10
5.2.7.3
With the step control board full on, individually enable each step. The respective pilot light should light
and the contactor(s) applying power to the respective elements.
5.2.7.4
For units with ON/OFF/PREHEAT switches, only part (approximately 25% may be activated when the
switch is down in the ‘PREHEAT’ position. This PREHEAT option is now rarely used.
5.2.7.5
If PREHEAT switch is used. When the outlet water temperature (or steam pressure) is at or near
setpoint,
turn the ON/OFF/PREHEAT switch ‘OFF’, then to ‘ON’ again. The controls will recycle to the no-load
condition. The steps should then begin to cycle on as needed, and all of the steps may come on if necessary
.
6.0 SEQUENCE OF OPERATION
6.1 Activation of Heating Element Circuits
6.1.1 Contactors
The heating elements are energized by pilot operation. That is, the power to an element circuit is supplied
through the contacts of a contactor. The contactor is activated (on a call for heat) by the closure of a
temperature (or pressure) switch or by a contact in the step control circuit
.
6.1.2 Fuses
All power leads to the elements are fused. The fusing is on the line side of the contactors
.
6.2 Sequencing Controls
6.2.1 Staged Control Circuit
6.2.1.1
In this type circuit, the contractor coils are energized directly by the contracts of immersion
thermostats (or pressuretrols). There is usually on thermostat (or pressuretrols) for each stage; or a multi-stage
thermostat (or pressuretrols) may be provided in some cases
.
6.2.1.2
The safety limit devices (high temperature, low water, etc.) interrupt the power to the contactor coil
circuits.
6.2.2 Proportional Step Controls
6.2.2.1 Step Control Sequence
The controller senses the unit’s water temperature (either via 135-ohm device or thermistor) or steam pressure
(either via 135-ohm device or 4-20 ma transducer). The output signal or the controller causes the step control
to sequence the steps on, or off, depending on whether the unit’s output is below or above setpoint.
When one of the switches closed on the step control, a contractor coil (or coils) is energized in a step-wise
fashion. This type of step control will mid-position (bring on half of the steps) when the unit’s water
temperature (or steam pressure) is at setpoint.
For example:
an 8 step steam boiler with the pressure controller set at 12 PSIG, will have 4 steps on when the
boiler steam pressure is at 12PSIG
.
6.2.2.2 Progressive Sequence Step Controls
These controls are provided as standard and include Selectronix SLC series and Viconics’ model R851B. They
provide first-on, first-off staging of the element circuits
.
As the unit’s temperature (or pressure) drops below setpoint, the control brings on more steps. As the
temperature (or pressure) increases and approaches setpoint, the step control drops out stages. The first step
to drop off is the one that has been on the longest. This provides even usage of the system’s components.
6.2.2.3 Linear Sequence Step Controls
This type of step control is limited to the some Selectronix models and Viconics’ model R851B. In this sequence
progression, the control applies power by progressing from Step 1 to the maximum number required to satisfy
the load, and then decreases power by retracing this sequence down toward the first step.
NOTE: Refer to the applicable vendor literature provided on the step control installed in your unit.
