[Equest-users] Summary of Chilled Water Modeling - Secondary Condenser Water Loops
Darryl Kasun via Equest-users
equest-users at lists.onebuilding.org
Mon Aug 22 16:05:23 PDT 2016
Below is the summary of a discussion regarding Chilled Water Modeling;
specifically secondary condenser water loops. This topic was discussed
between Aug 16 2016 and Aug 18 2016. It is organized in reverse
chronologically.
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Darryl,
I think Coles is leading you in the right direction. I did not really look
through your schematics, etc., but this sounds like the right work around.
Just check the plant equipment reports to make sure your “virtual” CW loop
pump and cooler don’t use energy when they shouldn’t, and that the annual
COP of the chillers is reasonable. (Look at PS-C and the sum of equipment
cooling load and sum of electric use.)
Fred
fporter at noresco.com
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Darryl,
I agree the dry coolers will provide the function of condenser relief,
though I haven’t seen condenser relief accomplished this way before. I’m
used to seeing either a completely separate condenser water loop, or an
air-cooled condenser for relief. As such, I can’t speak from direct
experience as to the best modeling approach. My initial thought is that you
would still model a water-cooled HRC, which requires a separate condenser
water loop, and assign the dry-coolers to the condenser water loop. This is
an approximation, but the closest readily available that I can think of.
Thanks,
*Coles Jennings,* PE, BEMP, LEED AP BD+C
Sr. Energy Engineer, Building Sciences Manager
Sr. Energy Engineer, Building Sciences Manager | Mason & Hanger*(formerly
Hankins & Anderson) *
A Day & Zimmermann Company
*D *804.521.7045
A Day & Zimmermann Company
*D *804.521.7045 | *O *804.285.4171 | *F *804.217.8520
4880 Sadler Road, Suite 300
4880 Sadler Road, Suite 300 | Glen Allen, VA 23060
ha-inc.com <http://www.ha-inc.com/> *(soon to be masonandhanger.com
<http://masonandhanger.com/>) *
ha-inc.com <http://www.ha-inc.com/> *(soon to be masonandhanger.com
<http://masonandhanger.com/>) *
*We do what we say.®*
____________________________________________________________
____________________
Hi Coles,
So I'm good on the Max Recover T now.
I have not found anything yet that would indicate that the HRCs have
condenser relief. There are dry coolers on the condenser side of the HRCs
that operate when entering condenser water temperature is above 100 degF
(design entering water temps of the HRC). The dry coolers would take
provide the function of a condenser relief correct?
Thanks
Darryl
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Darryl,
Max Recovery T is the maximum temperature at which the HRC can contribute
heat to the system. In the situation you described below, you can set this
to 110 and rest assured that all available heat from the HRC can be
recovered to the HWS loop.
Condenser relief indicates how the balance of heat is handled. For the
majority of the year there will be an imbalance between the heating load
and cooling load in the building, so the load needs to be balanced (either
via air-cooled or water-cooled condenser). Let’s say on a mild day, you
have 2x building cooling load as compared to the heating load. If you only
have an HRC supplying chilled water for the cooling load, it’s going to be
generating more than twice the heat you need to meet your heating load (
*more* than twice because the compressor heat is also contributing to the
heat available to the condenser). So you need somewhere for this extra heat
to go, which is what condenser relief accomplishes. Multi-stack makes HRCs
that have air-cooled condenser relief, or water-cooled condenser relief
which would have six piping connections (two for chilled water, two for hot
water, and two for condenser water to relieve the balance of heat).
If you don’t have any other chillers attached to this cooling system, my
strong hunch is that you do have condenser relief. I would check the
equipment schedules to confirm, or best case if you have a cut-sheet it
should be clearly indicated.
Luckily eQUEST models condenser relief fairly readily. It’s when you don’t
have it that things get tricky.
Thanks,
*Coles Jennings,* PE, BEMP, LEED AP BD+C
Sr. Energy Engineer, Building Sciences Manager
Sr. Energy Engineer, Building Sciences Manager
Sr. Energy Engineer, Building Sciences Manager | Mason & Hanger*(formerly
Hankins & Anderson) *
A Day & Zimmermann Company
*D *804.521.7045
A Day & Zimmermann Company
*D *804.521.7045
A Day & Zimmermann Company
*D *804.521.7045 | *O *804.285.4171 | *F *804.217.8520
4880 Sadler Road, Suite 300
4880 Sadler Road, Suite 300
4880 Sadler Road, Suite 300 | Glen Allen, VA 23060
ha-inc.com <http://www.ha-inc.com/> *(soon to be masonandhanger.com
<http://masonandhanger.com/>) *
ha-inc.com <http://www.ha-inc.com/> *(soon to be masonandhanger.com
<http://masonandhanger.com/>) *
ha-inc.com <http://www.ha-inc.com/> *(soon to be masonandhanger.com
<http://masonandhanger.com/>) *
*We do what we say.®*
____________________________________________________________
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Coles,
Thanks for your input.
Couple questions:
What is Max Recovery T.? I know the design condenser leaving temp. is 110
degF which is also the design HWS temp. of the heat recovery loop.
There are non-heat recovery chillers but they are separate from this
cooling system.
Regarding condenser relief - how does this affect operation and/or energy
use?
I'll start working on this configuration and will almost likely come back
with some questions.
Thanks
Darryl
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Darryl,
I’ve been following this thread and the configuration described by your
colleague makes a lot more sense to me. I can understand the initial
confusion because the heat rejection side of the heat recovery chiller acts
as the condenser, but also as the heating source for the terminal unit
reheat.
We do a lot of HRC models, so I just wanted to throw in a few words of
advice:
· Make sure that the Max Recovery T is compatible with the supply
temperature of your hot water loop. For example, if the max recovery T is
only 100°F, but your hot water loop is configured to supply 180°F water
with a 40°F delta, you will never recover any heat from the HRC in the
model. We’ve done multi-stack projects with recovery temps up to 140°F,
have not gone higher than this. The hot water system needs to be designed
to be able to make use of this heat. It’s especially beneficial in this
case to have a hot water reset schedule that lowers the supply temp during
milder outdoor temperatures, as the HRC will get more efficient the lower
the recovery temp is.
· Do you have standard, non-heat-recovering chillers on this
project as well? If so, you’ll need to pay special attention to the
equipment control (EQUIP-CTRL) sequences in your model, to make sure you
prioritize the HRCs when there is a heating load.
· Do you have condenser relief on the HRCs? This is an option on
multi-stacks. If you don’t have condenser relief, you’ll need to pay even
more attention to the EQUIP-CTRL sequences. To my knowledge, eQUEST can’t
model HRCs without condenser relief, but you can make more detailed control
sequences to limit relief in the model.
Thanks,
*Coles Jennings,* PE, BEMP, LEED AP BD+C
Sr. Energy Engineer, Building Sciences Manager
Sr. Energy Engineer, Building Sciences Manager
Sr. Energy Engineer, Building Sciences Manager | Mason & Hanger*(formerly
Hankins & Anderson) *
A Day & Zimmermann Company
*D *804.521.7045
A Day & Zimmermann Company
*D *804.521.7045
A Day & Zimmermann Company
*D *804.521.7045 | *O *804.285.4171 | *F *804.217.8520
4880 Sadler Road, Suite 300
4880 Sadler Road, Suite 300
4880 Sadler Road, Suite 300 | Glen Allen, VA 23060
ha-inc.com <http://www.ha-inc.com/> *(soon to be masonandhanger.com
<http://masonandhanger.com/>) *
ha-inc.com <http://www.ha-inc.com/> *(soon to be masonandhanger.com
<http://masonandhanger.com/>) *
ha-inc.com <http://www.ha-inc.com/> *(soon to be masonandhanger.com
<http://masonandhanger.com/>) *
*We do what we say.®*
____________________________________________________________
____________________
If this approach is reasonable, the CW loop can perhaps be defined as a
WWHP loop, and then it will accept a scheduled process “heating” load.
A “virtual” boiler might need to be added to that loop make it work, but if
your loads are reasonable then it shouldn’t activate, or maybe you want it
to activate to “simulate” this series or seasonal steam heat. I’m not sure
if there would be other “side effects.”
And then there is also the heat-recovery chiller option. The CW and HW
loops won’t be the same, but perhaps it might better represent the action
better, and will certainly allow supplementary heating of the reheat/etc.
loop based on some criteria, though perhaps not the exact criteria in the
real building.
Good Luck,
Fred
*Fred Porter, BEMP, LEED**©** AP*
Principal Engineer
Sustainability Services
*NORESCO*
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Part of the issue is that it can’t be a condenser loop to perform this
function. I was originally hoping to load the condenser loop in this way as
well.
How about if the load is modeled as a WSHP on a dummy zone? Then it can
both add or subtract heat from a condenser loop?
My other question though would be what proportion of load these have on the
overall system and they might be modeled as “negligible” in the scheme of
the overall capacity of the condenser system.
I also may have missed the part where it describes what is using 100F
reheat, which isn’t a lot for most reheat applications.
Darryl, for your benefit – the more typical approach for this type of heat
recovery in parts “South” of your location would be to size one of the
modular chillers for the heat recovery application and reject even warmer
water, while the rest of the chillers can operate at a more efficient ECWT
through the towers or dry-coolers.
To make the whole plant run at 100F means that the designer may be
expecting a large amount of heat recovery load in proportion to the cooling
plant capacity.
David
David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
*Grumman/Butkus Associates*
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Well, I guess from a logical point of vue, a negative load on the condenser
water loop doesn’t make much sense.. can’t really blame eQUEST on this I
guess.
At this point the only way I see left is to post-process all that’s
happening on the condenser water side.. which is not really ideal.
[image: Bouthillette Parizeau] <http://www.bpa.ca/>
*Patrick* *Lapierre**_ing.*
plapierre at bpa.ca
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I also did a test. Add a positive process load to the condenser loop
increases the heat rejection energy – the process load itself is not
reported as misc. energy – as expected. In the case of the condenser loop
serving the reheat coil loop, you would need to enter a negative process
load to reduce the cooling tower load/energy. Unfortunately, Equest/DOE22
thinks it is smarter than you so it changes the sign of the load to
positive if you enter a negative load. Therefore, this idea won’t fly.
*Christopher Jones, P. Eng.*
*Rowan Williams Davies & Irwin Inc.*
*Consulting Engineers & Scientists*
901 King Street West, Suite 400, Toronto, Ontario, M5V 3H5
*T:* (519) 823-1311 ext 2052
*M:* (416) 697-0056
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Darryl, I made a quick test to figure out about how the process load on the
condenser loop reports as a endues and it seems like it does not report at
all.
I’m quite surprised but it’s the result I got… In any case, just double
check to make sure the artificial process load (C) doesn’t get counted in
your results.
[image: Bouthillette Parizeau] <http://www.bpa.ca/>
*Patrick* *Lapierre**_ing.*
plapierre at bpa.ca
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It’s a pleasure to help. I’m always greatful when the community solves one
of my problems so it’s only natural to help out in turn.
The idea with the load (C) as I described it below is that once you plug it
as a process load on the condenser water loop, it will change the condenser
water loop temperature entering the dry coolers.
What you want to calculate is Qc = 500*GPMscnd1/2*(T3-T1) and add that load
to the condenser water loop.
In other words, you want the condenser leaving temperature calculated by
eQUEST to become T3 instead of T1. For that to happen, you plug Qc on the
condenser water loop. That way, the flow going to the dry coolers heat
exchanger will be at the right temperature.
As Nick mentioned also, be mindful that plugging a load on a loop as
process load will be reported as an endues (in the misc equip category I
believe.. not 100% sure on this). You’ll need to manually subtract it from
your results.
Don’t hesitate if something’s still fuzzy.
[image: Bouthillette Parizeau] <http://www.bpa.ca/>
*Patrick* *Lapierre**_ing.*
plapierre at bpa.ca
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Thank you very much Nick and Patrick for your responses!
So I've gone thru the steps suggested by Pat.
I am good with formulas (A) & (B) but am having a tougher time with (C).
Pat you wrote "Calculate the energy required to bring the actual condenser
leaving temperature to the calculated (A) global return temperature of the
loop after the reheat coils. (C)".
Is it possible that instead of actual condenser *leaving* temperature you
meant actual condenser *entering* temperature? That seems to make sense to
me, but again I could be missing something.
Thanks again for your help everyone!
Darryl Kasun
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Clever! I really like the idea of dumping a process load onto the CW loop,
and am curious how this might report as an enduse. This would result in
appropriate loop temps for the coolers and chillers, but be mindful in
output post processing to not double count the artificial "process"
energies.
Also I could see your intermediate processing growing more complex if
considering reheat temperature resets and/or seasonal HX bypass... I would
want to quiz the designer(s) thoroughly to ensure nothing like this that
would affect my calculations is missed.
Keep us posted - you might yet be covering new ground with this project!
~Nick
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As Nick pointed out, I don’t think eQUEST can model this particular case
without some intermediate processing or post-processing.
My first idea would be to try to model loads on the condenser water loop to
simulate the real terminal reheat return temperature (right before it goes
to the dry cooler exchangers). That way the dry cooler would see the real
load.
Here’s my take on the problem:
1- Model the reheat coils on its own hot water loop and plus a steam
meter to it, along with a dummy pump. Now I’m not sure if the flow going
into the coils is variable or constant.. the loop’s pumps have VFD but it
seems like the reheat coils have a bypass with a 3-way valve (so constant
gpm flow?). Either way can be modelled but it’s important to model to right
case.
2- Look up the supply temperature of the steam heat exchanger feeding
the terminal reheat coils (i.e. 120 F).
3- Here’s the intermediate processing step (on an hourly basis):
a. Using the terminal reheat load (1), the actual loop’s flow and the
steam exchanger supply temperature (2), calculate the global return
temperature of the loop after the reheat coils. (A)
b. Using the actual condenser leaving temperature and the actual flow,
calculate the energy required to increase the condenser leaving temperature
to the steam exchanger supply temperature (2). (B)
c. Calculate the energy required to bring the actual condenser leaving
temperature to the calculated (A) global return temperature of the loop
after the reheat coils. (C)
d. Using EMIT, create an hourly load schedule for (B) and (C).
4- Plug the load schedule (C) on the condenser water loop as process
load.
5- Plug the load schedule (B) on the steam loop.
If all goes well, step (4) should simulate the real terminal reheat return
temperature so that the dry coolers see the real load and step (5) should
get you your real steam demand for the terminal reheat.
All in all this strategy requires intermediate processing but no
post-processing. It’s not too complicated either but you’ll need to update
the 2 hourly schedules every time you make a change to the model.
That is unless I missed something.. it happens to all of us sometimes!
Don’t hesitate to comment/edit the strategy if that’s the case.
[image: Bouthillette Parizeau] <http://www.bpa.ca/>
*Patrick* *Lapierre**_ing.*
plapierre at bpa.ca
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*Fun stuff* - You didn’t even mention the snow melt!
What novel/nifty/weird here is that the CW loop is *also* the terminal
reheat heating loop (*Note: condenser temps are boosted up with steam HX’s
– critical point*), before coming back to hit the dry coolers tasked with
bringing things down to within reason for the chillers’ ECT. To my own
terminology… I might consider your chillers “preheat” for your steam HHW
reheat loop.
As an aside, this is NOT common to my personal design experience and I
wouldn’t feel shy about sharing that with the designers to open up a
conversation and be sure everyone is on the same page. Even if this is
standard practice in your region/company, it never hurts to review the
intent and function of the mechanical design with those putting it together
& simulating performance. You might both learn something!
There are a number of hard stops bounding this problem for doe2/eQUEST.
Airside reheat coils can only be assigned to circulation loops of type HW
and PIPE-2. From context and a few more clues in the diagram, I suspect
you have a bunch of simultaneous heating/cooling to deal with, so 2-pipe is
probably out of the running (can only deliver heating or cooling any given
hour, and has lots of other attachment limitations regardless).
I toyed in a dummy model with a possibility for re-purposing a GLHX of type
“Lake/Well” to force-feed a “return from the reheat” temperature profile to
the dry coolers on a CW loop – but some piece of that Frankenstein’s
monster (when I gave it a temperature schedule of straight 140’s) resulted
in negative cooling consumption in the outputs… Here’s a picture if anyone
is interested in carrying that ball further – it might actually work if you
built this from scratch:
Now I’m going to do something VERY WEIRD from my typical contributions here
on [eQUEST-users]: Frankly, this may be a case where *eQUEST/doe2 simply
isn’t a workable platform*. I’m *really* stretching my brain here and
having trouble coming up with something that wouldn’t involve a very
complex set of intermediate and post-simulation output processing
assumptions…
How bad would it be to suggest no simultaneous heating/cooling? That could
open up some doors for specific circulation loops & plant equipment… I
gather however you probably have a bunch of cooling+heating happening from
clues in the shared sequencing.
The only other hypothetical path I might consider is some odd approximation
using LOOP-TO-LOOP-HP chillers… but then my brain starts to hurt…
I’d be very interested to hear if anyone else would come to a different
conclusion!
~Nick
*Nick Caton, P.E.*
Senior Energy Engineer
Energy and Sustainability Services
Schneider Electric
D 913.564.6361
M 785.410.3317
E nicholas.caton at schneider-electric.com
F 913.564.6380
15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States
[image: cid:image001.png at 01D189AB.58634A10]
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Hi David - thank you for the reply!
I went through the heating schematic and there are a lot of reheat coils
this loop serves. I don't know how to hook up this heating demand to the
condenser water loop and maybe that's not directly possible in eQuest?? If
I add up the heating load, I guess I'll have to make some assumption as to
what that load will be and what it's schedule would be. Is that what you
meant?
Now regarding the dry cooler, looks like that loop doesn't activate until
entering condenser water temp hits 100 degF. Any thoughts on how to model
this?
Darryl
darryl.kasun at gmail.com
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What information do you have about these reheat coils that are using the
100F secondary condenser water? These might need to be modeled as a load on
the loop rather than explicitly as heating coils.
Setup-wise you may need to take some liberty with the primary/secondary
classification of the condenser water loops in model space to allow the
needed connections. It would help to know what proportion of the load the
designers envision going to the heating coils, and what proportion will go
to the dry-coolers.
David
DEldridge at grummanbutkus.com
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Hi Everyone,
I am having difficulty setting up this part of my model & looking for some
direction.
The project has a modular chiller (CH-1,2,3) that serves the bldg’s cooling
needs with the exception of the AHUs. I’m good on that side of things.
On the condenser side, a secondary condenser loop serves terminal reheat
and there are also dry coolers that serve the condenser loop. First off,
just want to confirm I know how this system works. Please correct me if
I’m wrong.
So the chiller is working to provide cooling to the bldg. On the condenser
side, one secondary loop provides reheat as required. The dry coolers
provide additional cooling to this loop as required. Is it as simple as
this? Please provide any or all insight that you can!
If I am correct on how this system works – if so, any suggestions on how do
I model it?
Thanks!
Darryl Kasun
darryl.kasun at gmail.com
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