Structural heat transfer surfaces

Modern steam plants are quite elaborate structures that can recover 80% or more of the heat of combustion of the fuel. The simplified sketch of Example 1.2 identifies several zones of heat transfer in the equipment. Residual heat in the flue gas is recovered as preheat of the water in an economizer and in an air preheater. The combustion chamber is lined with tubes along the floor and walls to keep the refractory cool and usually to recover more than half the heat of combustion. The tabulations of this example are of the distribution of heat transfer surfaces and the amount of heat transfer in each zone. 

Layer or layers of minerals (especially calcium carbonate) deposited, by the throwing down, or precipitation, onto a heat-transfer surface, reducing its U value. Scales are often hard and dense and difficult to remove. The scale can be crystalline in nature (a solid body having a characteristic internal structure, with symmetrically arranged plane surfaces and definite angles), or amorphous (lacking any characteristic crystalline shape). 

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. 

To prevent or reduce the risk of robust crystalline structures forming on heat-transfer surfaces, it is helpful to employ a crystal modifier. The action of these chemical compounds is to interfere with the crystal structure, so that the scale is more susceptible to the removal forces created by the liquid flow across the deposit. Crystal modifiers including polymaleic acid are widely used in cooling water circuits. 

Plate evaporators may be constructed of flat plates or corrugated plates, the latter providing an extended heat transfer surface and improved structural rigidity. Two basic types of heat exchangers are used for evaporation systems plate-and-frame and spiral-plate evaporators. Plate units are sometimes used because of the theory that scale will flake off such surfaces, which can flex more readily than curved tubular surfaces. In some plate evaporators, flat surfaces are used, each side of which can serve alternately as the liquor side and the steam side. Scale deposited while in contact with the liquor can then be dissolved while in contact with the steam condensate. There are still potential scaling problems, however. Scale may form in the valves needed for cycling the fluids and the steam condensate simply does not easily dissolve the seale produced. 

High-temperature corrosion problems are experienced mainly by boilers firing residual fuel oils. The corrosion is due primarily to the presence of vanadium, sodium, and sulfur compounds in the fuel oil (vanadium can be as high as 500 ppm, Na 300 ppm, and sulfur 40,000 ppm). During combustion the presence of these compounds react and give rise to complex low-melting-point materials that deposit on heat-transfer surfaces and supporting structures see reactions (17.1)—(17.3) (Niles and Sanders, 1962) ... 

Other than the particle dimension d, the porous medium has a system dimension L, which is generally much larger than d. There are cases where L is of the order d such as thin porous layers coated on the heat transfer surfaces. These systems with Lid = 0(1) are treated by the examination of the fluid flow and heat transfer through a small number of particles, a treatment we call direct simulation of the transport. In these treatments, no assumption is made about the existence of the local thermal equilibrium between the finite volumes of the phases. On the other hand, when Lid 1 and when the variation of temperature (or concentration) across d is negligible compared to that across L for both the solid and fluid phases, then we can assume that within a distance d both phases are in thermal equilibrium (local thermal equilibrium). When the solid matrix structure cannot be fully described by the prescription of solid-phase distribution over a distance d, then a representative elementary volume with a linear dimension larger than d is needed. We also have to extend the requirement of a negligible temperature (or concentration) variation to that over the linear dimension of the representa-... 

FIGURE 1L5 Pool boiling from smooth and structured surfaces on the same apparatus [40]. (a) sketch of cross sections of three enhanced heat transfer surfaces tested (b) boiling curves for three enhanced tubes and smooth tube. 

SLUDGE - A deposit on a heat-transfer surface that does not have the hard, crystalline structure of a scale but is softer and less dense. 

FIGURE 18.3 Heat exchange tube with the microstructured surface. The tube is characterized by high surface area, large volume of microcavities, and tight contact between the tube body and the array of whiskers. (Reprinted from Nucl. Instrum. Methods Phys. Res. B, 236, Schultz, A., Akapiev, G.N., Shirkova, YM. et al., A new method of fabrication of heat transfer surfaces with micro-structured profile, 254—258, Copyright 2005, with permission from Elsevier.)... 

Schultz, A., Akapiev, G. N., Shirkova, V. V. et al. 2005. A new method of fabrication of heat transfer surfaces with micro-structured profile. Nuclear Instruments and Methods in Physics Research B 236 254-258. 

Often complex geometries are used for the design of MSR. An example is the channel structure developed by the chemical company LONZA Ltd., Switzerland [16], where mixing elements are integrated in the microchannels (Figure 5.11). In these cases, the estimation of the active heat transfer surface area and the heat transfer coefficients are hardly possible. Therefore, the introduction of an overall volumetric heat transfer coefficient (U ) is useful. 

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