Primary-Secondary Loop vs. Primary only Loop (with VFD)

[ksechler]

All:
First let me apologize if this has been addressed. I tried the search feature and didn't come up with anything so I figured I would just ask.

I am working with an energy consultant to try to optimize the chiller plant at my facility which consists of 3 Trane 1500 ton chillers feeding a primary-secondary chilled water loop. The chillers are variable, the primary pumps are constant speed, and the secondary pumps are VFD. Chillers are staged to meet demand based on flow and the unfortunate conseqence is that our chillers spend most of the summer running at 50-60% which is the worst place on their efficiency curves.

I was wondering if someone which chiller experience could explain some of the pros and cons of the primary-secondary vs. primary only chilled water loop arrangement.

Thanks.

 

[DRWeig]

Nearly all pros, not cons. See attached from Gil Avery.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[marcoh]

You haven't given any information why the chillers are running at 50%-60% load. This is typically the most efficient section on a chiller load curve, but you are also implying that this is due to the flow based control strategy, which implies that you are referring to a low return water temperature temperatures? IF the return chilled water temperatures from the building is less than the design return chilled water temperature, you will never be able to load the chillers to 100% and is more likely to be a cooling coil and control issue than a pumping issue.

 

[MechEngNCPE]

You should stage the chillers such that one is at full load when the next stages on. The owner should hire an engineer to modify the sequence of operation of the chiller plant. When you say "Chillers are staged to meet demand based on flow" is this not a constant volume primary arrangement? They should be staged to meet demand based on the total demand load from the building (given from CHWR temp).

 

[Drazen]

more specific information is needed...

 

[KiwiMace]

I don't feel the need for any more specific info, personally...

If you are running variable speed compressors on your chillers and staging them correctly, I highly doubt that your 50-60% load is an inefficient operating point and I even more doubt that converting to primary-only will provide payback in any realistic time frame or building engineer's lifetime. Tell your energy consultant to hire a payback consultant and focus on optimizing the existing plant.

 

[urgross]

Definitely need more info in order to make any life cycle cost analysis. As KiwiMace clearly states, this would be along payback to try to justify converting to variable primary, unless the chiller plant is at end of useful life. Payback would also depend on your electrical rate schedule, the weather, and whether you are running the chillers year round or using economizers, etc. If going that route (variprime vs. primary/secondary) the best recommendation I could give is to start with as ASHRAE Appendix G analysis for baseline (existing conditions) vs upgraded variable primary/secondary and variprime. The IPLV analysis would also be useful in sizing the chillers-three same size chillers might not be the optimal arrangement for providing space conditioning (if that is the intent?). If you think 50-60% loading is an inefficient setpoint, how are the chillers doing on evenings, mornings and shoulder seasons?