Controllers; General - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

PNEUMATIC CONTROL FUNDAMENTALS
maintains a fixed branchline pressure for each temperature
within the throttling range (Fig. 17). The forces within the
nozzle-flapper-bimetal assembly always seek a balanced
condition against the nozzle pressure. If the setpoint is changed,
the forces in the lever system are unbalanced and the room
ambient temperature must change in a direction to cause the
bimetal to rebalance the lever system.
90
55
20
0
THROTTLING RANGE
NOTE: SETPOINT IS AT MIDDLE OF
THROTTING RANGE
Fig. 17. Relationship between Setpoint,
Branchline Pressure, and Throttling Range.
For example, if the setpoint of a direct acting thermostat is
increased, the bimetal reduces the force applied to the flapper

CONTROLLERS

GENERAL

A controller is the same as a thermostat except that it may
have a remote sensing element. A controller typically measures
and controls temperature, humidity, airflow, or pressure.
Controllers can be reverse or direct acting, proportional or two-
position, single or two pressure, and bleed, feed and bleed, or
pilot bleed.
A two-position controller changes branchline pressure rapidly
from minimum to maximum (or from maximum to minimum)
in response to changes in the measured condition, thus providing
ON/OFF operation of the controlled device.
A proportional controller changes branchline pressure
incrementally in response to a change in the measured condition,
thus providing modulating operation of the controlled device.
A proportional-integral (PI) controller adds to the
proportional controller a component that takes offset into
account. The integral component eliminates the control point
offset from the setpoint.
Bleed-type controllers can be used in one-pipe or two-pipe
configurations. In a one-pipe system (Fig. 18), the main air
goes through a restrictor to the controller and actuator in the
most expeditious routing. In a two-pipe system (Fig. 19), the
SETPOINT
C4218
and raises the flapper off the nozzle. This movement causes
the branchline pressure to bleed down and a heating valve to
open. Heat enters the space until the temperature at the
thermostat increases and the force of the bimetal is again in
equilibrium with the opposing force of the pressure at the nozzle.
Decreasing the setpoint causes the reverse to occur.
The throttling range adjustment provides the means for
changing the effective length of the cantilever bimetal in the
lever system. When the throttling range adjustment is positioned
directly over the nozzle, the force of the bimetal increases and
a narrow throttling range or very high sensitivity results. For
example, a change in temperature of 0.5 kelvins could result in
a branchline pressure change of 35 kPa.
When the throttling range adjustment is moved toward the
end of the bimetal and away from the nozzle, the force of the
bimetal is reduced. This reduction requires a greater temperature
change at the bimetal to throttle the flapper over the nozzle.
The result is a wider throttling range or very low sensitivity.
For example, a temperature change of 0.5 kelvins could result
in a branchline pressure change of only 7 kPa.
main air goes into the controller, through an internal restrictor
in the controller, and out of the controller through a branch line
to the actuator. All pilot-bleed and feed-and-bleed controllers
are two pipe.
MAIN
M
Fig. 18. One-Pipe Controller System.
MAIN
M
Fig. 19. Two-Pipe Controller System.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
70
CONTROLLER
BRANCH
VALVE
C2342
CONTROLLER
M B
BRANCH
VALVE C2343