Auxiliary air hoods

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. 

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... 

Other methods that have been used to try to cut the loss of air that has been heated or cooled include use of the auxiliary air hood, which introduces a separate air supply at the hood face, and use of heat recovery units positioned in the exhaust system. Unfortunately, both of these methods have a high initial cost and have other drawbacks as well. Auxiliary air hoods require two duct systems and two fans, thereby increasing installation and maintenance costs. The operator may find that working at such a hood is similar to working at an open window with the wind blowing in, especially if the supplemental air is untempered. If the auxiliary air is tempered for eom-fort, however, most of the cost savings are lost, and in some instances, costs may exceed the operating cost of a standard installation (275). 

The main forms of this type of hood are shown in Figs. 10.100-10.102. The difference between this type of hood and workbenches is that for fume cupboards the auxiliary air is only a part of the total exhaust flow rate and the flow is not intended to cover the person standing in front of the exhaust opening. The differences between this type of hood and an air curtain are that the air curtain is thinner, has a higher velocity covering the whole opening, and... 

FIGURE 10.100 Principle for auxiliary air supply to a laboratory fume hood. ... 

I FIGURE 10 101 Auxiliary air supplied directly into the hood volume. ... 

Comments on the temperature of the auxiliary air are common. Auxiliary cupboards were first introduced to save energy and unconditioned outside air has often been used for the auxiliary supply air. This is usually not acceptable because unconditioned supply air will be too cold during the heating season for a person to work at the hood. It may also be too hot during the cooling season. Today, most auxiliary air systems operate at temperatures close to room temperature. 

Fuller claims that an auxiliary cupboard has better performance than an ordinary cupboard, if properly designed and used. The auxiliary flow rate should be 50% to 75% of the total exhaust flow from the cupboard. Below 50% there is no beneficial effect and above 75% the auxiliary air will aspirate contaminants out of the cupboard. The auxiliary air should enter the hood through the upper one-half to two-thirds of the opening. This should fill the volume between the cupboard operator and the opening and assist in the containment. The auxiliary air should be distributed uniformly across the length of the cupboard for a vertically sliding sash or above only the open sash of a cupboard with a horizontally sliding sash. It should also have a temperature within 1.5 °C of the room temperature and have a constant flow rate without pulsations. 

A well-designed auxiliary-air-supplied hood reduces air-conditioning costs and improves hood capture efficiency. The disadvantages of this hood include higher initial cost and maintenance due to the additional replacement air system, the possible introduction of airborne dust into the laboratory, discomfort... 

Figure 3.18 Auxiliary or add-air hood, sash open and closed. 

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. 

The Auxiliary-Air fume hood system allows make-up air to be brought directly to the point of use fume hood. Supply air ducting and remote blower required. The Auxiliary-Air fume hood is 263 available in 4-, 5-, 6- and 8-ft. widths. Equipped with vapor proof-light and switch, aU electrical components and services are UL and CSA listed. HEMCO Corp., www.hemcocorp. com, (800) 779-4362. Circle 263... 

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 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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