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Equipment sizing errors

asked 2018-01-04 16:49:43 -0500

Yael gravatar image

updated 2018-01-05 08:23:55 -0500

I have an HVAC system consisting of 2 parallel air-cooled chillers, each with a designated pump. For each of the pumps I assigned flow rate of 15 kg/s . On each of the chillers I specified Reference Chilled Water Flow Rate of 15 kg/s and Condenser Fluid Rate of 15.234 m3/h. Also, on the water cooling coil I put 15 kg/s on the Design Water Flow rate and Design Air Flow Rate of 4.342 m3/h.

However, I receive the following errors:

Water cooling coil:

* ~~~ * User-Specified Design Air Flow Rate [m3/s] = 4.34200 * ~~~ * differs from Design Size Design Air Flow Rate [m3/s] = 4.87859 User-Specified Design Water Flow Rate [m3/s] = 1.50000E-002 * ~~~ * differs from Design Size Design Water Flow Rate [m3/s] = 1.02900E-002 * ~~~ * This may, or may not, indicate mismatched component sizes.

For both chillers:

*** SizeChillerElectricEIR: Potential issue with equipment sizing for CHILLER - AIR COOLED 1 * ~~~ * User-Specified Reference Chilled Water Flow Rate of 1.50000E-002 [m3/s] * ~~~ * differs from Design Size Reference Chilled Water Flow Rate of 3.00000E-002 [m3/s] * ~~~ * This may, or may not, indicate mismatched component sizes. * ~~~ * Verify that the value entered is intended and is consistent with other components. *** SizeElectricEIRChiller: Potential issue with equipment sizing for Chiller:Electric:EIR CHILLER - AIR COOLED 1 * ~~~ * User-Specified Reference Condenser Fluid Flow Rate [m3/s] = 15.23400 * ~~~ * differs from Design Size Reference Condenser Fluid Flow Rate [m3/s] = 33.19680 * ~~~ * This may, or may not, indicate mismatched component sizes.

The question is, what am I doing wrong here and how can I fix these warnings? And where did it get the idea that the Chillers' Design Size Reference Condenser Fluid Flow Rate should be 33.19680 [m3/s]? Or that on the water cooling coil the Design Size Design Water Flow Rate should be 1.02900E-002 [m3/s]?

Thank you!

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answered 2018-01-05 08:02:56 -0500

You did not do anything wrong. What these messages are telling you is that you specified sizing information (i.e,. those inputs that can be autosized by the program) that is different from what the simulation calculated.

You must have included sizing objects in your input file, for example Sizing:Zone, Sizing:System and Sizing:Plant.

You also must have told the simulation to execute a sizing run in the SimulationControl object:

SimulationControl,
  Yes,                     !- Do Zone Sizing Calculation
  Yes,                     !- Do System Sizing Calculation
  Yes,                     !- Do Plant Sizing Calculation
  Yes,                     !- Run Simulation for Sizing Periods
  Yes;                     !- Run Simulation for Weather File Run Periods

When the input file is set up this way, these messages will show up when you hard-size equipment so that you can see if your inputs match, or closely match, the simulation results. It's OK to provide your own data, but at times the information may not be accurate with respect to the simulation (e.g., lighting/equipment loads are off from the actual building, infiltration/ventilation is different from the actual building, etc.). Given this information you can investigate why these data are different.

You can turn these messages off by setting to No the Do xxxx Sizing Calculation input in the SimulationControl object.

Regarding the condenser water flow rate difference, the Engineering Reference shows the calculation for Design Condenser Volume Flow Rate (V8.8, section 10.7.17.3, pg 539 - or search for "Design Condenser Volume Flow Rate"). You can review the Eng. Ref. for any of the objects in E+ to see how those models calculate sizing data.

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Thank you! You've been very helpful.

Yael gravatar image Yael  ( 2018-01-07 00:49:34 -0500 )edit

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Asked: 2018-01-04 16:49:43 -0500

Seen: 137 times

Last updated: Jan 05 '18