Multiple Fan Systems; Types; Fume Hoods - Honeywell AUTOMATIC CONTROL Engineering Manual

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
Essentially, the increase of outdoor air above that required to
maintain building pressurization is done the same way as mixed
air control except outdoor air is controlled by flow rather than
mixed air temperature (Fig. 42). In colder climates, overrides
must be included to avoid freezing coils.

MULTIPLE FAN SYSTEMS

Multiple fan systems are a form of zone airflow control
systems. The same concepts for zone pressurization using
airflow tracking or direct space static pressure control apply to
multiple fan systems. A return fan is modulated instead of the
zone return damper to control zone pressurization.
EXHAUST SYSTEM CONTROL

TYPES

Local exhausts are individual exhaust fan systems used in
toilets, kitchens, and other spaces for spot removal of air
contaminants. These fans are generally off/on types. They
should be controlled or at least monitored from a central location
as the exhaust airflow can significantly affect energy efficiency.
STANDARD
SASH OPEN
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ENGINEERING MANUAL OF AUTOMATION CONTROL
General exhausts route contaminants into common ducts
which connect to a common exhaust fan. If the airflow is
manually balanced, the exhaust fan runs at a fixed level.
However, if the airflow is controlled at each entry to vary the
airflow in response to the local need, duct pressurization control
of the exhaust fan is required. It may also be necessary to
introduce outdoor air prior to the general exhaust fan in order
to maintain a minimum discharge velocity.

FUME HOODS

Fume hoods are the primary containment devices in most
chemical-based research venues. The lab envelope itself
becomes the secondary containment barrier. In all cases, the
basic use of the fume hood is for the safety of the worker/
researcher. Because no air is recirculated to the lab, the fume
hood is also the primary user of energy in most labs. The
continuing control challenge is to provide the safest possible
environment while minimizing operating costs.
There are three types of general purpose fume hoods (Fig. 43):
bypass, auxiliary, and standard. Bypass and auxiliary air hoods
approximate a constant exhaust airflow rate as the fume hood
sash opens and closes. Operation of the standard hood causes the
face velocity to increase or decrease with the up and down
movement of the sash as a fixed volume of air is exhausted
(constant volume).
BYPASS
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SASH CLOSED
Fig. 43. General Purpose Fume Hoods.
288
AUXILIARY AIR
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