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Chiller Oversizing and Chilled Beams

asked 2018-01-24 18:08:06 -0500

JCR562 gravatar image

updated 2018-04-08 08:57:38 -0500

I'm modeling a large office building that will be using chilled beams. There will be two water-cooled chillers equally sized. I've created a baseline model per App G System 8 (VAV with PFP boxes) and each chiller comes out at 350 tons.

I then used that model to create the building model using chilled beams. Instead of a dedicated chiller supplying 58°F CHW for the chilled beams and a dedicated chiller supplying 42°F CHW for the OA units, we're only producing a CHWT of 42°F. Then we'll use a heat exchanger to create a tempered CHW loop at 58°F for the chilled beams. When I run the model, my chillers become grossly oversized at 734 tons each. When I look at my total coil load (OA units plus chilled beam) the design load is only 679 tons so I'm not sure why the chillers are being oversized so much. My oversize factor is set at 15% (per App. G) and all the coils & chillers are set to autosize.

Has anyone run into this issue? Much thanks in advance for any input.

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Please add the software you are using as a tag.

__AmirRoth__ gravatar image__AmirRoth__ ( 2018-01-24 18:24:40 -0500 )edit

Sorry about that Amir. Added. I'm using OpenStudio v2.4.0.

JCR562 gravatar imageJCR562 ( 2018-01-24 18:27:38 -0500 )edit

3 Answers

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answered 2018-01-26 03:34:02 -0500

updated 2018-01-26 03:35:05 -0500

For sizing, like @Lyle K justly pointed out: each component on the loop is sizing to meet the full loop load multiplied by the component-level sizing factor. See for example for a Chiller:Electric:EIR

Sizing Factor allows the user to size a component to meet part of the design load while continuing to use the autosizing feature.

The full loop load is calculated by what's on the demand side of the plant loop, so in general that's driven by the zone cooling loads or airflow (for a coil placed on an AirLoopHVAC). Both are affected by the Sizing:Parameters Cooling Sizing Factor.

So what happens for a chiller really amounts to

Full loop load * chiller Sizing Factor  * global cooling sizing factor

So if you want two equally sized chillers, and a global cooling oversizing of 15% you can:

  • Create two Chillers, each with a Sizing Factor of 0.5
  • Assign 1.15 Cooling Sizing Factor on the Sizing:Parameters object

You can see that this is what openstudio-standards does for eg around here

(Note that you could also do sizing parameters of 1, and 0.575 sizing factor for each chiller... depends on what behavior you want for other objects... for appendix G, do what's above)

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answered 2018-01-27 17:27:38 -0500

JCR562 gravatar image

I appreciate everyone's help. Hard sizing the chillers worked.

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This is not answer, you should use a "add a comment", perhaps on the initial question since you're thanking multiple people, then delete this non-answer. Thank you!

Julien Marrec gravatar imageJulien Marrec ( 2018-01-29 03:02:11 -0500 )edit

answered 2018-01-25 14:03:58 -0500

In EnergyPlus, all chillers on a loop are sized to meet the full loop load.

You can define a PlantEquipmentOperation object to partition the load on your chillers, though I don't think EnergyPlus considers that during sizing. Someone can correct me on this.

You'll need to hardsize the chiller capacity based on your initial run.

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Chillers are sized to meet the full loop load multiplied by a component-level sizing factor

Lyle K gravatar imageLyle K ( 2018-01-25 15:33:13 -0500 )edit

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Asked: 2018-01-24 18:08:06 -0500

Seen: 541 times

Last updated: Jan 27 '18