Honeywell AUTOMATIC CONTROL, Engineering Manual

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PNEUMATIC CONTROL FUNDAMENTALS
ENGINEERING MANUAL OF AUTOMATIC CONTROL
70
Fig. 18. One-Pipe Controller System.
C2342
MAIN BRANCH
VALVE
CONTROLLER
M
C2343
MAIN BRANCH
VALVE
CONTROLLER
M B
M
THROTTLING RANGE
0
3
8
13
SETPOINT
NOTE: SETPOINT IS AT MIDDLE OF
THROTTING RANGE
BRANCHLINE PRESSURE (PSI)
C1091
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
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 1 degree F
could result in a branchline pressure change of 5 psi.
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 1 degree F
could result in a branchline pressure change of only 1 psi.
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
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.
Fig. 19. Two-Pipe Controller System.
CONTROLLERS