Forms of Heating Surface

The heat transfer term envisions convection to an external surface, and U is an overall heat transfer coefficient. The heat transfer area could be the reactor jacket, coils inside the reactor, cooled baffles, or an external heat exchanger. Other forms of heat transfer or heat generation can be added to this term e.g, mechanical power input from an agitator or radiative heat transfer. The reactor is adiabatic when 7 = 0. 

The simplest and cheapest form of heat transfer surface for installation inside a vessel is a helical coil see Figure 12.73. The pitch and diameter of the coil can be made to suit the... 

The photogenerated electrons and holes may recombine in the bulk of the semiconductor or on its surface within a very short time, releasing energy in the form of heat. However, electrons and holes that migrate to the surface of the semiconductor without recombination can respectively reduce and oxidise the reactants adsorbed by the semiconductor. Both surface-adsorption and photochemical-reaction rates are enhanced by use of nano-sized semiconductor particles, as a greatly enhanced surface area is made available. 

Some of the energy imparted to the film of material appears in the form of heat, and a jacketed shroud is frequently fitted round the periphery of the working surfaces so that some of the heat may be removed by coolant. This jacket may also be used for circulation of a heating medium to maintain a desired temperature of the material being processed. 

The photogenerated electrons and holes can recombine in bulk or on the semiconductor surface releasing energy in the form of heat or a photon. The electrons and holes that migrate to the semiconductor surface without recombination can, respectively, reduce and oxidize water (or the reactant) and are the basic mechanism of photocatalytic hydrogen production, see Fig. 6.3. 

An immense amount of energy from the sun strikes the surface of the earth every day. This energy may be captured and used in the form of heat in solar thermal applications, or it may be converted directly into electricity to power electrical devices using photovoltaic cells. 

From the three distinct 2D cross-sectional views (7.41a), (7.42), (7.43) of the P-T-x surface, we can now visualize the full 3D form of the surface as shown in Fig. 7.8. The surface is seen to resemble a curved envelope, clipped at each end to reveal the inside of the envelope through the hatched holes. Viewed toward the P—T plane, only the curved edge of the envelope is seen, as in (7.41a). However, viewed toward the P-xB plane or the T-xB plane, the inside of the envelope is seen as the hatch marks in (7.42) or (7.43), respectively. The upper P-T-x surface of the envelope is called the bubble-point surface, in reference to the first vapor bubbles that are seen as the liquid is heated to its boiling point. The P-T-xBap underside of the envelope is correspondingly called the dew-point surface, in reference to the first dewy droplets of liquid as the vapor is cooled to its condensation temperature. Although we normally see only the flat P-T, P-xB, or T-xb projections on the blackboard or book page, it is useful to keep in mind the full 3D form of the P-T-xB surface that underlies these 2D projections of the / = 3 system. 

It has long been known that heat is generated when fine particles are wetted. This is a surface phenomenon which does not necessarily depend upon chemical activity. The latter is important and markedly affects observations but entirely apart from chemical action the creation of a film requires a certain amount of work which is expended in the form of heat. 

F, 1.05 lb/h of condensate will be formed per square foot of heating surface in still air. With forced air circulation, the condensate rate is (5)(1.05) = 5.25 lb/h per square foot of heating surface. 

Sensible heat loss from the skin depends on the temperatures of the skin, die environment, and the surrounding surfaces as well as the air motion. The latent heat loss, on the other hand, depends on the skin wettedness and the relative humidity of the environment as well. Clothing serves as insulation and reduces both the sensible and latent forms of heat lo.ss. The heat transfer from the lungs through respiration obviously depends on the frequency pf breatlrlng and the volume of the lungs as well as the environmental faplors that affect heat tran.sfcr from tlie skin. 

Finally, noting that the direction of heat transfer is alv/ays from high to low temperature, all forms of heat transfer determined above are in the same direction, and the total rate of heat loss from the water to the surrounding air and surfaces is... 

The simplest and cheapest form of heat transfer surface for installation inside a vessel is a helical coil see Figure 12.75. The pitch and diameter of the coil can be made to suit the application and the area required. The diameter of the pipe used for the coil is typically equal to Dj,/30, where Dj, is the vessel diameter. The coil pitch is usually around twice the pipe diameter. Small coils can be self-supporting, but for large coils some form of supporting structure will be necessary. Single- or multiple-turn coils are used. 

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