Honeywell AUTOMATIC CONTROL Engineering Manual page 301

Figure 49 shows a similar example of negative space
pressurization utilizing direct pressure control. If the airflow
through the hood is 1000 cfm and the pressure control reduces
the supply airflow when the door is opened, the average velocity
through openings drops from 400 fpm to 48.8 fpm.
LAB AIRFLOW AIRFLOW
SENSOR
CONTROLLER PANEL
SUPPLY AIR
DAMPER
ACTUATOR
S U P
P L Y
A I R
G E
E X
D I F
F E R
P R E
E N T
S S U I A L
S E N
S U P
R E
S O
P L Y
A I R R
T O
C O S U
R R
A I R
I D O
R
L A B
Fig. 48. Direct Pressure Control.
CRACK AREA = 0.5 FT 2
SUPPLY
0-800 CFM
2
DOOR 20 FT
DOOR CLOSED
DIFFERENTIAL = EXHAUST– SUPPLY
= 200 CFM
÷
VELOCITY = 200 0.5
= 400 FPM
Fig. 49. Direct Space Pressure Control Example
with Door Closed and Open.
EXHAUST AIR
DAMPER ACTUATOR
E X H
A U S
A I R
T
F U M
E
H O
O D
D A M
L P E R
R A
A C T
N E
U A T
T
U S
O R
H A
A I R
L Y
P P
T O
VELOCITY
SENSOR
M12216
EXHAUST
1000 CFM
FUME HOOD
DOOR OPENED
DIFFERENTIAL = EXHAUST– SUPPLY
= 1000 CFM
VELOCITY =1000 20.5 ÷
= 48.8 FPM
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
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.
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.
C2639
291
ENGINEERING MANUAL OF AUTOMATION CONTROL