Airflow capture velocity

Local exhausts arc designed to capture air pollutants and heat at the source, and thus their location and the exhausted airflow rate should ensure sufficient capture velocity. 

Numerical simulation of hood performance is complex, and results depend on hood design, flow restriction by surrounding surfaces, source strength, and other boundary conditions. Thus, most currently used method.s of hood design are based on experimental studies and analytical models. According to these models, the exhaust airflow rate is calculated based on the desired capture velocity at a particular location in front of the hood. It is easier... 

Gases, vapors, and fumes usually do not exhibit significant inertial effects. In addition, some fine dusts, 5 to 10 micrometers or less in diameter, will not exhibit significant inertial effects. These contaminants will be transported with the surrounding air motion such as thermal air current, motion of machinery, movement of operators, and/or other room air currents. In such cases, the exterior hood needs to generate an airflow pattern and capture velocity sufficient to control the motion of the contaminants. However, as the airflow pattern created around a suction opening is not effective over a large distance, it is very difficult to control contaminants emitted from a source located at a di,stance from the exhaust outlet. In such a case, a low-momentum airflow is supplied across the contaminant source and toward the exhaust hood. The... 

The capture velocity of a hood is defined as the air velocity created by the hood at the point of contaminant generation. The hood must generate a capture velocity sufficient to overcome opposing air currents and transport the contaminant to the hood. For enclosing hoods, capture velocity is the velocity at the hood opening. In this case, the velocity must be sufficient to keep the contaminant in the hood. In practice, hood shape and the influence of crossdrafts on the measured capture velocity have to be considered. All three velocity components should be measured and used to calculate the magnitude and direction of the total velocity. Other methods used, not as good as the previous one, are to measure the velocity with a directional velocity sensor towards the hood or to measure the net velocity by an omnidirectional velocity sensor. In the last method the main airflow direction should be viewed and evaluated by means of a smoke test (see Sections 10.2.1 and 10.2.2.1). 

Whether the emission source is a vacuum-pump discharge vent, a gas chromatograph exit port, or the top of a fractional distillation column, the local exhaust requirements are similar. The total airflow should be high enough to transport the volume of gases or vapors being emitted, and the capture velocity should be sufficient to collect the gases or vapors. 

Exhaust air must enter the hood to carry the contaminants with it and convey them to the exhaust point. The required air velocity at the point the contaminants are given off to force these contaminants into an exhaust hood is called capture velocity and should be at least 0.5 m/sec. An airflow meter can be used to measure the air velocity. A static pressure gauge can be installed to continuously monitor the air velocity in the hood by pressure drop. 

A laboratory study of surface-treatment tanks by Braconnier et al." showed the effects of cross-drafts and obstructions to airflow on capture efficiency. They found that, without obstructions, capture efficiency decreased with increasing cnrss-draft velocity but the importance of this effect depended on freeboard height. In their study, cross-draft direction was always perpendicular to the hood face and directed opposite to the hood suction flow. Follow cro.ss-draft velocities (less than 0.2 m s ), efficiency remained close to 1.0 for the three freeboard heights studied. With higher cross-draft velocities, efficiency decreased as freeboard height decreased. For example, when the crossdraft velocity was 0.55 m s , efficiency decreased from 0.90 to 0.86 to 0.67 as freeboard height decreased from 0.3 m to 0.15 m to 0.1 m, respectively. 

It is necessary to check that no outward-directed velocity occurs across the open face of the booth. In such a case, another capture principle must be chosen or the exhaust airflow has to be increased until the needs are fulfilled. The possibility of spilling contaminated air out of the booth could also be reduced by the use of a flexible curtain covering the open face of the booth. 

Exterior hoods intended to capture contaminants should be placed as close to contaminant sources as possible. In actual practice, however, the hoods can not always be placed close to the source due to circumstances such as working conditions. In such cases, to enhance the exhaust efficiency of exterior hoods, it is useful to use a low-momentum air supply directed toward the exhaust outlet. The supply airflow, which functions to transport contaminants emitted from sources located at a distance from the exhaust outlet,. should be relatively low with a uniform velocity but high enough so that it is not disturbed by the. surrounding air motions. The advantages of using low-momentum supply with exterior hoods are that (1) a lower supply airflow rate to the workspace is possible, (2) a lower exterior hood exhaust flow rate is possible, and (3) it is possible to supply clean air to the breathing zone of the worker. 

TTie ability of the ventilation system to protect the worker efficiently can readily be determined by personal samples. The PIMEX method (see Chapter 12) can be used to determine the worker s exposure during various work phases. The capture efficiency as well as the supply air fraction can be measured using tracer gas techniques. Simple evaluation is carried out visually with smoke tube or pellet tests. Daily system evaluation is recommended using airflow or static pressure measurements at appropriate parts of the system. The air velocities, turbulence intensities, air temperature, mean radiant temperature, and air humidity should also be measured to provide an assessment ol thermal comfort. 

Two or more plane jets can be placed above and outside the rim (all sides) of a canopy hood and directed downward. Fhe exhaust flow into the hood makes the down-directed jets turn inward and upward when the jet velocity has slowed down enough to be influenced by the exhaust flow. In many cases, the aim is to diminish the general supply airflow rate into the room and sometimes to use the jets as separators. lliis method is quite often used on large kitchen hoods to increase their capture efficiency. If the jet is directed toward the front of the fireplace and just reaches the front before turning inward, a high capture efficiency can be achieved. 

Example 6.4 A 30- xm-diameter unit-density sphere (t = 2.75 x 10-3 s) falling at a terminal settling velocity of 2.7 cm/s is captured by a horizontal airflow of 100 ft/min which is flowing into a hood. Find its velocity 1 ms later, relative to the point at which it was captured. 

Field measurements of clearance between the east-west skips and the pressure over the surfaces of the east skip during the travels were carried out for a production shaft of a potash mine in August 2010. The acceleration, velocity and displacement of the east skip were captured with a three-dimensional motion sensor. The measured displacements of the skips at current 3300 fpm (16.76 m/s) velocity were the resultant movements under the combined influences of airflow, Coriolis effect and guide rope vibration. 

A reasonable face velocity for airflow at the front of the hood is about 100 fpm (feet/minute) ( 20%). This airflow velocity alone is not a defining criterion for adequate hood performance. Containment of airborne contaminants is the essential and primary function of the hood as pictured in Figure 7.1.3.3. Hood effectiveness is its ability to capture and contain airborne chemicals within its boundaries. Several factors are interrelated in achieving this containment. In addition to face velocity, other equally important factors that contribute to containing airborne chemicals include what is in the hood, how you carry out operations, and the design of the hood. Thus, the mild draft of the face velocity ideally minimizes any aerosols or gases from escaping from the hood into the lab but the three other factors also have a marked influence on a hood s performance... 

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