Loads heat transfer within

Rg. 2.21 The many concurrent modes of heat transfer within a fuel-fired furnace. Some refractory surfaces, r, and charged loads, c, are convection-heated by hot poc flowing over them. Triatomic molecules of the combustion gases, g, and soot particles, p, radiate in all directions to refractories, r and loads, c. The surfaces of r and c in turn radiate in all possible directions, such as r to r, r to c, c to c, and c to r. 

To have temperature uniformity within each load piece and among the pieces, furnace gases and solids must have low temperature differences. All heat supplied by the combustion reaction flows either (1) directly from the hot poc gases to the load or (2) from the poc gases to the refractory, and is then re-radiated to the load. Heat transfer is a form of potential flow, moving from high temperature to low temperature. Thus, the flame and poc gases must be hotter than the refractory, and the refractory must be hotter than the load. 

With an effective thermal model of the cells, modules and overall system, an analysis of the performance under different situations and load conditions can be evaluated. This proves to be a very useful tool in the development of the pack as these thermal models can be input into computational fluid dynamic (CFD) models to determine how the cells will heat during operation. A good CFD model can be used to determine flow rates, turbulence, and heat transfer within a pack. In addition, it is possible to use a lumped parameter model to develop a simplified model where the external parameters are basically ignored and the model is designed using fully adjustable parameters to do high-level evaluations of the thermal effectiveness of a system. 

The removal of heat within an enclosed space must be considered as a multi-step heat transfer process. Heat passes from the occupants or equipment to the air within the space, and from there to the refrigerant or chilled water. It follows that the temperature differences at each step are a reciprocal function of the air mass flow. Where there is a high latent heat load within the space, the relative humidity will also vary with the air flow - the variation being higher with low air flow. 

The intent of this study is to determine the coolest points within a specihed load and conhguration. Cool points originate because of the varied rate of heat transfer throughout the load. It is therefore imperative that heat penetration studies be conducted to determine cool points within a loading pattern and ensure that they are consistently exposed to sufficient heat lethality. 

Anchor Mixers Anchor mixers are the simplest and one of the more common types of high-viscosity mixers (Fig. 18-42). The diameter of the anchor D is typically 90 to 95 percent of the tank diameter T. The result is a small clearance C between the rotating impeller and the tank wall. Within this gap the fluid is sheared by the relative motion between the rotating blade and the stationary tank wall. The shear near the wall typically reduces the buildup of stagnant material and promotes heat transfer. To reduce buildups further, flexible or spring-loaded scrapers, typically made of polymeric material, can be mounted on the rotating blades to move material physically away from the wall. 

Semitransparent Material. When the load is not opaque, radiative heat transfer occurs within the material in conjunction with conduction and/or advective transfer (if the load is moved with respect to a coordinate system). The thermal response of the load is, therefore, determined in part by volumetric radiation heat transfer, necessitating prediction or measurement of the relevant radiative properties. 

Direct Fired Furnaces. For direct fired furnaces, radiative heat transfer from the flame and combustion products as well as from the walls to the load is usually the dominant heat transfer mode. Convection from the combustion gases makes a much smaller contribution. The radiative transfer within the furnace is complicated by the nongray behavior of the combus-... 

Load configurations and packaging material.s have a significant effect on the rate of heat transfer into the product. Since it is microorganism.s within the product that are to be inactivated, it is to the product that the critical process conditions must be delivered. Multiprobe temperature penetration profiles over replicate cycles are necessary. A wider tolerance of 5 C can be expected [6) due to slower response limes. Cold spots (if any) should be identified and corrected, or specifically examined in the biological phase of process validation. 

Conduction heat transfer is molecule-to-molecule transfer of vibrating energy, usually within solids. Heat transfer solely by conduction to the charged load is rare in... 

The soot particles are confined within the visible flame. The triatomic molecules are everywhere within the furnace, but can absorb and emit radiation only within narrow wavelength bands. Interference among the several modes of heat transfer can make calculation of net heat transfer in a fuel-fired furnace difficult. Some of the many variables that must be considered are composition, velocity, temperature, and beam thicknesses of the poc and well as emissivities, absorptivities, conductivities, densities, and specific heats of the refractory wall and load materials. 

In most heating applications, temperature uniformity is a major player in product quality. Furnace users have insisted that temperature differences from thermocouples in gridlike racks should be within 25°F, or 10°F with no loads in the furnace. After the loads are placed in a furnace, the thermocouple grid uniformity check should be replaced by T-sensors strategically attached to the loads because the following heat transfer variables become dominant. 

Because ample heat can usually be released at sufficiently high temperatures in industrial furnaces, the next problem to be studied in calculation of furnace capacity should be heat transfer to the furnace load and temperature equalization within the load. With adequate heat release at sufficiently high temperature assured, note the following factors that affect furnace capacity. 

Generally, (a) the rate of heat transfer to the load determines the best possible heating rate for thin loads whereas (b) temperature equalization within the load(s) determines heating capacity rates for thick loads, especially those having low thermal conductivity. 

The preceding two questions cause one to wonder how to evaluate a log mean temperature difference for the purpose of calculating the heat transfer to the load. There is a practical answer to this and to how to get the most even temperature distribution within the load Use enough blower power and velocity to assure a temperature drop in the gas stream less than the allowable temperature difference within the load, in which case use a simple average temperamre drop for the calculation (see table 3.2). 

To keep the temperature differences small within the load(s) across the furnace, heat transfer beneath the load from the gas blanket to piers and product must be kept relatively low. To minimize heat transfer from the gas stream, the thickness of the stream must be very small (8 to 12 in., or 200 to 300 mm), and the percentage of diatomic gases in the products of combustion must be low. Excess air will lower the percentage of diatomic gases and reduce the temperature drop of the gas sdeam under the load from the burner wall to the opposite wall. 

Examples of nonuniform heating-control problems above 10(X) F (538 C) are (1) nonuniform scale formation with carbon steels, (2) questionable completion of the combustion reaction (pic contact the load surface), (3) sticky scale with resultant rolled-in scale, (4) spotty decarburization of high carbon steels, (5) some stainless steels may not tolerate contact with the reducing atmosphere within the flames, and (6) using impingement heating for steel pieces of heavy cross section could cause formation of reflective scale with resultant reduction of heat transfer. 

If the burner firing rate is increased, the gas volume and temperature increases thus, the gas flow velocity increases. The cumulative heat transfer from hot gases to loads (directly, and indirectly via refractory to loads) is a function of time. Higher velocity shortens the time for heat transfer to be accomplished within a given flow path length (furnace size) thus, the gases remain at higher temperature. 

A perfect heating situation would have each load piece completely surrounded (360 degrees in all planes) by equally high heat transfer rates to all its surfaces. That is often impossible or impractical because of (a) load shape and size, (b) handling and support problems, and (c) lack of appropriate piers, posts, or kiln furniture. The resultant uneven heating necessitates a long soak time to let the temperatures even out within the load, with possible increased fuel costs. Long soak times may cause excessive surface oxidation, and they surely cause lowered furnace productivity. 

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