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How to model process cooling with ASHRAE 90.1 App G Baseline system 5 or 6

asked 2017-05-15 19:08:16 -0500

edixon16 gravatar image

updated 2017-05-16 11:18:46 -0500

I have a LEED project that I am evaluating using the Performance Rating Method in Appendix G of ASHRAE 90.1. This building meets the space, height, and primary heating criteria for the baseline model to have HVAC system 5, Packaged DX VAV w/HW reheat. As such the cooling is done via DX cooling coils and there is no CHW loop. However, there is a process cooling load for some laboratory equipment within the building design that requires CHW for cooling. How do I accomplish this with the baseline model? I see two options.

  1. Use HVAC system 7 instead of HVAC system 5 and follow appendix G accordingly.
  2. Keep using HVAC system 5 for the space conditioning and model a chw loop only for the process load using a minimally compliant water cooled chiller and pumping power according to appendix G.

I'm leaning towards option 1. If anyone has experience with this issue in their LEED submission I would be grateful to hear from you.

Edit: The proposed case hvac system uses CHW coils for space cooling and the process cooling is served by the same cooling plant consisting of air cooled chillers and dry coolers for free cooling in the colder months.

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answered 2017-05-15 21:09:40 -0500

Anna gravatar image


It is a bit unclear how the space cooling is provided in the actual design (proposed model)? I assume that the space cooling is provided by the VAV systems with chilled water coils, and the same chiller plant is used for both space cooling and process cooling. Right?

Actually, I think Option 2) would be more appropriate. The Baseline HVAC system type should be consistent with Appendix G ( DX) and the process cooling should be modeled the same between the Baseline and Proposed models. You can simply run the Proposed model without process cooling and with the process cooling to calculate the energy difference, and charge the Baseline model with that value.

If you want to take some credit for the process cooling , then you would need to do exceptional calculations (ECM).

Have you checked these LIs?

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I am going to second @Anna 's approach that Option 2 is more appropriate. Only modification I'd make is that the process chiller EER/COP should match the Proposed chiller's efficiency. This would help make the process loads equal between the baseline and proposed. May have to look at loading curves as well...

Only other element might be if there is a DES. Nothing indicates that in the question, but in many cases our models with Labs are on a DES/Campus system.

dradair gravatar imagedradair ( 2017-05-16 09:07:34 -0500 )edit

Agreed. Modeling the same rated cooling efficiency with an auto-sized chiller capacity in the Baseline model should be good enough in most cases. In some cases, when the cooling load dynamic is much different between the Baseline and Proposed models, I would calculate the average cooling efficiency in the Proposed model and apply that to the cooling process load in the Baseline model. This may not be necessary though.

Anna gravatar imageAnna ( 2017-05-16 09:44:11 -0500 )edit

Yes, the proposed case space cooling is met with a system with CHW coils and the process cooling is served by the same cooling plant. No DES.

Following the logic from the comments above in keeping the efficiency of the chiller the same as in the proposed case, would I also want to use the same W/gpm as the proposed model?

Also, the project is located in a cold climate and will use dry coolers in the colder months for free cooling. Would I be justified in excluding the free cooling from the baseline cooling plant when modeling either explicitly or using an average cooling efficiency?

edixon16 gravatar imageedixon16 ( 2017-05-16 11:08:01 -0500 )edit

That's a tough one. For the pumps I would match the kW/GPM of the proposed, in the effort to keep them as close as possible. For the dry cooler, I think you have two options. The first would be to model the same CHW configuration as the proposed, Chiller & dry cooler, at the same efficiencies. This makes it the easier to directly compare the two, though with chiller curves & loading the two plants will (most likely) run differently. The more difficult path would be to determine the average Proposed chiller efficiency and model that chiller in the Baseline. (cont)

dradair gravatar imagedradair ( 2017-05-16 12:11:44 -0500 )edit

Then implement a 'flat' performance curve. Similar to the DES Option 1 in LEEDv2009 OR DES Option 1 Path 3 in LEED v4. (Streamlined, averaged efficiency DES plant simulation). This method would use the Proposed chiller plant average load efficiency, ignoring how the comfort vs process cooling demand might load the plant differently. The overall, average efficiency for the plant. The down fall of this is that it makes the baseline & proposed different (and moderately difficult to explain). Personally...I'd take the first approach and match the efficiencies of chiller & drycooler...and run...

dradair gravatar imagedradair ( 2017-05-16 12:19:11 -0500 )edit

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Asked: 2017-05-15 19:08:16 -0500

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Last updated: May 16