Exhaust buoyancy

The effective stack height (equivalent to the effective height of the emission) is the sum of the actual stack height, the plume rise due to the exhaust velocity (momentum) of the issuing gases, and the buoyancy rise, which is a function of the temperature of the gases being emitted and the atmospheric conditions. 

In the stratification strategy the supply air is used to substitute the outgoing air from the ventilated (in most cases occupied) zone, thus preventing circulation patterns between the zones. The supply air has to be distributed in such a way that the buoyancy flows are not disturbed. Exhaust air openings are to be located downstream in order to avoid reverse currents within the room. The location of the contaminant sources and the heat sources causing density differences must be the same in order to carry out the contaminants with equal or higher density than air. 

Each method has its own design criteria, but common to most ol the methods is that air supply is located close to or inside the controlled zone and the exhaust openings are located inside the uncontrolled zone. The location and power of the buoyancy sources in relation to the supply air jets have a remarkable influence on the accumulations of heat, contaminants, and humidity within the room. 

FIGURE 10.4 Example of how a natural force (buoyancy) is used when exhausting generated contaminants. 

Use of warm processes on a downdraft table should be avoided since the air velocity created by the exhaust is often lower than the velocity due to buoyancy effects. Effective use of a downdraft table for welding requires velocities high enough to counteract the buoyancy, which could result in disturbances of the welding process. 

The final example is shown in Fig. 10.86. Several workers are breaking gates off of castings on the conveyor by hand. Much dust is generated by this operation and the dust rises due to buoyancy. To remove the dust, an exterior hood was placed beside the conveyor and a supply inlet was placed above the workers. The supply airflow is blown toward the breathing zone of the workers and the dust source. In this case, as the workers and the dust source are located within the supply airflow, the airflow functions to supply the workers with clean air and to transport the dust toward the exhaust inlet. The velocity of supply air is relatively low, 1.1 m s , and the exhaust velocity at the hood face is 2.75 m s . The dimensions of the system are indicated in the figure, and the depth of the device is 6.0 m (compare with Sections 10.3.3 and 10.4.6). 

Outside air entering the space through openings near the ground spreads over the floor and absorbs energy from the floor surface. The resulting air temperature increase leads to buoyancy and forces the air up into the upper hall zone. This results in a temperature stratification in the hall. Due to this vertical temperature gradient, the air in the occupied zone does not reach the exhaust air temperature (see Fig. 11.37). 

The factors affecting the performance of a local exhaust system are well known. For fume control, an added factor is the effect of heat release or buoyancy. Important design parameters are process heat release and the size and geometry of air-supply openings and their location relative to major surfaces of the enclosure, lire kxation of the fume off-take is usually only of secondary importance. 

In the set of conservation equations described earlier, the Reynolds number and the Froude number must be the same for the model and the prototype. Since most industrial operations involve turbulent flow for which the Reynolds number dependence is insignificant, part of the dynamic similarity criteria can be achieved simply by ensuring that the flow in the model is also turbulent. For processes involving hot gases (i.e., buoyancy driving forces), the Froude number similarit) yields the required prototype exhaust rate as follows. 

The first term on the right-hand side is known as the buoyant force, the second is known as thrust. If this were just a puff of hot air without the balloon exhaust, we would only have the buoyant force acting. In this case we could not ignore (d/dt) Jjf pvxdV since the puff would rise (vx +) solely due to its buoyancy, with viscous effects retarding it. Buoyancy generated flow is an important controlling mechanism in many fire problems. 

Finally a column of buoyant combustion products, ash, embers, and smoke are produced which rise above the flames. This often spectacularly visible plume of exhaust products can itself be very hazardous since it contributes to pyrolysis, firebrands, suffocation, and loss of visibility. The behavior of this plume has been the subject of extensive analysis and research, but in most cases the plume is presumed to rise independent of any surrounding terrain, structures or porous surroundings. Little attention has been given to the motion of the plume in the immediate vicinity of a fire as it is modified by surrounding forest or building structures. Once the plume penetrates the surface layer above the canopy it is presumed to follow conventional plume/jet mixing, trajectory, and kinematic under the influence of buoyancy and cross flow winds [82, 180, 363,498, 635],... 

When the buffering capacity of the Bornholm Basin is exhausted, weak inflows of saline and oxygen-rich water can pass that basin in depths of 50-60 m. This water frequently interleaves just below the halocline at the level of neutral buoyancy and propagates through... 

These are not dyed by exhaust methods. Primary environmental concerns are suspended solids and color. The suspended soUds result from the foaming and coagulation of the binder, antimigrant, and thickeners. Due to their large hydro-dynamic radius and neutral buoyancy, these solids are difficult to treat as they will not settle nor float for efficient skimming. 

A dry suit or other buoyancy-changing equipment not directly connected to the helmet or mask shall be equipped with an exhaust valve. 

The flow configuration is oriented upward so that the buoyancy effect on the stagnation-flow field is diminished. This configuration provides a stagnation-flow field with a radially uniform velocity profile at the inlet. Gas fines are also heated to prevent the condensation of liquids. The gases are exhausted through an annular pipe and burned in a Bunsen burner which is also housed in the reactor. 

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