Stack designs

The stack must provide enough draft to hold the firebox at a slight negative pressure and to overcome frictional losses through the convection section and stack. First, calculate the draft caused by the firebox itself. Determine whether the limiting factor is pressure drop across the burners or maintenance of negative pressure throughout the firebox. 

Assume a stack temperature 100°F lower than the flue gas leaving the convection section. Select a stack diameter to give a stack velocity of about 30 feet per second, and calculate the velocity head. Estimate the stack height and calculate the friction Loss through it, allowing 1.5 velocity heads for the inlet and exit losses, 1.5 velocity heads for the damper and 1 velocity head for each 50 diameters Df stack height. 

Try a heater 20 feet wide by 25 feet high with 96 tubes 40 feet long overall, 38.5 feet exposed, 8-inch centers. Estimate 6 shield tubes. 

Since there is no preheated air or fuel, the heat rate of the combustion air, qa, equals the heat rate of the fuel — which equals zero. Assume that the heat loss from the tubes, q l divided by the rate of heat combustion, or Qi/g = 0.02. Estimate that the average firebox temperature equals 1,500°F. 

For this type of furnace, assume that the exit gas temperature is the same as the average firebox temperature. 

Most fired heaters operate with natural draft, and the stack height must be sufficient to achieve the flow of combustion air required and to remove the combustion products. 

It is normal practice to operate with a slight vacuum throughout the heater, so that air will leak in through sight-boxes and dampers, rather than combustion products leak out. Typically, the aim would be to maintain a vacuum of around 2 mm water gauge just below the convection section. 

The stack height required will depend on the temperature of the combustion gases leaving the convection section and the elevation of the site above sea level. The draft arises from the difference in density of the hot gases and the surrounding air. 

The draft in millimetres of water (mm H20) can be estimated using the equation  

Because of heat losses, the temperature at the top of the stack will be around 80°C below the inlet temperature. 

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Table 3.1 Results table for a paper-based analysis of the solenoid assembly stack design...

Figure 3.9 Chart showing that capable tolerances cannot be optimized for solenoid tolerance stack design...

The inadequacy of the worst case approach to tolerance stack design compared to the statistical approach is evident, although it still appears to be popular with designers. The worst case tolerance stack model is inadequate and wasteful when the capability of each dimensional tolerance is high > 1.33). Some summarizing comments on the two main approaches are given below. 

When the discs are of the shrunk-on design, they are made up individually and stacked onto the shaft by first heating the disc to dilate the bore. They are then allowed to cool and thus attach themselves to the shaft. Keys are normally not used. When the discs are of the stacked design, the discs are equipped with rabbet fit to radially lock the discs to maintain concentricity of assembly. The through-bolts are usually tensioned by stretching hydraulically to a precise value to ensure the mechanical integrity of the assembly. 

The flare stack design shall be in accordance with Specifications ME-0-JBOOI and ME-O-JSOIO. 

This chapter explores the design of stacks from the point of view of the downwind observer whose task is to determine the connection between stack design, process emissions, meteorology, and, most important, environmental effects. Stacks must be designed to specifications based on meteorological conditions and environmental air quality standards, which may be quite umelated to process requirements... 

This example points out one of the central problems in stack design for pollution control local, short-term effects may be the most important stack design consideration, but will usually be the aspect of the problem about which the least information is available. 

The seal tank/pot is not a separator but a physical liquid seal (Figure 7-70) to prevent the possibilities of backflash from the flare from backing into the process manifolds. It is essential for every stack design. 

Figure 7-77. Diagrams for alternate flare stack designs of Straitz. By permission, Straitz, J. F. iii and Aitube, R. J., NAO, inc. [62].

Rain caps at the end of the pipe are intended to keep rain from entering the system. Builders use various cap designs for this purpose. The use of rain caps can cause a loss of airflow in a system, which may lessen the effectiveness of the system. It is advisable to use a rain cap that is designed in such a way as to not seriously impede airflow. For more information on rain caps and stack design, see the ASHRAE Fundamentals Handbook.19... 

FIGURE 5.1 Schematics of edge sealing of planar cells (above) and external gas manifold seals (below) used for a simple cross-flow SOFC stack design. 

Other important parts of the cell are 1) the structure for distributing the reactant gases across the electrode surface and which serves as mechanical support, shown as ribs in Figure 1-4, 2) electrolyte reservoirs for liquid electrolyte cells to replenish electrolyte lost over life, and 3) current collectors (not shown) that provide a path for the current between the electrodes and the separator of flat plate cells. Other arrangements of gas flow and current flow are used in fuel cell stack designs, and are mentioned in Sections 3 through 8 for the various type cells. 

Manufacturing details of the Ballard Power Systems cell and stack design are proprietary (18), but the literature provides some information on the cell and stack design. An example schematic of a manufacturer s cell is shown in Figure 3-1. 

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