Application Considerations
Figure A-8 - Typical Dual-Path
MCC/SDU Stacked Dehumidification Unit
Dehumidification. ASHRAE Standard 62-1989
observes that "high humidities can support the
growth of pathogenic or allergenic organisms" and
suggests that the relative humidity of the occupied space not exceed 60 percent. Higher humidities also require lower supply
air temperatures for thermal comfort. Most climates
require dehumidification to achieve this design goal. That can be accomplished either by removing moisture from the air with a
desiccant wheel or by condensing the water vapor on
cooling coils. Of these approaches, cooling-coil
dehumidification is most commonly used in commercial HVAC applications.
However, cooling coils can overcool the occupied space
when dehumidifying at sensible part-load conditions.
Placing a reheat device — usually a "new
energy" hot-water or electric coil — downstream of the cooling coils solves this problem, but usually at an increased
operating cost.
The Modular Climate Changer’s stackable design
enables a more effective means of dehumidification by
combining the respective functions of an outdoor air unit and a standard unit into the same air handler. Referred to as a stacked
dehumidification unit or SDU, this unique
Modular Climate Changer design eliminates or minimizes the need for reheat. As
Figure A-8 indicates, the upper deck or "secondary
unit" of the SDU controls space humidity using a
cooling coil with special water management features. It treats only the outdoor
air required for ventilation. The SDU’s lower deck
or "primary unit" maintains the sensible
temperature in the space, and handles return airflow
as well as any economizer outdoor airflow. As a "rule of thumb," size the upper deck at 25 percent of the lower deck’s
capacity when designing an SDU air handler. Insulated
channel spacers may be required to provide proper water management and prevent interdeck heat transfer, as in stacked dehumidification
unit (SDU) applications.
Humidification. Low relative humidities (below
30 percent) in the occupied space are also undesirable
for they necessitate higher supply air temperatures for thermal comfort, promote
static electricity and favor the transmission of
airborne infections such as influenza. Raising the
space humidity to an appropriate level requires a humidifier to inject water particles into the passing air stream. To avoid
promoting microbial growth, the unit’s design must
assure that the injected water is fully absorbed within
the air handler without collecting on its walls or components.
Three types of commercial humidifiers are generally
used in central air handling systems: wetted media,
atomized water and steam. Of these, ASHRAE Standard 62-1989 prefers steam "as
the moisture source for humidifiers." Its low
temperature and pressure make it easy to introduce
directly into the passing air stream, and encourage complete absorption in a short distance.
Among the Modular Climate Changer’s "building
blocks" is a custom module with a factory-mounted
"dry" steam humidifier. Refer to "AHU
Components: Humidification Module" for specific application guidelines.
AHU Layouts
Having determined the required airflows and functions, the designer must now
decide whether the application is best served by one or
two outdoor air paths. See Figure A-2.
Single-Path Layout Single-path air handler
designs — whether for outdoor air units, standard
units or some combination of these — rely on one outdoor air path. Depending
on application requirements, that path either provides
ventilation air only or supplies both ventilation air and "economizing" air for natural, nonmechanical cooling. Components for cleaning and tempering the air
are arranged in series.
This layout accommodates passive or powered return-
and/or exhaust-air paths as well as energy recovery.
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