Basic Pneumatic Control System
70-6925
129
H
u
m
idi
st
at
s
an
d
Th
er
m
o
st
at
s
Controller
s
Sensors
Relay
s
Switch
es
Actuators
Va
lv
e
s
Ac
ces
sor
ie
s
En
gi
ne
eri
ng
Gu
ide
Cr
o
s
s
Refere
nc
e
Fig. 6. Bimetal Sensing Element.
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.
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.
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
diaphragm chamber. The expansion causes the diaphragm pad
to push the pin toward the lever, which moves the flapper to
change the branchline pressure.
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.)
HIGH COEFFICIENT OF EXPANSION
LOW COEFFICIENT OF EXPANSION
METALS:
C1087
C. DECREASED TEMPERATURE
B. INCREASED TEMPERATURE
A. CALIBRATION TEMPERATURE
M
FLAPPER
BRANCH
NOZZLE
SETPOINT
SCREW
C1088
CONTACT POINT FOR
THROTTING RANGE
ADJUSTMENT
BIMETAL
TUBE
C1089
ROD
CONNECTION
TO FLAPPER
SPRING
DIAPHRAGM
PAD
PIN
DIAPHRAGM CHAMBER
CONTROLLER
BULB
LIQUID FILL
CAPILLARY
C1090-1
Summary of Contents for VP525C
Page 58: ...50 customer honeywell com 70 6925 ...
Page 120: ...112 customer honeywell com 70 6925 ...