Laboratory chemicals often require storage in locations that keep them isolated from the laboratory environment. The chemicals may also give off fumes that are noxious or dangerous. When these chemicals are used or stored, they must remain in a fume hood. An important specification for a fume hood is its capture velocity. Fume hood capture velocity is the speed at which fumes must be moving at a specific distance in front of the hood opening in order for the fumes to move to the hood and exhaust out of the room. This ensures that no other air currents in the laboratory redirect the fumes to other parts of the laboratory. This means the air at a given distance in front of the hood must be moving at the fume hood velocity. There are different equations for calculating fume hood velocity, depending on the configuration of the hood.

Calculate the area of the hood opening, assuming that your hood is circular in shape. Use this equation: area = pi x hood-radius^2. Your hood radius is circular 1/2 of the hood diameter. Pi is approximately equal to 3.14. For example, if your hood has a diameter of 16 inches, your equation will be pi x 8^2 = 200.96. The area of this hood is 201 square inches. Other configurations and shapes of hoods will require a different equation.

Determine the capture velocity for a particular pollutant by using the equation Q = VH x (10 D^2 + A). "A" represents fume hood area; "D" is the distance from the hood where the pollutant is released (assume 12 inches); VH is recommended hood capture velocity for a pollutant (300 feet per minute); and Q is the volumetric flow rate. Solving for Q specifies the volumetric flow rate required to achieve a capture velocity within a distance D inches from the hood opening. Rearrange the equation to solve for VH and you can determine the capture velocity for your hood at D inches from the hood opening. VH = Q / (10D^2 + A) with the variables plugged into the equation yields a capture velocity, VH for your hood, is the volumetric flow rate of exhaust divided by 1640 for the values in this example. The value of VH is not dependent on the shape of the hood but only on the particular pollutant released. The volumetric flow rate of the hood will determine the exhausting ability of the hood for pollutants in the laboratory.

Remember that only the surface area of the fume hood opening has any effect on the capture velocity of contaminants. As you lower the shield of the fume hood, the fume hood velocity increases in direct proportion to the area of the hood that is open. Note that the volumetric air flow of the hood relates to the area of the hood opening and not to the capture velocity of the contaminant. The equation used illustrates this: Q = VH x (10D^2 + A). Lowering the fume hood door to leave only a small, vertical opening changes the type of hood from an exhaust hood to a slot hood. Slot hoods differ from exhaust hoods in that they have a vertical-to-horizontal ratio of 0.2 or less.

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