Two-stage Boiler (v1.4)

Lib.Mervis.v1_4.TwoStageBoiler

Block used for controlling the performance of a boiler with a two-stage burner and a circulation pump.

• Lib.Mervis

program test
    var
        twoStageBoiler: lib.mervis.v1_4.TwoStageBoiler;
    end_var

    twoStageBoiler();
end_program

The block is used for controlling a two-stage boiler. Both stages are controlled separately in the following way:

• 1st stage
  

Its control uses a thermostat, whose input is either the common or own output and the setpoint from the cascade controller. The hysteresis of this thermostat is dynamically set to meet the minimum boiler runtime condition and at the same time maintain the regulated temperature within the range from the setpoint to the setpoint plus hysteresis (the actual value is at least as high as the setpoint). The following two cases may occur:

• Runtime is shorter - the hysteresis is increased by 1/3 of its size - up to a maximum size of fifteen degrees Celsius.
• Runtime is longer than twice the minimum - the hysteresis is decreased by 1/3 of its value - down to a minimum value of five degrees Celsius.
• 2nd stage
  

Its control uses the integral criterion, whose inputs are the same as for the first stage. The second stage is switched by the AttachCmd and DettachCmd requests - it is activated upon attachment, and deactivated upon detachment.

The circulation pump is started together with the command for the first or second stage, only when the boiler is the first in line is the pump continuously operated (if in automatic mode).

The block also evaluates the following fault conditions:

• Burner non-operation - based on the difference between the command and the feedback signal or directly using the fault input. If this fault occurs, the error counter is incremented, and if it exceeds the FailureCountLimit, the boiler is permanently marked as faulty and removed from cascade control. The counter is reset by a successful start.
• Circulation pump non-operation - evaluated in the same way as the burner non-operation, its fault also results in removal from cascade control.