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MODULATION OPERATION DESCRIPTION
DETAIL OF THROTTLE VALVE FOR GAS FLOW
REGULATION BY MEANS OF SERVOMOTOR
When the burner is ignited at the minimum flow-rate, if the
modulation probe allows it (adjusted to a temperature or pressure
which is greater than that present in the boiler) the air/gas
adjustment servomotors activate,
•
in a clockwise direction the air flow increases;
•
anticlockwise rotation: the air flow decreases.
Thus gradually increasing gas supply and combustion air until the
maximum supply level set for the burner is reached. The burner
remains in the maximum flow position until the temperature or
pressure is high enough to trip the modulation regulator, which
reverses the rotation of the air adjustment servomotor. The
inverse rotation and therefore the air and gas supply reduction
occurs shortly thereafter. With this procedure, the modulation
system attempts to balance the amount of heat supplied to the
boiler with respect to the amount the boiler releases for use. The
modulation probe installed on the boiler measures any variation
demand and automatically adjusts fuel and the air/gas flow rate
with an increasing or reducing rotation. If the (temperature or
pressure) limit value at which the stop device is set (thermostat or
pressure switch) is reached even with gas flow rate at minimum
level, the burner is shut down by the device. As the temperature or
pressure drops below the shut-down device's set point, the burner
will be turned on again as described above.
A
B
A Graduated scale.
B Gas throttle valve position reference index.
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