DX Coil Rated Inlet Air Humidity Ratio value is not consistent

EnergyArchmage commented 8 years ago

Issue overview

DX cooling coil performance curve input definitions are based on coil inlet wet bulb of 67F/19.44C wetbulb. Inside the code, the DX model uses RatedInletAirHumRat instead of RatedInletWetBulbTemp. However from inspection it does not appear that a value of w = 0.01125 is consistent with the two temperatures of drybulb = 26.6667C and wetbulb = 19.44C

Real64 const RatedInletAirTemp( 26.6667 ); // 26.6667C or 80F
Real64 const RatedInletWetBulbTemp( 19.44 ); // 19.44 or 67F, cooling mode
Real64 const RatedInletAirHumRat( 0.01125 ); // Humidity ratio corresponding to 80F dry bulb/67F wet bulb

The result from the following is w = 0.011178906890772281 kg-water/kg-dryair

    PsyWFnTdbTwbPb( RatedInletAirTemp, RatedInletWetBulbTemp, DataEnvironment::StdPressureSeaLevel, RoutineName );

The result from the following is inlet wetbulb = 19.494277671676496C (67.0897F)

   Psychrometrics::PsyTwbFnTdbWPb( RatedInletAirTemp, RatedInletAirHumRat, DataEnvironment::StdPressureSeaLevel );

It isn't clear what elevation (pressure) is defined for the rating point.

For the code to be consistent with the documentation, it appears that RatedInletAirHumRat should be set to 0.0111789. This occurs in several different places in the code and unit tests.

The impact is that program will not really hit 1.0 on coil performance modification curves at the rating point. The independent wetbulb variable is silently shifted by 0.089F when applying the curves, which could cause confusion and small errors.

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rraustad commented 7 years ago

From ANSI/AHRI Standard 210/240 2008 the guidance for conversion from volume to mass is: 3.13 Standard Air. Air weighing 0.075 lb/ft3 [1.2 kg/m3] which approximates dry air at 70°F [21°C] and at a barometric pressure of 29.92 in Hg [101.3 kPa].

At 29.92 pressure, 80/67 equates to 0.01122 lb h2o/ lb da and specific humidity 0.0111 lb h2o /lb where specific volume is 13.75 here and 0.0111705 where specific volume is 13.84 here

In my mind there is some confusion between rating and operating conditions and use of dry air versus moist air properties.

Nigusse commented 7 years ago

I think for consistency let the function PsyWFnTdbTwbPb initialize the "RatedInletAirHumRat" using the "RatedInletAirTemp", "RatedInletWetBulbTemp", and "StdPressureSeaLevel" any where in the code. This way we have a state point defined based on three variable values that match.

NREL / EnergyPlus