NIH Takes a 3-D Look at Fume Hood Containment
By Susan English
Bethesda, MD–The National Institutes of Health (NIH), Division of Engineering Services, in conjunction with Flomerics Ltd. of England, the Building Services Research and Information Association (BSRIA) of England, and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) is three-quarters of the way through a research project that will produce a foundation for standards that will maximize fume hood containment in research laboratories.
The project, initiated last March, has a targeted completion date of February, 1996, when it is expected to produce guidelines that will significantly reduce health and safety hazards associated with working in laboratories where fume hoods and/or safety cabinets are used. Research for the project was based on more than 160 different case studies, tabulated data from which will be presented at the project`s 75 percent completion meeting to be held at the NIH on November 28 and 29.
The presentation will include a background of the project, description of the analyzed cases, methodology used for evaluating the performance of each analyzed case, and discussion of results. Also, animated computer-generated models of air movement will be used to illustrate the different cases, showing in detail the interaction of the many parameters and their effect on the functioning of laboratory fume hoods. The study encompassed various combinations of the following: laboratory geometry and layout, location and type of supply diffusers and exhaust registers; discharge air temperature, discharge air velocity, source, location, and type of heat generation; fume hood face velocity, type and location of contaminant source; and contamination release rate. Principal Investigator and Project Director of the research is Dr. Farhad Memarzadeh, senior mechanical engineer in the engineering division of the NIH`s Office of Research Services. Research participants comprise an impressive list of attendees. Among those participating: NIH`s Office of Research Services; NIH`s National Institute of Environmental Health Science; Howard Hughes Medical Institute; the Centers for Disease Control and Prevention; Massachusetts Institute of Technology; Harvard University; Pennsylvania State University; Johns Hopkins University; Duke University; the University of North Carolina at Chapel Hill; and the American Institute of Architects.
Computational Fluid Dynamics
The project, entitled “Laboratory Air Distribution Analysis for Optimum Fume Hood Performance Using Computational Fluid Dynamic Modeling and Empirical Field Verification,” uses computational fluid dynamics (CFD), a mathematical 3-dimensional modeling technique, to predict ventilation performance for various laboratory configurations. It is no secret that room geometry, diffuser placement, and operational procedures within the laboratory affect the containment performance of fume hoods and safety cabinets. Therefore, the impetus for the NIH study was the need to study laboratory ventilation as a system and not just at the fume hood level.
However, literature on the subject of fume hood containment is sparse, says Dr. Memarzadeh, and what is available does not address the complexities of laboratory layout and equipment placement at the level that exists at facilities like the NIH. What does exist is some empirical information about the effect of airflow velocities near the hood face on fume hood containment, as derived from a generic laboratory. But sophisticated laboratory research usually requires a great deal of equipment, which compromises the ability of the hoods to perform. It also tends to interfere with the air from the supply diffusers, sabotaging their design effectiveness, and thus aggravating the problem of contaminant containment. It is the resulting laboratory air movement (from these complex interactions) near the hood that compromises fume hood containment performance. In conducting detailed experiments on air flow and hood containment performance with multiple configurations of laboratory ventilation, Dr. Memarzadeh found CFD an invaluable tool. He was able to calculate air flow patterns without physically rearranging equipment or space. “It was expensive to demolish and rebuild rooms,” says Dr. Memarzadeh, “so in the early 90s, I started to use CFD.”
To predict airflow, temperature, and the distribution of airborne contaminants in any given point in laboratory space, room designers, given a set of design parameters such as the geometry and layout of the lab, the sources of heat and contaminants and the position of exhausts and ventilation ducts, must consider the fundamental laws of physics describing fluid flow and heat transfer. CFD simulations use a method called “space discretisation,” by which the 3-dimensional space of the laboratory is subdivided into a large number of control volume cells. The level of discretisation will influence the detail and accuracy of the final results. In some cases, the number of cells could run into tens or perhaps hundreds of thousands. Each cell is used to represent the mathematical definition of the boundaries contained in it. For example, a cell could encompass the volume enveloped by a corner of a solid piece of furniture, the surrounding air and some heat source. The CFD software will then attempt to solve the complex equation for a predetermined set of variables for each cell: velocities in three directions, temperature, pressure, and the turbulence quantities that simulate random behavior of the air.
The mathematics governing these phenomena is fully contained in the century-old Navier-Stokes equation, which includes the full interaction of the three basic modes of heat transfer–convection, conduction and radiation–simultaneously at every point, and in three dimensions. Deceptively simple, only the emergence of ever faster computers over the past two decades has made it possible to solve the real-world problems governed by this equation.
Over the past 25 years, CFD techniques have been used extensively and successfully mainly in high-end technology sectors, such as the nuclear and aerospace industries. In its raw form, CFD has always been the province of fluids experts. However, recently, the concept of tailoring CFD software and combining it with R&D expertise targeted to the appropriate market segment has made it possible to apply these powerful methods to provide fast and accurate results to designers under severe time and budgetary constraints.
One CFD-based attempt to address the needs of the building services industry has been launched by U.K.-based Flomerics Limited. Flomerics joined forces with the U.K.`s leading experts in the field, England`s Building Services Research and Information Association (BSRIA), to develop a CFD-based software. Called Flovent, it features a specifically tailored menu structure to make the complex techniques more accessible to the needs of the HVAC industry.
At present, Flomerics is taking a low-key approach to the U.S. market, taking time to build what it sees as the appropriate support infrastructure to help users avoid costly errors. Its strategy is to sell the software together with the appropriate technological know-how in the form of a 3-D computer simulation service, Flovent Modeling Services (FVMS). The long-term goal of Flomerics is to introduce the software into appropriate consultancy organizations. “We believe the impetus for this will come from the end-users, who can see the benefit of having a scientifically-based assessment of their ventilation systems performance,” says Mark Seymour, manager for FVMS and Flovent Modeling Services.
A Standard for the Industry
The accessibility of CFD as a design tool for predicting ventilation performance, along with the rapidly increasing power of modern computer technology needed for this type of analysis, has provided designers with the required set of tools to examine the interaction of the many parameters and their effect on the fume hood containment system. The NIH`s Dr. Memarzadeh and Flomerics` Mark Seymour and Hassan Moezzi expect that the information provided by this project will be used to form the basis of industry guidelines for controlling ventilation performance, maximizing local containment, and minimizing the risk of contaminant dispersion within the laboratory or other areas. As an immediate result of this research, building owners and managers will be able to utilize the library of cases accumulated during the project to identify the safety-related characteristics of a lab and thus be able to develop safety guidelines that meet their own requirements. Ultimately, these techniques will become a standard for designing in sensitive environments. Building designers will be able to design lab space and specify operating conditions for the desired or required level of safety, much as today`s cleanrooms are quantified. n