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PNEUMATIC CONTROL FUNDAMENTALS
From the PRV, the air flows through the main line to the
controller (in Figure 1, a thermostat) and to other controllers or
relays in other parts of the system. The controller positions the
actuator. The controller receives air from the main line at a constant
pressure and modulates that pressure to provide branchline air at
a pressure that varies according to changes in the controlled
variable, as measured by the sensing element. The controller signal
(branchline pressure) is transmitted via the branch line to the
controlled device (in Figure 1, a valve actuator). The actuator
drives the final control element (valve) to a position proportional
to the pressure supplied by the controller.
When the proportional controller changes the air pressure to
the actuator, the actuator moves in a direction and distance
proportional to the direction and magnitude of the change at
the sensing element.
RESTRICTOR
The restrictor is a basic component of a pneumatic control
system and is used in all controllers. A restrictor is usually a
disc with a small hole inserted into an air line to restrict the
amount of airflow. The size of the restrictor varies with the
application, but can have a hole as small as 0.08 millimeters.
NOZZLE-FLAPPER ASSEMBLY
The nozzle-flapper assembly (Fig. 3) is the basic mechanism
for controlling air pressure to the branch line. Air supplied to
the nozzle escapes between the nozzle opening and the flapper.
At a given air supply pressure, the amount of air escaping is
determined by how tightly the flapper is held against the nozzle
by a sensing element, such as a bimetal. Thus, controlling the
tension on the spring also controls the amount of air escaping.
Very little air can escape when the flapper is held tightly against
the nozzle.
SENSOR
FORCE
NOZZLE
RESTRICTOR
M
AIR SUPPLY
Fig. 3. Nozzle-Flapper Assembly with Restrictor.
FLAPPER
SPRING
BRANCH
C1084
To create a branchline pressure, a restrictor (Fig. 3) is
required. The restrictor and nozzle are sized so that the nozzle
can exhaust more air than can be supplied through the restrictor
when the flapper is off the nozzle. In that situation, the
branchline pressure is near zero. As the spring tension increases
to hold the flapper tighter against the nozzle, reducing the air
escaping, the branchline pressure increases proportionally.
When the spring tension prevents all airflow from the nozzle,
the branchline pressure becomes the same as the mainline
pressure (assuming no air is flowing in the branch line). This
type of control is called a "bleed" control because air "bleeds"
continuously from the nozzle.
With this basic mechanism, all that is necessary to create a
controller is to add a sensing element to move the flapper as
the measured variable (e.g., temperature, humidity, pressure)
changes. Sensing elements are discussed later.
PILOT BLEED SYSTEM
The pilot bleed system is a means of increasing air capacity
as well as reducing system air consumption. The restrictor and
nozzle are smaller in a pilot bleed system than in a nozzle-
flapper system because in a pilot bleed system they supply air
only to a capacity amplifier that produces the branchline
pressure (Fig. 4). The capacity amplifier is a pilot bleed
component that maintains the branchline pressure in proportion
to the pilot pressure but provides greater airflow capacity.
NOZZLE
BRANCH
CHAMBER
M
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
ENGINEERING MANUAL OF AUTOMATIC CONTROL
62
FLAPPER
PILOT
CHAMBER
VENT
BLEED
VALVE
BRANCH
SPRING
FEED
VALVE
CAPACITY
DISC
AMPLIFIER
C1085
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