Throttling Range Adjustment - Honeywell AUTOMATIC CONTROL Engineering Manual

PNEUMATIC CONTROL FUNDAMENTALS
A temperature controller consists of a bimetal element linked
to a flapper so that a change in temperature changes the position
of the flapper. Figure 7 shows a direct-acting thermostat
(branchline pressure increases as temperature increases) in
which the branchline pressure change is proportional to the
temperature change. An adjustment screw on the spring adjusts
the temperature at which the controller operates. If the tension
is increased, the temperature must be higher for the bimetal to
develop the force necessary to oppose the spring, lift the
flapper, and reduce the branch pressure.
BIMETAL
SETPOINT
SCREW
M
Fig. 7. Temperature Controller with
Bimetal Sensing Element.
ROD AND TUBE
The rod-and-tube sensing element consists of a brass tube
and an Invar rod, as shown in Figure 8. The tube expands and
contracts in response to temperature changes more than the
rod. The construction of the sensor causes the tube to move
the rod as the tube responds to temperature changes. One end
of the rod connects to the tube and the other end connects to
the flapper spring to change the force on the flapper.
CONNECTION
TO FLAPPER
SPRING
Fig. 8. Rod-and-Tube Insertion Sensor.
On a rise in temperature, the brass tube expands and draws the
rod with it. The rod pulls on the flapper spring which pulls the
flapper closed to the nozzle. The flapper movement decreases
the air-bleed rate, which increases branchline pressure.
REMOTE BULB
The remote-bulb sensing element has as measuring element
made up of a capillary and bulb filled with a liquid or vapor
(Fig. 9). On and increase in temperature at the bulb, the liquid
or vapor expands through the capillary tubing into the
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTACT POINT FOR
THROTTING RANGE
ADJUSTMENT
FLAPPER
NOZZLE
BRANCH
C1088
TUBE
ROD
C1089
diaphragm chamber. The expansion causes the diaphragm pad
to push the pin toward the lever, which moves the flapper to
change the branchline pressure.
DIAPHRAGM PAD
PIN
CONTROLLER
Fig. 9. Remote-Bulb Temperature Sensor.
Remote-bulb temperature sensors are used in bleed-type
controllers. Capillary length of up to 2.5 meters are normally
used for inserting the bulb in duct, tank, or pipe.
AVERAGING ELEMENT
The averaging-element sensor is similar to the remote-bulb
sensor except that it has no bulb and the whole capillary is the
measuring element. The long, flexible capillary has a slightly
wider bore to accommodate the equivalent liquid fill that is
found in a remote-bulb sensor. The averaging-element sensor
averages temperatures along its entire length and is typically
used to measure temperatures across the cross section of a
duct in which two air streams may not mix completely.
Averaging element sensors are used to provide an input signal
to a controller.

THROTTLING RANGE ADJUSTMENT

A controller must always have some means to adjust the
throttling range (proportional band). In a pneumatic controller,
the throttling range is the change at the sensor required to
change the branchline pressure 10 psi. The setpoint is usually
at the center of the throttling range. For example, if the
throttling range of a temperature controller is 4F and the
setpoint is 72F, the branchline pressure is 3 psi at 70F, 8 psi at
72F, and 13 psi at 74F for a direct acting controller.
In all pneumatic systems except the sensor-controller system,
the throttling range is adjusted by changing the effective length
of a lever arm. In Figure 7, the throttling range is changed by
moving the contact point between the bimetal and the flapper.
(For information on adjusting the throttling range in a sensor-
controller system, see SENSOR-CONTROLLER SYSTEMS.)
RELAYS AND SWITCHES
Relays are used in control circuits between controllers and
controlled devices to perform a function beyond the capacity
of the controllers. Relays typically have diaphragm logic
64
LIQUID FILL
CAPILLARY
BULB
DIAPHRAGM CHAMBER
C1090-1