Honeywell VP525C, Engineering Manual, Page: 136 / 228

Basic Pneumatic Control System
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Fig. 4. Pilot Bleed System with Amplifier Relay.
The pilot pressure from the nozzle enters the pilot chamber of
the capacity amplifier. In the state shown in Figure 4, no air
enters or leaves the branch chamber. If the pilot pressure from
the nozzle is greater than the spring force, the pilot chamber
diaphragm is forced down, which opens the feed valve and
allows main air into the branch chamber. When the pilot
pressure decreases, the pilot chamber diaphragm rises, closing
the feed valve. If the pilot chamber diaphragm rises enough, it
lifts the bleed valve off the feed valve disc, allowing air to
escape from the branch chamber through the vent, thus
decreasing the branchline pressure. Main air is used only when
branchline pressure must be increased and to supply the very
small amount exhausted through the nozzle.
Signal Amplifier
In addition to the capacity amplifier, pneumatic systems also
use a signal amplifier. Generally, modern amplifiers use
diaphragms for control logic instead of levers, bellows, and
linkages.
A signal amplifier increases the level of the input signal and
provides increased flow. This amplifier is used primarily in
sensor-controller systems where a small signal change from a
sensor must be amplified to provide a proportional branchline
pressure. The signal amplifier must be very sensitive and
accurate, because the input signal from the sensor may change
as little as 0.06 psi per degree Fahrenheit.
Another use for a signal amplifier is to multiply a signal by two to
four times so a signal from one controller can operate several
actuators in sequence.
Feed and Bleed System
The “feed and bleed” (sometimes called “non bleed”) system of
controlling branchline pressure is more complicated than the
nozzle-flapper assembly but theoretically uses less air. The
nozzle-flapper system exhausts some air through the nozzle
continually, whereas the feed and bleed system exhausts air
only when the branchline pressure is being reduced. Since
modern nozzle-flapper devices consume little air, feed and
bleed systems are no longer popular.
The feed and bleed system consists of a feed valve that
supplies main air to the branchline and a bleed valve that
exhausts air from the branchline (Fig. 5). Each valve consists of
a ball nested on top of a tube. Some pneumatic controllers use
pressure balance diaphragm devices in lieu of springs and
valves. A spring in the tube continually tries to force the ball up.
The lever holds the ball down to form a tight seal at the end of
the tube. The feed and bleed valves cannot be open at the
same time.
Fig. 5. Feed and Bleed System.
A force applied by the sensing element at the sensor input point
is opposed by the setpoint adjustment spring and lever. When
the sensing element pushes down on the lever, the lever pivots
on the bleed ball and allows the feed ball to rise, which allows
main air into the chamber. If the sensing element reduces its
force, the other end of the lever rises and pivots on the feed
ball, and the bleed ball rises to exhaust air from the system. The
sensor can be any sensing element having enough force to
operate the system.
Sensing Elements
Bimetal
A bimetal sensing element is often used in a temperature
controller to move the flapper. A bimetal consists of two strips of
different metals welded together as shown in Figure 6A. As the
bimetal is heated, the metal with the higher coefficient of
expansion expands more than the other metal, and the bimetal
warps toward the lower-coefficient metal (Fig. 6B). As the
temperature falls, the bimetal warps in the other direction
(Fig. 6C).
M
FLAPPER
NOZZLE
BLEED
VALVE
SPRING
VENT
C1085
BRANCH
BRANCH
CHAMBER
FEED
VALVE
DISC
CAPACITY
AMPLIFIER
PILOT
CHAMBER
M
SENSING
FORCE
PRESSURE
CHAMBER
BRANCHLINE
PRESSURE
EXH
C2382-1
SETPOINT
ADJUSTMENT
BLEED VALVE
FEED VALVE