Heat transfer thermal resistance coefficient

The heat-transfer quaUties of titanium are characterized by the coefficient of thermal conductivity. Even though the coefficient is low, heat transfer in service approaches that of admiralty brass (thermal conductivity seven times greater) because titanium s greater strength permits thinner-walled equipment, relative absence of corrosion scale, erosion—corrosion resistance that allows higher operating velocities, and the inherently passive film. 

High Temperature. The low coefficient of thermal expansion and high thermal conductivity of sihcon carbide bestow it with excellent thermal shock resistance. Combined with its outstanding corrosion resistance, it is used in heat-transfer components such as recuperator tubes, and furnace components such as thermocouple protection tubes, cmcibles, and burner components. Sihcon carbide is being used for prototype automotive gas turbine engine components such as transition ducts, combustor baffles, and pilot combustor support (145). It is also being used in the fabrication of rotors, vanes, vortex, and combustor. 

In most mixers, the metal wall has a negligible thermal resistance. The paste film, however, usually has high resistance. It is important, therefore, while minimiziug the resistance of the heating or coohng medium, to move the paste up to and away from the smooth wall surface as steadily and rapidly as possible. This is best achieved by having the paste flow so as to follow a close-fitting scraper which wipes the film from the wall with each rotation. Typical overall heat-transfer coefficients are between 25 and 200 J/(m -s-K) [4 to 35 Btu/(h-fF-°F)j. 

By virtue of its chemical and thermal resistances, borosilicate glass has superior resistance to thermal stresses and shocks, and is used in the manufacture of a variety of items for process plants. Examples are pipe up to 60 cm in diameter and 300 cm long with wall tliicknesses of 2-10 mm, pipe fittings, valves, distillation column sections, spherical and cylindrical vessels up 400-liter capacity, centrifugal pumps with capacities up to 20,000 liters/hr, tubular heat exchangers with heat transfer areas up to 8 m, maximum working pressure up to 275 kN/m, and heat transfer coefficients of 270 kcal/hz/m C [48,49]. 

In the case of a temperature probe, the capacity is a heat capacity C == me, where m is the mass and c the material heat capacity, and the resistance is a thermal resistance R = l/(hA), where h is the heat transfer coefficient and A is the sensor surface area. Thus the time constant of a temperature probe is T = mc/ hA). Note that the time constant depends not only on the probe, but also on the environment in which the probe is located. According to the same principle, the time constant, for example, of the flow cell of a gas analyzer is r = Vwhere V is the volume of the cell and the sample flow rate. 

In most cases where convective heat transfer is taking place from a surface to a fluid, the circulating currents die out in the immediate vicinity of the surface and a film of fluid, free of turbulence, covers the surface. In this film, heat transfer is by thermal conduction and, as the thermal conductivity of most fluids is low, the main resistance to transfer lies there, Thus an increase in the velocity of the fluid over the surface gives rise to improved heat transfer mainly because the thickness of the film is reduced. As a guide, the film coefficient increases as (fluid velocity)", where 0.6 < n < 0.8, depending upon the geometry. 

In such a vessel, the thermal resistances to heat transfer arise from the water film on the inside of the coil, the wall of the tube, the film on the outside of the coil, and any scale that may be present on either surface. The overall transfer coefficient may be expressed by ... 

Typical values of thermal resistances and individual and overall heat transfer coefficients are given in Tables 9.15-9.18. 

The temperature difference, T1 - T2, is the driving force in this case, the quantity 1 jh S (= R), is known as the thermal resistance for convective heat transfer, and h is called the surface coefficient. 

When a fluid is heated, the hot less-dense fluid rises and is replaced by cold material, thus setting up a natural convection current. When the fluid is agitated by some external means, then forced convection takes place. It is normally considered that there is a stationary film of fluid adjacent to the wall and that heat transfer takes place through this film by conduction. Because the thermal conductivity of most liquids is low, the main resistance to the flow of heat is in the film. Conduction through this film is given by the usual relation (74), but the value of h is not simply a property of the fluid but depends on many factors such as the geometry of the system and the flow dynamics for example, with tubes there are significant differences between the inside and outside film coefficients. 

On the other hand, it has been argued that the resistance to heat transfer is effectively within a thin gas film enveloping the catalyst particle [10]. Thus, for the whole practical range of heat transfer coefficients and thermal conductivities, the catalyst particle may be considered to be at a uniform temperature. Any temperature increases arising from the exothermic nature of a reaction would therefore be across the fluid film rather than in the pellet interior. 

A katharometer is employed to determine the concentration of H2 in a H2/CH4 mixture. The proportion of H2 can vary from 0 to 60 mole per cent. The katharometer is constructed as shown in Fig. 6.54 from four identical tungsten hot-wire sensors for which the temperature coefficient of resistance ft, is 0.005 K. The gas mixture is passed over sensors R, and R whilst the reference gas (pure CH4) is passed over sensors R2 and R,. The total current supplied to the bridge is 220 mA and it is known that the resistance at 25°C and surface area of each sensor are 8 Q and 10 mm2 respectively. Assuming the heat transfer coefficient h between gas and sensor filaments to be a function of gas thermal conductivity k only under the conditions existing in the katharometer and that in this case h = k x 10 (h in W/m2K and k in W/mK), draw a graph of the output voltage V0 of the bridge network as a function of mole per cent H2. 

We take the heat transfer coefficient a to be independent of the jet velocity and of the residence time in the vessel. Physically, this assumption together with the assumption of complete mixing of the substance in the reaction vessel and of a constant mean temperature throughout the vessel corresponds to the idea that for heat transfer the governing factor is the thermal resistance from the internal wall of the vessel to the outside space in which the temperature is kept at T0. In other words, our assumption corresponds to the concept of a vessel which is thermally insulated from outside. 

The practical heat-transfer coefficient is the sum of all the factors that contribute to reduce heat transfer, such as flow rate, cocurrent or countercurrent, type of metal, stagnant fluid film, and any fouling from scale, biofilm, or other deposits. The practical heat-transfer coefficient ((/practical) is, in reality, the thermal conductance of the heat exchanger. The higher the value, the more easily heat is transferred from the process fluid to the cooling water. Thermal conductance is the reciprocal of resistance (/ ), to heat flow ... 

Thus, an apparently slower mass uptake is observed as compared with the isothermal uptake or the case when heat transfer is controlled by the external film resistance. Consequently, ignoring the internal thermal resistance may lead to erroneously low mass transfer coefficient. 

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