Stacking Subject

Figure 48-6 The effect of vortex shedding on a stack subjected to a steady wind is oscillation of the cantilevered cylinder in a direction transverse to that of the wind (left). Theory says that vortices are shed intermittently from each side of the stack, causing the motion. Studies of such dynamic wind effects show only the first vibration mode to be significant...

FIG. S-39 External reactions and bending moment diagram for exhaust stack subjected to design wind load. (Source Altair RIters International Limited.)... 

A Hquid is a material that continues to deform as long as it is subjected to a tensile and/or shear stress. The latter is a force appHed tangentially to the material. In a Hquid, shear stress produces a sliding of one infinitesimal layer over another, resulting in a stack-of-cards type of flow (Fig. 1). 

Since electrodialysis membranes are subject to fouling, it is sometimes necessaiy to disassemble a stack for cleaning. Ease of reassembly is a feature of ED. 

Alternatively, reactant and product gases can be distributed to and removed from individual cells through internal pipes in a design analogous to that of filter presses, (iare must be exercised to assure an even flow distribution between the entiv and exit cells. The seals in internally manifolded stacks are generally not subject to electrical, thermal, and mechanical stresses, but are more numerous than in externally manifolded stacks. 

Limits on emissions are both subjective and objective. Subjective limits are based on the visual appearance or smell of an emission. Objective limits are based on physical or chemical measurement of the emission. The most common form of subjective limit is that which regulates the optical density of a stack plume, measured by comparison with a Ringelmann chart (Fig. 25-1). This form of chart has been in use for over 90 years and is widely accepted for grading the blackness of black or gray smoke emissions. Within the past four decades, it has been used as the basis for "equivalent opacity" regulations for grading the optical density of emissions of colors other than black or gray. 

The efficiency for a boiler or heater is improved by lowering its stack temperature. The stack minimum temperature is frequently limited by SO3 gas dew point. References 6, 7, and 8 discuss this important subject. A stack as hot as 400°F (or perhaps higher) can have problems if the SO3 concentration is high enough. Reference 9 states that SO, condensation will produce a blue-gray haze when viewed against a clear blue sky. 

The key here was the theory. The pioneers familiarity with both the kinematic and the dynamic theory of diffraction and with the real structure of real crystals (the subject-matter of Lai s review cited in Section 4.2.4) enabled them to work out, by degrees, how to get good contrast for dislocations of various kinds and, later, other defects such as stacking-faults. Several other physicists who have since become well known, such as A. Kelly and J. Menter, were also involved Hirsch goes to considerable pains in his 1986 paper to attribute credit to all those who played a major part. 

Derrick Type Stack - This type of unit is well-suited for tall structures subject to strong winds. However, derrick type stacks are the most expensive to erect and maintain. 

Flare systems are subject to potential flashback and internal explosion since flammable vapor/air mixtures may be formed in the stack or inlet piping by the entry of air, and the pilot constitutes a continuous ignition source. Flares are therefore always provided with flashback protection, which prevents a flame front from travelling back to the upstream piping and equipment. Design details are described later. 

Composite materials must survive in the environment to which they are subjected at least as well as the conventional materials they replace. Some of the harmful environments encountered include exposure to humidity, water immersion, salt spray, jet fuel, hydraulic fluid, stack gas (includes sulfur dioxide), fire, lightning, and gunfire as well as the combined effects of the space environment. 

Suppose we replace the 90° layers with a laminae in an attempt to increase the axial stiffness and to increase the first-ply failure load as in Figure 7-61. The load-deflection curve slope after first-ply failure also increases when a laminae replace the 90° layers. However, the energy absorption decreases with such a stacking sequence change. The associated fatigue lives are not known unless both laminates are made and subjected to fatigue loading. 

Humans inside collapsing buildings are subjected to the impact of very heavy stmctural parts. Pictures taken after earthquakes or bomb attacks reveal that vertical members usually fail, leaving a stack of floors on top of another. Although a... 

Stack emissions can include particulates as well as dense gases (heavier tlian air, e.g., chlorine). These are subjected to a downwash settling tlirough tlie alinosphere due to tlie action of gravity. For tlie particles, especially large ones, an additional external force term must be included in the analysis. 

The maximum ground concentration of stack gases subjected to atmospheric diffusion occurs about 5-10 effective stack heights dow tiw ind from the point of emission. 

When stack gases are subjected to atmospheric diffusion and building turbulence is not a factor, gromid level concenlrations on the order of 0.001-1 % of the stack conccnuniion are possible for a properly designed stack. 

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