. 387]. This heat will be used to heat the ceramic to the temperature where a portion of the material will melt. The effective heat of melting is given by... 

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