Basic Documentation

Chapter 6–Laboratory Containment Units - Ventilation
When applying a HOPEC type of fume hood, the ventilation system’s
exhaust provision is configured for about 50% less exhaust airflow from a
HOPEC fume hood than from a constant volume bypass fume hood of the
same size. Of course, this results in about a 50% reduction in the room
makeup air required which yields a saving in the energy required to condition
and move the air. Although this savings is attained without any additional
ventilation system fume hood control equipment, it is limited to 50% of the
consumption of a similar size of conventional constant volume fume hood.
Although a HOPEC type of fume hood is a very easy way to save ventilation
system energy and even perhaps achieve a downsizing of the ventilation
system, it may not be suitable in every application. It has a more complex
sash arrangement and various other design aspects make it somewhat more
expensive than a basic fume hood. Secondly, it is not available in all sizes
and types. If the fume hood application very often requires a larger face area
access opening while full containment is in effect, the HOPEC fume hood
might not be suitable since its face velocity drops to around 60 fpm when the
sash is fully open. You must also be made very aware of its usage limitation
which may make its application in chemical teaching labs somewhat
impractical.
• Variable air volume fume hoods–The VAV fume hood is being used more
and more, especially in large research facilities, because it offers significant
advantages over constant air volume type fume hoods. An advantage of a
VAV fume hood is that the VAV fume hood incorporates an active control
arrangement to ensure that the face velocity of the air entering the open sash
area is always maintained at set value (typically 100 fpm. Figure 8 illustrates
how a VAV fume hood uses an active control element in its exhaust to vary
the exhaust airflow and thus maintain a constant average face velocity
regardless of sash position.
As a result, the containment of a VAV fume hood is potentially superior to a
constant air volume type of fume hood where the face velocity can
appreciably vary. Since the exhaust and the subsequent makeup air needs
of a VAV fume hood are reduced whenever the sash is at least partially
closed, its operational cost becomes significantly less than for a comparable
size of constant air volume fume hood. Laboratories that have incorporated
VAV control arrangements on existing ventilation systems and constant air
volume fume hoods have normally been able to achieve a 50% to 60%
reduction in annual air usage and thus fume hood operational cost.
Ventilation system designers should therefore consider VAV ventilation
systems for laboratory facilities particularly when there will be a significant
number of chemical fume hoods present.
• Variable air volume fume hoods - face velocity control–As indicated in
Figure 8, a VAV fume hood incorporates a control element to vary the
amount of airflow through the fume hood to maintain a constant face velocity
regardless of sash position. Figure 9 shows a typical VAV fume hood control
arrangement that automatically adjusts the exhaust airflow from the fume
hood to provide the desired face velocity of the incoming air.
50 Siemens Building Technologies, Inc.