Humidification - Affiliated Engineers, Inc.

The increasing globalization of scientific research, advanced health care, and other specialty facilities
is expanding the range of climates in which facilities with specialized HVAC needs are sited. At the
same time, rising energy prices and the volatility of global energy markets have driven some
rethinking of traditional means of humidification and dehumidification of buildings. This article
examines a number of tools that may be used alone — or, in select cases, more effectively
together — to reduce energy consumption, be it in Jeddah or Seattle, in Hyderabad or Hyde Park.
Humidification is typically required in dry climates yearround
and during winter in cold climates. Facilities
where humidification is either required or beneficial
include hospitals, laboratories, certain types of process
facilities, and museums. Humidification reduces
microbial growth (the growth of microorganisms is minimized in the
50% rh range), and prevents static electricity buildup that occurs at
low relative humidity levels, which can harm electronics.
Dehumidification is typically applied in warm humid climates,
in summertime in most middle latitude areas, in certain functionspecific
facilities such as cold rooms, specialty research spaces that
require dry air, and spaces that have high latent gains (e.g., indoor
swimming pools). Dehumidification is applied for some of the
same reasons as humidification, including reduction of microbial
growth. Special process needs drive other uses, and of course some
dehumidification is driven by the need to provide outside air to
buildings during humid weather.
THE TOOLS
Although the variety of tools available to humidify and/or dehumidify
air in building HVAC systems is broad, the use of an isothermal
humidifier for humidification (where required) and reliance on
the cooling coil for dehumidification represent the default design
approach in North America, and other technologies are often not
considered. Given the now more widely varying demands of locale,
and the imperative of optimizing functional efficiency, understanding
the unique capabilities and capacities of each is essential to truly
effective application, since one size most assuredly does not fit all.
BY BRUCE A. MCLAY, P.E., LEED ® AP
ISOTHERMAL HUMIDIFIERS
These have been the most commonly used type of humidifier in
HVAC systems. Isothermal humidifiers inject steam into the airstream,
increasing the absolute humidity level in the air (i.e., moisture
content), without significantly changing the air temperature.
The source of the steam varies — it may be generated by an electric
source, a gas-fired source, a direct plant steam source, or an indirect
steam-to-steam exchange. The water source for the steam also
varies. City water may be used, but softened water or pure water
may be required based on water quality or process needs. Steam is
typically injected directly into the airstream via a jacketed manifold
type humidifier or a dispersion tube array.
Advantages of isothermal humidifiers include the ability to controllably
approach complete saturation of the airstream, relatively
short absorption distances, and minimal concerns over microbial
growth. Disadvantages include the energy required to create steam.
(In many systems, this energy would ultimately have to be added to
the airstream. For instance, adiabatic humidifiers cool the air as they
humidify, so winter humidification applications in 100% outside
air buildings require the heating coil to
provide all of the energy that would have AS SEEN IN
been used to create steam in an isothermal
humidifier.)
Isothermal humidifiers are best applied
in 100% outside air systems, in systems
where microbial growth concerns are high
(e.g., hospitals), and where available AHU
or duct length for absorption is limited.

ADIABATIC HUMIDIFIERS
Adiabatic humidifiers are available in several
forms. The category includes wetted pad type
humidifiers, spray nozzle humidifiers (which
may use either pressurized water or water
and compressed air for atomization), and
ultrasonic humidifiers. Adiabatic humidifiers
evaporate moisture into the air by presenting
a large water surface area to the airstream.
This is achieved by either wetting a medium
with a large surface area (wetted pad type) or
by atomizing the water into very small droplets
(spray nozzles or ultrasonic). The water
source is often city water, although fouling
may be reduced through the use of either
softened or pure water. Ultrasonic humidifiers
require pure water.
Advantages of adiabatic humidifiers
include the low energy required to humidify
the air (which is typically negated in winter
100% outside air applications as noted
above) and the associated cooling effect,
which can be used to great advantage in
warm dry climates. Disadvantages include
a somewhat increased microbial growth
risk due to the ambient temperature water
source, particularly in the wetted media
type where water is essentially stagnant.
Absorption distances also tend to be longer
(for spray type) and approach to full saturation
is generally more difficult.
Adiabatic humidifiers are best applied in
dry climates, particularly if humidification
is required during cooling season, and in
systems with airside economizers, which
can also benefit seasonally from the reduced
energy use of adiabatic humidifiers.
SOLID DESICCANT DEHUMIDIFIERS
Solid desiccant dehumidifiers usually take
the form of desiccant wheels, rotary heat
exchangers either coated or impregnated
with a desiccant material (typically silica
gel or lithium chloride). Desiccant wheels
are designed to remove moisture from the
airstream, and operate nearly adiabatically,
dependent on the heat of sorption of the
desiccant and the mass of the matrix used to
present the desiccant to the air. The path followed
by the supply air on a psychrometric
chart is nearly the opposite of the adiabatic
humidifier, but the path tends to be slightly
flatter due to the effects mentioned above.
Desiccant wheels are divided into a
process side and a regeneration side. The
process (supply) side air is dried by the
desiccant material, while on the regeneration
side, another airstream is preheated
and runs in counterflow through
FIGURE 1. Typical panel-type isothermal humidifier and associated psychrometric path.
(AEI Illustration.)
FIGURE 2. Typical wetted-media-type adiabatic humidifer and associated psychrometric
path. (AEI Illustration.)
FIGURE 3. Typical desiccant wheel process and regeneration sides and associated psychrometric
paths. (AEI Illustration.)
FIGURE 4. Typical enthalpy wheel and associated outside air and exhuast air psychrometric
paths. (AEI Illustration.)

Humidification and Dehumidification
FIGURE 5. Typical cooling coil and associated psychrometric path. (AEI Illustration.)
FIGURE 6. Dehumidification section of Seattle Biotech AHU and associated psychrometric
path. (AEI Illustration.)
FIGURE 7. The Pennington Cycle, shown diagrammatically, and on the psychrometric
chart. (AEI Illustration.)
the opposite side of the desiccant wheel,
driving moisture back out of the wheel.
The potential for drying of the supply
side is determined by the relative humidity
of the regeneration air. The hotter the
regeneration air is heated, the lower the
relative humidity and the greater the drying
potential in the supply side. Outside
air and building exhaust air are the most
common sources of regeneration air. The
heat source for regeneration is typically
hot water, ideally from a waste source.
Advantages of desiccant dehumidifiers
include the ability to dry air to very
low dewpoints. The ability to dry using
a heat source may also be an advantage,
particularly where a waste heat source is
available. Disadvantages include first cost
and elimination of the sensible heat that
must be removed in most cases (typically
through the use of a supplemental rotary
heat exchanger).
Desiccant wheels are best applied when
required dewpoint temperature approaches
32°F, making it impractical to use cooling
coils due to coil frost issues; when the
cost of heat is low relative to electricity; or
where very high levels of moisture reduction
are necessary.
LIQUID DESICCANT DEHUMIDIFIERS
Liquid desiccant dehumidifiers operate in
manner similar to solid desiccants but use a
spray tower rather than a wheel to present
the desiccant to the airstream. The process
side air runs in counterflow with a dry
desiccant spray (usually lithium chloride
or lithium bromide), the desiccant is then
regenerated by a heated airstream in a
regeneration side spray tower.
Liquid desiccant systems tend to be
more expensive than solid desiccants but
offer the benefit of enhanced microbial
decontamination due to the bacteriacidal
and virucidal nature of lithium chloride.
Advantages and disadvantages are similar
to solid desiccant wheels. Liquid desiccants
offer the advantage over solid wheels of
slightly better isolation of supply and exhaust
airstreams, since wheel carryover is not possible.
Applications are also similar, but liquid
desiccants may offer benefits where greater
microbial decontamination is required or
where the liquid desiccant’s capabilities closely
match process requirements.
ENTHALPY WHEELS
Enthalpy wheels are similar in nature to
desiccant wheels — both are rotary heat
exchangers either coated or impregnated
with a desiccant material — but differ in their
focus. While desiccant wheels manipulate the
regeneration side airstream to achieve deep
drying, enthalpy wheels focus on extraction
of the available moisture, or lack thereof,
in the building exhaust airstream. Enthalpy
wheels are also intended to operate as heat
exchangers. Accordingly, the sensible heat
transfer capability of enthalpy wheels is

high, whereas the heat transfer capability of
desiccant wheels is low, since the transfer of
sensible heat from the hot regeneration side
air is an unwanted side effect.
Advantages of enthalpy wheels include
the ability to simultaneously transfer both
heat and moisture.
Enthalpy wheels are best applied in high
outside air percentage humidified buildings
in cold climates, where recovery of
the energy invested in both heating and
humidification is possible, and in high
outside air percentage very humid climates
where enthalpy wheels can recover both
cooling and lack of dehumidification from
the outgoing air.
COOLING COILS
Although dehumidification is typically not
the primary purpose of a cooling coil, cooling
coils act as summer dehumidifiers in
most commercial buildings by limiting the
dewpoint of the supply air to no more than
the discharge air temperature of the air
handling unit (typically 55°).
Advantages of cooling coils include
simplicity and familiarity. Disadvantages
include inability to dehumidify easily at
or below 32° and inability to handle large
loads without multiple coils.
Cooling coils are best applied where cooling
loads are modest and where process
requirements do not drive unusually low
humidity.
EXAMPLE 1
The King Abdullah University of Science
and Technology (KAUST) is sited in the
high heat and humidity of Jeddah, in Saudi
Arabia’s desert coastal climate. The requirement
for 100% outside air in the laboratory
facilities presented a real challenge given
the design conditions. The logical approach
of the enthalpy wheel (recommended by
Gary Kuzma of HOK) can salvage a high
percentage of the dehumidification and
cooling that has been invested in the building
exhaust air.
As the psychrometric chart shows, the
burden on a cooling coil acting alone in
an attempt to cool and dehumidify the
incoming outside air would be extreme.
The enthalpy wheel recaptures a high percentage
of the cooling and dehumidification
already performed on the exhaust air,
leaving the cooling coil with a much more
modest and manageable load. The reduction
in cooling load handled by the cooling
coil is 64% compared to a cooling coil
operating alone, and the effectiveness of the
enthalpy wheel was 81.4% (e.g., the wheel is
able to transfer 81.4% of the enthalpy difference
between the supply and exhaust air
from the exhaust to the supply).
EXAMPLE 2
This biotech research facility is located in
the mild marine climate of Seattle. The
laboratory facility included a process space
with very specific temperature and humidity
requirements. The conditions presented
a challenge because the desired humidity
was just low enough that subcooling by a
cooling coil would be difficult.
The solution was the use of a solid desiccant
wheel to assist with moisture removal.
Since the desiccant wheel’s ability to remove
moisture was much greater that the required
level of reduction, only a portion of the supply
air was directed through the desiccant
wheel. As seen on the psychrometric chart,
by bypassing approximately 20% of the air
the through a desiccant wheel, the total
air absolute humidity was reduced from
51 grains/lb to approximately 42 grains/lb
by mixing 7-grain desiccated air with the
bypassed 51-grain air.
EXAMPLE 3
While the tools discussed here are often used
alone, they may also be used in combination
to achieve a particular psychrometric
effect or to take advantage of available
resources. An example is the Pennington
Cycle. The Pennington Cycle is a 100% outside
air cycle that incorporates a desiccant
wheel, a sensible heat exchanger wheel, and
an adiabatic humidifier. Supply side air is
first dehumidified, then cooled sensibly in
the heat exchanger, and finally humidified
(and cooled) adiabatically by the adiabatic
humidifier (evaporative cooler).
On the exhaust side, the air is first adiabatically
humidified (and cooled) to enhance
the cooling effect on the supply side air
as the return passes the cool to the supply
in the rotary heat exchanger. The exhaust
air is then heated by a low grade heat
source (often solar or waste heat), and dries
(regenerates) the desiccant material.
As the psychrometric chart shows, the combination
of desiccation, sensible cooling,
and adiabatic humidification of the supply
air creates a circuitous path around the
psychrometric chart, but the beginning and
end points are very close to those that a
cooling coil alone could have achieved.
Ordinarily, this would seem to be an overly
complex means to achieve cooling, but the
cycle is intended to take advantage of waste
heat (or solar heat) for cooling. By applying
this particular series of components, waste
heat is converted into cooling energy.
CONCLUSION
Best available technologies for humidification
and dehumidification vary according
to system type, climate, process requirements,
energy use expectations, space availability,
and contamination concerns. The
needs of the system, the efficiency of the
technology, and the cost of available energy
sources must be considered as systems are
selected.
Some general conclusions:
As reported elsewhere, adiabatic humidifiers
may offer energy savings over isothermal
humidifiers in systems with less than
50% outside air.
Enthalpy wheels offer great benefits for
pre-conditioning of outside air in hot
humid climates (and the same holds true
for high outside air percentage-humidified
buildings in cold climates).
Desiccant wheels and liquid desiccant
systems provide deep drying capability in
applications that require air at dew points
less than 42° to 45°.
Humidification and dehumidification
technologies can be used in series (or parallel)
to achieve a net effect equivalent to
other technologies, with potential energy
savings benefits. ES
McLay is a senior
engineer and project
manager with AEI/
Affiliated Engineers,
Inc., specializing in
technically complex
facilities.
Reprinted from Engineered Systems Magazine © September 2009 P.O. Box 4270 Troy, MI 48099