Heat transfer exposed surfaces

The technique most often employed in combustion systems to maintain efficient heat transfer across surfaces exposed to the flue gas, is the technique of soot blowing. The method was originally devised, as the name implies, to remove accumulations of soot from the internal surfaces of combustion plant, but it now has a more universal application for the removal of any deposits that may interfere with the heat transfer process. The use of additives (see Chapter 14) to modify the structure of slags and foulants, may improve the effectiveness of soot blowing equipment. 

When radiating and receiving surfaces are not in parallel, as in rotary-ldln devices, and the sohds burden bed may be only intermittently exposed and/or agitated, the calculation and procedures become veiy complex, with photometric methods of optics requiring consideration. The following equation for heat transfer, which allows for convective effects, is commonly used by designers of high-temperature furnaces ... 

In rotary devices, reradiation from the exposed shelf surface to the solids bed is a major design consideration. A treatise on furnaces, including radiative heat-transfer effects, is given by Ellwood and Danatos [Chem. Eng., 73(8), 174 (1966)]. For discussion of radiation heat-transfer computational methods, heat fliixes obtainable, and emissivity values, see Schornshort and Viskanta (ASME Paper 68-H 7-32), Sherman (ASME Paper 56-A-III), and the fohowing subsection. 

There is a large difference between the total surface of the particles (as determined by their size and shape) and the effective surface actually exposed to the passing gas stream. In practice, it has been estimated tnat as little as 10 to 25 percent of the total surface is effective in heat transfer when unscreened particles are treated. 

One manner in which size may be computed, for estimating purposes, is by employing a volumetric heat-transfer concept as used for rotary diyers. It it is assumed that contacting efficiency is in the same order as that provided by efficient lifters in a rotaiy dryer and that the velocity difference between gas and solids controls, Eq. (12-52) may be employed to estimate a volumetric heat-transfer coefficient. By assuming a duct diameter of 0.3 m (D) and a gas velocity of 23 m/s, if the solids velocity is taken as 80 percent of this speed, the velocity difference between the two would be 4.6 m/s. If the exit gas has a density of 1 kg/m, the relative mass flow rate of the gas G becomes 4.8 kg/(s m the volumetric heat-transfer coefficient is 2235 J/(m s K). This is not far different from many coefficients found in commercial installations however, it is usually not possible to predict accurately the acdual difference in velocity between gas and soRds. Furthermore, the coefficient is influenced by the sohds-to-gas loading and particle size, which control the total solids surface exposed to the gas. Therefore, the figure given is only an approximation. 

The boiler designer must proportion heat-absorbing and heat-recovery surfaces in a way to make the best use of heat released by the fuel. Water walls, superheaters, and reheaters are exposed to convection and radiant heat, whereas convection heat transfer predominates in air preheaters and economizers. The relative amounts of these surfaces vary with the size and operating conditions of the boiler. 

An alternate means of reboiler control is to remove the control valve from the steam line and provide a condensate level controller for the chest cascaded from the tray temperature. The alternate method uses A tube surface for control, with the condensate covering more or less tube surface to vary the area exposed to condensing stream. Condensing area is many times more effective for heat transfer than area covered by relatively stagnant condensate. The reboiler must have extra surface to allow part of its surface to be derated for control purposes. 

A = net external surface area of tubes exposed to fluid heat transfer (notjust the length of the individual tubes), ft . 

Required effective outside heat transfer surface area based on net exposed tube area. Note. Later in text = A. 

A = total exchanger bare tube heat transfer, ft or, net external surface area of tubes exposed to fluid heat transfer, ft or, area available for heat transfer, ft (for conduction heat transfer, A is a cross-sectional area, taken normally in the direction of heat flow, ft2). 

Corrosion may also occur under deposits, and the removal of a deposit from a heat transfer surface typically may expose a pit or other form of metal wastage. 

The cooling coil in a reactor has a surface area of 10,000 ft2. Under the most severe conditions the coils can contain water at 32°F and can be exposed to superheated steam at 400°F. Given a heat transfer coefficient of 50 Btu/hr-ft2-°F, estimate the volumetric expansion rate of the water in the cooling coils in gpm. 

Fire protection systems achieve exposure protection by absorption of heat through application of extinguishing agents to structures or equipment exposed to a fire. The application of some extinguishing agents removes or reduces the heat transferred to the structures or equipment from the exposing fire, as well as limits the surface temperature of exposed structures and equipment to a level that will minimize damage and prevent failure. 

Nickel-platinum bimetallic catalysts showed higher activity during ATR than nickel and platinum catalysts blended in the same bed. It was hypothesized that nickel catalyzes SR, whereas platinum catalyzes POX and, when they are added to the same support, the heat transfer between the two sites is enhanced [59, 60]. Advanced explanations were reported by Dias and Assaf [60] in a study on ATR of methane catalyzed by Ni/y-Al203 with the addition of small amounts of Pd, Pt or Ir. An increase in methane conversion was observed, ascribed to the increase in exposed Ni surface area favored by the noble metal under the reaction conditions. 

The UL 2085 tank construction is intended to limit the heat transferred to the primary tank when the AST is exposed to a 2-h hydrocarbon pool fire of 1093°C (2000°F). The tank must be insulated to withstand the test without leakage and with an average maximum temperature rise on the primary tank not exceeding 2TC (260 F). Temperatures on the inside surface of the primary tank cannot exceed 204°C (400°F). 

The bigger the radiator, the more heat is provided to a room. The bigger the radiator, the faster the steam condenses to water inside the radiator. A larger radiator has more heat-transfer surface area exposed to the condensing steam. Unfortunately, the radiator shown in Fig. 13.1 is suffering from a common malfunction. Water-hardness deposits have partly plugged the condensate drain line. Calcium Carbonate is a typical water-hardness deposit. 

As the hot-vapor bypass valve opens, the condensate level in the shell side of the condenser increases to produce cooler, subcooled liquid. This reduces the surface area of the condenser exposed to the saturated vapor. To condense this vapor, with a smaller heat-transfer area, the pressure of condensation must increase. This, in turn, raises the tower pressure. This then is how opening the hot bypass pressure-control valve increases the tower pressure. 

Upper section control methods are shown on Figure 3.15. They all incorporate control of the pressure on the tower, either by throttling some vapor flow rate or by controlling a rate of condensation. In the latter case this can be done by regulating the flow or temperature of the HTM or by regulating the amount of heat transfer surface exposed to contact with condensing vapor. 

Sublimation temperatures are in the range of —10 to —40°C and corresponding vapor pressures of water are 2.6-0.13 mbar. Cabinet tray dryers are the most commonly used type. The trays are lifted out of contact with hot surfaces so the heat transfer is entirely by radiation. Loading of 2.5 lb/sqft is usual for foodstuffs. Drying capacity of shelf-type freeze dryers is 0.1-1.0kg/(hr)(m2 exposed surface). Another estimate is 0.5-1.61b/(hr)(sqft). The ice surface has been found to recede at the rate of 1 mm/hr. Freeze drying also is carried out to a limited extent in vacuum pans, vibrating conveyors, and fluidized beds. Condensers operate as low as —70°C. 

Other factors being constant, the rate of vaporization from a large exposed liquid surface is proportional to the area of the surface. This may also be true for droplets 67, 96) if radiant heat transfer is predominant 17). Under normal circumstances, however, the rate of vaporization of a droplet at rest with respect to its environment is proportional to the droplet diameter 3). This comes about because the vaporization rate is controlled by the rate of conduction of heat or of mass through the gas film surrounding the droplet. The appropriate equation is ... 

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