
P a g e 5 2 • D R I - S T E E M D e s i g n G u i d e
D R I - S T E E M D e s i g n G u i d e • P a g e 5 3
Isothermal humidifier basics
Isothermal humidifiers use an energy source to boil water into
steam for dispersion either directly into an occupied space or
through an HVAC system. All isothermal humidifiers have a
makeup water fill valve, a drain valve for periodic and/or end-of-
season draining, and a water level control mechanism. Our systems
also have a water surface skimmer for reducing particulates at the
high-water level. Isothermal humidifiers use a variety of water
types, from purified water such as deionized (DI) or water filtered
by reverse osmosis (RO), to softened or tap water. DRI-STEEM
humidifiers can use any type of water.
Float-fill valve allows best water control
As a humidifier boils off water to supply humidification steam,
that water must be replaced. When makeup water is introduced
into the tank, it stops the boil, and steam output ceases until the
makeup water reaches boiling temperature. However, if water
replenishment modulates to match the rate that steam boils off,
there is no interruption in steam production, and no fluctuation
in steam output. Float-fill valves are currently the only humidifier
water level control mechanisms that provide fully modulated water
replenishment (see Figure 53-1).
Float-fill valves are not effective in tap or softened water systems
because precipitated minerals collect on the float-fill assembly and
interrupt float movement. Float-fill valves, therefore, are used only
with DI or RO filtered water systems.
Replenishing water levels using conductivity probes
Conductivity probe systems (used in our standard-water
models) replace boiled-off water periodically and therefore cause
interruption in steam output (see Figures 53-2 and 53-3).
In these systems, a probe with three rods of three lengths detects
water levels. When water reaches the top rod, the fill valve stops
filling; when water reaches the middle rod, the fill valve starts
filling. Once water no longer touches the bottom rod, heaters
are de-energized for low-water protection. The vertical distance
between the ends of the top and middle rods defines how much
water is replaced when the fill valve is on, which corresponds to the
time period of reduced or no steam output. Conductivity probe
systems require a minimum level of minerals in the water to operate
and therefore do not work with DI or RO water.
An advantage of this type of water level control is that it provides
accurate water level readings, allowing a predictable skim cycle
for removing precipitated minerals by water surface skimming
with each fill cycle. Skimming flushes floating precipitates to drain
Figure 53-1:
Humidifier with float-fill valve
Fill valve
Low-
water
cutoff
oat
switch
Drain
valve
Heating
element
Vapor hose
or pipe to
dispersion
assembly
Float
ball
Figure 53-2:
Humidifier with conductivity probe
Fill valve
Conductivity
probe
Heating
element
Vapor hose
or pipe
Drain
Skimmer
port
Fill valve closes
Fill valve opens
Low-water cutoff
Figure 53-3:
Detail of conductivity probe
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