Basic Documentation

Chapter 6–Laboratory Containment Units - Ventilation
Since an active airflow control arrangement is not often applied to this type of
fume hood, the interaction between the sash and the bypass area is the only
means of keeping the face velocity from increasing or decreasing as the
sash is repositioned. Because of inherent differences in the aerodynamics
between the bypass area (covered by a grill or set of louvers) and the open
sash area, the average face velocity through the open sash area will still vary
as the sash is repositioned. The exhaust system static pressure also affects
the amount of exhaust from this type of fume hood so some variation in the
face velocity can be expected even when the sash remains in a given
position. As a result of these factors, the ability of a constant volume fume
hood to provide containment is somewhat dependent upon the extent that
the face velocity will vary which is not always a known factor.
As previously mentioned, a consideration with regard to constant air volume
ventilation systems and constant volume fume hoods, is their high energy
consumption due to the high air consumption rate. Since the makeup air
supplied to a fume hood is conditioned room air, the operation of a multitude
of constant volume fume hoods with no provision to limit their exhaust rate
results in a considerable energy expense. The factors that contribute to the
cost include the fan electrical power to provide the exhaust and makeup
airflow and the energy required to heat, cool and humidify the makeup air.
As a result, each typical 6-foot wide fume hood can easily cost a facility
between $3,000 and $6,000.
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per year in ventilation system energy
expenditure.
• Auxiliary air fume hoods–An auxiliary air type of fume hood refers to the
use of an additional source of outside makeup air (auxiliary air) that is
arranged to discharge into a laboratory room near the upper front area of a
fume hood. (The discharge is typically just above where a fume hood user
normally stands.) This arrangement was introduced as a reaction to the
1970’s energy crises when energy costs underwent a sharp rise and energy
availability was curtailed. The auxiliary air provision was intended as a way to
utilize unconditioned outside air as a large portion of a fume hood’s makeup
air rather than using so much conditioned room air. However, the utilization
of auxiliary fume hoods has continually declined as users and ventilation
system designers have become aware of the problems with their
implementation.
Among the drawbacks to auxiliary air fume hoods is the higher first cost due
to the need to add additional HVAC equipment to supply and distribute the
auxiliary outside air to the fume hoods. Another problem with the concept is
that in most locations, the outside air must be at least partially conditioned
prior to being introduced into the area where the fume hood user will be. (It
has been found to be unacceptable to allow unconditioned outside air that is
significantly cooler or warmer than room air to descend upon a fume hood
user.) Lastly, the downward auxiliary airflow often becomes an undesirable
cross current that exceeds 50% of the average face velocity (as mentioned
previously) and adversely affects the ability of a fume hood to provide
containment and user safety. For these reasons it is not recommended to
employ auxiliary air type fume hoods when designing new or renovating
existing laboratory facilities.
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This cost is based upon a typical yearly cost of between $3 to $5 (U.S.) to supply each cfm of conditioned air. A 6 foot
wide constant air volume type of fume hood will normally require a constant exhaust and corresponding makeup air
supply of approximately 1250 cfm to maintain a nominal 100 fpm face velocity at all times.
48 Siemens Building Technologies, Inc.