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P a g e 3 6 • 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 3 7

How water type affects humidifier performance

Isothermal systems — systems that boil water to make steam

(vapor) — typically maintain relative humidity (RH) levels within

1%-5% of an established set point, with the ability to maintain a

specific level of control directly dependent on the system's ability

to respond to changing environmental conditions. Responsiveness

is affected by two things: delivery of the energy source and the

amount of water discarded (through skim, drain, and flush cycles)

to remove minerals.

In combination with a programmable controller, using high quality

valves or substituting electronic heater controllers such as SSRs for

mechanical contactors allow responsive steam production.

Water hardness, however, plays a critical part in an isothermal

humidifier’s ability to maintain RH set point. As water hardness

increases, so does the need for skimming, draining, and flushing.

Skimming removes precipitated minerals before they attach to

humidifier tank walls and elements as scale. As water is skimmed

off, cold water is introduced into the tank. In some cases, this

introduction of cold water causes a delay in steam output until the

cold water is heated to boiling. Drain and flush cycles, automated

on most systems, completely drain the humidifier and then

typically flush the tank with cold water. In this situation, not only

is the humidifier off-line for a period of time, but the tank needs

to be filled and heated to boiling before it can produce steam.

In the meantime, the RH level can drop 5% or more until the

humidifier is producing steam again. In certain applications, such

as office buildings or other environments humidified to improve

comfort, RH fluctuation is not a major issue. In process-critical

environments, however, a 5% RH fluctuation can affect processes.

Humidifiers in these environments typically use softened or DI/RO

water, depending on the level of control required. The fewer the

minerals in the water, the better the control capability.

Low mineral content means low maintenance

From a maintenance point of view, the lower the mineral content

in the water, the less maintenance required. Mineral buildup in

improperly-maintained isothermal systems can cause humidifiers

to malfunction: heater coils can fail prematurely, heat exchanger

output is reduced by scale buildup, conductivity probe systems

that measure water levels quit working, and drain valves become

plugged. DI/RO water has the lowest mineral content, but its use

is cost-prohibitive unless needed for high purity humidification

or to meet very strict performance requirements (such as in

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