Heater design

In the vei tical-tube single-row double-fired heater, a single row of vertical tubes is arrayed along the center plane of the radiant section that is fired from both sides. Usually this type of heater has an overhead horizontal convec tion bank. Although it is the most expensive of the fired heater designs, it provides the most uniform heat transfer to the tubes. Duties are 21 to 132 GJ/h (20 to 125 10 Btu/h) per cell (twin-cell designs are not unusual). 

Von Wiesenthal, P. and Cooper, H. W., Guide to Economics of Fired Heater Design, Chemical Engineering, April 6, 1970, pp. 104-112. 

Figure 2.37 Temperature profiles across the membrane covering the reaction channel of the T micro reactor for a silicon and a silicon nitride membrane and two different heater designs, as discussed by Quiram et al. [128].

GP 1] [R 10] By proper heater design, membrane-based reactors with internal heaters allowed one to reach quasi-uniform temperatures at the membrane, which determines the catalyst temperature [19]. This thermal uniformity was checked during reaction, i.e. when large heats were released in the oxidation of ammonia and needed to be transferred out of the reaction zone (Figure 3.29). Thin-film-coated temperature sensors in the center and at the edges of the membrane served to monitor the lateral temperature difference. 

In such cases, radiant heat transfer is used from the combustion of fuel in a fired heater ox furnace. Sometimes the function is to purely provide heat sometimes the fired heater is also a reactor and provides heat of reaction. The special case of steam generation in a fired heater (a steam boiler) will be dealt with in Chapter 23. Fired heater designs vary according to the function, heating duty, type of fuel and the method of introducing combustion air. However, process furnaces have a number of features in common. A simple design is illustrated in Figure 15.19. The chamber where combustion takes place, the radiant section... 

Process furnace or direct-fired heater Design type, absorbed heat duty, pressure, tube material, capacity... 

The third microhotplate introduced in Sect. 4.3 was designed to extend the operation temperature limit imposed by the CMOS-metallization contacts in the heated area. A new heater design was devised, and a microfabrication sequence that enables the realization of Pt temperature sensors and Pt-electrodes was developed. This microhotplate was also monolithically integrated with circuitry as presented in Sect. 5.2, and operating temperatures of up to 500 °C have been achieved. 

The circular heater design also perfectly matches the shape of the sensitive tin-oxide droplet No excess area is heated, and the heat losses to ambient air are reduced. A SEM (Scanning Electron Microscope) micrograph of a microhotplate coated with a Sn02-droplet is shovm in Fig. 4.1. The grainy structure of the nanocrystaUine oxide material is clearly visible. 

Internal resistive heating, where a miniature heater is placed inside the pressure cell, requires considerably less power than external heaters. A number of internal heater designs in which the heating element is either attached to the gasket [203], is part of the gasket [204], or is inside the sample chamber itself [205-207] have been described. Care must be taken to ensure that the heater is electrically insulated from the gasket and other metallic components. Resistively heated cells can attain temperatures as high as 2,700 K [207], particularly if both internal and external heaters are employed. 

Scaling treatment by injection of polymer scale inhibitor is common to North Sea operators and is normally included in the crude heater design package. 

Burners may be located in the floor or on the ends of the heaters. Liquid fuels are atomized with steam or air or mechanically. A particularly effective heater design is equipped with radiant panel (surface combustion) burners, illustrated in Figure 17.16(a), (b). The incandescent walls are located 2-3 ft from the tubes. The furnace side of the panel may reach 2200°F whereas the outer side remains at 120°F because of continual cooling by the air-gas mixture. Radiant panel burners require only 2-5% excess... 

TABLE 8.16. Equations and Other Relations for Fired Heater Design... 

Heat-Transfer Analysis Thermal-Capillary Models. Numerous analyses of various aspects of heat transfer in the CZ system have been reported many of these are cited by either Kobayashi (143) or Derby and Brown (144). The analyses vary in complexity and purpose, from the simple one-dimensional or fin approximations designed to give order-of-magni-tude estimates for the axial temperature gradient in the crystal (98) to complex system-oriented calculations designed to optimize heater design and power requirements (145,146). The system-oriented, large-scale calculations include radiation between components of the heater and the crucible assemblies, as well as conduction and convection. 

The first alternative simplifies the heat integration of the chemical process, and the second simplifies the interface between the chemical process and the secondary loop. CEA (Leybros, 2009) has designed their S-I process for the first alternative, as the requirements for the reactor expected to be used are most suitable for it. GA has developed flow sheets for both alternatives. Sandia National Laboratories (SNL) is a partner of both CEA and GA in the operation of a demonstration loop for the S-I cycle. SNL is charged with design and operation of the sulphuric acid decomposition section. They have developed a bayonet-heater design for the decomposer which incorporates internal heat recovery. As a result, the outlet temperature of the bayonet heat modules is too low to use in the HI decomposition section. Thus, helium is utilised in the HI decomposition section, as in the CEA flow sheets. 

De Bliedc, J.. Mol, How to choose the best eraddng heater design for an olefin plant", Petroleum Imemat.,... 

Clamp-on band heaters, for use up to 700°C, are specified when high-watt densities are required or the external vessel area is limited. Heating elements can be woven into a quartz fabric to permit flexibility and ease of removal. These heaters are typically specified for lower-temperature demands because of their lower watt density. More rigid heater designs employ fully exposed, radiant heating elements or heating elements embedded within a ceramic fiber matrix. The latter are the heater of choice for high-temperature applications. 

Hydrocarbon and steam mixture flows from the convection section into the radiant section at approximately 525-700°C, depending on the feedstocks and the heater design. This is called the crossover temperature. For ethane and propane thermal cracking, the crossover temperature is approximately 700° C. For vacuum gas oil (VGO) and AGO cracking. 

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