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How do you model Desiccant Regen Air from Adjacent Zone?

asked 2018-06-27 09:19:43 -0500

RCulham gravatar image

I am modelling a fan powered desiccant dehumidifier to control the humidity in a zone adjacent to a large zone with predominately high temperature and relatively lower humidity. The desiccant dehumidifier features face and bypass dampers on the desiccant wheel with its own supply fan rated at 2250 cfm. The conditioned air enters a return air mixer connected to a air handling unit with heating and cooling coils an its own supply fan rated at 3500 cfm. The air handling unit heats or cools to maintain the zone setpoint temperature. In the off mode the desiccant dehumidifier is isolated from the air handling unit air loop. The desiccant dehumidifier maintains the zone humidity by modulating the face and bypass dampers. As there are no supply side splitters in and mixers in the supply side equipment in EnergyPlus other than an outdoor air mixer, I have simplified the model of the system such that there is no face and bypass on the desiccant wheel and there is only when supply fan rated at 2250 cfm and there are no isolation dampers and the there is no return air mixer between the dehumidifier and the air handling unit.

How do I model the regen air from the adjacent zone where the standard method is to use outdoor air? I am currently looking at the AirflowNetwork objects in EnergyPlus possibly using the RoomAirNode:AirflowNetwork object for the adjacent zone.C:\fakepath\Image (231).jpg

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answered 2018-06-27 15:37:27 -0500

AirflowNetwork objects will not help with this. Simply(?) connect the regen fan inlet to a Zone Exhaust Node in the adjacent zone. Taking the DesiccantCVRhZoneRHCtrl.idf example file, I connected the "Regen Air Inlet Node" to Zone3, NORTH ZONE:

ZoneHVAC:EquipmentConnections,
NORTH ZONE,              !- Zone Name
Zone3Equipment,          !- Zone Conditioning Equipment List Name
Zone3Inlets,             !- Zone Air Inlet Node or NodeList Name
Regen Air Inlet Node,     !- Zone Air Exhaust Node or NodeList Name
Zone 3 Node,             !- Zone Air Node Name
Zone 3 Outlet Node;      !- Zone Return Air Node or NodeList Name

Fan:VariableVolume,
Desiccant Regen Fan,     !- Name
<snip>
Regen Air Inlet Node,!- Air Inlet Node Name
Regen Coil Inlet Node;   !- Air Outlet Node Name

Dehumidifier:Desiccant:NoFans,
Desiccant 1,             !- Name
<snip>
Regen Coil Out Node,     !- Regeneration Air Inlet Node Name
Regen Air Inlet Node,!- Regeneration Fan Inlet Node Name

Now, if the adjacent zone does not have any HVAC connected to it, you will be required to add something so that you can connect the exhaust node from that zone. If necessary, add a simple baseboard electric heater and schedule it off all the time.

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Thank you for your response MJ Witte. Using the Zone Air Exhaust Node from the adjacent zone is a simple fix. I now will consider enhancing the model to account for the face and bypass dampers as the zone humidity is too low in the shoulder seasons.

RCulham gravatar image RCulham  ( 2018-06-28 08:12:23 -0500 )edit

While it isn't exactly the same performance as face and bypass, you can control the dehumidifier based on a control zone's RH. That's how this particular example file is set up.

MJWitte gravatar image MJWitte  ( 2018-06-28 12:33:24 -0500 )edit

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Asked: 2018-06-27 09:19:43 -0500

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Last updated: Jun 27 '18