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Correct VAV maximum air flow fraction during reheat for heating dominated use cases #10763

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Correct VAV maximum air flow fraction during reheat for heating dominated use cases #10763

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90f2846
Correct VAV maximum air flow fraction during reheat
rraustad Sep 24, 2024
73fcfff
Revised based on using DesHeatVolFlowMax as limit
rraustad Sep 24, 2024
d29faa1
Update 2 unit tests
rraustad Sep 24, 2024
3473759
Fix reheat coil air flow rate used for coil sizing
rraustad Oct 23, 2024
1129a59
Merge branch 'develop' of https://github.com/NREL/EnergyPlus into 107…
rraustad Oct 23, 2024
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12 changes: 9 additions & 3 deletions src/EnergyPlus/SingleDuct.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -2877,8 +2877,10 @@ void SingleDuctAirTerminal::SizeSys(EnergyPlusData &state)

CheckZoneSizing(state, this->sysType, this->SysName);

MaxAirVolFlowRateDes = max(state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesCoolVolFlow,
state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow);
Real64 heatingMaxFlow = (this->DamperHeatingAction == Action::ReverseWithLimits)
? state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax
: state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow;
MaxAirVolFlowRateDes = max(state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesCoolVolFlow, heatingMaxFlow);
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If ReverseWithLimits uses a different heating air flow rate from the zone design heating air flow rate then that flow rate should be used as the value for the VAV box heating air flow rate.

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@rraustad rraustad Sep 24, 2024 •
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Well, I was expecting DesHeatVolFlowMax to always be less than DesHeatVolFlow. For 5ZoneBoilerOutsideAirReset that's not the case. The diffs show that Zone 5 TU max air flow rate increased, I was only expecting decreases in max air flow rate for heating dominated zones. Zone cooling design air flow is greater than zone heating design air flow, but DesHeatVolFlowMax is larger than both of these.

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// set the zone maximum heating supply air flow rate. This will be used for autosizing VAV terminal unit
// max heating flow rates
zsFinalSizing.DesHeatVolFlowMax = max(zsFinalSizing.DesHeatMaxAirFlow,
                                      zsFinalSizing.DesHeatMaxAirFlow2,
                                      max(zsFinalSizing.DesCoolVolFlow, zsFinalSizing.DesHeatVolFlow) * zsFinalSizing.DesHeatMaxAirFlowFrac);

zsFinalSizing.DesHeatMaxAirFlow2 = zsFinalSizing.DesHeatMaxAirFlowPerArea * floorArea * zoneMult;
                                 = 0.002032 * 182.49 * 1 = 0.37082

N8 , \field Maximum Flow Fraction During Reheat
     \type real
     \autosizable
     \default autosize
     \note Used only when Reheat Coil Object Type = Coil:Heating:Water and Damper Heating Action = ReverseWithLimits
     \note When autocalculating, the maximum flow fraction is set to the ratio of
     \note 0.002032 m3/s-m2 (0.4 cfm/sqft) multiplied by the zone floor area and the
     \note Maximum Air Flow Rate.
     \note This optional field limits the maximum flow allowed in reheat mode.
     \note At no time will the maximum flow rate calculated here exceed the value of
     \note Maximum Air Flow Rate.

So is DesHeatVolFlowMax just an upper limit? or is it the expected heating air flow rate?

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@rraustad rraustad Sep 24, 2024 •
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So, since DesHeatVolFlowMax looks like a limit and not a flow rate, maybe I should change this line and line 3224 to?:

Real64 heatingMaxFlow = (DesHeatVolFlowMax > DesHeatVolFlow) ? state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow :
    state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax);

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Based on this discussion I changed to treat DesHeatVolFlowMax as an upper limit. This should still correct the issue and result in fewer diffs.


if (MaxAirVolFlowRateDes < SmallAirVolFlow) {
MaxAirVolFlowRateDes = 0.0;
Expand Down Expand Up @@ -2927,7 +2929,11 @@ void SingleDuctAirTerminal::SizeSys(EnergyPlusData &state)
}
} else {
CheckZoneSizing(state, this->sysType, this->SysName);
MaxHeatAirVolFlowRateDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow;
if (this->DamperHeatingAction == Action::ReverseWithLimits) {
MaxHeatAirVolFlowRateDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax;
} else {
MaxHeatAirVolFlowRateDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow;
}
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Below at line 3224, ReverseWithLimits uses DesHeatVolFlowMax. That value should also be used here.

3224: MaxAirVolFlowRateDuringReheatDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax;

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@rraustad rraustad Sep 25, 2024 •
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As discussed above, these 2 areas of code, above (2937) and below (3231), were changed to use DesHeatVolFlowMax as a limit.

if (MaxHeatAirVolFlowRateDes < SmallAirVolFlow) {
MaxHeatAirVolFlowRateDes = 0.0;
}
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39 changes: 34 additions & 5 deletions tst/EnergyPlus/unit/SingleDuct.unit.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -2737,23 +2737,52 @@ TEST_F(EnergyPlusFixture, VAVReheatTerminal_SizeMinFrac)
EXPECT_TRUE(compare_err_stream(""));

int SysNum = 1;
auto &thisSys = state->dataSingleDuct->sd_airterminal(SysNum);

// First test - design min flow < max flow
state->dataSize->ZoneSizingRunDone = true;
state->dataSize->CurZoneEqNum = 1;
state->dataSize->CurTermUnitSizingNum = 1;
state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin = 0.5;
state->dataSingleDuct->sd_airterminal(SysNum).SizeSys(*state);
EXPECT_EQ(0.5, state->dataSingleDuct->sd_airterminal(SysNum).ZoneMinAirFracDes);
thisSys.SizeSys(*state);
EXPECT_EQ(0.5, thisSys.ZoneMinAirFracDes);

// Second test - design min flow > max flow
state->dataSize->ZoneSizingRunDone = true;
state->dataSize->CurZoneEqNum = 1;
state->dataSize->CurTermUnitSizingNum = 1;
state->dataSingleDuct->sd_airterminal(SysNum).ZoneMinAirFracDes = AutoSize; // need to reset this so it sizes again
thisSys.ZoneMinAirFracDes = AutoSize; // need to reset this so it sizes again
state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin = 1.5;
state->dataSingleDuct->sd_airterminal(SysNum).SizeSys(*state);
EXPECT_EQ(1.0, state->dataSingleDuct->sd_airterminal(SysNum).ZoneMinAirFracDes);
thisSys.SizeSys(*state);
EXPECT_EQ(1.0, thisSys.ZoneMinAirFracDes);

// test Maximum Flow Fraction During Reheat for heating dominated
thisSys.MaxAirVolFlowRate = DataSizing::AutoSize;
thisSys.ZoneMinAirFracDes = DataSizing::AutoSize;
thisSys.MaxAirVolFlowRateDuringReheat = DataSizing::AutoSize;
thisSys.MaxAirVolFractionDuringReheat = DataSizing::AutoSize;
state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlow = 1.7;
state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax = 1.6;
thisSys.SizeSys(*state);
Real64 expectedZoneMinAirFracDes = std::min(1.0, state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin / thisSys.MaxAirVolFlowRate);
Real64 expectedMaxAirVolFractionDuringReheat = state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax / thisSys.MaxAirVolFlowRate;
EXPECT_EQ(expectedZoneMinAirFracDes, thisSys.ZoneMinAirFracDes);
EXPECT_EQ(1.0, thisSys.MaxAirVolFractionDuringReheat);
EXPECT_EQ(expectedMaxAirVolFractionDuringReheat, thisSys.MaxAirVolFractionDuringReheat);

// test Maximum Flow Fraction During Reheat for cooling dominated
thisSys.MaxAirVolFlowRate = DataSizing::AutoSize;
thisSys.ZoneMinAirFracDes = DataSizing::AutoSize;
thisSys.MaxAirVolFlowRateDuringReheat = DataSizing::AutoSize;
thisSys.MaxAirVolFractionDuringReheat = DataSizing::AutoSize;
state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlow = 1.4;
state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax = 1.3;
thisSys.SizeSys(*state);
expectedZoneMinAirFracDes = std::min(1.0, state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin / thisSys.MaxAirVolFlowRate);
expectedMaxAirVolFractionDuringReheat = state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax / thisSys.MaxAirVolFlowRate;
EXPECT_EQ(expectedZoneMinAirFracDes, thisSys.ZoneMinAirFracDes);
EXPECT_EQ(1.0, thisSys.MaxAirVolFractionDuringReheat);
EXPECT_EQ(expectedMaxAirVolFractionDuringReheat, thisSys.MaxAirVolFractionDuringReheat);
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Added 3 new tests for maximum air flow fraction during reheat to this unit test.

}

TEST_F(EnergyPlusFixture, setATMixerSizingProperties_Test)
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