I have a question regarding 90.1 2004 Appendix G baseline chilled water pump
control. Section 3.1.3.10 says that baseline CHW systems must be
primary/secondary, with a constant flow primary loop and a variable flow
secondary loop. The secondary pump will either ride the curve or be
controlled by a VSD depending on square footage. In the case I currently
have in my hands, the pump rides the curve. No questions yet.
Section 3.1.3.11 goes on to state that each chiller shall have its own
condenser water and chilled water pumps, interlocked to operate with the
chiller. This is clear and reasonable for the condenser water and primary
chilled water pumps, but what about secondary chilled water pumps? For my
current application (one chiller, pump riding curve), my question simply
boils down to whether I should turn the secondary pump off when the chiller
is off, or if I should run it all of the time.
But to get into it a little more, here are three approaches I see:
1. Model one secondary pump for each chiller, and turn each pump off
when its associated chiller turns off. Section 3.1.3.10 is titled Chilled
Water Pumps, and that section includes both primary and secondary pumps, so
when the next section references "chilled water pumps" that should include
both primary and secondary. However, this does not reflect the typical
reality of one secondary pump serving multiple chillers, and the common
interpretation that only a primary pump has one single associated chiller.
2. Model one secondary pump regardless of how many chillers there are,
and turn it off when all chillers are off. This is perhaps most similar to
what you might expect in a 'real' system. If you only have one chiller (as
I currently have), this is exactly the same as approach 1.
3. Model one secondary pump regardless of how many chillers there are,
and never turn it off. Many buildings are unfortunately operated this way.
This approach significantly increases baseline energy when the pump is
riding the curve.
I have always used approach 2 without putting much thought into it. But now
my results are highly dependent on which approach I take, and I see enough
ambiguity that arguments could be made for approach 3.
Your thoughts?
Thanks,
Christian Kaltreider, LEED AP
Christian,
I always model one secondary pump for the baseline. I see no reason to do otherwise.
My approach to loop activation has been to model it the same as the proposed design. Using DOE-2 for example, it could be SCHEDULED to run 24/7 during cooling months, or DEMAND-ONLY to stay off until a coil or process load exists. If the proposed design has no CHW plant, but has a boiler/HHW plant, you could model the baseline CHW loop activation similarly to the proposed HHW loop. If the proposed design has no analogous loop operation, you just have to make a judgement call, but I am not aware of any guidance in 90.1 or LEED. Also keep in mind that 22W/gpm is the total pump power allowance for all your primary/secondary CHW pumps, and not the power of each pump.
Regards,
Bill
Bill,
Thanks. Yes, the 22W/gpm total requirement is still fresh in my mind from
the recent discussion on this forum!
I understand your approach to pump control. But.would you be able to defend
that approach if someone (like your client) pushed you about why you aren't
getting them better savings by running your secondary CHW pump 8760?
Certainly the 90.1 folks had some specific intent, whether they made it
clear or not.
Thanks,
Christian
To clarify an email I sent to the last thread on the 22 W / gpm: I
recently received the updated documentation spreadsheet for a v4 pilot
project, and they have updated the notes. Not sure if this image will
come thru to everybody, so here's what it says:
"The sum of primary and secondary must be 22 W / gpm (349 KW/1000 L/s)
per G3.1.3.10. Recommended that the pump power be split as one third
(primary) and two-thirds (secondary)."
James Hansen, PE, LEED AP, BEMP
Big Brother GBCI has eyes and ears everywhere... =)
I for one am very happy for clearer direction coming about on this point. For reference, I think that same recent conversation brought about a reference I kept note of for ASHRAE interpretation IC 90.1-2007-14, which ruled that 22 W/GPM is a "whole system" power requirement, similar to Pfan for baseline fan calculations.
The one-third / two-thirds split is to my knowledge new direction, (I have previously simply dumped it all baseline pumping power onto secondary pump for simplicity of documentation - perhaps to my own detriment in certain cases), but I can empathize the reasoning behind providing such direction where distribution of pumping power could (in some cases) affect overall baseline performance.
Thanks very much for sharing James!
~Nick