Fume hoods airflow

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 class I biological safety cabinet is intermediate between a fume hood and a closed glove box. This cabinet can be used with the front open or be fitted with gloves. Since the front access opening is normally only 8 inches high, the cabinet requires less ventilation than a fume hood. Class I cabinets are used with an airflow at the face of 100 linear feet per minute. 

Many operations within the laboratory start with weighing. It is difficult to undertake precise weighing in conventional laboratory fume hoods, as the airflow and vibration inside an operating fume hood often disturb the balance. In order to accommodate the need for an engineered control at this scale of operation, dust control systems have evolved known as ventilated weighing safety enclosures or powder weighing hoods. Key characteristics of a ventilated weighing safety enclosure include ... 

Regularly check fume hoods for proper airflow. Ensure that fume hood exhaust is not drawn back into the intake for general building ventilation. 

Fume hood fulfilling the present internationally recognized requirements set for fume hoods, such as DIN, OSHA (US) and BS standards. Normal linear airflow velocity at the face should be over 0.5 m/s (measured in compliance with BSI DD80 or equivalent) with approx, size 90 x 90 x 90 cm. 

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. 

Portions of the HCF structure, along with the steel confinement boxes (SCBs), gloveboxes, fume hoods, and the ventilation systems perform confinement functions. These confinement systems provide defense in depth by ensuring that hazardous materials are retained in specific designated areas within the HCF. They accomplish this function by maintaining an air pressure differential hierarchy from regions of greater contamination to those of lesser contamination within the facility. These differentials are described later in this section. This pressure differential controls the movement of contamination by diffusion and by adverse airflows. The Identified contamination zones in the HCF are as follows ... 

The fume hood is perhaps the most misused item of laboratory equipment. Fume hoods are not designed for chemical storage the presence of many containers of various chemicals and other material that accumulates in hoods makes their operation inefficient and unsafe. Objects blocking baffle slots, operating the unit with the sash raised, inadequate airflow, and obstmction of the front airfoil are only some of the problems typically found in laboratory fume hoods. Satisfactory operation of the fume hood depends upon three factors proper use of the chamber and work area, correct hood location, and adequate airflow capacity. These are discussed in detail below. 

Although the location of the fume hood is not under the control of the user, proper location is very important in both the efficiency and safety of the unit. The fume hood should be located out of the traffic patterns of the laboratory, away from ventilation crossdrafts, doors, and windows. Two fume hoods should not be facing one another across an aisle. Crossdrafts from ventilation and laboratory traffic can completely alter the airflow characteristics of a fume hood. 

If these conditions occur in an existing installation, take steps to avoid production of crossdrafts by restricting traffic during fume hood use, baffling air ducts (but not to the point where room air balance is completely upset), closing windows or doors, or taking whatever steps may be necessary to preserve the unit s airflow characteristics. 

The total airflow capacity of a fume hood is proportional to the cross-sectional area of the hood face and the face velocity. For a 1.5-meter (5-foot) hood with a sash height opening of 70 centimeters (28 inches), the sash opening area is 1.05 square meters. If the face velocity... 

In this example, the total exhaust is 32 cubic meters per minute. In order for the hood to function properly, the supply air delivered to the room when the hood is operating must equal this exhaust volume plus the general room air exhaust volume. This fact has been neglected in some installations, and has led to operational problems (175). We will discuss airflow in more detail below in the context of fume hood efficiency. 

Do not permit a fume hood to be used for storage of chemicals. An accident in a hood used for chemical storage can become a major problem. Also, storage in the hood defeats its airflow characteristics. 

Daily and before-use inspections/tests. At a minimum, daily/before-use and quarterly inspection and testing should be conducted on fume hoods. The daily or before-operation inspection should consist of an airflow check. A simple qualitative check that can be used to check for proper airflow, exhaust, is the ribbon or tissue paper check. During this test, a ribbon or piece of tissue paper (be careful not to lose control of these for they could gum up the system) is placed at the hood opening to determine if it reflects directional airflow. Daily and before each use the hood gauges and monitors should be checked for proper operation within a predetermined range. 

Quarterly inspection and testing. Face velocities for laboratory fume hoods should be measured on a quarterly basis. These tests should be conducted by qualified personnel using a properly calibrated velometer. Smoke tube or smoke candle tests should be performed quarterly to evaluate airflow patterns. 

Gases, vapors, and fumes usually do not exhibit significant inertial effects. In addition, some fine dusts, 5 to 10 micrometers or less in diameter, will not exhibit significant inertial effects. These contaminants will be transported with the surrounding air motion such as thermal air current, motion of machinery, movement of operators, and/or other room air currents. In such cases, the exterior hood needs to generate an airflow pattern and capture velocity sufficient to control the motion of the contaminants. However, as the airflow pattern created around a suction opening is not effective over a large distance, it is very difficult to control contaminants emitted from a source located at a di,stance from the exhaust outlet. In such a case, a low-momentum airflow is supplied across the contaminant source and toward the exhaust hood. The... 

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