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Primary-Secondary Loops - Variable Speed Pumping with Minimum Flow

asked 2017-08-23 01:23:41 -0500

bobw's avatar

updated 2017-08-23 18:29:20 -0500

I'm trying to set up autosized primary-secondary heating and chilled water loops, with variable speed primary and secondary flow pumps.

I am modelling as recommended elsewhere in this forum (see here). Only difference is:

  • I am using headered pumps because it lets me set a minimum pump flow rate as a fraction of maximum (useful when autosizing)

  • I have bypass on all supply / demand components (if I get it working I will remove the ones that wouldn't normally be in the system)

  • I have multiple coils on the loop

I need minimum flow rates on both primary and secondary otherwise the energy estimation will be unrealistically low (in the order of 60% flow on primary and 5%-25% flow on secondary). I have tried setting a minimum flow on the pumps (fraction) and on the loop object (flow rate), neither works. I suspect the control hierarchy does not allow these limits to be implemented - but given I've only built a couple of models in energy plus I could be way off.

Has anyone found a way to model this? Preferably in a way that I can include in a measure.

I've tried and abandoned the 'common loop' function - doesn't model variable flow on primary.

Sorry can't upload because I'm new.

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Btw, you should have enough reputation to upload a file now.

Julien Marrec's avatar Julien Marrec  ( 2017-08-24 08:19:43 -0500 )edit

Thanks for your answer Julien. To clarify what I am after is an intermittent pump which modulates at higher loads, runs at a minimum defined flow rate when there is a small load, and switches off when there is no load. I am having all sorts of problems with my model at the moment which probably contributes my problems with the pumps. I will upload when I have troubleshooted a bit more (probably Monday).

bobw's avatar bobw  ( 2017-08-25 00:17:40 -0500 )edit

I was recently trying to do the same - constant primary and variable secondary. Or are you doing a variable primary and variable secondary (in which case why not do variable primary only?). Anyway, I found that headered pumps do not work here and went back to regular pumps. Seems to work really well with the Common Pipe simulation. The drawback is, I have to estimate the minimum secondary pump flow in gpm rather than %. No minimum plant loop flow needed.

Also see

Matt Koch's avatar Matt Koch  ( 2017-12-06 13:38:42 -0500 )edit

@Matt Koch, note that common pipe for primary/secondary is kinda deprecated and it's recommended to actually use two separate loops with a HeatExchanger:FluidToFluid in between. See Plant Application Guide - Example 3 (2nd paragraph) and Andrew's answer here

Julien Marrec's avatar Julien Marrec  ( 2017-12-11 02:37:42 -0500 )edit

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answered 2017-08-23 09:12:43 -0500

updated 2017-08-23 09:12:52 -0500

I'm a bit unclear about what you mean by "neither works", but check your Pump Control type. If you set it as "Intermittent", and if there is no load on the loop, it'll shut down. If you want to have run all the time (as minimum flow fraction), use "Continous". Note that this is going to both consume more electricity and impose a heat load on your loop from the inefficiencies of the pump, if your loop has no load for extended periods of time, pump heat build up can occur. In which case, you could use an availability manager to turn off the loop itself.

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Asked: 2017-08-23 01:16:03 -0500

Seen: 835 times

Last updated: Aug 23 '17