OPTIONAL EQUIPMENT FOR CHROMALOX STEAM BOILERS
AUXILIARY LOW WATER
Operation of this control is accomplished by sensing a minute AC current flowing between submerged contact probe in the boiler shell.
When this minute AC current is conducted through an external cir- cuit resistance up to 100,000 ohms or less, a signal of sufficient mag- nitude is present to trigger the SCR and, in turn, energize the control relay.
As the water level in the boiler drops below the level of the probe. the AC circuit is broken and the control relay is
Specifications |
|
Input supply | 120 |
Delectable Range | 100.000 ohms |
Probe Voltage | 24 vac |
Probe Current | 10 milliamps |
Control Relay | Single pole double throw |
CAUTION: Control will not work with
PROPORTIONING PRESSURE CONTROL (See Figure 17) Typical Operation
Pressure variations cause the bellows to expand or contract. Linkage between the bellows and the potentiometer wiper causes the wiper to move across the windings on the potentiometer. This varies the resistance between R and B, and between R and W, causing an imbalance in the circuit connected to the controller.
A proportioning pressure control is used to regulate a motor driven or solid state sequencer. The controller potentiometer, the feedback potentiometer in the motor and a balancing relay in the motor form an electric bridge circuit. As long as the pressure of the controlled medi- um remains at the set point of the controller, the circuit is balanced; i.e., equal currents flow through both sides of the balancing relay and the relay contacts are open. When the circuit is balanced, the motor does not run.
If the pressure of the controlled medium rises, the wiper in the con- troller moves toward W. This unbalances the circuit, so a larger current flows through one side of the balancing relay. The “close” contacts in the relay make, causing the motor to drive toward its closed position. As the motor runs, the wiper on the feedback potentiometer moves in a direction balance, the balancing relay contacts open and the motor stops.
Similarly, if the pressure of the controlled medium falls, the wiper on the controller potentiometer moves toward B, and the “open” con- tacts in the balancing relay make. The motor drives toward its open position until circuit balance is achieved.
The slightest change in the pressure of the controlled medium will cause a change in the number of elements energized to compensate for it, thus keeping the pressure constant. This process is called modula- tion.
PROPORTIONAL PRESSURE CONTROL ONLY SUPPLIED WITH SEQUENCER
Main Setting - Turn the adjustment screw until the indicator is opposite the low point of the desired throttling range. That is, if the pressure is to be held to a minimum of 50 psi, set the indicator at 50 psi. The pressure will then be maintained between 50 psi and a higher pressure equal to the 50 psi plus the throttling range.
THROTTLING RANGE SETTING L918
After setting the indicator for the minimum pressure, turn the throt- tling range adjustment screw until the throttling range indicator points to the desired throttling range on the scale. This scale is graduated from
“min” to “F”. The value of each division varies with the scale range of the instrument.
Pressure | Value Each |
Scale Rating | Division on Scale |
2.2 psi | |
3.6 psi |
Pressure scale rating will vary depending on pressure control supplied.
CHECKOUT
After the controller has been installed, wired, and set, it should be tested with the system in operation. First allow the system to stabilize. Then observe the operation of the controller while raising and lower- ing its set point. Pressure should increase when the set point is raised and decrease when the set point is lowered. Use accurate pressure test- ing equipment when checking out the controller. Do not rely on inex- pensive gauges. The controllers are carefully calibrated at the factory.
If the motor or actuator runs in the proper direction when the set point is adjusted, it can be assumed that the controller is operating properly. If it runs in the wrong direction, reverse the B and W wires. Observe the action of the motor to see if it stabilizes. If the motor is moving constantly, widen the proportioning range a little at a time, until the system is stable.
Adjusting Screw | Differential |
Adjusting Screw |
R
W
B
Figure 17
(L91B) Proportioning Pressure Control Used with Sequencer Control
IFA CONTROLLER SEEMS
TO OPERATE IMPROPERLY (See Figure 18)
If the controller is suspected of operating improperly, it may be further checked as follows.
1.Leave the controller installed where it is, but disconnect all power to the controlled motor.
2.Loosen the cover screw below the main scaleplate and remove the cover.
3.Disconnect the wires from the controller.
4.Connect an ohmmeter between controller terminals 8 and W to measure the resistance of the potentiometer in the controller. The ohmmeter should read about 135 ohms on an L91B.
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