Humidification Systems Summary

Ellen Carrlee, internal notes, Alaska State Museum Feb 22, 2007

ISOTHERMAL HUMIDIFICATION
Uses external heat source to boil water into vapor as steam.  Often used
in buildings that already have a boiler.  Direct steam injection comes
right from a steam source.  Steam-to-steam involves a heat exchanger to
purify steam from a boiler or other source that uses untreated water or
water with chemicals in it.  Steam can also be made in gas-fired or
electric humidifiers.  Electrode humidifiers rely on conductivity to
make steam.  Ionic bed humidifiers use cartridges immersed in water for
resistance heating to create steam.

PROS
* Little or no change in air temperature
* Allows very large air capacity in a small air stream (shorter air
handling systems)
* Available in many forms using many heat sources
* Sterile because of the high heat needed to make the steam
* Ionic bed humidifiers don’t utilize conductivity, so various water
qualities can be used.
* Steam droplets are the smallest water droplet size, and are thus
absorbed into the air quicker than the mist made in adiabatic systems.
* Electronic steam and ionic bed methods can be programmed not to
promote bacteria
* Electronic steam and ionic bed methods require fairly simple
maintenance less frequently than all other methods but direct steam
injection.

CONS
* Easy to over-saturate the air, leading to condensation in duct work
* Placement is critical…must not locate upstream of condensing
surfaces (cooling coil, duct vanes etc)
* Operating costs vary widely with fuel cost
* Cannot use the central steam that comes directly from your boiler
since it is usually full of harsh chemicals that are used to prevent
corrosion in the boiler.  Must process the water first to make “clean
steam” in a steam-to-steam process.
* Except with direct steam injection, the need to produce the steam
results in slower response time to fluctuations.  Direct steam is the
fastest response.
* Electrode humidifiers cannot use purified water because it is not
conductive enough, but hard water requires more frequent maintenance and
soft water means a shorter electrode life.
* Ionic bed humidifiers must have cartridges replaced as impurities from
water accumulate.  (can be less frequent if better water is used.)

OTHER FACTORS
* Have to have clear distance downstream to avoid condensation
* Multiple units allows staging, zoning, and reduced energy use
* Experts say a common problem is choosing an oversized unit…when in
doubt, better to use next-smaller unit, not next-larger.

ADIABATIC HUMIDIFICATION
Uses heat from the surrounding air to evaporate water into vapor.
Evaporative pan humidification from blowing air across water to pull it
into the air.  Some use a large wetted pad, known as pad-type.
Air-blast or atomizing systems use compressed air to blow water from
tiny spigots, mixing the air and the water vapor.  Ultrasonic
humidification uses high frequency sound waves to break tiny droplets
off a pan of water.

PROS
* Low maintenance compared with steam-to-steam systems
* Low operating costs when working properly
* Pad-types cannot over saturate the air

CONS
* Cools the air while it humidifies it, requiring more heating of the
air
* If enough heat is not available fast enough, the water remains liquid
and can lead to bacteria, algae, and corrosion issues
* Pad-types: Large capacity requires very wide equipment and large
supply air flow
* Atomization types easily over-saturate the air
* Long lengths of duct work for adequate mixing because the air is
cooled and cooler air won’t hold as much humidity.  Ultrasonic may need
10 feet, compressed air may need 12 feet, evaporative systems may need 4
feet.  If duct is too short, moisture hits far wall and condenses,
throwing off the RH and leading to mold and bacteria growth.
* Evaporative pan systems require frequent cleaning of the pan and risk
breeding and distributing bacteria.
* Adiabatic systems can be difficult in a retrofit because they require
a longer evaporation distance than steam humidifiers.  When mounted in
the air handler, a fog chamber needs to be put before the cooling coil,
which lengthens the footprint of the air handler.
* In ducts, airflow velocities are often wrong and allow water particles
to be carried downstream, requiring a duct expansion, mist eliminator
and drain pan.
* If tap water is used, nozzles can clog and mineral dust can be carried
with the vapor and settle out on surfaces in the building.  Use of
purified water helps, but can increase costs.
* Common mistake: not heating the air before adding moisture.  If you
add moisture to cold air, the moisture just falls out.

OTHER FACTORS
* If you place upstream of cooling coils, you reduce cooling load in
shoulder seasons
* If you place downstream of cooling coils, the warm air allows higher
capacity in the same air and reduces potential for condensation.

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