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BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
CRACK AREA = 0.05 m 2
SUPPLY
3
0-0.4 m /s
2
DOOR
2m
DOOR CLOSED
DIFFERENTIAL = EXHAUST– SUPPLY
3
= 0.1m /s
VELOCITY = 0.1
0.05
= 2 m/s
Fig. 49. Direct Space Pressure Control Example
with Door Closed and Open.
When a door is opened, the space pressure control responds
by reducing the supply airflow to zero and/or increasing general
exhaust flow. Replacement air for the space that is being
exhausted migrates from adjacent areas through the doorway
and cracks. The supply system for the adjacent area must replace
this air in order to maintain a positive building pressurization.
REFERENCES
The following references were useful in preparing this section
on Building Airflow System Control Applications. Selected
material was included from:
Design of Smoke Control Systems for Buildings
ISBN 0-910110-03-4
ASHRAE Handbooks—1995 Applications and 1996 HVAC
Systems and Equipment
American Society of Heating, Refrigerating and Air
Conditioning Engineers, Inc.
1791 Tullie Circle, N. E.
Atlanta, GA 30329
Trane Air Conditioning Manual
The Trane Company
Educational Department
3600 Pammel Creek Road
LaCrosse, WI 54601
EXHAUST
3
0.5 m /s
FUME HOOD
DOOR OPENED
DIFFERENTIAL = EXHAUST– SUPPLY
3
= 0.5m /s
VELOCITY = 0.5
2.05
= 0.24 m/s
C4077
The significant issues are 1) how fast can the room
pressurization system respond to upset (a door opening or
several hoods being closed at once) and 2) what is the impact
on adjacent areas and the rest of the building. Because of the
inherent lag of direct pressure control systems (the time it takes
the differential pressure sensor to know that several hoods have
been closed) the lab can go into a positive pressure mode for a
short period of time. Further, with extended door openings and
other breaches it is possible for a direct pressure based system
to call for amounts of exhaust air which may be drawn
excessively from the adjacent spaces. This has the potential for
cascading air flow and pressure effects throughout the building.
For reasons of speed and stability, volumetric tracking control
is becoming the more accepted method of pressurization control
in lab spaces.
Direct pressure control remains a viable alternative, especially
in lab spaces that are sealed tightly, where there is sufficient
building supply air and good lab operation protocols.
Engineering Fundamentals of Fans and Roof Ventilators
Plant Engineering Library
Technical Publishing*
1301 S. Grove Avenue
P. O. Box 1030
Barrington, IL 60010
Industrial Ventilation Manual
Committee on Industrial Ventilation
P. O. Box 16153
Lansing, MI 48901
Modern Development In Fluid Dynamics
Dover Publications Inc.
180 Variek Street
New York, NY 10014
* A Division of Dun-Donnelley Publishing Corp., a Company
of the Dun & Bradstreet Corporation.
290
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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