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# What is the difference between standard density flow rate and current density flow rate?

In DesignBuilder HVAC system node oututs, I came across these two flow rates but I don't know the difference. Would you please help me understand the difference?

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The node volume flow rate can be viewed as mass flow rate using standard density (20 C and 0.0 humidity ratio) or actual density (using actual temp and humidity ratio). Each of these evaluated at the current barometric pressure (weather file altitude).

state.dataEnvrn->StdBaroPress = DataEnvironment::StdPressureSeaLevel * std::pow(1.0 - 2.25577e-05 * state.dataEnvrn->Elevation, 5.2559);
state.dataEnvrn->StdRhoAir = Psychrometrics::PsyRhoAirFnPbTdbW(
state, state.dataEnvrn->StdBaroPress, DataPrecisionGlobals::constant_twenty, DataPrecisionGlobals::constant_zero);


or

this->stdRhoAir = Psychrometrics::PsyRhoAirFnPbTdbW(state, barometricPressure, 20.0, 0.0);


versus actual air density calculated as:

Real64 rhoair = Psychrometrics::PsyRhoAirFnPbTdbW(state, state.dataEnvrn->StdBaroPress, ActualTemp, ActualHumRat);


There are several component models in EnergyPlus that convert volumetric air flow rate to mass flow rate using actual air density (e.g., DX coils) and others that convert volume air flow rate to mass using standard air density. Depending on which model is used, these report variables are available when post processing calculations are desired. If post processing is not desired, then these report variables are not all that useful except to see the component flow rate throughout the simulation and that the flow rate is what is expected.

Output:Variable,*,System Node Mass Flow Rate, hourly; !- HVAC Average [kg/s]
Output:Variable,*,System Node Standard Density Volume Flow Rate, hourly; !- HVAC Average [m3/s]
Output:Variable,*,System Node Current Density Volume Flow Rate, hourly; !- HVAC Average [m3/s]
Output:Variable,*,System Node Current Density, hourly; !- HVAC Average [kg/m3]

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@rraustad Is the standard density reported as an Output:Variable? This would be handy when developing a Python Plugin that interacts with HVAC flows. Alternatively, users could calculate the standard density using the reported barometric pressure and a Psychrometrics function call. Another thing I've noticed is that when polling the "System Node Current Density" from a Python plugin, it always reports zero (0.0). I've tried this from several calling points, and for several keys. This may be an issue on my end, or a bug in the Python API. Other variables are working fine.

( 2021-09-08 10:38:42 -0600 )edit

Yes, both of the density reports are available using Output:Variable. Regarding the Py results, that would have to be investigated, either by you or the E+ team preferably using your input file. Let me know if you can't get that working.

( 2021-09-08 11:51:09 -0600 )edit

After some more investigation, it appears the density for a node is 0.0 during simulation warmup, but is then correctly reported to python during normal runtime. I didn't catch this before because the logged Output:Variable always showed a value, but doesn't log the warmup data. Is this behavior expected / normal?

( 2021-09-09 08:39:41 -0600 )edit

I am checking with the author.

( 2021-09-10 17:10:45 -0600 )edit

Response: There is a function in Python called api_data_fully_ready that lets users check if the simulation is passed kickoff...when that is false the user should simply return from the python function without doing anything. Once that turns true then the simulation is "ready". This is how we have most (all?) of our PyEMS examples set up.

( 2021-09-13 06:44:58 -0600 )edit

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