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100% unmet hours in chilled water system

asked 2017-12-11 13:09:26 -0500

Yael gravatar image

updated 2017-12-12 03:23:13 -0500

I have an HVAC system consisting of 2 electric chillers in a college. For some reason, my reported unmet hours are 100% although the system is operating (chillers producing 100 kW cooling load).

I've put Setpoint Manager: Warmest on the demand side for Supply Air Temperature, and also set schedules for cooling thermostat of 20°C on the Thermal Zone tab. Checking the outputs I see some zones temperatures are rising as high as 30°C.

What can I be missing here?

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@Yael Could you post a link to your model?

Avi gravatar imageAvi ( 2017-12-12 03:00:51 -0500 )edit

You two chillers are perhaps just undersized. Try increasing the capacity ten folds, see if you still have the same problem.

Julien Marrec gravatar imageJulien Marrec ( 2017-12-12 03:24:05 -0500 )edit

@Avi How can I post a link of my model? I have two chillers that are big enough,however not all their capacity is used (on all operation modes). I suspect it may have something to do with my part-load ratio characteristics: I'm using 2 chillers, each of them contains 2 circuits with 4 scroll compressors (2 compressors aligned on each circuit). I have 4 steps of capacity: 25%, 50%, 75% and 100%. The way I defined the ratios: minimum PLR 0.1, max PLR 1, optimum PLR 0.25, minimum unloading ratio 0.25. Can it be that not all the compressors are operated because I defined something incorrectly?

Yael gravatar imageYael ( 2017-12-12 06:10:37 -0500 )edit

@Yael you could upload it to some cloud service (Dropbox, google drive etc.) and share the link.

Avi gravatar imageAvi ( 2017-12-12 06:25:48 -0500 )edit

Maybe start by exposing the plant output variables and examine how the plant loop performs

Avi gravatar imageAvi ( 2017-12-12 06:27:07 -0500 )edit

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answered 2017-12-13 01:06:23 -0500

Yael gravatar image

The problem was solved, first by changing the type of chilled beams from passive to active. In addition I put setpoint managers: single zone cooling to each zone and that solved the problem completely.

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Asked: 2017-12-11 13:09:26 -0500

Seen: 72 times

Last updated: Dec 13 '17