How maximum outdoor air flow rate is calculated in Controller:OutdoorAir?

asked 2017-10-19 04:07:28 -0600

2085 ●1 ●4 ●14

updated 2017-10-19 19:45:53 -0600

I am trying to implement PAU+FCU with chilled water loop.
Since maximum flow rate specified in Fan:VariableVolume is bigger than minimum flow rate autosized by Controller:OutdoorAir, I expected that maximum flow rate specified in Fan:VariableVolume will be used for maximum flow rate in Controller:OutdoorAir. However, checking the result, maximum flow rate is same as minimum flow rate as shown below.
image description
Then I start to wonder how maximum flow rate for economizer is calculated.
Can anyone tell me how maximum outdoor air flow rate is calculated in Controller:OutdoorAir?

Followings are some of the objects I mentioned.

Controller:OutdoorAir,
    A_LZ_Office_sys OA Controller,  !- Name
    A_LZ_Office_sys Relief Air Outlet,  !- Relief Air Outlet Node Name
    A_LZ_Office_sys Air Loop Inlet,  !- Return Air Node Name
    A_LZ_Office_sys Mixed Air Outlet,  !- Mixed Air Node Name
    A_LZ_Office_sys Outdoor Air Inlet,  !- Actuator Node Name
    autosize,                !- Minimum Outdoor Air Flow Rate {m3/s}
    autosize,                !- Maximum Outdoor Air Flow Rate {m3/s}
    DifferentialEnthalpy,    !- Economizer Control Type
    ModulateFlow,            !- Economizer Control Action Type
    ,                        !- Economizer Maximum Limit Dry-Bulb Temperature {C}
    ,                        !- Economizer Maximum Limit Enthalpy {J/kg}
    ,                        !- Economizer Maximum Limit Dewpoint Temperature {C}
    ,                        !- Electronic Enthalpy Limit Curve Name
    ,                        !- Economizer Minimum Limit Dry-Bulb Temperature {C}
    NoLockout,               !- Lockout Type
    ProportionalMinimum;     !- Minimum Limit Type

Fan:VariableVolume,
    A_LZ_Office_sys Supply Fan,  !- Name
    Office HVAC On (BEAM),   !- Availability Schedule Name
    0.8,                     !- Fan Total Efficiency
    1600,                    !- Pressure Rise {Pa}
    145.2,                   !- Maximum Flow Rate {m3/s}
    Fraction,                !- Fan Power Minimum Flow Rate Input Method
    0.0,                     !- Fan Power Minimum Flow Fraction
    ,                        !- Fan Power Minimum Air Flow Rate {m3/s}
    0.9,                     !- Motor Efficiency
    1,                       !- Motor In Airstream Fraction
    0.0015302446,            !- Fan Power Coefficient 1
    0.0052080574,            !- Fan Power Coefficient 2
    1.1086242,               !- Fan Power Coefficient 3
    -0.11635563,             !- Fan Power Coefficient 4
    0,                       !- Fan Power Coefficient 5
    A_LZ_Office_sys Heating Coil Outlet,  !- Air Inlet Node Name
    A_LZ_Office_sys Supply Fan Outlet;  !- Air Outlet Node Name

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Comments

Are you working from the same file you posted here? In those posted objects you have used "NoEconomizer" in the setting for the Economizer Control Type field in the Controller:OutdoorAir. Did you change that setting for the run results shown here?

Molly Curtz ( 2017-10-19 17:33:58 -0600 )edit

The one you mentioned is different model. I added the Controller:OutdoorAir object to the post.

katsuya.obara ( 2017-10-19 19:41:32 -0600 )edit

Where is your Controller:MechanicalVentilation? DId you read the I/O, the intro section is pretty explanatory.

Julien Marrec ( 2017-10-20 02:32:48 -0600 )edit

I didn't model Controller:MechanicalVentilation since I thought that it is not required to model air side economizer. I read the I/O. They explain how the outdoor air volume is decided according to outdoor and indoor condition. However, cannot find how Maximum Outdoor Air Flow Rate is calculated when we set to autosized

katsuya.obara ( 2017-10-20 03:48:28 -0600 )edit

What is the design flow rate of the system when you look at the output reports and/or in the Sizing:System object? I suspect that max flow rate of the supply fan is not enforced in any way to be the same as the system max flow rate.The fan flow rate in the Fan:VariableVolume object would just be used to determine part load ratio of the fan needed calculate the power use in simulation.

Molly Curtz ( 2017-10-20 12:41:47 -0600 )edit

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answered 2017-10-26 11:21:05 -0600

MJWitte

9760 ●6 ●10 http://www.gard.com/

As @Molly Curtz points out in the comment above, the hard flow rate in the supply fan object (or any other component) is unknown to the system sizing calculations. The OA controller Maximum Outdoor Air Flow Rate autosizes to the design flow rate for the system. If you want to size components on an air loop to a specific system flow rate, use these Sizing:System object inputs.

Sizing:System,
VAV Sys 1,               !- AirLoop Name

Flow/System,             !- Cooling Supply Air Flow Rate Method
145.2,                   !- Cooling Supply Air Flow Rate {m3/s}
,                        !- Cooling Supply Air Flow Rate Per Floor Area {m3/s-m2}
,                        !- Cooling Fraction of Autosized Cooling Supply Air Flow Rate
,                        !- Cooling Supply Air Flow Rate Per Unit Cooling Capacity {m3/s-W}
Flow/System,             !- Heating Supply Air Flow Rate Method
145.2,                   !- Heating Supply Air Flow Rate {m3/s}
,                        !- Heating Supply Air Flow Rate Per Floor Area {m3/s-m2}
,                        !- Heating Fraction of Autosized Heating Supply Air Flow Rate
,                        !- Heating Fraction of Autosized Cooling Supply Air Flow Rate
,                        !- Heating Supply Air Flow Rate Per Unit Heating Capacity {m3/s-W}

Note that this flow rate will also be used to scale any autosized air terminal flow rates.