Summary

The first step in understanding what the design goals for fume hoods are is to conduct a risk assessment of the processes that need to be controlled. Design options include:

  • The type of ventilated enclosure to be specified
  • Appropriate functions and associated options to be asked of the equipment (adjustable baffles, automatic sash closure, sash style (vertical/horizontal/combination sash), HEPA filtration)
  • Specific monitoring protocols for assessing equipment performance
  • Other accessories to be provided (sinks, gases, shelves, rod lattice systems)

Discussion Group led by Greg Muth
President, I2SL, New England Chapter
Greg Muth

Discussion Questions

Prudent selection of appropriate local ventilation devices requires a risk assessment of the work to be done in the space being designed. Unfortunately, the information required to fully develop such an assessment may not be available as plans for facility construction or renovation proceed. In this context, it is important to have an informed discussion between the various stakeholders in a laboratory project. Questions to be considered include:

  1. What are the required internal and external materials of construction (stainless, PVC, FRP, cement board, etc.)?
  2. How should sashes be constructed (options include framed, not framed, horizontal, vertical or combination)?
  3. How important are airfoil sills to the aerodynamic design?
  4. Should baffles be adjustable, fixed or automated?
  5. Should automatic sash closure devices be installed and what are the desirable attributes? Do they really save enough money to justify the cost of installation, maintenance and training?
  6. What is the purpose of the bypass and how much bypass is required to minimize velocities, ensure hood static pressure, and promote dilution within the hood?
  7. Where should use feedback mechanisms such as  face velocity monitors and signage be located?
  8. How bright should the lights be and how can they be accessed to replace bulbs without entering the hood?
  9. What is the best outlet collar: square, rectangular or round?
  10. What is the necessary duct transport velocity and internal airflow patterns required to minimize accumulation and deposition of materials within the hood or duct?
  11. Where should sinks and faucets be located, if they are necessary?
  12. Can shelves be used in hoods and if so, how should they be designed and how might they affect hood performance?

 

Discussion by Small Group Participants

The group discussed a number of topics related to Construction and Maintainability. Key topics that arose and thoughts collected relative to them are enumerated below.

Materials of Construction

  • Should have a 20-50 year life cycle
  • Exterior: Powder coat steel; Specialty Hoods may require polypropylene or stainless steel
  • Must be suitable for widest range of applications
  • Interior should have the following characteristics:
    Work
    Surface
    Liners Sash
    • Epoxy Resin
    • Stainless Steel
    • Polypropylene
    • Phenolic Resin
    • Fiberglass Reinforced Polyester (FRP)
    • Epoxy Resin
    • Stainless Steel
    • Polypropylene
    • Powder Coated Steel
    • Phenolic Resin
    • Laminated Safety Glass
    • Tempered Glass
    • Lexan
  • UL 1805 Certification
  • Tests for chemical resistance, flame spread smoke generation, etc.Materials

Sashes – Ergonomics and Safety

    • Combination Sash

      • Use of horizontal panels as safety shield is impacted by the width of panel
      • High and low access to the workspace is necessary
      • For all sash types, it is important to train users on expected sash use
      • Some uses require an extra height vision panel
      Hood sash styles
    • Cost Implications of Combination Sashes

      • Premium for sash
      • Additional horizontal sensors for VAV controls
      • Additional sash positions for testing & balancing
    • Sash Drive Mechanism Options to Consider

      • Cable & pulley
      • Chain & sprocket
      • Belt
      • Sash stop 18 inches vs. 16 or 12 inches, etc.
      • Auto sash return
      • Adminstrative controls and training
    • Air foils should be required on all hoods
    • Airflow rates is a key design criteria
    • Ergonomics is a secondary design consideration, but critical in its impact on how the hood is used

Electric Cord Management Options

  • Flip foil
  • Post pass through

Service Access

  • Front Panel controls
  • Interior access panels (gaskets are a problem in corrosive atmospheres)
  • Use of front-load plumbing valves

Baffle Adjustment

  • Users should not be expected to adjust baffles to maintain containment; design must not rely on user adjustment.
  • The degree of baffle adjustment available may be manufacturer specific, so these should be designated in the hood specifications if they are expected to be operated by the hood user.

Automatic Sash Closers

  • There are financial cost/payback questions around these devices.
  • When used, they require safety features to prevent closure on objects and training for users.
  • User acceptance is key to successful use of these systems.
  • The cost savings are conditional based on the type of VAV system, hood density, etc..
  • This is not needed if an effective sash management training program exists.

Bypass

  • In Constant Air Volume hoods, the bypass reduces increase in face velocity as sash is closed.
  • Typical design target is that face velocity shall not increase more than 2.5 to 3.5 times from velocity at full open sash (i.e. up to 250 FPM to 300 FPM)
  • In Variable Air Volume hoods, the bypass area is based on minimum setback flow controls;  bypass should not be a factor until sash closes to point where minimum flow is reached.

Face Velocity Monitors Location

  • Place at 48” from the floor; spell out AFF for ADA hoods and place on the hood post.
  • 60” is the typical height for placing monitor on hood.
  • Digital or analog monitors should be specified as part of the hood standard.
  • Should purge buttons be provided? The effectiveness of purge for containing spills is questionable.
Color Coded Hood Monitor

User Instruction Signage Options

  • On front panel
  • On sash glass

Other features

Hood Lights

  • Must be accessible from outside the hood for maintenance purposes
  • 80 foot-candles average at the work surface
  • T8, T5 bulbs or LED are the most energy efficient

Duct and Duct Velocity Considerations

  • For the collar connection, a Bell Mouth is preferred
  • Round ducts are best if space on hood roof allows
  • Rectangular ducts are possible if space considerations on the roof or to match the plenum behind baffle make them necessary
  • If non-round duct is provided, then the manufacturer should provide transition piece to round
  • Duct Velocity requirements can be found in the current ACGIH Industrial Ventilation Manual

Sinks

  • Should only be provided when specific use is identified
  • A cup sink have berms to prevent a spill going down the drain
  • An alternative is to wall mount cup sinks
  • Sink location is user preference; typical location is at hood rear

Shelves

  • Must be designed to not disturb airflow patterns in hood chamber
  • Shelf designs should be validated through performance testing
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