Exhaust buoyancy


Transport Disengaging Height. When the drag and buoyancy forces exerted by the gas on a particle exceed the gravitational and interparticle forces at the surface of the bed, particles ate thrown into the freeboard. The ejected particles can be coarser and more numerous than the saturation carrying capacity of the gas, and some coarse particles and clusters of fines particles fall back into the bed. Some particles also coUect near the wall and fall back into the fluidized bed.  [c.79]

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.  [c.2183]

Gases leaving the tops of stacks rise higher than the stack top when they are either of lower density than the surrounding air (buoyancy rise) or ejected at a velocity high enough to give the exit gases upward kinetic energy (momentum rise). Buoyancy rise is sometimes called thermal rise because the most common cause of lower density is higher temperature. Exceptions are emissions of gases of higher density than the surrounding air and stack down wash, discussed next. To estimate effective plume height, the equations of Briggs (1-5) are used. The wind speed u in the following equations is the measured or estimated wind speed at the physical stack top.  [c.321]

Fine powders can exist over a wide range of bulk densities and, therefore, exhibit substantial differences between incipient buoyancy and incipient bubbling. For coarser granular solids, no distinction can be made between incipient buoyancy and incipient bubbling, as illustrated qualitatively in Figure 30. From a practical point, the incipient bubbling velocity is the more significant one in reactor design. The terms particulate and aggregative were coined to differentiate between bubbling beds (aggregative) and non-bubbling beds (particulate). In general, liquid-fluidized beds are non-bubbling, whereas gas-fluidized beds bubble. Bubbling is related to fluid and particle properties in a manner permitting the prediction of a system s maximum attainable bubble size, which, if negligible, leads to the observation of so-called particulate fluidization. Rather than employ the terms aggregative and particulate, it is more correct to refer to the maximum stable bubble size for a particular system.  [c.478]

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.  [c.634]

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.  [c.636]

Thermal stratification in the room air greatly influences the spreading and dispersion of contaminants (Fig. 8.47). See Section 7.5.4, Plumes in Confined Spaces. In such cases, an exhaust opening might be placed at, or close to, the equilibrium height for the main contaminant. In addition, there should be a general exhaust either at ceiling level or at floor level, depending on the buoyancy of the contaminants. A temporary wall (can be canvas can separate different zones, so that a polluted zone is separated from a clean zone (e.g., the welding zone separated from the mechanical assembly zone, I ig. 8.48).  [c.661]

When some natural forces exist, it is essential to utilize, and not to counteract, these forces. Some examples are buoyancy forces from hot sources or contaminant jets from grinding or spray painting (see Fig. 10.4). To completely isolate a volume from its surroundings only using air is impossible. To achieve  [c.815]

BEOs can be used for both warm and cold sources depending on available space. When used for warm sources (e.g., welding fumes or fumes from pouring molten metal) it is much better (usually necessary) to place the exhaust above the source, since the buoyancy force is strong enough to counteract an exhaust flow from below or on the side. See Chapter 7.5 for descriptions of airflow from thermal sources. Sometimes a source could make the contaminants move first upward and when reaching the rim move downward (e.g., during degreasing operations). Usually processes release contaminants in one main direction and the exhaust must be placed in a way that does not try to counteract these natural forces. Whenever possible, the.  [c.828]

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.  [c.876]

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).  [c.968]

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).  [c.1077]

In still air conditions, the source of pressure difference to drive ventilation is buoyancy due to the decrease in density of heated air. In any occupied building, there will be a higher temperature inside than outside due to heat gains from people, plant and solar radiation. The lighter heated air will try to rise, causing an increase in internal pressure at high level and a reduction at low level with a neutral plane between the two conditions. Any opening above the neutral plane will therefore exhaust air and any opening below the neutral plane will provide inlet air. Under steady heat load conditions, a balance will be achieved with a throughput of air dependent upon the heat load and the size and location of the openings. Conditions at this balance point can be readily calculated using one of the following formula  [c.421]

Externa.1 Floa.ting Roofs. Pontoon roofs are common for floating roofs from diameters of approximately 30—100 ft (10—30 m). The roof is simply a steel deck having an annular compartment that provides buoyancy (Fig. 5a). Double-deck roofs (Fig. 5b and 5c) are built for very small floating roofs up to about 30 ft (10 m) in diameter. These are also used on diameters that exceed about 100 ft (30 m). These roofs are strong and durable because of the double deck and are suitable for large-diameter tanks.  [c.314]

Mean airflow velocities approach zero as the inspired airstream enters the lung parenchyma, so particle momentum also approaches zero. Most of the particles reaching the parenchyma, however, are extremely fine (< 0.5 pm MMAD), and particle buoyancy counteracts gravitational forces. Temperature gradients do not exist between the airstream and airway wall because the inspired airstream has been warmed to body temperature and fully saturated before reaching the parenchyma. Consequently, diffusion driven by Brownian motion is the only deposition mechanism remaining for airborne particles. Diffusivity, can be described under these conditions by  [c.224]

In rixims with ventilation systems creating temperature and/or contaminant distribution (see Section 7.5.4), one might consider placing an exhaust opening at, or close to, the equilibrium height for the mam contaminant (Fig. 7.16). In addition, there should be a general exhaust either ar ceiling level or at floor level, depending on the buoyancy of the contaminants. When It is not certain if the contaminants have negative or positive buoyancy, one should place exhaust openings both under the ceiling and at floor level (Fig. 7.17). More about general exhaust location selection is covered in Section 8.10.  [c.442]

Studies of nonisothermal mam stream and horizontal directing jet mterac-non were conducted to evaluate the maximum heat load that can be eltectively supplied by such HVAC systems. To summarize experimental data both in free and confined conditions, it was suggested that the above limiting condition is achieved when the current Archimedes number Ar ratio of rhe buoyancy forces over ineiTia forces along the resulting jet axis) does not exceed s[c.502]

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.  [c.1277]

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.  [c.1278]


See pages that mention the term Exhaust buoyancy : [c.1550]    [c.320]   
Industrial ventilation design guidebook (2001) -- [ c.660 ]