Fix #6050 - Return Fan:VariableVolume Fan Power Minimum Flow Fraction should not impose air loop flow #8401
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jmarrec
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Defect
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jmarrec
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Dec 1, 2020
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| // Test for #6050 - Return Fan 'Fan Power Minimum Flow Fraction' enforces loop flow, while it should not | ||
| // I am simulating a very simple shoebox building with 1 zone. | ||
| // There is an airLoopHVAC with a Return Fan:VariableVolume, with a 'Fan Power Minimum Flow Fraction' = 0.3 | ||
| // The Supply fan has a fraction of zero | ||
| // | ||
| // The supply side of the loop is has follows: only has a Return Fan, a Mixing box, and a SupplyFan | ||
| // o o | ||
| // | | | ||
| // --- (Return Fan) ---- o-/-o ---- (SupplyFan) ---- | ||
| // | | | ||
| // o o | ||
| // | ||
| // The Demand side has only one ATU:VAV:NoReheat with a Minimum flow Fraction of 0 so that it doesn't drive the airlfow either |
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Gist of the unit test./ This is basically exactly the MCVE I created on EnergyPlusDevSupport.
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| // NOTE: Terminal has a min flow of 0 | ||
| "AirTerminal:SingleDuct:VAV:NoReheat,", | ||
| " VAV No Rht, !- Name", | ||
| " Always On Discrete, !- Availability Schedule Name", | ||
| " Zone1 ATU Inlet Node, !- Air Outlet Node Name", | ||
| " ATU VAV No Reheat Inlet Node, !- Air Inlet Node Name", | ||
| " 1.0, !- Maximum Air Flow Rate {m3/s}", | ||
| " Constant, !- Zone Minimum Air Flow Input Method", | ||
| " 0; !- Constant Minimum Air Flow Fraction", // IMPORTANT |
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Important to set the ATU Min Air Flow Fraction to zero, so that it's not the one driving the flow (I got bit by this initially)
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| // NOTE: The supply has as a min flow fraction of of 0.3 | ||
| "Fan:VariableVolume,", | ||
| " VSD Return Fan, !- Name", | ||
| " Always On Discrete, !- Availability Schedule Name", | ||
| " 0.6045, !- Fan Total Efficiency", | ||
| " 1017.592, !- Pressure Rise {Pa}", | ||
| " 1.0, !- Maximum Flow Rate {m3/s}", | ||
| " Fraction, !- Fan Power Minimum Flow Rate Input Method", // IMPORTANT | ||
| " 0.3, !- Fan Power Minimum Flow Fraction", // IMPORTANT |
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Return Fan with a Fan Power Minimum Flow Fraction of 0.3
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| EXPECT_EQ(0, DataLoopNode::Node(returnFanNode).MassFlowRateMin); | ||
| EXPECT_EQ(0, DataLoopNode::Node(supplyOutletNode).MassFlowRateMin); | ||
| EXPECT_EQ(0, DataLoopNode::Node(returnFanNode).MassFlowRate); | ||
| EXPECT_EQ(0, DataLoopNode::Node(supplyOutletNode).MassFlowRate); |
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Ensures that the flow rate is indeed zero, and not corresponding to 0.3 Min flow frac
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| // According to the IO Ref guide: | ||
| // "Note that this field is only used to calculate the fan power. | ||
| // This field does not enforce the system air flow rate during simulation" | ||
| // Node(OutNode).MassFlowRateMin = Fan(FanNum).MinAirMassFlowRate; |
regression.5ZoneGeometryTransformThis is a warning about small diffs in EIO. It seems there is a very minimal change in supply airflow rates. ---
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@@ -204,7 +204,7 @@
Component Sizing Information, Coil:Cooling:DX:TwoSpeed, MAIN COOLING COIL 1, User-Specified Low Speed Gross Rated Sensible Heat Ratio, 0.80000
Component Sizing Information, Fan:VariableVolume, SUPPLY FAN 1, Design Size Maximum Flow Rate [m3/s], 2.55461
! <VAV DX Cooling Coil Standard Rating Information>, DX Coil Type, DX Coil Name, Fan Type, Fan Name, Standard Net Cooling Capacity {W}, Standard Net Cooling Capacity {Btu/h}, IEER {Btu/W-h}, COP 100% Capacity {W/W}, COP 75% Capacity {W/W}, COP 50% Capacity {W/W}, COP 25% Capacity {W/W}, EER 100% Capacity {Btu/W-h}, EER 75% Capacity {Btu/W-h}, EER 50% Capacity {Btu/W-h}, EER 25% Capacity {Btu/W-h}, Supply Air Flow 100% {kg/s}, Supply Air Flow 75% {kg/s},Supply Air Flow 50% {kg/s},Supply Air Flow 25% {kg/s}
- VAV DX Cooling Coil Standard Rating Information, Coil:Cooling:DX:TwoSpeed,MAIN COOLING COIL 1,Fan:VariableVolume,SUPPLY FAN 1,41609.72,141978.25,13.58,2.60,3.51,4.55,5.44,8.89,11.97,15.54,18.55,2.8853,2.2646,1.6075,0.7192,
+ VAV DX Cooling Coil Standard Rating Information, Coil:Cooling:DX:TwoSpeed,MAIN COOLING COIL 1,Fan:VariableVolume,SUPPLY FAN 1,41609.72,141978.25,13.58,2.60,3.51,4.55,5.43,8.89,11.97,15.54,18.54,2.8853,2.2648,1.6076,0.7197,
Component Sizing Information, Coil:Heating:Fuel, MAIN HEATING COIL 1, Design Size Nominal Capacity [W], 19315.62546
! <Environment>,Environment Name,Environment Type, Start Date, End Date, Start DayOfWeek, Duration {#days}, Source:Start DayOfWeek, Use Daylight Saving, Use Holidays, Apply Weekend Holiday Rule, Use Rain Values, Use Snow Values, Sky Temperature Model
! <Environment:Special Days>, Special Day Name, Special Day Type, Source, Start Date, Duration {#days} |
… Flow Rate) to the outlet node's MassFlowRateMin
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The UnitarySystem regression tests that use a Fan:VariableVolume are affected by this change 5Zone_Unitary_HXAssistedCoilThe Unitary System has a Fan:VariableVolume in it, with does have a non zero minimum flow fraction The Loop is serving 5
Confirming that the deviation is a result of the Fan Min Flow no longer passed to outlet nodeI have taken the file, and produced 8 cases, by varying two variables and the executable used
As can be seen in the graphs, the deviation is indeed from the changes here.
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The defect and fix are confirmed. Results look good here. Merging. |
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Pull request overview
MinAirMassFlowRateto the outlet node'sMassFlowRateMinThe original defect file is on EnergyPlusDevSupport and upgraded to v9.4. I also created a MCVE that exhibits the same behavior. Please see #6050 (comment), but for convenience the AirLoopHVAC in question is here:
https://user-images.githubusercontent.com/5479063/100435961-fcd95680-309e-11eb-991e-491f9c761438.png
Here is a graph showing three cases:
Fan Power Minimum Flow Fractionis 0.0Fan Power Minimum Flow Fractionis 0.3Fan Power Minimum Flow Fractionis 0.0You can see that the Before-Frac03 is forcing the flow.
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