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Part Two

Optimizing Controller Performance

The

Model FHC50

controller uses both integral and PI control methods. Integral control (slower

control signal) is used when the controller is near set point. Integral control provides stability
when natural system fluctuations occur such as operators working at hoods and lab doors
opening. PI control (fast control) is used when responding to large disturbances to face velocity
such as sash movements. PI control rapidly returns the face velocity to set point, thus assuring
containment. Once the

controller is in PI control, it continues to control in this mode until the

operating set point is met.

There are four menu items that change the characteristics of the control output signal;

1) SENSITIVITY

2) SPEED
3) Kc VALUE

4) Ti VALUE

TSI recommends only adjusting the

SENSITIVITY

and

SPEED

to fine tune the control signal.

Only when the

SPEED

and

SENSITIVITY

items cannot provide a stable system should

VALUE

and

Ti VALUE

be adjusted. The role of each menu item is covered in the

Menu and

Menu Items

section of the manual. This section provides some guidance of when a menu item

should be changed.

The controller is shipped with PI values that are appropriate for 95+ % of the fume hoods
installed. In fume hoods where some adjustment is needed, minor changes to the

SENSITIVITY

and

SPEED

menu items will yield excellent control. The

SENSITIVITY

item selects when the unit

goes into PI control. Each percent of the setting from 100% indicates that the controller must be
1 ft/min away from control set point prior to activating PI control. If the

SENSITIVITY

setting is

60% (40% missing), the face velocity must be 40 ft/min off set point before PI control is activated.
Conversely, if the

SENSITIVITY

setting is 80% (20% missing), the face velocity must only be

20 ft/min off set point before PI control is activated. The default of 80% is usually a good
compromise between PID and integral control.

The

SPEED

menu item slows down the control output. The controller is shipped with a control

signal capable of rotating the damper 90 degrees in 1.5 seconds. This may be too fast if the
damper is in an unstable flow area (very near the exhaust fan), or there are competing air flows in
the laboratory. Controllers modulating a VFD system will probably need to be slowed down, since
the control signal is substantially faster than the VFD/fan can respond.

The remaining menu items,

Kc VALUE

and

Ti VALUE

should not be adjusted unless severe

stability problems exist. Adjusting these variables may improve the response and stability, but the
exact opposite may happen causing the controller to become unstable, hunt substantially, or
have very slow response. If controller performance cannot be improved by adjusting the

SPEED

and

SENSITIVITY

, the two menu items can be manually set to their default values.

Maintenance and Repair Parts

The Model FHC50 Fume Hood Controller requires minimal maintenance. Periodic inspections of
system components as well as an occasional velocity sensor cleaning are all that are needed to
ensure that the Model FHC50 is operating properly. The Model FHC50 should be calibrated
annually. Refer to the

Calibration

section for further information.