Surface overall temperature difference

AT, At Temperature difference Atj, Ato, and At temperature difference across bodies 1, 2, and 3 or at points 1, 2, and 3 AT, , At, , for overall temperature difference Ati, for temperature difference between surface and boihng hquid K ... 

In designing and evaluating heat exchangers. Equation 5.1 cannot be used directly, as the temperatures ofthe wall surface and are usually unknown. Thus, the usual practice is to use the overall heat transfer coefficient U (kcal h m 2 °C or Wm K" ), which is based on the overall temperature difference - that is, the difference between the bulk temperatures of two... 

Here the heat-transfer rate is related to the overall temperature difference between the wall and fluid and the surface area A. The quantity h is called the convection heat-transfer coefficient, and Eq. (1-8) is the defining equation. An analytical calculation of h may be made for some systems. For complex situations it must be determined experimentally. The heat-transfer coefficient is... 

Two 3.0-cm-diameter 304 stainless-steel bars, 10 cm long, have ground surfaces and are exposed to air with a surface roughness of about I fim. If the surfaces are pressed together with a pressure of 50 atm and the two-bar combination is exposed to an overall temperature difference of I00°C. calculate the axial heat flow and temperature drop across the contact surface. 

In these equations, and h are the inside and outside lihn heat-transfer coefficients. A, and A are the inside and outside surface areas of the clean tube, and Rf are the inside and outside surface fouling resistances, is the interface temperature between the fluid and the surface of the fouling deposit inside the tube, tf is the interface temperature between the fluid and fouling surface on the outside, and are the inside and outside tube wall temperatures, and is the thermal conductivity of the tube material. Each of the terms in the denominator is a resistance to the heat flow, and their sum is the total resistance to heat transfer between the two fluids. These equations can be combined to eliminate the intermediate temperatures and find the heat-transfer rate using only the overall temperature difference (T- t) ... 

The evaporation of an organic solvent is a common conunercial process. For example, using an SDR to evaporate toluene, aim diameter disc could evaporate approximately 1.5 t/h of toluene, again with a temperature different of 20°C. Although the use of a 20°C overall temperature difference is typical of industry, the very high process film heat transfer coefficient of an SDR means that the temperature difference between the process fluid and the disc surface is very low. For an overall temperature difference of 20°C the film to process temperature difference is only about 5°C. This can be very important when processing heat sensitive materials where low delta-Ts can reduce product degradation. 

The overall thermal transmittance, t/, is used to calculate the total heat flow. For a plane surface of area A and a steady temperature difference AT, it is... 

Conduction of heat through plain surfaces under steady-state conditions is given by the product of the area, temperature difference, and overall conductance of the surface (see Section 1.8) ... 

To evaluate the required condenser area, point values of the group UAT as a function of qc must be determined by a trial and error solution of equation 9.181. Integration of a plot of qc against 1/17AT will then give the required condenser area. This method takes into account point variations in temperature difference, overall coefficient and mass velocities and consequently produces a reasonably accurate value for the surface area required. 

This section is concerned with the UA xtiT — Text) term in the energy balance for a stirred tank. The usual and simplest case is heat transfer from a jacket. Then A xt refers to the inside surface area of the tank that is jacketed on the outside and in contact with the fluid on the inside. The temperature difference, T - Text, is between the bulk fluid in the tank and the heat transfer medium in the jacket. The overall heat transfer coefficient includes the usual contributions from wall resistance and jacket-side coefficient, but the inside coefficient is normally limiting. A correlation applicable to turbine, paddle, and propeller agitators is... 

U = overall heat transfer coefficient in Btu/hr ft2oF A = tube surface area in ft2 ATM = mean temperature difference in °F. For our... 

The planets nearest the Sun have a high-temperature surface while those further away have a low temperature. The temperature depends on the closeness to the Sun, but it also depends on the chemical composition and zone structures of the individual planets and their sizes. In this respect Earth is a somewhat peculiar planet, we do not know whether it is unique or not in that its core has remained very hot, mainly due to gravitic compression and radioactive decay of some unstable isotopes, and loss of core heat has been restricted by a poorly conducting mainly oxide mantle. This heat still contributes very considerably to the overall temperature of the Earth s surface. The hot core, some of it solid, is composed of metals, mainly iron, while the mantle is largely of molten oxidic rocks until the thin surface of solid rocks of many different compositions, such as silicates, sulfides and carbonates, occurs. This is usually called the crust, below the oceans, and forms the continents of today. Water and the atmosphere are reached in further outward succession. We shall describe the relevant chemistry in more detail later here, we are concerned first with the temperature gradient from the interior to the surface (Figure 1.2). The Earth s surface, i.e. the crust, the sea and the atmosphere, is of... 

Here the index i distinguishes the different regions, Q is the overall heat flux from outside into the system through the i-th region, which is maintained at the temperature Ij. If we have only two surfaces with temperatures Tj and Tn, then Qj = —Qn by the conservation of energy. Referring the heat transfer to some surface S0, we determine the heat transfer coefficient... 

When processing is controlled by heat transfer variables, a log mean temperature difference (ATlmtd) and heat transfer surface area will predominate over the agitation variables. Provided it is sufficient to give a homogeneous process fluid temperature, increased agitation can only reduce the inside film resistance, which is one of a number of resistances that determines the overall heat transfer coefficient. 

The heat-transfer coefficient for the water flow on the inside of the pipe is determined from the flow conditions with properties evaluated at the bulk temperature. The free-convection heat-transfer coefficient on the outside of the pipe depends on the temperature difference between the surface and ambient air. This temperature difference depends on the overall energy balance. First, we evaluate /i,and then formulate an iterative procedure to determine h . 

Forced convection heat transfer is probably the most common mode in the process industries. Forced flows may be internal or external. This subsection briefly introduces correlations for estimating heat-transfer coefficients for flows in tubes and ducts flows across plates, cylinders, and spheres flows through tube banks and packed beds heat transfer to nonevaporating falling films and rotating surfaces. Section 11 introduces several types of heat exchangers, design procedures, overall heat-transfer coefficients, and mean temperature differences. 

Figure 3.10 illustrates a qualitative dependence of the overall heat AuxJq from the bottom to the upper surface on the temperature difference AT. 

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