Exhaust hood 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... 

This type of dependence of capture efficiency on the exhaust flow rate and cross-draft velocity has also been seen by Fletcher and Johnson who determined the capture efficiency of a flanged square exhaust hood in a cross flow. 

FIGURE >0.70 The capture of tracer gas released at five times the diameter of the exhaust inlet when the radius of the exhaust hood is 0. tS m, the radius of the exhaust hood inlet is 0.037 m, and the width of the exhaust jet is 8.0 mm. fej Suction alone with average inlet velocity of 12.7 m s. (b) Combined suction and Injection with the average inlet and exhaust velocities of 12 8 rn s and 7.7 m s . respectively. (Figures are courtesy of the Health and Safety Executive, Research Division. Sheffield. UK.)... 

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... 

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. 

Capture Velocity - The air velocity that generates sufficient air flow to remove contaminated air being given off from the source and force it to flow into an exhaust hood. 

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. 

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. 

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