Thomas C. Smith, Exposure Control Technologies, Inc. and Nick Paschke, DuctSox, Inc.
Laboratory fume hoods are installed to protect laboratory personnel from exposure to hazardous airborne materials. A fume hood protects personnel by containing, capturing, and exhausting the materials generated within the hood enclosure. However, it is well documented that containment can be affected by numerous variables including the aerodynamic design of the hood, the average face velocity, and cross draft velocities produced near the hood opening. Numerous tests by Knutson, Hitchings, Smith, and others have indicated that improper supply of air to the laboratory can produce cross drafts that negatively affect hood performance. However, there is little information that directs laboratory designers toward proper selection of air supply diffusers, proper location of the diffusers with respect to the hood, and determination of the proper discharge characteristics.
A study was conducted in the test laboratory at Exposure Control Technologies to evaluate the affects of six different air supply diffusers on performance of a 6-foot high-performance fume hood operating at an average face velocity of approximately 60 feet per minute with the vertical sash open to a height of approximately 28 inches. The diffusers included a 2' x 4' radial flow fabric diffuser, a 2' x 4' laminar flow fabric diffuser, a 2' x 4' flat perforated metal diffuser; a 2' x 4' perforated radial flow diffuser, and a commercial type, 2' x 2', four-way, high-velocity, high-aspirating, diffuser. The study was undertaken to evaluate the affects of each diffuser on hood performance. With the exception of the 2' x 2' diffuser, the laboratory configurations and supply volumes were chosen by the authors to follow “good” or “recommended” laboratory design practices in terms of location with respect to the hood and supply discharge volume.
The diffusers were located in the 10-foot high ceiling approximately six feet to the right side of the hood (Laboratory Configuration 1) and approximately five feet directly in front of the hood (Laboratory Configuration 2). The supply volume for each diffuser was held constant at approximately 650 cubic feet per minute during the tests. Two sequences of tests were conducted for each diffuser at each of the two laboratory configurations. The first sequences of tests (“Normal Challenge”) were conducted with the supply discharge temperature equivalent to the room air temperature (approximately 72 degrees Fahrenheit). The second sequences of tests (“Thermal Challenge”) were conducted to produce a thermal stratification by cycling the supply discharge temperature to cold (less than room air temperature) and then to warm (above room air temperature).
Three series of five-minute performance tests were conducted for each of the six diffusers under the two laboratory configurations and two temperature challenges. Tracer gas tests were conducted to evaluate containment according to methods described in the ASHRAE 110 “Method of Testing Performance of Laboratory Fume Hoods.” The mannequin was placed at the center of the hood opening at a breathing zone height of approximately 22 inches above the work surface. Output from the calibrated leak detector was recorded at a rate of one sample per second using the ECT HoodPRO test system. The system was used to simultaneously record supply static pressure, exhaust static pressure, room differential pressure, supply discharge temperature, room temperature, horizontal cross draft velocity, vertical cross draft velocity, perpendicular cross draft velocity, and face velocity. The simultaneous collection of data produced more than 216,000 data points.
Figure 1: Test Lab (Configuration 1) with Experimental Apparatus and HoodPRO Data Collection System
As expected, the results indicate that good hood design, selection of appropriate diffusers and proper laboratory design are imperative to proper fume hood performance. The results also indicate that there may be numerous variables affecting performance that are beyond our current understanding and beyond those studied during this project. The DuctSox Fabric diffusers and the 2' x 4' metal diffusers resulted in acceptable containment, using a criterion of 0.05 parts per million concentration for an average 5-minute tracer gas test. However, the 2' x 2' diffusers produced cross drafts that caused the hood to fail to meet the containment criterion particularly when located in front of the hood opening. The data suggests that locating the diffusers to the side of the hood had the least impact on hood performance and confirms the well know assertion that cross drafts in excess of 50 percent of the average face velocity can have deleterious affects on hood performance. Finally, the data indicates that discharge temperatures may not have a significant impact on hood performance when diffusers are properly selected and located with respect to the hood.
Table 1: Tabulated Summary of Test Results
The study indicates that hood performance can be affected by diffuser selection, location of the diffuser in the laboratory, and supply volume. However, the study falls short of establishing performance envelopes for the diffusers in terms of the proximity to the hood, affect on different hood types, and allowable flow volumes. These topics should be the subject of further study to ensure provision of safe, dependable, and efficient laboratories.
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Thomas C. Smith is the President of Exposure Control Technologies, Inc. Mr. Smith is a leader in laboratory safety and energy management. He specializes in helping laboratories provide safe, dependable, and energy efficient operation of laboratory hoods and ventilation systems. He holds a Bachelor of Science degree in mechanical engineering from North Carolina State University and a Master of Science degree in environmental engineering from the University of North Carolina.
Mr. Smith is active in developing national and international standards for laboratory ventilation and serves as Chairman of ASHRAE TC9.10 Laboratory Systems and Vice Chairman of ANSI/ASHRAE 110 Fume Hood Testing. He is also a member of ANSI/AIHA Z9 Standards for Ventilation and Health.
Since 1985, Mr. Smith has participated in hundreds of laboratory ventilation projects and evaluated thousands of laboratory hood systems. His work has improved the safety of laboratory environments and saved millions of dollars in energy costs. He currently provides technical consultation to numerous Fortune 100 companies, top research universities, and government agencies on the forefront of environmental safety and energy conservation.