variable speed loop delta T

I am modeling a simple district cooling "rig" on the supply side and several chilled water coils on the demand side of a chilled water loop (cooling). There is also a variable speed pump on the supply side, just before the chilled water coils, because the model won't run otherwise. I have set the chilled water loop temperature difference to 10 oF. In contrast to a constant speed pump, where the chilled water delta T may fluctuate wildly, I would expect the variable speed pump to ramp up or down to maintain the 10 oF pretty closely at almost all times. However, what I observe is a similarly wild fluctuation, and it is essentially never at 10 oF, but a few oF above. Any clues as to what I might be missing?

EDIT <<<

OK, so here are the screenshots for the chilled water loop:

image description

The loop is to maintain a constant chilled water supply temperature setpoint of 44 oF at all times.

As to the tags, did TomB add "openstudio" and "chilled-water-loop", or did I do that initially? Apologies if I forgot.

Matt Koch

asked 2017-11-30 18:29:06 -0500, updated 2017-12-01 10:54:01 -0500

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Are you using OpenStudio? Add tags accordingly. Also, screenshots of your chilled water loop and a description of the setpoint managers will enable assistance.

TomB (2017-11-30 20:44:37 -0500) edit

Please see my edited original question. Also, the district cooling plant is set to auto-size. I do not think these come any much simpler than what I show. All the more am I puzzled why the delta T is not maintained? Unless, of course, the OS:Sizing:Plant entry for Loop Design Temperature Difference is used only once, i.e. to convert cooling load to flow rate on the design day. But that then would bring up the question of how to enforce a certain delta T during regular simulations? Just setting the chilled water supply temperature with the setpoint manager might be insufficient?

Matt Koch (2017-12-01 11:00:20 -0500) edit

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See this thread for position of pumps positioning-of-pumps-plant-loopcondenser-loop

Gard analytics energy plus presentation also has useful guidance on detailed HVAC

I've found that hardsizing the Loop Volume to a large number helps create some stability in the chilled water loop.

The chilled water return temperature may be affected, at times, by the bypass branch. Inspect flow rates on that branch to see if they rare non-zero, and whether they coincide with return temperature fluctuations.

Sizing the Loop with a delta T does not guarentee that delta T is maintained during the simulation. The setpoint manager controls the dynamics during the simulation, and it's job is merely constant supply temperature. Have a look at the flow resolver for the plant loops

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TomB

answered 2017-12-01 14:34:01 -0500