Chimney effects

Figure 17-6. When the control valve was removed, a chimney effect caused air to enter the system, and an explosion occurred.

Electrical isolation Heat radiation Cooling coils Recent incidents Vacuum relief valves Accidents at sea Fires Problem sources Emulsion breaking Chimney effects Interlock failure Choosing materials. 

The availability of very small areas of control can be advantageous where electronic apparatus with mild chimney effect would have its natural cooling upset by strong downdrafts, by confining air supply to gangways. 

Top spray systems During top-spray cooling of an overheated core, the wall temperature is usually higher than the Leidenfrost temperature, which causes water to be sputtered away from the wall by violent vapor formation and then pushed upward by the chimney effect of the steam flow generated at lower elevations (as shown in Fig. 4.25). A spray-cooling heat transfer test with BWR bundles was reported by Riedle et al. (1976). They found the dryout heat flux to be a function of spray rate and system pressure. The collapsed level required to keep the bundle at saturation for various pressures compared reasonably well with that in the literature (Duncan and Leonard, 1971 Ogasawara et al., 1973). 

Introduction into a DC plasma requires rather more care and attention owing to its inherent design features. As the hydride is being introduced into the plasma, it is necessary to provide a controlled sheath of argon to contain the hydride and direct it into the plasma. This chimney effect significantly improves the sensitivity for hydride-forming elements. This interface has also formed the basis of an introduction system for mercury vapour into an atomic-fluorescence spectrometer as described by Godden and Stockwell [12]. 

Figure 16.24 illustrates the vertical transport processes and complex interactions that can occur between urban areas, downwind regions, and the free troposphere (Fast, 1998). Emissions from urban regions into the surface layer can be transported into the mixed layer as well as into the free troposphere by several mechanisms, including venting up mountain slopes due to solar heating of the surface, which creates a chimney effect. As shown in Fig. 16.24, clouds also play a role in these vertical transport processes. 

A hurricane operates as a heat pump. Its evaporator is at its core where the vacuum sucks in the moist and warm air from the ocean s surface. Its condenser is on the perimeter, where the moisture condenses into rain and where its energy is released. The driving force for this circulation is the chimney effect created by the pressure and temperature differences between the core and the perimeter. 

Hurricanes are formed by the development of hot areas in the ocean. This high temperature zone is the eye of the hurricane, where the oceanls heat vaporizes the water and the moist air is pulled up by the chimney effect, resulting in low pressures at the core. The hot air moves to the perimeter where it releases its energy content as its moisture condenses. 

Eliminate chimney effect Optimize start-up timing Optimize air makeup (COz)... 

In high-rise buildings in the winter when the cold air on the outside is heavier than the conditioned air on the inside, the chimney effect tends to pull in ambient air at ground elevation, and this cold air adds an additional load to the building s heating system. Eliminating the chimney effect can lower the operating cost by approximately 10%. 

Figure 2.9 shows the controls required to eliminate the chimney effect. The key element of this control system is the reference riser, which allows all pressure controllers in the building to be referenced to the barometric pressure of the outside atmosphere at a selected elevation. Using this pressure reference allows all zones to be operated at the same pressure of about 25 Pa (0.1 in. H20) pressure and permits this constant pressure to be maintained at both ends of all elevator shafts. 

Chimney effects in high-rise buildings can be eliminated by using the proper pressure controls. 

Water sprays from monitor nozzles and hose lines can be used for vapor mitigation. Tests have been conducted in which monitor nozzles and hose lines have been used to create a chimney effect through which the gas is forced upward and dispersed at a high elevation (Beresford, 1981). Application techniques and flow rates are facility-, installation-, and material-specific. Careful planning, analyses, and testing should be conducted prior to deciding on the use of a mobile water spray as a proven means of mitigation. Preventive maintenance of this equipment is key to reliable operation. Hose lines, typically, are hydrostatically tested annually. Flow tests should also be conducted periodically. 

To give an example of the dramatic influence which the geometric parameters can have on coalescence behavior, Fig. 77 shows Y(X) correlations for the industrial-size slot injector which were obtained in a vessel of 30 x 8 m water height. The injector was positioned 1 m above the bottom at the vessel wall in such a way that its axis formed an angle of 0°, + 35° resp. - 35° with the horizontal. Only in the last case, the free jet was pointed towards the floor and decomposed into the bubble swarm just above it. Near the floor, the suction of the free jet is weakest on account of bottom friction. Furthermore, the bubble swarm which has formed does not exert a chimney effect there. Consequently, liquid entrainment into the free jet is suppressed at exactly that point at which it would be particularly supportive of coalescence on account of the weakened kinetic energy of the free jet. 

As discussed in Chap. 7, bubbles change in shape from spherical to ellipsoidal to lens-shaped as their diameter increases. Larger bubbles often rise in spiral paths, at terminal velocities that are almost constant and independent of their size (see Fig. 7.8). In clouds or swarms of bubbles there may be considerable coalescence the rate of rise of clouds of small bubbles is considerably less than that of single bubbles if the bubbles are distributed uniformly over the cross section of the vessel. A cloud of bubbles rising at one location creates an upflow of liquid, and this chimney effect may greatly increase the bubble velocity. 

Most of the kilns prior to 1935 operated on natural draught , i.e. the combustion products were drawn through the kiln by the chimney effect of vertical kilns, often supplemented by a small chimney. As a result, output rates were low (e.g. 1.5 t/d per m of cross-section). Horizontal kilns operated on the draught produced by tall chimneys. 

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