Sash, fume hood

If possible, position the fume hood sash so that work is performed by extending the arms under or around the sash, placing the head in front of the sash, and keeping the glass between the worker and the chemical source. The worker views the procedure through the glass, which will act as a primary... 

Avoid opening and closing the fume hood sash rapidly, and avoid swift arm and body movements in front of or inside the hood. These actions may increase turbulence and reduce the effectiveness of fume hood containment. 

Open any windows/doors and the fume hood sash to help dissipate vapor. 

Chemical Fume Hood. The chemical fume hood should have an average linear face velocity of 100 linear feet per minute (Ifpm). The window sash height that gives this measured inflow should be marked on the edge wall of the cabinet. The inflow velocity to a fume hood with the sash fully opened should be 85 Ifpm or more. Figure 7. 

Construction Materials Fume Hood Bases Sashes... 

In any evacuation procedure, standard operating procedures for closing down operations should be included, if there is sufficient time to implement them. Gas should be turned off, along with electric and other types of heaters. Valves on gas cylinders should be turned off, especially if they contain flammable or toxic materials. High voltage equipment should be turned off Closing sashes on fume hoods may be desirable. Certainly any flammable material storage cabinets should be closed. 

The most popular sash configuration is the vertically sliding type. The sash must be counter weighted, especially if the sash window is made of heavy glass. In order to avoid a guillotine effect should the counterweight cable break, a safety device must be incorporated in the sash. As for transparent materials used for the sash window, only a few are used in good quality, currently available commercial fume hood models. 

There are several different types of fume hoods (1) conventional hood, vertical sash, (2) conventional hood, horizontal sash, (3) bypass hood, (4) auxiliary air hood, (5) walk-in hood, and (6) self-contained hood. The differences in types 1,4, and 6 are especially important in terms of the amount of tempered air lost during operations, while 1,2, and 3 differ primarily in the airflow patterns through the sash openings. Figures 3.14 to 3.19 illustrate each of these types and the air currents through them during typical operations. In addition, there are specialty fume hoods for perchloric acid and radioisotopes, which will be treated separately. All of the hoods discussed in this section will be updraft units, where the exhaust portal is at the top of the hood, with of course, the exception of the self-contained type. 

Receptacles that provide electric power for operations in hoods should be located outside the hood. This location prevents the production of electrical sparks inside the hood when a device is plugged in or disconnected, and it also allows a laboratory worker to disconnect electrical devices Irom outside the hood in case of an accident. Cords should not dangle outside the hood in such a way that they can accidentally be pulled out of their receptacles or tripped over. Simple, inexpensive plastic retaining strips and ties can be used to route cords safely. For fume hoods with airfoils, the electrical cords should be routed under the bottom airfoil so that the sash can be closed completely. Most airfoils can be easily removed and replaced with a screwdriver. 

Laboratory fume hoods are the most important components used to protect laboratory workers from exposure to hazardous chemicals and agents used in the laboratory. Functionally, a standard fume hood is a fire-and chemical-resistant enclosure with one opening (face) in the front with a movable window (sash) to allow user access into the interior. Large volumes of air are drawn through the face and out the top to contain and remove contaminants from the laboratory. 

Exterior windows with movable sashes are not recommended in laboratories. Wind blowing through the windows and high-velocity vortices caused when doors open can strip contaminants out of the fume hoods and interfere with laboratory static pressure controls. 

Do not modify fume hoods in any way that adversely affects the hood performance. This includes adding, removing, or changing any of the fume hood components, such as baffles, sashes, airfoils, liners, and exhaust connections. 

For hoods without face velocity controls (see section 8.C.6.3.2), the sash should be positioned to produce the recommended face velocity, which often occurs only over a limited range of sash positions. This range should be determined and marked during fume hood testing. For hoods with face velocity controls, it is impraative to keep the sash closed when the hood is not in use. 

Fume hoods connected to a common exhaust manifold offer an advantage. The main exhaust system will rarely be shut down hence, positive ventilation is available to each hood on the system at all times. In a constant air volume (CAV) system (see section 8.C.6.3.1), "shutoff dampers to each hood can be installed, allowing passage of enough air to prevent fumes from leaking out of the fume hoods and into the laboratory when the sash is closed. It is prudent to allow 10 to 20% of the full volume of the hood flow to be drawn through the hood in the off position to prevent excessive corrosion. 

Laboratory fume hoods and the associated exhaust ducts should be constructed of nonflammable materials. They should be equipped with either vertical or horizontal sashes that can be closed The glass within the sash should be either laminated safety glass that is at least 7/32 inch thick or other equally safe matraial that will not shatter if there is an explosion within the hood. The utility control valves, electrical receptacles, and other fixtures should he located outside the hood to minimize the need to reach within the hood proper. Other specifications regarding the construction materials, plumbing requirements, and interior design will vary, depending on the intended use of the hood. (See Chapter 6, sections 6.C.1.1 and 6.C.I.2.)... 

Airfoils built into the fume hood at the bottom and sides of the sash opening significantly reduce boundary turbulence and improve capture performance. All fume hoods purchased should be fitted with airfoils. 

A constant air volume (CAV) fume hood draws a constant exhaust volume through the hood regardless of sash position. Because the volume is constant, the face velocity varies inversely with the sash position. The fume hood volume should be adjusted to achieve the proper face velocity at the desired working height of the sash, and then the hood should be operated at this height. (See section 8.C.4.)... 

A common misconception is that the volume of air exhausted by this type of hood decreases when the sash is closed. Although the pressure drop through the hood increases slightly as the sash is closed, no appreciable change in volume occurs. This does not mean that these fume hoods should not be closed when... 

The auxiliary air fume hood was developed in the 1970s primarily to reduce laboratory energy consumption. It is a combination of a bypass fume hood and a supply air diffuser located at the top of the sash. These hoods wo-e intended to introduce unconditioned or tempered air, as much as 70% of the air exhausted from the hood, directly to the front of the hood. Ideally, this unconditioned air bypasses the laboratory and significantly reduces air conditioning and heating costs in the laboratory. In practice, however, many problems are caused by introducing unconditioned or slightly conditioned air above the sash, aU of which may produce a loss of containment. 

Each chemical fume hood tested within last year Sash closed when not in active use Chemical-fume hood vents (baffles) unobstructed Chemical-fume hood used with sash in appropriate position Chemical storage limited in actively used hood Chemicals and equipment at least 6 in. firom the sash... 

The use of procedural controls should be avoided and considered as the "last resort" if appropriate engineering controls cannot be designed into the facfiity, process, or product. An example of the use of personal protective equipment (PPE) in lieu of engineering controls would be the use of a fume hood. While the hood has appropriate engineering controls such as a monitored exhaust flow and a physical barrier (the sash), the separation of the user... 

Most of the chemical fume hoods considered here consist of a cabinet or enclosure set at waist level (above a table or storage cabinet) that is connected to a blower located above the hood or external to the hood through a duct system. The cabinet has an open side (or sides) to 2illow a user to perform work within. A movable transparent sash separates the user from the work. Most chemical fume hoods have a sill that functions as an airfoil at the work surface below the sash. The connection to the blower might be by use of a v-belt, or it may be direct drive. This allows provision of a smooth flow of air with minimal turbulence. In some installations, axially mounted blowers are used, especially if multiple hoods are ducted into a common blower. Baffles located in the rear of the cabinet provide control of the air flow patterns, and can usually be adjusted to provide the best air flow around the experiment or procedure being performed. Many chemical fume hoods are equipped with air flow indicators, low flow monitors and alarms, and differenti2d pressure sensors to allow the user to operate safely. The major types of chemical fume hoods include the standard/conventional, W2dk-in, bypass, variable air volume, auxiliary air, or ductless types. Additional types include snorkels and canopies that are portable. Each type must be understood to be operated most efficiently within specifications (see the section below on safe operation). 

The standard chemic2d fume hood utilizes a constant speed motor, and for this reason the volume of air drawn into the hood will change with movement of the sash position. As the sash is lowered, the velocity of the air drawn into the hood will increase. 

The bypass chemic2d fume hood is very similar to the standard/ conventional hood except that as the sash is lowered, a vent is opened above the sash to 2Jlow additional air flow into the hood. This prevents a large increase in velocity in the working area inside the hood. 

The variable volume chemical fume hood controls the volume of air drawn into the hood as a function of sash position, while maintaining the face velocity of the air at a constant rate, within the specifications required. These types of chemical fume hoods are more energy efficient than the standard or bypass hoods because they minimize costs incurred by laboratory heating and cooling. 

The walk-in hood is a chemical fume hood that is mounted directly on the laboratory floor or a slightly raised chemical resistance platform. It is used for the ventilation of larger pieces of equipment, with the advantage that these pieces of equipment can be wheeled in and out of the walk-in hood. The walk-in hood typically uses two separate sashes. 

The user should avoid placing his/her head inside the chemical fume hood, beyond the plane of the sash. 

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