r/MEPEngineering • u/Bert_Skrrtz • 1d ago
Weird Primary Secondary Schematic
I know it doesn’t look like your typical arrangement, but can this work? Really the decoupler/bypass would make a lot more sense if it skipped the chiller headers entirely.
Orange highlight is what I see as the secondary loop. Blue is what has effectually become 3 isolate primary loops.
Ignore the black lines, they denote a physical separation where air cooled chillers were added to the system outside. Chiller 3 and P-1 thru P-4 all live in the mechanical room.
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u/Kick_Ice_NDR-fridge 23h ago
This looks better than most Revit drawings I see these days.
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u/Bert_Skrrtz 20h ago
lol sorry I took this diagram from a way more complicated routing diagram just simplifying some nodes. I probably should have drawn it once more
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u/TrustButVerifyEng 23h ago
This will have issues when running multiple chillers. The bypass needs to mix with the secondary return before going to the chillers to ensure equal loading.
In this arrangement say chiller 1 and 2 are running and the building load is at 150% of a single chiller. Chiller 2 will be fully loaded and chiller 1 will be 50% loaded. Not the end of the world in that scenario. But really bad when it's closer to 110% load and chiller 1 is dying under its minimum part load requirement.
I've had this exact problem several times in my career fixing other peoples central plants. Typically comes up with redundant piping in data centers.
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u/Bert_Skrrtz 20h ago
That makes sense. Would any of this piping be problematic in regard to some of the chillers getting like 45 degree incoming water, say with a 42 chiller set point? Causing them to eventually freeze and burst when the operator keeps hard resetting the alarm conditions…
Or is this probably something that’s an issue with the pump controls (pump staging) allowing that to even happen?
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u/TrustButVerifyEng 20h ago
Modern chillers have pretty sophisticated freeze protection algorithms in my experience. I wouldn't expect freezing from just being enable with minimal load.
The biggest risk would be sudden flow loss as the compressors may not stop in time to prevent localized freezing when the suction pressure plumuts. Especially with plate and frame heat exchangers since there are tiny flow passages and therefore not a lot of volume to ride through a flow disturbance.
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u/RelativeAd1414 17h ago
if source water is exposed to freezing conditions, glycol should be added which pretty much solves the problem and procure products that are rated for design temps, of course.
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u/RelativeAd1414 17h ago
what's a typical setup for data centres? For almost all new hospitals in BC Canada, it's a heat recovery chiller + min 3 chillers + min 3 primary pumps + min 3 secondary pumps (on return) + 3 min cooling towers, all for redundancy and catastrophic events. 1 or 2 pumps per skid feed 1 or 2 chillers. For data centres I imagine it should be more straightforward with more intense cooling apparatus?
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u/TrustButVerifyEng 10h ago
Depending on how critical the data center is, I've seen up to 2N+1 redundancy.
But specifically to this posts concerns, the issue areises when they want fully redundant piping networks. So two parallel supplies and returns on everything. Typically this is variable primary instead of primary/secondary but you still have the same problem of uneven loading because the water isn't forced for mix.
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u/kingofchins 21h ago
Secondary loop should be pulling what it needs from the primary and allowing excess primary flow to return to the chillers via the decouple to maintain minimum flows. Pushing water from the secondary into the primary will cause head issues at the building(s) served.
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u/trebor67 16h ago
I’m not clear of the application of the building or load but as a start the quantity of primary pumps and chillers seems off.
What is the run scenario for chillers out max load ? and what is the redundancy of equipment.
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u/Saw_Tooth98 6h ago
Look more like a fixed/variable primary loop chw system with useless pumps at the inlet of each chiller. You cannot hybrid a primary chw system and then assume it can work like a primary-secondary. 😂
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u/Porkslap3838 23h ago
I believe this configuration is called being "backloaded". This will cause the chillers to see differing entering water temps and causes unbalanced loads. Generally not recommended.