Basic Documentation

Page 2 of 8 Siemens Industry, Inc.
Document No. 149-975
Differences between the two methods
In Figure 1, the sash position based control uses a
Sash Position Sensor as well as an Exhaust Airflow
Measurement Sensor as inputs to the fume hood
face velocity controller. Side wall sensing based
control only uses the Side Wall Air Velocity Sensor
as a controller input.
Sash Position Based Control
In the sash position based control process, the face
velocity controller continuously calculates the fume
hood's total open sash area based upon the sash
position sensor and other stored fume hood data
1
.
Then, knowing the fume hood's total open area, the
controller mathematically determines the required
exhaust airflow necessary to maintain the incoming
makeup air at the average face velocity setpoint.
For instance, if a fume hood's fixed open area
amounts to 0.5 square feet, and at a given point in
time the sash opening is 7.5 square feet, the
controller would combine these to yield a total fume
hood open area of 8.0 square feet. If the face
velocity setpoint was 100 fpm, the calculation for the
required airflow becomes:
8.0 ft
2
× 100 ft/min = 800 ft
3
/min
Thus, 800 cfm is the required exhaust airflow rate.
Then, by means of the airflow control element and
the exhaust airflow measurement sensor, the
controller maintains 800 cfm of exhaust airflow as
long as the sash remains in that same (7.5 square
foot open area) position. If the sash is repositioned,
the controller immediately determines the new total
fume hood open area, calculates the new required
exhaust airflow and positions the airflow control
element to attain the new required exhaust airflow.
Side Wall Sensing Based Control
The side wall sensing based control process is not
based upon any mathematical calculations. Rather,
the fume hood face velocity controller senses the
velocity of the makeup air entering the fume hood by
means of the side wall air velocity sensor. Then, the
1. Note that the open sash area does not usually represent the
total open area of a fume hood. Fume hoods typically also
have a slot opening under their airfoil shelf. In such
instances the fume hood face velocity controller must add
this (and any other fixed fume hood open areas) to the sash
open area in order to determine the fume hood's total open
area.
controller compares this sensed air velocity with the
desired face velocity setpoint. If the side wall air
velocity is below the setpoint (which happens
whenever the sash opening is increased), the
controller modulates the exhaust airflow control
element to increase fume hood exhaust airflow. This
results in a corresponding increase in the incoming
makeup air velocity through the sash opening. The
controller will continue to increase the exhaust
airflow until the air velocity sensed by the side wall
sensor matches the face velocity setpoint.
If the air velocity at the side wall sensor increases
above the face velocity setpoint (which happens
whenever the sash opening is decreased), the
controller modulates the exhaust airflow control
element to decrease fume hood exhaust. This
results in a corresponding decrease in the incoming
makeup air velocity through the sash opening. The
controller will continue to decrease the exhaust
airflow until the air velocity sensed by the side wall
sensor matches the face velocity setpoint.
Which method provides better control of fume hood
average face velocity?
The answer to that question is best addressed by
examining the primary attributes by which any
control process is evaluated. These typically include:
 Control Accuracy
 Response Time
 Control Stability
This report discusses each of these attributes in
detail with respect to both sash position sensing
based control and side wall sensing based control.
However, to better appreciate the factors involved in
the appraisal, let's begin by considering a common
control analogy that we're all very familiar with and
one that we experience most every day—control of
room temperature. In its simplest form, room
temperature can be controlled by linking a
thermostat or temperature sensor on a wall with a
means to heat or cool the incoming room air. But,
whether or not the room temperature control process
will really provide satisfactory room conditions is
really dependent upon the same attributes listed
above: Control Accuracy, Response Time, and
Control Stability.
Control Accuracy
If the room thermostat or temperature sensor is not
accurate, this will adversely affect the control
process and the room temperature won't be
maintained at the setpoint. Thus, the ability to