Document byBZqbq0kQY0v3MgeqD9kBoyZ

4S i i m Interoffice Communication to Distribution From J* J Hall Date May 22, 1978 subject Laboratory Hood Classification Schedule ,r*r" ' 3 Properly functioning laboratory hoods are an essential element in controlling exposures to chemicals in our laboratories. Face velocity in linear feet per minute is the critical parameter of hood performance. Much controversy has revolved around the search for a "magic number" suitable for all hoods under all conditions. Unfortunately such a number does not exist. There are a number of variables to be considered In selecting the proper face velocity, including: 1. Characteristics of the substance to be handled (quantities handled, toxicity, vapor pressure, etc.). 2. The amount of air disturbance expected at, or near the hood. (For example, the proximity of the hood to a door, walkway, or air conditioning grill). 3. The physical configuration of the hood including the size and shape of the normal opening, the presence or absence of internal baffles. 4. The uniformity of face velocities. For maximum flexibility In the use of hoods, the ideal situation is to pro` vide all hoods with air flows which ensure complete capture of all materials released within the hood. However, a savings in energy to condition make-up air, can be realized by tailoring each hood's performance to its specific use Outlined below is a hood classification schedule which can be used as a guideline, in conjunction with the variables mentioned above, to arrive at proper performance criteria for your laboratory hoods. i CLA--SSES OF HOODS ! i. Class A Hoods: for handling highly toxic materials where complete control Is required. For work falling into this cagegory, hoods should be designed to give an actual performance of 125 feet per minute. Such hoods should have special design characteristics depending on each Individual situation. Materials handled In these hoods would Include tetraethyl lead, radioactive materials, beryllium compounds, carbonyls, benzene, VCM, and materials having a similar high order of toxicity. Class B Hoods: would be hoods designed to give an actual performance of jtfL feet per minute face velocity. These could be used for any operations with the exception of those offering potentially severe hazards due to a high order of toxicity of materials to be handled. Basically these hoods would be used for materials having from a moderate to a high order of toxicity. DTH OC0023900 'a'it* 2 Ha.- 22, 978 3. Class C Hoods: would be designated as those designed to give an actual performance of 50 feet per minute face velocity. These could be used for all operations where the hazard Is not unusual due to a moderate or low order of toxicity of materials to be handled. This would Include essentially all hydrocarbons and most likely would represent a great portion of the work required under a hood In most of the laboratories. For assistance in determining proper classification, a list of typical com pounds in each class is attached. COMMENTS 1. Velocity readings should be taken In the center of twelve or more equal area sections over the hood face with the doors wide open. A visual check of the air flow pattern at the hood face with a ventilation smoke tube is also helpful. The smoke tube test can show where distrubing air room currents cause reverse air flow or where there are obstructions to air flow in the hood. 2. In all cases the face velocities mentioned represent minimum average velocities. To assure reasonable uniformity across the hood face, no area should have a velocity less than 80% of the average value. At n point should there be a negative flow or air flow from the hood into the room. This can be easily checked visually through the use of smoke tubes. 3. In addition to the ventilation tests, there should be a regular program for checking fan speed and the condition of the ducts, fan belts, and lubrication. If you have a particular problem or question, give me a call. Distribution Aberdeen - R. Martin A. H. Sather Baltimore - K. W. Resh J. A. Werneke Rod Simmons ^ A. E. Russell 3~. Bo*.**- LCCP - S. F. Pitts D. Zimmer Ponca City - O.P. Steffey LC-VCM - P. L. Fetzer W. V. Henry Newark - G. P. Haberman H. Pltera OKC - K. L. Fogg DTH 000023901