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How Should VAV Equipment Be Specified on Laboratory Hoods?

How Should VAV Equipment Be Specified on Laboratory Hoods?

Dr. Robert K. Haugen, Director of Product and Technology Development

Flow Sciences, Inc. 

2025 Mercantile Drive

Leland, North Carolina 28451

12/29/2017

Preface:

Recently, there has been an increasing interest in Variable Air Volume (VAV) fume hood control systems.1 This paper will define such systems in the most elemental sense.  The scope of HVAC systems affected by fume hood VAV will then be detailed. Finally, I will make recommendations regarding the best way to specify such systems to get the best overall finished product from all building suppliers involved.

VAV Systems Defined and Explained:

VAV chemical fume hoods vary the volume of room air exhausted while maintaining the face velocity at a set level down to the lowermost sash openings. Different VAV hoods change the exhaust volume using different methods; exhaust dampers may open or close based on sash position, or a variable speed blower can change fan RPM to meet air-volume demands. The purpose of reducing hood exhaust volume as a function of sash position is to give lab designers an opportunity to exhaust less conditioned air while maintaining proper face velocity, saving energy expense, and promoting sustainability.

ASHRAE 110-2016 addresses VAV hoods in the following manner in Section B3.9: 2

“While the majority of laboratory hoods are installed on constant-volume systems, a growing number of laboratories use VAV supply and exhaust systems. These VAV systems can reduce the volume of air exhausted from the laboratory. It is important to ensure that the VAV systems are properly designed, calibrated, and maintained. The VAV systems, just like all other components of the HVAC system, must function correctly to ensure that fume hood performance is not compromised.”

VAV hoods are connected electronically to the lab building heating, ventilating, and air-conditioning system (HVAC), so hood exhaust and room supply are balanced. In rooms with multiple hoods, labs may become positively or negatively pressurized if this balance in flow is not accomplished.

An “unbalanced lab” scenario is where air exhaust and air supply are significantly different. A lab area at a large positive pressure can be potentially dangerous, particularly when this pressurized lab contains an accident and fumes are “blown out” of the lab into other critical spaces.  Most newer VAV hoods feature monitors and/or alarms that warn the operator of such positive situations as they can lead to unsafe containment conditions.2    The boundary level for an unacceptable positive lab pressure is typically set at 0.1” w.g. (about 0.00036 pounds per square inch [PSI]).3  While measurable, this number is a challenge to accurately measure and not knowing this value will leave lab personnel without warning.

Lab pressurization is only one example of why flows in a VAV system must be accurately measured. The elements shown in the lab schematic below all must be measured and regulated with accuracy for a safe and functional VAV fume hood exhaust system 4:

The four boxes in figure 1 highlight four key elements of any VAV system:

1) Red box shows fume hood and its VAV damper.

2) Blue box shows room exhaust VAV damper. In some cases, this damper is not included because room exhaust is all originated by the fume hoods.

3) Green box is the room make-up air damper, room differential pressure sensor, and supply duct.

4) Purple box (rounded corners) is the control module.

5) It should be added that the depicted system uses dampers rather than VFD multi-RPM exhaust/supply fans. VFD designs are less frequently used because often they have a response time which cannot keep up with rapid and unpredictable sash movements.

Assuming boxes 1-3 are properly designed and yield exhaust response times within a 1-2 second range, the purple box becomes the real challenge. This pressure control module must vary exhaust and supply CFM to provide design face velocity to the fume hood while increasing or decreasing room make-up air to achieve a slightly negative room static pressure differential. Large accidental toxic discharges inside a lab should NEVER be allowed to be oozed into neighboring labs by a lab space wrongly under positive pressure.

Over the past thirty years of increasing applications of VAV technology, a variety of operational problems have been documented with fume hood VAV systems similar to that shown in Fig. 1:

1) Elements within a VAV system must have sensor/valve lag times adjusted in such a way that the system will not degenerate into a condition where labs cycle between over and under pressurization.

2) To accomplish stability, response times of each element must be rapid enough to assure changing sash positions will quickly yield a steady-state, balanced system with acceptable fume hood face velocities.

3) Given the defining control element of any VAV system is the actual fume hood face velocity achieved at a certain sash position, the face velocity measurement technique should not inferentially measure this variable without proof such an inference is valid under lab conditions. The most plausible criticism of VAV is some relevant variables critical to operator safety and air exhaust/delivery stability are frequently not directly measured. These variables may include:

a) Face velocity of the fume hood

b) Exhaust CFM of the fume hood

c) Total room exhaust

d) Total room supply

e) Room static pressure differential

In spite of the issues reviewed above, VAV fume hoods are a fact of life and are increasingly becoming a way of life in 21st century lab design.5,6   For this reason, all companies that manufacture fume hoods must offer high efficiency VAV hoods and be prepared to compete in the open marketplace to sell them and effectively install them into HVAC systems largely furnished to the jobsite by others.

From our product’s perspective (fume hoods), this market reality requires clear performance specifications and subcontractor responsibilities that have been established before VAV system elements are specified, purchased, and meshed together. Architects, mechanical engineers, and HVAC contractors should assume the responsibility for stating the duties of each subcontractor toward achieving a functional, stable, and safe fume hood exhaust system.  This system must be far more than a collection of autonomous independently tested components. At least three key elements in such a system must work together:

1) A high efficiency fume hood with great containment and controlled face velocity 7

2) Make-up air room supply and exhaust properly located for room air flow stability

3) An automated control system that maintains each lab at the proper negative pressure, regardless of fume hood sash position(s).

Practical Requirements for Specifying a VAV exhaust System:

The researcher makes the following recommendations regarding specifying fume hood VAV systems.  These steps apply to all subcontractors who are together responsible for providing an effective laboratory ventilation system:

1) The top priority for any fume hood VAV system must be safety. The Author has participated in the development of fume hood containment standards for the last 30 years using the periodically updated ASHRAE 110 standard.  The most recent version of this fume hood containment standard is ASHRAE 110-2016 8.  The standard has specifically stated ways to evaluate VAV systems which should be followed (Sections 6.2 and 6.3). While the ASHRAE test has no containment pass-fail standard, ANSI/AIHA Z 9.5-2012 has set a pass-fail factory as manufactured (AM) standard of 0.05 PPM 9 using the ASHRAE 110 procedure.

This pass-fail level has created some controversy. Lab Planner Jerry Konigsberg thought the pass-fail number too large, while consultant and lab designer Gregory Muth regarded it as too small. Mr. Muth states that many applications of fume hood containment should not require the 0.05 PPM control level standard, but “something higher.” 10

Making the specified containment level higher is problematic.  First, if hood application determines where the containment performance bar is to be set, what happens when the hood application changes? Second, no hood smoothly transitions from a lower to a higher control number as one lowers the design face velocity.  Instead, containment level abruptly fails at some point and yields new performance levels thousands of times higher. Like concrete bridge supports, we are taking chances if we set the acceptable test level barely above the failure point.

2) While fume hoods are assumed to be “furniture” in a traditional specification format, their performance must be evaluated as part of the larger lab ventilation picture in a modern VAV specification. To gauge fume hood containment performance, a proper air flow environment must be established and then hood-validated using ASHRAE 110 containment testing. Many poorly designed or calibrated components of a VAV system might cause severe system containment issues in field commissioning tests. This would generally mean the hoods are not working for reasons located elsewhere in the exhaust system and the reason for the subpar performance needs to be identified and corrected using other trades.

For example, if exhaust fans are not running, the hoods won’t work. If sensors are not properly measuring flow parameters, the hoods may not work.  Each VAV component must be properly synched with all other VAV containment system input elements before containment is evaluated.

3) Finally, the overall HVAC contract should include work necessary to assure system stability and fume hood containment. System commissioning is a team sport. Any team needs a leader. All pieces of the HVAC puzzle should function together properly. A poor lab designer who has not thought out the HVAC overall structure may not understand how it should work and will be in an impossible position to supervise and determine fixes when building commissioning reveals new performance issues.

Such a stability assessment should start at local control boxes and work its way out into the tentacles of the system. Knowledge of system dynamics is essential.

In conclusion, lab VAV fume hoods must be specified in coordination with other vital elements of the fume exhaust system.  Air dampers/valves, exhaust fans, sensors, thermostatic controls, and the BMS controlling lab pressures are all critical in producing a safe lab exhaust system.

If it is not the contractual responsibility of a single party to “get the system running”, the building HVAC system can easily create migraines at the end of a lab project.

 

  1. R&D Magazine, 10/21/2009, The Great Fume Hood Debate, Paul Livingstone
  2. Dale T. Hitchings, PE, CIH, Laboratory Space Pressurization Control Systems, 2001, http://www.safelab.com/FACT_SHEETS/Pressure.pdf
  3. ANSI/ASHRAE Standard 110-2016, Methods of Testing Performance of Laboratory Fume Hoods, March 31, 2016,
  4. Wikipedia, Fume Hood, Variable Air Volume.
  5. http://ateam.lbl.gov/Design-Guide/DGHtm/vavsystems.htm
  6. Greg Muth, Laboratory Design News, Sustainable Lab, 10/12/2015, https://www.labdesignnews.com/article/2015/10/fume-hoods-lab-design
  7. Gordon Sharp and Others, R&D Magazine, 2010, Airing Out Laboratory HVAC, pp1-3, http://www.aircuity.com/wp-content/uploads/RD-Magazine_Airing-Our-Lab-HVAC_2010_08_Eprint.pdf
  8. ASHRAE 110-2016 and ANSI/AIHA Z9.5 together establish testable minimum safety standards.
  9. ANSI-AIHA Z 9.5, Section2.4.1, explanation column.
  10. R&D Magazine, 10/12/2015, Fume Hoods in Lab Design, Hock, Lindsay, p 1 of 7, https://www.rdmag.com/article/2015/10/fume-hoods-lab-design

 

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