Dual-Path Air Handler (Split Dehumidification Unit)
A dual-path air handler (split dehumidification unit) separately conditions the outdoor and recirculated airstreams within the same unit. Each airstream has its own cooling coil, but a single constant-volume fan serves both paths.
Sample scenario
A large meeting room is served by a dual-path air handler (split dehumidification unit). During the cooling season, the dew point of the air leaving the outdoor-air cooling coil is adjusted to prevent the relative humidity in the room from rising above 60%. The return-air cooling coil is then modulated to maintain the space dry-bulb temperature at setpoint. During the heating season, the outdoor-air heating coil heats the entering outdoor air to 55°F dry bulb.
■ The system controller determines the highest possible dew point leaving the outdoor-air cooling coil that avoids the room relative humidity from rising above the maximum upper limit (60%, in this example).
■ When the outdoor dew point is greater than the current cooling supply-air dew point setpoint, the outdoor-air cooling coil will dehumidify the entering outdoor air to the cooling supply-air dew point setpoint. The return-air cooling or heating coil modulates to maintain the space temperature at setpoint.
minimum Figure 4–16 Split dehumidification unit
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■ When the outdoor dew point is less than the current supply-air dew point setpoint and the outdoor dry-bulb temperature is greater than the cooling supply-air dry-bulb setpoint, the outdoor-air cooling coil will cool the entering outdoor outdoor-air to the cooling supply-air dry-bulb setpoint. The return-air cooling or heating coil modulates to maintain the space temperature at setpoint.
■ When the outdoor dew point is less than the current cooling supply-air dew point setpoint and the outdoor dry-bulb
temperature is less than the cooling supply-air dry-bulb setpoint, the outdoor-air heating coil will heat the entering outdoor air to the heating supply-air dry-bulb setpoint. (If no heating coil exists in the outdoor-air path, input the heating supply-air dry bulb colder than the winter outdoor design temperature.) The return-air cooling or heating coil modulates to maintain the space temperature at setpoint.
This example will cover how to model the air-distribution system (steps 1–8), model the cooling and heating equipment (steps 9–
11), and assign the loads from the coils to the equipment (step 12).
1 Start by selecting Single Zone as the system type.
2 Click Advanced and select (or verify) Draw Thru fan configuration.
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Dual-Path Air Handler (Split Dehumidification Unit) 3 On the Dedicated OA
tab, select Split Dehumidification Unit.
4 Select the control method for the unit. Because the ventilation cooling coil dehumidifies the outdoor air to the cooling supply-air dew point that is reset based on room relative humidity, Optimize Supply Air Dew Point will be used.
5 Input the Cooling and Heating Design Setpoints. In this scenario, 65°F should be input for the cooling supply-air dry bulb, 55°F for the heating supply-air dry bulb, 65°F for the cooling supply-air dew point high limit, and 55°F for the cooling supply-air dew point low limit.
Split Dehumidification Unit – a dual-path air handler brings outdoor air through a dedicated coil to cool the outdoor air to the user-input cooling dew point (not reheated) during the cooling season, or heat the outdoor air to the user-input heating dry bulb during the heating season, before it is mixed with recirculated return air and delivered through a common supply fan.
Note: For additional information related to the other Control Methods, press F1 and click Control Method.
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6 Select the schedule for the coils and fan in the split dehumidification unit.
7 On the Temp/Humidity tab, select Direct Dehumidification...
as the Dehumidification Method and specify a Maximum room relative humidity of 60%.
If the ventilation air is to be precooled, the Dedicated Ventilation Cooling Coil schedule defines the hours of available operation of the ventilation cooling coil. The optional ventilation cooling coil is available for any hour that this schedule reads 1 percent or greater.
If the ventilation air is to be preheated, the Dedicated Ventilation Heating Coil schedule defines the hours of available operation of the ventilation heating coil. The optional ventilation heating coil is available for any hour that this schedule reads 1 percent or greater.
The Optional ventilation fan schedule defines the hours that the ventilation unit can operate. If the Optional ventilation fan schedule reads 1 percent or greater, the fan is available that hour.
Note: If a custom schedule is created for the fan or coils, it is recommended to use values of 0 percent for hours off and 100 percent for hours available to run to avoid confusion.
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Dual-Path Air Handler (Split Dehumidification Unit) the fan type and the total
static pressure for the primary fan in the dual-path air handler.
Next, define the cooling and heating plants represented by the dual-path air handler system and the rooftop air conditioner:
9 Add a new cooling plant and a new heating plant to the Configuration pane.
10Drag the appropriate icons from the Equipment Category section to define each plant.
11To finish defining this equipment, enter the information from Table 4–10 on page 4–72 on the appropriate equipment tab.
For cooling, use water-cooled chiller and air-cooled unitary.
For heating, use boiler and gas-fired heat exchanger.
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Finally, assign the coil loads of the air-distribution system to the proper cooling and heating plants:
12Drag each coil in the Unassigned Systems/
Coils pane to the appropriate plant in the Systems and Plants pane.
The illustration labeled After shows the completed assignments.
Before…
After…
Table 4–10 Equipment types
Cooling Equipment tab Heating Equipment tab Water-cooled chiller 3-stage centrifugal
chiller gas-fired hot water
boiler