Design Guidance for Exhausts

Edwina Wong and Michel Ratcliff, Rowan Williams Davies & Irwin

Objectives:

Exhausts from laboratory fume hoods have been known to cause odors and adverse health effects if the exhaust from the stacks is re-entrained into a building or impinges upon outdoor pedestrian areas. The proper design of chemical fume hood exhaust stacks requires a target dilution criterion based on estimates of the concentrations of chemical vapors in the stack. However, the level of chemical vapors in the stack is dependent on many variables that are difficult to quantify, including the type of process being undertaken in the fume hood, the face velocity through the hood and the flow rate of the hood exhaust. In addition, the emission rate of the chemical is dependent on the physical state and volatility of the chemical in question. The evaluation of acceptable levels of chemical fumes is also dependent on the toxicity or odor potential of the particular substance.

The objective of this paper is to present a practical methodology for establishing a dilution criterion for a worst-case release of commonly used laboratory chemicals and gases within a fume hood, and to provide guidance on the application of these criteria for achieving good exhaust stack design. The criteria will evaluate the potential advantages of large manifolded exhausts over individually exhausted fume hoods.

Findings:

A methodology for developing a dilution criterion for fume hood exhaust stacks will be presented. A general criterion that works for most cases will be presented along with a methodology for modifying the criterion as necessary for light chemical usage, or for extraordinary chemical emissions. The criteria are partly based on calculations of chemical emissions and partly on other factors including experience, feasibility of stack designs, and comparison to other related laboratory criteria. A summary of calculations for estimating emissions from a worst-case release scenario (i.e., chemical spills) for each of approximately 400 commonly used liquid and gaseous chemicals will be presented.

Examples of the methodology will be applied for selected chemicals. Most cases are served well with a simple criterion based on previous experience. An example detailed analysis will be shown for a case where the simple criterion is too strict, resulting in lower stack heights or flow rate requirements. Another example will be shown for extraordinary chemical usage and what risks may be present.

Labs21 Connection:

This presentation directly reflects the following aspects of the Labs21 approach:

• Minimize overall environmental impacts - Meeting the fume hood exhaust dilution criteria will help to mitigate or avoid odors and health impacts at surrounding areas.
• Protect occupant safety - Meeting the dilution criteria will also help assure safety of building occupants from toxic emissions.
• Energy efficiency - An exhaust system designed on the basis of a well reasearched and accurate criterion may avoid over-design of the system, possibly resulting in lower stack heights and lower exhaust flow rates. Lower exhaust flow rates greatly reduce energy use and operational energy costs over the building lifetime.

Biographies:

Edwina K. Wong, M. Sc.(Eng), P.Eng., has a B. Sc. (Eng). and M.Sc (Eng) in Environmental Engineering from the University of Guelph. Her research work for her Masters Degree investigated the field of micrometeorology and its use in measuring air contaminants from bioremediation sites. During her studies, Ms. Wong completed field measurements of hydrocarbons and developed a screening level numerical dispersion model to assist in the assessment of bioremediation sites.

Ms. Wong is currently the Team Leader and a Project Engineer in the Environmental Modeling Division at Rowan Williams Davies & Irwin Inc. in Guelph, Ontario, Canada. Edwina focuses primarily on numerical and physical air quality modeling for different types of studies including exhaust re-entrainment and roadway/railway emissions assessments. She specializes in exhaust re-entrainment studies for the design of building exhaust and air intake systems for laboratory and other related facilities.