ZoneVentilation:DesignFlowRate fluctuates

asked 2024-08-07 02:39:52 -0600

Keigo
4021 ●1 ●47 ●21 https://www.linkedin.c...

updated 2024-08-07 02:52:30 -0600

I tested with an ExampleFile VentilationSimpleTest.idf (V24-1-0). I changed the Coefficients as follows so that the ventilation rate is not affected by outdoor air temperature and velocity. I expected the ventilation rate to be constant.

ZoneVentilation:DesignFlowRate,
    ZONE 1 Ventl 1,          !- Name
    ZONE 1,                  !- Zone or ZoneList or Space or SpaceList Name
    Constant,                !- Schedule Name
    flow/zone,               !- Design Flow Rate Calculation Method
    6.131944,                !- Design Flow Rate {m3/s}
    ,                        !- Flow Rate per Floor Area {m3/s-m2}
    ,                        !- Flow Rate per Person {m3/s-person}
    ,                        !- Air Changes per Hour {1/hr}
    NATURAL,                 !- Ventilation Type
    0,                       !- Fan Pressure Rise {Pa}
    1,                       !- Fan Total Efficiency
    1,                       !- Constant Term Coefficient
    0,                       !- Temperature Term Coefficient
    0,                       !- Velocity Term Coefficient
    0;                       !- Velocity Squared Term Coefficient

I output Zone Ventilation Standard Density Volume Flow Rate, Zone Ventilation Current Density Volume Flow Rate and Zone Ventilation Mass Flow Rate, but all fluctuate. Why? What kind of ventilation rate is supposed to be constant other than current density volume, standard density volume and mass? How can I keep the ventilation rate constant?

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The same bug was found in ZoneInfiltration:DesignFlowRate.

Keigo ( 2024-08-07 02:59:31 -0600 )edit

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answered 2024-08-07 10:25:55 -0600

shorowit

11811 ●200 ●9 https://www.nrel.gov/b...

updated 2024-08-07 13:29:22 -0600

The ZoneInfiltration:* and ZoneVentilation:* objects use outdoor current density to convert from volume flow to mass flow. So, for example, you will obtain a constant flow rate for your object if you use this calculation:

Flow Rate [m3/s] = (Ventilation Mass Flow Rate [kg/s]) / (Environment:Site Outdoor Air Density [kg/m3])

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Thanks. The above calculation is constant, but other questions came up.

What's the difference between the calculation and the output variable Zone Ventilation Current Density Volume Flow Rate? I thought Zone Ventilation Current Density Volume Flow Rate = (Ventilation Mass Flow Rate) / (Environment:Site Outdoor Air Density).
Other OA related objects e.g. DesignSpecification:OutdoorAir and ZoneHVAC:OutdoorAirUnit specify not Current Density Volume but Standard Density Volume Flow Rate. Why do input fields for Air Flow Rate have different meanings? They should be consistent.

Keigo ( 2024-08-07 21:03:14 -0600 )edit

Current density is based on the current density of the zone air. (And Standard Density is based on the standard density of air, which in EnergyPlus corresponds to 20ºC drybulb, dry air, and nominally adjusted for elevation.) If there are inconsistencies between objects, I don't know why it is. I'm not really an EnergyPlus developer, just the messenger.

shorowit ( 2024-08-07 21:44:51 -0600 )edit

As @shorowit has explained, the volume flow is constant at the outdoor air density, which is not directly reportable. This is how it was designed from day one. There has been a long-standing suggestion to add input fields to give the user more control over this. https://github.com/NREL/EnergyPlus/is...

MJWitte ( 2024-08-08 08:17:23 -0600 )edit

@MJWitte Sorry that I'm new to the community and did not know the history. My opinion is that ideally, users should be able to chose which density is used for calculating OA volume flow rate, Current OA density or Standard density. For Mechanical Ventilation without cooling/heaitng and AHUs, the OA volume flow rate is controlled by a volume damper or a VAV box located upstream of cooling/heating coils, so the Current OA density should be used. For DOAS, the OA volume flow rate is normally controlled at the air terminal (CAV/VAV) after the OA is handled, so the Standard density should be used.

Keigo ( 2024-08-08 11:23:16 -0600 )edit

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