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Sizing Fluid to Fluid Heat Exchanger. Energyplus results interpretation.

asked 2016-08-03 12:30:11 -0500

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Hi! I need help to interpret the results that I am getting of a model I´m doing in openstudio. This is a chilled water secondary plant loop diagram. The supply side is a fluid to fluid heat exchanger and the demand side are the different cooling coils located in a multi-story office building. The primary loop is a typical chilled water primary loop with chillers and pumps (not shown in diagram). The plant sizing result for the primary chiller water loop is 1,366 tons, which is an expected result for me. The plant sizing for the secondary chilled water loop coincides with the primary in terms of design water flows for both supply and demand side (both have the same design temperature difference). But, for the heat exchanger, the sizing results are different: 1,590 tons if I choose an ideal heat exchanger type (16.3% over the primary plant size); and 735 tons if I choose a counterflow heat exchanger type (53.8% of the primary plant size). I have been reading energyplus engineering reference and Input/Ouput manuals to see if I find an explanation about why I am getting these results but so far I have not. Any help will be appreciated. Thank you!

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answered 2016-08-03 14:26:18 -0500

The problem could be related to this documented defect in Fluid to Fluid Heat Exchanger capacity reporting.

What do you think, @Archmage?

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Yes, I agree. The nominal capacity in watts that is reported for the HX seems to be in error. But this is just a reporting issue and doesn't affect the modeling. The idea was to put out a number in Watts that would be more convenient that a UA value for describing how big the HX is, but since the Watts number is wrong it confuses more than helps.

Archmage gravatar imageArchmage ( 2016-08-04 14:58:31 -0500 )edit

Understood. Then the report sizing for the HX in ideal mode is correct. I will keep using that setting until the issue is resolved. Final sizing in ideal mode then has to be adjusted based on HX effectiveness.

Carlos Vazquez gravatar imageCarlos Vazquez ( 2016-08-04 22:10:16 -0500 )edit

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Asked: 2016-08-03 12:30:11 -0500

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Last updated: Aug 03 '16