4.6.1 Operating principle of a gas-fired humidifier
In a gas-fired humidifier, the production of steam is obtained inside a boiler containing water that is
heated to and then held at boiling temperature. The heat required to boil the water is provided by a
heat exchanger, heated by a modulating premixed gas burner. Burner operation is completely automatic
and there is no pilot flame. All stages of burner operation are managed by an electronic board that,
through ionisation control, constantly checks the flame. The continuously-delivered heat output reflects
heat demand, across a wide modulation ratio (1: 4). The variable-speed fan (driven by the control board),
combined with a proportional gas valve, ensures modulation of heat output (the gas flow- rate is
proportional to the air needed for combustion). The water that evaporates over time is automatically
replenished from the mains supply. When fully operational, the required level of production is achieved
automatically by adjusting the heat output of the burner. The salts introduced by automatically refilling
the water partly deposit in the form of scale inside the boiler, causing a progressive decline in
performance, and partly remain dissolved in the water. To avoid excessive accumulation of salts, a certain
amount of water is periodically and automatically drained, and then replaced with fresh water.
Control principles
The humidifier can be configured to ensure the desired humidity or temperature value. It can be
controlled in the following ways:
4.6.2 ON/OFF control
Operation is all or nothing, and is activated by an external contact; the maximum percentage of unit
production can be set.
4.6.3 Proportional to an external signal (modulating control)
Steam production is proportional to the value of an external signal Y (selectable by programming one of
the following standards: 0-1Vdc; 0-10Vdc; 2-10Vdc; 0-20mA; 4-20mA). The maximum production Pmax
corresponds to the maximum value of the external signal Y and will be the humidifier’s rated output. The
activation hysteresis hy is settable by the user and refers to the external signal Y.
Fig.4.f
Key:
Steam_pr Steam production Y External signal
Pmax Max production hy Activation hysteresis
Pmin Min production
Note: the above graph applies if the preheating function is disabled.
4.6.4 Autonomous control with humidity probes
The production of steam is related to the % rH reading made by the relative humidity probe, and
increases as the value read deviates from the set point St. The maximum production Pmax is delivered
when the relative humidity read by the probe is equal to the value of BP (proportional band) from the set
point. The activation hysteresis hy is settable by the user.
34|4. Electrical connections
gaSteam - UG +0300090EN rel. 1.2– 09.10.2018
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