Desiccant dehumidification
Desiccant dehumidification describes the use of a moistureattracting material (solid or liquid) in the air stream. As Figure 4–1
suggests, desiccant dehumidification requires a process air
stream and a regeneration air stream. In this example, the
moisture in the process (outdoor) air stream is absorbed by the
desiccant wheel, which then releases the collected moisture in
the regeneration (exhaust) air stream.
Figure 4–1 Air-distribution system with desiccant dehumidification
Continued operation of the desiccant requires a regeneration
process. Regeneration is usually accomplished by applying heat
to the regeneration air stream before it reaches the desiccant.
Consideration of desiccant dehumidification is warranted
whenever one or more of the following conditions is true of a
prospective application:
■ The latent load is significantly larger than the sensible load
■ The cost to regenerate the desiccant is low when compared to
the cost associated with overcooling the space
■ The humidity-control level in the space would require chilling the
supply air to subfreezing dew points when vapor-compression
refrigeration alone is used
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System Design Options
Desiccant dehumidification
TRACE 700 User’s Manual • CDS-PRM001-EN
Typical applications for desiccant dehumidification include ice
rinks, supermarkets, retail clothing stores, and clean rooms, as
well as installations in humid southern climates. In each instance,
desiccant dehumidification provides independent control of
sensible and latent heat without the need for reheat. It also
dehumidifies the supply air stream without lowering the dry-bulb
temperature.
Application considerations
■ Desiccant regeneration commonly requires high temperatures,
which incurs a significant energy cost.
■ Exposure to contaminants, clogging, and thermal stress causes
desiccants to degrade over time.
■ One should consider space requirements, cost, and added
pressure drop when evaluating whether desiccant
dehumidification is practical for a particular application.
Related reading
■ Dehumidification in HVAC Systems Applications Engineering
Manual (Trane literature number SYS-APM004-EN)
Sample scenario
The ventilation deck of the air-distribution system for a building
includes a solid-desiccant wheel with an integral run-around coil
loop. The wheel only operates from 6 a.m. until 6 p.m.
To model desiccant
dehumidification, which is a
means of removing latent
heat from an air stream,
begin by modeling the airdistribution system.
1 Choose the desired
system type from the
categories listed.
For this example, select
Series Fan-Powered VAV
from the Variable
Volume category.
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System Design Options
Desiccant dehumidification
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2 Specify the type of air-toair energy recovery /
transfer and the location
of the process air stream.
This example uses a solid
desiccant located on the
ventilation deck.
Click Yes to activate or include the
desiccant device for design and energy
analysis. Click No to ignore the desiccant
device during the design calculation and
include it only during the energy-analysis
simulation.
Note: This message will only appear
whenever an energy-transfer device is
added and Allow energy recovery /
transfer during design calculations is
turned off.
3 A message will appear
that asks whether the
user wants to account for
energy recovery / transfer
during the design and
energy analysis
simulations or only during
the energy analysis
simulation. Select Yes for
this example.
■ Select Ventilation upstream if the
device is located in the outdoor air
stream and upstream of the optional
ventilation coil.
■ Select Ventilation downstream if the
device is located in the outdoor air
stream and downstream of the optional
ventilation cooling coil.
■ Select Return / outdoor air
downstream if the device is located
after the mixed air stream, downstream
of the main cooling coil.
■ Select Return / outdoor air upstream if
the device is located after the mixed air
stream, upstream of the main cooling
coil.
4 Select the appropriate
supply-side deck for the
device. For this example,
Ventilation upstream
will be selected as the
supply-side deck. The
desiccant device will be
used to dehumidify the
outdoor air stream.
Note: If no optional ventilation system has been input, then Ventilation
upstream and Ventilation downstream will function identically. If the main
cooling coil has been deleted, or if the ventilation-deck location on the
Advanced window in Create Systems is not Return/Outdoor deck, then
Return / outdoor air upstream and Return / outdoor air downstream
will function identically.
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System Design Options
Desiccant dehumidification
TRACE 700 User’s Manual • CDS-PRM001-EN
5 Select the exhaust-side
deck for the device. This
will determine which
airflow will be used as the
regeneration air stream.
The airstreams typically
used are building exhaust
air, outdoor air, or a
mixture of the two.
Note: In this case, the airflow
that is being exhausted from
the building at the system
level will be used as the
regeneration airstream for the
desiccant device. Because of
this, System Exhaust will be
used.
■ Select Outdoor & room exhaust mix if the regenerator will first try to
use room exhaust as the sole source of regeneration airflow. If the room
exhaust airflow is insufficient, the rest of the regeneration airflow will be
made up of outdoor air.
■ Select Outdoor & system exhaust mix if the regenerator will first try to
use system exhaust as the sole source of regeneration airflow. If the
system exhaust airflow is insufficient, the rest of the regeneration airflow
will be made up of outdoor air.
■ Select Outdoor if outdoor air is used exclusively as the regeneration
airstream.
■ Select System Exhaust if main system exhaust is used exclusively as the
regeneration airstream. If a liquid desiccant is used and the system
exhaust varies in volume (due to varying ventilation rates), do not use this
input for the exhaust-side deck— use Outdoor & system exhaust mix to
get the required constant regeneration airflow.
■ Select Room Exhaust if the room exhaust airflow is used exclusively as
the regeneration airstream. Room exhaust airflows are input on the
Airflows tab of Create Rooms. If no room exhaust is input, then all
exhaust air will be at the system level. Also, if a liquid desiccant is used
and the room exhaust varies in volume (due to varying ventilation rates or
room exhaust schedules), do not use this input for the exhaust-side
deck—use Outdoor & room exhaust mix to get the required constant
regeneration airflow.
Note: If the proper airstreams are not selected for the energy recovery /
transfer device, then the device will not function correctly or may not
function at all. The Airflows section of the System Checksums report
can be used to verify that sufficient airflow is available to be used for
energy recovery / transfer. Room Exhaust = Rm Exh on the System
Checksums report and System Exhaust = Exhaust on the System
Checksums report.
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6 Choose the schedule that
describes when the
desiccant device is
permitted to operate. For
this example, a custom
schedule was created
that allows the desiccant
device to operate only
from 6 a.m. to 6 p.m.
7 Now that the desiccant
subsystem has been
input, the coil loop needs
to be added as the
second stage of energy
recovery. The coil loop
will preheat the
regeneration airstream,
reducing the amount of
energy required to
regenerate the desiccant
and cool the outdoor air
after it leaves the
desiccant device,
therefore reducing the
amount of cooling energy
required. Refer to “Coil
loop for exhaust-air
energy recovery” on
page 4–8 for step-by-step
instructions on how to
input a coil loop.
The default schedule Available 100% will allow the energy recovery / transfer
device to operate year-round. Liquid-desiccant systems should typically only be
made available to operate during occupied hours, because the liquid-desiccant
system will try to maintain the ERD (energy-recovery device) leaving-air humidity
ratio at its design value. (This is auto-calculated, based on the design conditions,
unless overridden by the user on the Desiccant Dehumidification Options
screen that appears when the Options button is clicked.)
Note: For instructions on how to create custom schedules, refer to “Creating
schedules” on page 6–137. Within the custom schedule, 0 percent would
indicate that the desiccant device is deactivated, and 100 percent would
indicate that it is available to operate.
Note: If both stage 1 and
stage 2 energy recovery/
transfer devices are given
the same exhaust-side deck,
then stage 1 will be
upstream of stage 2.
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System Design Options
Desiccant dehumidification
TRACE 700 User’s Manual • CDS-PRM001-EN
Note: To further refine the model of the desiccant dehumidification
device, the Options button on the Options tab of Create Systems can
be used to define such items as parasitic energy consumers, supplyside design air conditions, coolant types, approach temperatures, heatsource temperatures, economizer lockout, and so on.
Next, define the
heating plant that serves the
airside heating loads.
8 Add a gas-fired, hot-water
boiler to serve the main
heating load.
To provide the heat
needed to regenerate the
desiccant, add a gas-fired
heat exchanger.
Not shown: Specify
equipment types,
capacities, schedules,
and the full-load
consumption of the hotwater pump on the
Heating Equipment tab.
9
Unassigned systems and
associated coils appear in
this pane. To assign a load
and/or system, drag the
relevant icon to the
appropriate cooling or
heating plant at right.
Assign the heating
coil loads to the heating
plants.
The Stage 1 desiccant
regenerator regenerates
the solid desiccant.
Additional item
1 When adding a desiccant subsystem to an airside system, it is
recommended that the minimum and maximum cooling supplyair dry bulbs be set equal to each other to fix the value for the
cooling supply-air dry bulb. This is suggested because
TRACE 700 cannot psychrometrically solve for the cooling supplyair dry bulb when an energy recovery / transfer device is attached
to the Return/Outdoor deck. Refer to “Frequently asked
questions” on page 6–20 for the ramifications of inputting a
cooling supply-air dry bulb.
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System Design Options
Desiccant dehumidification
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