Bypass Hoods

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 

Quantitative tracer gas testing of many auxiliary air fume hoods has revealed that, even when adjusted properly and with the supply air properly conditioned, significantly higher worker exposure to the materials used in the hood may occur than with conventional (non-auxiliary air) hoods. Auxiliary air hoods should not be purchased for new installations, and existing auxiliary air hoods should be replaced or modified to eliminate the supply air feature of the hood. This feature causes a 

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. 

Ductless fume hoods are ventilated enclosures that have their own fan, which draws air out of the hood and through filters and ultimately recirculates it into the laboratory. The filters are designed to trap vapors generated in the hood and exhaust clean air back into the laboratory. These hoods usually employ activated carbon filta-s. The collection efficiency of the filters decreases o er time. Ductless fume hoods have extremely limited applications and should be used only where the hazard is very low, where the access to the hood and the chemicals used in the hood are carefully controlled, and under the supervision of a laboratory supervisor who is familiar with the serious limitations and potentially hazardous characteristics of these devices. If these limitations cannot be accommodated, then this type of device should not be used. 

The California hood is a ventilated enclosure with a movable sash on more than one side. These hoods can usually be accessed through a horizontal sliding sash Irom the front and rear. They may also have a sash on the ends. Their configuration precludes the use of baffles and airfoils and therefore may not provide a suitable face velocity distribution across their many openings. 

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Another type of laboratory hood is the bypass hood, shown in Figure 3-7. For this design bypass air is supplied through a grill at the top of the hood. This ensures the availability of fresh... 

Internal Fixtures Types of Chemical Fume Hoods a. Conventional Fume Hood Bypass Hood Auxiliary Air Hoods Walk-ln Hood Self-Contained Hoods... 

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. 

Figure 3.17 Bypass hood. Sash open and closed.

The bypass hood (Figure 3.17) is designed so that a portion of the air entering the face of the hood may pass over the top of the sash opening as well as below it. This has two consequences. The first is that the air velocity near the work surface remains reasonably constant, so that excessive air speeds which could be detrimental to delicate apparatus or experiments will not occur. The second is that there is less static pressure and hence less frictional resistance to the flow of air than with the conventional hood, so that the volume of air through the hood remains more nearly the same at different sash heights, permitting better... 

A non-b)q)ass hood has only one major opening through which the air may pass into the hood, that is, the sash opening. The airflow pattern of this type of hood is shown in Figure 8.2. A CAV non-bypass 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. 

This figure presents only the so-called internal events. Not included are the "V" sequences (valve rupture that bypasses containment), earthquake, fire, Hood, tornado and air crash,... 

Figure 3-7 Standard bypass laboratory hood. The bypass air is controlled by the height of the sash. Source N. Irving Sax, Dangerous Properties of Industrial Materials, 4th ed. (New York ...

Place the flow hood opening completely over the filter or diffuser, seating the face of the hood against a flat surface to prevent air bypass and inaccurate readings. 

If a deep bed u used in a reflux extractor the exlractur can be analyzed tike fixed-bed extractors operating with dtiwnftow and yj0 0. In such a case V = Ee. However, if the bed is Hooded and consequently part of the reflux bypasses the bed. V should be based on an Eh that barely results in bed saturation rather than the actual Efe. If the refluxed solvent b not passed throngh a bed but is edded 10 a well-mixed batch of extract and solid and the rate of extract discharge is E . the extractor can be treated like a diflcrtmriel extractor. Reflux extractors in which very short beds are used can also be treated like differential extractors as a mesonablc approximation. 

Perchloric hoods are usually constructed with an integral liner of a single piece of stainless steel, such as 316 stainless, which will resist the effects of the acid, although P VC can also be used as a liner. The liner should have coved comers and as few seams as possible to allow ease of decontamination. In order to avoid buildup of perchloric precipitates in the hood and duct system, a hood intended to be used for perchloric acid work must he equipped with a rinse system which will make it possible to thoroughly flush the interior of the hood and duct work with water. This may be done with a manual control system or by an automatic system that will come on and rinse the system for 20 - 30 minutes at the end of a work session. A combination of an automatic system which can be bypassed for additional rinses is preferable so the researcher may choose to clean the system if necessary. 

The first fume hoods were simply boxes that were open on one side and connected to an exhaust duct. Since they were first introduced, many variations on this basic design have been made. Six of the major variants in fume hood airflow design are listed below with their characteristics. Conventional hoods are the most common and include benchtop, distillation, and walk-in hoods of the constant air volume (CAV), variable air volume (VAV), bypass and non-bypass variety, with or without airfoils. Auxihary air hoods and ductless fume hoods are not considered "conventional" and are used less often. Laboratory workers should know what kind of hood they are using and what its advantages and limitations are. 

The exhaust from the low pressure turbine cylinders flows to the main turbine condenser which has three shells, located under the exhaust hoods of the low pressure turbine cylinders. The condenser is designed to accept also the steam flow from the main steam bypass system on startup, hot standby and turbine trip. During normal power operation, the steam flow to the condenser amounts to about 60% of the total steam flow, but the condenser system is designed to accommodate the full steam flow for a limited time period the steam flow shall be reduced to 60% within 20 seconds to avoid a reactor trip due to too high condenser pressure. 

The turbine exhaust flows to a condenser which has three shells, located under the low pressure turbine exhaust hoods. The condenser also accepts the exhaust flow from the feed pump turbines and, on startup, hot standby and turbine trip, flow from the main steam and bypass system. 

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 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 auxiliary air chemical fume hood includes an additional blower that injects air into or at the face of the hood, providing additional flow inside the enclosed cabinet. These types of hoods are rarely installed in renovations or new construction, but may be encountered in older laboratories. They are less desirable than the standard/conventional, bypass or variable volume types because they require a great deal of energy to operate (although the early designs featured the addition of an auxiliary air stream that was not air conditioned). These devices are mechanically more complex than other types, and consequently more prone to maintenance problems. 

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