Once a fume hood has been determined to be the appropriate ventilated enclosure to install in a laboratory, defining the operating parameters for it must include consideration of how the performance of the fume hood is impacted by the physical environment it is located in. Key questions to be answered in this area:

  • Where in the room should the hood be placed?
  • Where are supply and exhaust diffusers to be located relative to the hood?
  • What is the proper minimum face velocity?
  • What should the design operating sash height be?
  • What are acceptable ranges and tolerances for operation of the flow monitors, static pressure, face velocity, VAV controls response time?
Mess and micro scale elements of lab vent
Discussion led by Jim Coogan
Engineer at Siemens
Jim coogan

An effective fume hood contains gases released inside the hood in order to protect the worker and the laboratory environment. It also removes those gases through the exhaust system to be dispersed from the building, preventing high concentrations in the hood. These containment and removal functions are described by a set of operating parameters.The primary operating parameters are:

  • exhaust flow rate
  • open area of the hood
  • average face velocity

The fume hood operating specification establishes values and ranges for the primary parameters, as well as other relevant quantities, both in steady state operation and in changing situations.  The work to establish those values includes defining the relationships among the physical parameters and their effect on the functions of containment and removal.  It also includes considering proper and improper use cases; both types of these can be expected to occur.

Questions to be addressed in this section include:

  • What is the required average face velocity and allowable range of variation, from the average of the readings and across the face of the hood (uniformity)?
  • Are specifications required for spatial and temporal (turbulent) variations of face velocity at the plane of the sash?
  • What defines the plane of the sash?
  • What is the maximum allowable cross draft velocity and is the direction and angle of incidence of concern (horizontal, vertical, perpendicular)?
  • What is the minimum flow when the sash is closed?
  • What is the maximum inlet face velocity when the sash is nearly closed?
  • What is the required hood static pressure (max and min)?
  • What is minimum VAV response time and how is it defined?
  • Is the stability of flow at a fixed opening important and what is the right metric (Coefficient of variation?)
  • What is the containment criteria and should it be associated with generation rate and the dilution factor (i.e. internal concentration versus external concentration)?
  • How is smoke containment and airflow patterns observed and rated?
  • What is the required tolerance for accuracy and precision of hood monitors?

Key take home points from the discussion are:

Start with the users of the hood

  • What is needed in the work station?  What size enclosure and openings will allow adequate access to the work area while maintaining chemical containment?
  • Hood makers can work with performance specifications more usefully than a specific face velocity requirement. As an  example, a specification of a ppm measurement relative to the ASHRAE containment test provides more specific guidance to lab designers. Is 0.01 ppm release acceptable? 0.05? What’s behind determining this value?
  • The design plans should include an air flow budget for the room as a whole.
    • Who would use such a budget?
    • The HVAC designer wants the number to appropriately size ventilation equipment.
    • The commissioning agents and operating staff of the facility also need to understand the design parameters to be implemented after the laboratory is built.
  • Some renovation jobs go that way, others use an air flow budget or cap, not a set value.
  • How sensitive is the ASHRAE 110 test to spatial details of the test?
    • Would we need a containment test on each hood?
    • If this is to be done that way at commissioning, follow with face velocity tests later.
    • Ways that a hood is likely to fail a containment test should be identified and corrective actions addressed as part of the design plan. This is a key element in determining whether lower flow hoods can be used in a particular laboratory.
    • Specifications for the test can be both as manufactured (AM) and as installed (AI).

Discussion Notes

The group discussed a number of topics related to Specifying Design and Operating Parameters:

Priority of Containment vs. Face Velocity

  • Priority Considerations for Control Systems
  • Airflow budget (max – affects house exhaust design)
  • Ordering of Specification Parameters
  • Access dimensions (Width, Height, and Depth)
  • Operating sash dimensions (W and H), and configuration
  • Containment criterion

ASHRAE 110 (As Manufactured) factory type – testing by width

  • ASHRAE 110 (As Installed) (Tracer Gas)
  • Limits on room environment
  • Cross drafts
  • Temperature gradient (Floor to Ceiling)
  • No internal heat sources in hood
  • Comprehensive Energy Performance
  • Automatic sash closers
  • User reminders
  • Occupancy sensors
  • VAV turn-down
  • Limit on Static Pressure drop

What is the Acceptable Face Velocity Range Over Face Area?

  • Historically this range is plus or minus 20% with the lower the face velocity the tighter
  • This requirement becomes critical for High Performance hoods (for example if the face velocity is set for 80 fpm average face velocity design, 75 fpm to 88 fpm is accepted).
  • Reminder: some hoods are non-uniform by design

Spatial and Time-Varying Face Velocity

  • Exhaust variations are commonly found over minutes, hours, and seasons due to varying electrical voltages
  • If the hoods are in a connected battery, moving one sash can affect other hoods
  • Room pressure variations can also be significant in impacting face velocities; can this concern be addressed within the Hood Specification?
  • Aerodynamic components: what part should be in Mechanical Spec?
  • Should flow volume regulation become integral to hood package, by specifying spatial variation
  • Relate to the “As Manufactured” test conditions