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Design Your Fume Hood

Fume Hood Selection
There are two main factors to consider when selecting a Fume Hood style.

First consider the type of work being performed in the Hood. The use of perchloric acid or radioactive materials require a Fume Hood specifically designed for these materials. Large apparatus or complex set-ups may require the use of a low bench or walk-in Fume Hood to accommodate the space requirements.

The second consideration in selecting a Fume Hood style is the air consumption of the hood. Operation of a Fume Hood requires a relatively large volume of room air.

If you determine that the amount of air being supplied to the room is adequate to accommodate the exhaust volume of the Fume Hood the Bypass style Fume Hood would be an excellent choice.

If there is not enough supply air available, the Add Air style may be the right choice.

The Variable Air Volume style Fume Hood is designed to be used with an adjustable damper system in the ductwork. The use of this Hood will, when used in conjunction with a VAV control system, drastically reduce the total amount of air consumed. This makes the Variable Air Volume style Hood a good choice when trying to reduce air consumption for energy cost savings or when there simply is not enough available supply air.

Fume Hood Operation
A fume hood must be properly installed and operated in order to achieve optimum performance.

When using a Fume Hood, keep all apparatus at least 6" back from the face of the Hood. Do not put your head in the Hood when contaminants are being generated. Do not store chemicals or apparatus in the hood. Following these guidelines will help insure proper performance of the Fume Hood.

OSHA requires laboratories, where hazardous chemicals are used, to develop and carry out a written chemical hygiene plan. This plan should include provisions for monitoring fume hoods for proper performance, as well as periodic performance testing of the Fume Hoods.

Fume Hood Locations
Location of the Fume Hood is of prime importance, since a variety of conditions will effect the performance of the Hood. When selecting a location for the Fume Hood, operator convenience, work flow and exhaust duct locations should all be considered. In addition, an attempt should be made to keep the hood away from the line of traffic. It is both inconvenient and dangerous to install a Fume Hood so that the operator is forced to work in the line of traffic movement. The presence of cross-drafts will adversely effect the performance or the Fume Hood. For this reason it is a good idea not to locate the Fume Hood near open doors and windows. High velocity air diffusers located directly in front of the Fume Hood could actually draw fumes out of the Hood and into the room. Although there is no single preferred method for delivery of make up air, it is a good idea to locate the Fume Hood as far from the air diffuser as possible so that the air first sweeps through the laboratory working area and then into the Fume Hood.

Ducts are excellent conductors of sound. Anchoring the ductwork securely to the building structure and sizing the ductwork so as not to exceed velocities in excess of 2000 FPM will help to reduce duct noise.

The exhaust discharge should be located at least 10' above the roof structure and have a zero pressure weather cap. Covered weather caps that increase static pressure and interfere with the flow of discharged air are not recommended.

As a rule, each Fume Hood should be exhausted by its own exhaust blower, as shown in Figure 3. When multiple Hoods are manifolded to a single exhaust blower, as shown in Figure 4, imbalance in the exhaust airflow can occur as Hoods are turned on and off. To overcome this situation, all Hoods could be turned on and off together. However, this may result in an unsatisfactory arrangement from the standpoint of energy efficiency.

Multiple Hoods which are controlled by a variable air volume (VAV) control system may be manifolded to a single exhaust blower. The VAV controls maintain the proper air balance and provide excellent energy efficiency, by reducing the amount of air being exhausted when the Hood is not in use.

Care should be taken in the design of the VAV system. The air being supplied to the laboratory must also be variable to meet the changing exhaust volume of the Fume Hoods. If not, over pressurization of the laboratory could occur causing contamination of adjacent areas.

Because the exhaust volume of the VAV system can vary greatly, it is a good idea to select an exhaust blower with some type of air bypass, as shown in Figure 5. This will keep the fan motor from overloading and maintain an acceptable stack velocity.

Fume Hood blowers and ductwork systems should be included in the maintenance schedule. Dust and other contaminants collected on the blades of the fan can greatly reduce its capacity. Blowers should be placed so that they are accessible for periodic cleaning. Periodic examination of the ductwork should also be made to detect any loose joints or porosity due to corrosion.

Flow Patterns
Airfoil Bypass Hoods
The Airfoil Bypass Hoods have a double bypass to provide constant exhaust volume and limit changes in face velocity.

The upper bypass functions automatically with the raising and lowering of the sash. The lower bypass provides a continuous air sweep of the work surface.

When hood top enclosures are used, a bypass grille must be specified

Airfoil Variable Air Volume Hoods
The Airfoil Variable Air Volume Hoods (VAV) are designed to be used with exhaust control systems provided by other manufacturers. In lieu of an upper bypass the VAV Hood has a lintel which restricts the air intake of the Hood to the sash opening and lower bypass.

The commercially available exhaust control system will detect the movement of the sash and adjust the volume of air required to maintain a constant face velocity.

Factory mounting of the exhaust controls is available when specified.

Airfoil Add Air Hoods
The Airfoil Add Air Hoods provide up to 70% of the Hood exhaust requirements. With a bypass similar to the Airfoil Bypass Hood, outside air is introduced through the add air plenum, thus minimizing the amount of conditioned room air required.

When outside temperatures are extreme, it may be necessary to temper the incoming air.

Hood Locations

Good Location - Fig. 1

Poor Location - Fig 2
Exhaust Blower Configurations

Fig. 3

Fig. 4

Fig. 5

Airflow Patterns
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