Loads effective heat transfer area

Situation a For products loaded in a two-high configuration on 12" high piers, the effective heat transfer area of the top load(s) would be their full projected top surface area. Because of the thinner gas cloud or blanket adjacent to the lower row of load pieces, their effective heat transfer area would be less. (See fig. 4.7.)... 

Positioning the loads to raise their effective heat transfer area not only improves heat transfer rates but also reduces the lag time (time it takes for the core or lowest %exposed area side to reach the temperature of the hottest surfaces). This benefit reduces thermal stresses in the product, resulting in shorter cycles (less fuel and higher productivity) plus higher quality products. 

AlO. When the spaces between the load pieces are perpendicular to the furnace gas flow, the gases between the loads are practically stationary, so their temperature will stay very near that of the loads. With essentially no temperature difference between these gases and the loads, little if any heat transfer takes place. If energy can be supplied to the stagnant area between the loads by small high-velocity burners (enhanced heating), the effective heat transfer area between the loads and the hearth will increase by more than 25%. 

In furnaces with top and bottom heat and preheat zones, there is greater resistance to poc gas flow below the loads and their conveyor. That resistance causes the bulk of the bottom gases to flow into the top zones, reducing the effective heat transfer exposure areas significantly. This movement of combustion gases into the top zones reduces productivity and lowers available heat, increasing fuel use per ton of product. 

The most effective way to control a condenser is to vary its heat transfer area. This is done by manipulating the flow of condensate so as to partially flood the condenser, thereby reducing the surface available for condensation. The level of condensate within the condenser is an indication of the heat load on the process. The system is described in Fig. 9.7. 

A triple-effect evaporator is fed with 5 kg/s of a liquor containing 15 per cent solids. The concentration in the last effect, which operates at 13.5 kN/m2, is 60 per cent solids. If the overall heat transfer coefficients in the three effects are 2.5, 2.0, and 1.1 kW/m2K, respectively, and the steam is fed at 388 K to the first effect, determine the temperature distribution and the area of heating surface required in each effect The calandrias are identical. What is the economy and what is the heat load on the condenser ... 

Effect of Mechanical Load. The influence of mechanical load on thermal conductivity of various evacuated multilayer insulations has been investigated. Figure 9 shows the effect of mechanical load on thermal conductivity for the above multilayer insulation. Assuming that the thermal conductivity of insulation under compression is a function of the heat transferred by solid conduction at the contact points, the thermal conductivity is a function of the size of the contact area. The size of the contact area is proportional to the f power of the contact pressure [ ]. The curve representing the f power of mechanical load has the same shape as the experimentally measured points for loads exceeding about 2 psi. 

With a load placed in a furnace or oven, its effective area for heat transfer is determined by its location relative to other loads, the sidewalls, and the end walls. 

Temperature differences in a loop stem provide the driving force for mass transport of corrosion products. Although no definitive experiments have been conducted, indications are that the magnitude of the AT in sodium has no effect above a minimum value of about 100 C [691 From a consideration of heat transfer properties, in any heat transfer system at a given flow velocity, different liquid metals would require different ATs to dehver the same heat load. If the surface area and geometry are fixed by heat flux considerations, the AT and/or flow velocity, and hence mass transfer fluxes, will change for liquid metals with different heat transfer characteristics. 

Double-effect evaporators tend to produce steam economies of 1.6-1.85 in backward-flow systems. Triple-effect steam economies are about 2.5, and flow arrangements are backward or mixed. Steam economy is variable and as a primary measure of performance may be unreliable. For example, the extent of use of steam condensate for heat interchange varies. This practice retains heat within the evaporator system and always improves the measured steam economy. The overall heat economy, however, may not improve. Recycling colder condensate to the boilers may simply transfer some of the load from the process to the utility area. 

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