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Technology Report

February, 2003

Siemens Industry, Inc. Page 1 of 6

VAV Fume Hood Energy Savings and Safety Considerations

Sash Closure vs. Automatic Face Velocity Setback

This report addresses the potential energy savings

and safety considerations associated with Variable

Air Volume (VAV) fume hood control systems that

offer automatic face velocity setback. Recall that the

purpose of a VAV fume hood control system is to

control fume hood exhaust as a function of the sash

opening in order to maintain a constant face velocity.

By maintaining a constant face velocity, fume hood

air consumption is reduced whenever the sash

opening is reduced, thereby saving ventilation

system energy. VAV fume hood air consumption and

ventilation system energy usage is minimized

whenever the sash is fully closed.

This contrasts with Constant Air Volume (CAV) fume

hoods, which do not change overall fume hood air

consumption in relation to the sash position. Thus,

CAV fume hoods consume significantly more

ventilation system energy than VAV fume hoods.

Automatic Face Velocity Setback

Aside from maintaining a constant face velocity at all

sash positions, some VAV fume hood control

systems incorporate a type of user detection sensor

to enable initiating an automatic reduction in face

velocity when the user is not at the fume hood. The

claim is made that when no one is at a fume hood, a

face velocity of 60 feet per minute (fpm) in lieu of the

commonly accepted 100 fpm will still provide

adequate fume containment. And subsequently, by

reducing the face velocity from 100 fpm to 60 fpm,

fume hood air consumption will be reduced to about

60% of what it would be at 100 fpm.

Safety Considerations

Before addressing the potential energy savings

claim, the safety issues associated with automatic

face velocity setback should be the first

consideration. Although face velocity setback has

been promoted for a number of years it continues to

be a subject of controversy within the laboratory

ventilation industry. The reservations against

implementing face velocity setback center around

whether 60 fpm will provide adequate fume

containment even if no one is in front of the fume

hood.

Research has revealed that room air currents or

cross currents in the vicinity of the fume hood can be

very detrimental to effective fume hood containment.

Even relatively mild air currents passing a fume

hood sash opening can draw out fumes from the

hood interior. Figure 1 illustrates chemical fumes

being d

rawn out of the fume hood by room air

currents (shown by dark arrows) passing the open

sash. This phenomenon can result when the room

air currents are horizontal, vertical, or even angular.

Thus, all room air currents pose the potential for

compromising effective fume hood containment.

ROOM AIR

FUME

ROOM AIR

Figure 1. Room Air Currents Can Draw Out

Fumes from the Interior of the Fume Hood.

Document No. 1

49- 976

Summary of content (6 pages)

PAGE 1
Technology Report February, 2003 VAV Fume Hood Energy Savings and Safety Considerations Sash Closure vs. Automatic Face Velocity Setback This report addresses the potential energy savings and safety considerations associated with Variable Air Volume (VAV) fume hood control systems that offer automatic face velocity setback. Recall that the purpose of a VAV fume hood control system is to control fume hood exhaust as a function of the sash opening in order to maintain a constant face velocity.
PAGE 2
To minimize these effects ANSI/AIHA Z9.5 states that room air velocities should be less than one-half and preferably less than one-third of the face velocity 1 . Standard NFPA 45 also states that room air currents should ideally be less than 30% of the fume hood face velocity.
PAGE 3
velocity rather than 100 fpm, an automatic face velocity setback provision will provide a near 40% 6 reduction in fume hood ventilation energy for most of the time. However the actual energy savings are diminished if: 1. Users close their sashes when they no longer need immediate access to the fume hood interior. VAV fume hood ventilation energy savings will then attain the maximum reduction of approximately 80% when the sash is closed.
PAGE 4
STOP 100 DID YOU REMEMBER TO CLOSE YOUR SASH ? AN OPEN SASH IS A HAZARD ! FUME HOOD ANNUAL ENERGY COST SI ZE SASH OPEN SASH CLOSED 5Foot $5,00000 $1,00000 6Foot $6,25000 $1,25000 KEEP FUME HOOD SASHES 8Foot $8,50000 $1,70000 CLOSED 10Foot $10,75000 $2,15000 Figure 2. Example Signage for Reminding Laboratory Workers to Close Fume Hood Sashes. the sash.
PAGE 5
Conclusion  Automatic Sash Closure Mechanisms– Mechanisms may be added to fume hoods to slowly close the sash after a pre-designated period in which there is no sash movement. Such devices typically incorporate appropriate safety features to ensure against personal injury and exerting excessive closing force on flexible hoses, electrical cords, or other apparatus that may extend into the sash opening.
PAGE 6
Product or company names mentioned herein may be the trademarks of their respective owners. © 2009 Siemens Industry, Inc. Siemens Industry, Inc. Building Technologies Division 1000 Deerfield Parkway Buffalo Grove, IL 60089-4513 USA Printed in the USA Document No.