Heat transfer to surface

Heat Transfer. One of the reasons fluidized beds have wide appHcation is the excellent heat-transfer characteristics. Particles entering a fluidized bed rapidly reach the bed temperature, and particles within the bed are isothermal in almost all commercial situations. Gas entering the bed reaches the bed temperature quickly. In addition, heat transfer to surfaces for heating and cooling is excellent. 

Xavier, A. M., and Davidson, J. F., Heat Transfer to Surfaces Immersed in Fluidized Beds, Particularly Tube Arrays, Fluidization, Proc. of Second Eng. Foundation Conf., pp. 333-338, Cambridge Univ. Press (1978)... 

Thai is, the net rate of radiation heal transfer from surface J to surface 2 must equal the net rate of radiation heal transfer from surface 1 and the net rate of radiation heat transfer to surface 2. 

From a radiative thermal network, normalizing the heat transfer to surface i of area A, and where all other surface resistances Rj Ri, then... 

Nonuniform Surface Temperature. Nonuniform surface temperatures affect the convective heat transfer in a turbulent boundary layer similarly as in a laminar case except that the turbulent boundary layer responds in shorter downstream distances The heat transfer to surfaces with arbitrary temperature variations is obtained by superposition of solutions for convective heating to a uniform-temperature surface preceded by a surface at the recovery temperature of the fluid (Eq. 6.65). For the superposition to be valid, it is necessary that the energy equation be linear in T or i, which imposes restrictions on the types of fluid property variations that are permitted. In the turbulent boundary layer, it is generally required that the fluid properties remain constant however, under the assumption that boundary layer velocity distributions are expressible in terms of the local stream function rather than y for ideal gases, the energy equation is also linear in T [%]. 

The heat transfer to surfaces at 76 or 20 K is materially reduced when cold surface wrapping is employed. The heat transfer is independent of the presence of solid CO2 in single- or multiple-wrap insulations. 

Now assume there is a uniform vertical temperature gradient, dT/dy in the system. Molecules crossing the surface from above will have a temperature given by T - - h) dT/dy, while molecules crossing the surface from below will have temperature given by T — h)dT/dy. Therefore, the net heat transferred to surface dA in time df will be given by... 

The stainless steel plates are separated (ca 3 pm between) by nonabsorbent vulcanized gaskets. Various profiles and configurations, including raised knobs, crescents, channels, or diamonds, provide a rapid, uniform heat-transfer plate surface. During operation the plates must be pressed together to provide a seal, and mounted and coimected in such a manner that air is eliminated and that the product drains from the plates without opening. 

The effect on the coolant temperature of latent and sensible heat transferred to the surface from the condensing vapor is as shown in equation 5 ... 

The presence of 0.004—0.0062% Te in a welding rod or wke produces a positive surface tension coefficient which enhances the heat transfer to the... 

For the radiative mechanism of heat transfer to solids, the rate equation for parallel-surface operations is... 

High heat transfer to reduce surface area. 

Wet-bulb temperature is the dynamic equilibrium temperature attained by a water surface when the rate of heat transfer to the surface by convection equals the rate of mass transfer away from the surface. At equilibrium, if neghgible change in the dry-bulb temperature is assumed, a heat balance on the surface is... 

Constant-rate Period In the constant-rate period moisture movement within the sohd is rapid enough to maintain a saturated condition at the surface, and the rate of diying is controlled by the rate of heat transferred to the evaporating surface. Drying proceeds by diffusion of vapor from the saturated surface of the material across a... 

When the heat for evaporation in the constant-rate period is supplied by a hot gas, a dynamic equilibrium establishes the rate of heat transfer to the material and the rate of vapor removal from the surface ... 

Under steady-state conditions the temperature of the evaporating surface increases until the rate of sensible heat transfer to the surface equals the rate of heat removed by evaporation from the surface. To calculate this temperature, it is convenient to modify Eq. (12-26) in terms of humidity rather than partial-pressure difference, as follows ... 

Heat transfer to or from active centers to the catalyst-particle surface... 

Limit temperature of heating medium Use split heating/cooling system to eliminate heat transfer to unwetted surface Heat with sparged steam/tempered water Avoid splashing of material onto unwetted heating surface... 

The initial moisture content is a determinant factor in the rate of heat transfer to the center of the core mat [226,227]. At short press closing times the rapid temperature rise occurs at the same time for both lower and higher moisture content percentages indicating that the steam condensation front reaches the core at the same rate and that this is then determined more by local permeability rather than local moisture content. The slope of the rise is similar as it is the balance of horizontal and vertical permeability which controls the rate of steam flow to the core layer. Furthermore, water remains in the surface layer in a quantity such as... 

Heat transfer firom surfaces to a fluidized bed is a strong function of the rate at... 

Gritzo, L. A, et al., 1995a, Heat Transfer to the Fuel Surface in Large Pool Fires, Transport Phenomenon in Combustion, S. H. Choa (ed.), Taylor and Francis Publishing, Washington, DC. 

The major reasons for the beluu ior of vertical temperature in water bodies are the low thermal condnctii ity and the absorption of heat in the first few meters. As tlie surface waters begin to heat, transfer to low er layers is reduced and a stability condition develops. The prediction of thermal behavior in lakes and reser oirs is an important power plant siting consideration and also is a major factor in preienting e.xcessive thermal effects on sensitive ecosystems. Furthermore, the extent of thermal stratification influences the vertical dissolved ox)gen (DO) profiles where reduced DO often results from minimal exchiuige with aerated water. ... 

The outer and inner tubes extend from separate stationary tube sheets. The process fluid is heated or cooled by heat transfer to/from the outer tube s outside surface. The overall heat transfer coefficient for the O.D. of the inner tube is found in the same manner as for the double-pipe exchanger. The equivalent diameter of the annulus uses the perimeter of the O.D. of the inner tube and the I.D. of the inner tube. Kem presents calculation details. 

The carbon/hydrogen ratio of gas is considerably lower than oil or coal, which results in a flame of very low luminosity. Radiation from the flame is therefore low and furnace design must allow for heat transfer to be primarily by convection and conduction, together with re-radiation from hot surfaces. 

Another approach is to improve the heat transfer conditions from the product. This can be accomplished in several ways. One way is to operate in a coolant medium that would also act as the lubricant for the system. The heat transfer to a liquid is usually much better than to air, and the liquid can be cooled by passing it through a heat exchanger device. A second approach is to improve the heat transfer to air. This can be done by increasing the surface area of the product by means of fins or other surface projections. The larger area will increase the heat flow... 

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