Air-cooled chiller sizing can cause RH > 100 warnings. - #11768
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rraustad
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rraustad
changed the title
rraustad
commented
Aug 28, 2026
| if (thisChiller.CondenserType == DataPlant::CondenserType::AirCooled || | ||
| thisChiller.CondenserType == DataPlant::CondenserType::EvapCooled) { | ||
| thisChiller.CondVolFlowRate = 0.0011; // set to avoid errors in calc routine | ||
| } |
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Given the issue, not a good choice. HindSight.
rraustad
commented
Aug 28, 2026
| // sizing of condenser flow for air/evap-cooled is not reported but the condenser side deltaT is calculated, so at least make it realistic | ||
| if (this->CondVolFlowRate == DataSizing::AutoSize && state.dataPlnt->PlantFinalSizesOkayToReport) { | ||
| this->CondVolFlowRate = this->NomCap * 0.000114; // m3/s/w (850 cfm/ton) | ||
| } |
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Size non-water-cooled condenser air flow to a reasonable value. This info is not reported but is used in a calculation and can trip RH > 100 warnings. So just give the engine a decent (rule-of-thumb) value for condenser air flow rate.
this->CondVolFlowRate = this->NomCap * 0.000114; // m3/s/w (850 cfm/ton)
this->CondMassFlowRateMax = rho * this->CondVolFlowRate;
Real64 CpCond = Psychrometrics::PsyCpAirFnW(state.dataLoopNodes->Node(this->CondInletNodeNum).HumRat);
this->CondOutletTemp = condInletTemp + this->QCondenser / this->CondMassFlowRate / CpCond;
Collaborator
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Example file is clean (of this issue) now: |
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Collaborator
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The 3 example files with new warnings for the air-cooled chiller should be fixed in this branch. |
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Collaborator
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The diffs in eso for SmallOffice_CentralDOAS made me look at air-cooled node data for the condenser side. It was not working at all. This last change seems to fix it. The condenser flow and conditions are not used for anything but the node info can still be reported, so why not just report the information correctly. 
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…d or blank condenser flow for heat recovery, size zero flow using the 850 cfm/ton estimate, update IDD, and add tests
mitchute
reviewed
Sep 9, 2026
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| if (thisChiller.CondVolFlowRate <= 0.0 && thisChiller.CondVolFlowRate != DataSizing::AutoSize && | ||
| !state.dataIPShortCut->lNumericFieldBlanks(10)) { |
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Added a couple of touch-ups here. This is because the IDD labels this field as \minimum 0.0, but it should probably be \minimum> 0.0
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mitchute
approved these changes
Sep 9, 2026
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| } else { | ||
| // sizing of condenser flow for air/evap-cooled is not reported but the condenser side deltaT is calculated, so at least make it realistic | ||
| if (state.dataPlnt->PlantFinalSizesOkayToReport) { | ||
| Real64 const desAirVolFlowRate = this->NomCap * 0.000114; // m3/s/w (850 cfm/ton) | ||
| if (this->CondVolFlowRate == DataSizing::AutoSize || this->CondVolFlowRate == 0.0) { | ||
| this->CondVolFlowRate = desAirVolFlowRate; | ||
| BaseSizer::reportSizerOutput( | ||
| state, "Chiller:Electric", this->Name, "Design Size Design Condenser Fluid Flow Rate [m3/s]", this->CondVolFlowRate); | ||
| } else if (std::abs((this->CondVolFlowRate - desAirVolFlowRate) / desAirVolFlowRate) > state.dataSize->AutoVsHardSizingThreshold) { | ||
| ShowWarningError(state, std::format("User-specified Design Condenser Fluid Flow Rate = {:.5f} [m3/s]", this->CondVolFlowRate)); | ||
| ShowContinueError(state, | ||
| std::format("differs from design size design condenser fluid flow rate = {:.5f} [m3/s]", desAirVolFlowRate)); | ||
| ShowContinueError(state, "This may, or may not, indicate mismatched component sizes."); | ||
| ShowContinueError(state, "Verify that the value entered is intended and is consistent with other components."); | ||
| ShowContinueError(state, std::format("Occurs in Electric Chiller object={}", this->Name)); | ||
| BaseSizer::reportSizerOutput( | ||
| state, "Chiller:Electric", this->Name, "Design Size Design Condenser Fluid Flow Rate [m3/s]", desAirVolFlowRate); | ||
| BaseSizer::reportSizerOutput( | ||
| state, "Chiller:Electric", this->Name, "User-Specified Design Condenser Fluid Flow Rate [m3/s]", this->CondVolFlowRate); | ||
| } | ||
| } |
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| state.dataLoopNodes->Node(this->CondInletNodeNum).MassFlowRate = this->CondMassFlowRate; | ||
| state.dataLoopNodes->Node(this->CondOutletNodeNum).MassFlowRate = this->CondMassFlowRate; | ||
| } |
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Looks good. The water-cooled nodes are set through PlantUtilities::SetComponentFlowRate.
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| } else { | ||
| // sizing of condenser flow for air/evap-cooled is not reported but the condenser side deltaT is calculated, so at least make it realistic | ||
| if (this->CondVolFlowRate == DataSizing::AutoSize && state.dataPlnt->PlantFinalSizesOkayToReport) { | ||
| this->CondVolFlowRate = this->NomCap * 0.000114; // m3/s/w (850 cfm/ton) | ||
| } | ||
| } |
Collaborator
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Thanks @rraustad. Merging. |
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Pull request overview
Description of the purpose of this PR
Plant chillers calculates a condenser fluid outlet temperature. If the condenser fluid (air) flow rate is not accurately represented then the outlet temperature is not correct. The air-cooled condenser water and outlet air temp are not reported but do set a NODE condition and therefore should be calculated in good faith.
Correction changes: this->CondMassFlowRate to a more reasonable value
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