[Equest-users] FW: [eSimForums] Custom Chiller Curves

[Nick Caton]

Hi Jeremy,

 

I’m going to pose a few suggestions in response – I’ll caution that for each question, I had to question what exactly you were questioning… so you may need to paraphrase / simplify your queries in a few cases:

 

1a.  When you specify a PLR for a data point in the EIR-fPLR (or EIR-fPLR&dT) curve, it’s value is “capacity at this data point” divided by “capacity at the reference point.”  Whether that reference point capacity matches what your manf. rep entered in their sizing software depends on your rep and how well you explained your needs to them.  Ultimately, whether that point represents your design capacity or your maximum capacity, you can work with that data provided you are consistent with using the same reference data point between all three curves and in your specification of the chiller in eQuest.  Note that if you are creating only the PLR curve you need to have that reference point be at ARI conditions as that’s what the other two curves are normalized to.

1b.  “% load(ing)” means the percentage of capacity from the maximum/design capacity (whatever is in context).  PLR means Part Load Ratio - you might also call it the “normalized capacity.”  “Part load capacity” is an unnecessary term you’re sprinkling in to confuse yourself (wink wink ^_^), but I would interpret it as the capacity at a given part load.

2.    To paraphrase the line in the EDR reference you brought up:  The part load data points used to make your EIR-f(PLR) or EIR-f(PLR,dT) curve need to be at matching conditions as the point(s) at 100% loading.

3.    You should not be asking for part load points for your curves that are a function of CWT and CHWT.  They should be run at either design or maximum capacity, depending on your chiller type.

4.    When developing a spreadsheet to do the math to come up with your data points for eQuest to do the coefficient regressions, I’d suggest ignoring the equations in the EDR guidelines and following the definitions for the variables in the DOE2 help entries – they make much more sense.  PLR as eQuest interprets it is just as I described under response 1a.  The reference point is should not change between the curves – review the help entry titled “Chiller Energy Consumption” carefully if you are not perfectly sure why.  Be consistent. 

4b.  If your curve using the same reference point between all three curves (and in the equest interface) is resulting in crazy chiller performance – have you checked your work?  Take the generated coefficients and plot them against each other as in excel as I demonstrated.  See if your curves make sense – can you use the curves to work backwards and read off an EIR for a set of PLR, CWT, CHWT conditions that matches manufacturer data?

 

There’s my end of the week effort =).  Hope this helps you and others!

 

~Nick

 

 

NICK CATON, E.I.T.

PROJECT ENGINEER

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

Check out our new web-site @ www.smithboucher.com 

 

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Jeremy McClanathan
Sent: Friday, November 05, 2010 4:40 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] FW: [eSimForums] Custom Chiller Curves

 

Hello,

 

I have a couple more questions to add to the chiller curve conversations going on.  I posted the question to eSimForums so the spreadsheet would stay with the topic and am forwarding the question along.  Below are the questions, and here is a link to the post:

http://www.esimforums.com/equest/viewtopic.php?f=13&t=105&view=unread#unread <http://www.esimforums.com/equest/viewtopic.php?f=13&t=105&view=unread%23unread> 

 

Thanks for any advice,

 

Jeremy

 

-----Original Message-----
From: admin at esimforums.com [mailto:admin at esimforums.com] 
Sent: Friday, November 05, 2010 2:18 PM
Subject: [eSimForums] Custom Chiller Curves

 

New post:

 

Hello all,

 

Attached is a spreadsheet I used to gather information from a manufacturer

and create custom chiller curves using the EDR guidelines for modeling

energy efficient chillers (link to the document at the bottom of the post).

 I have been testing the curves in two models. In a test wizard model (also

attached), both curves work.  In another real model (which I can't upload)

the curves that I expected to be correct cause the chiller to create

energy.  The difference between the curves is the PLR equation.  So, I have

a few questions regarding gathering information from manufacturers and the

PLR equation.

 

1)    

When I specify a %load for a part-load data point, does this automatically

dictate an operating capacity that a manufacturer must input into their

program?  For example, in the attached spreadsheet, I chose to ask for 90%

load with a CHWS & ECT of 42 & 78.  I take this to mean that I am asking

the manufacturer to input an operating capacity of 90% of the full load

capacity at the same CHWS & ECT’s (0.9*343=308.7 tons in the attached

spreadsheet) into their software and give me the corresponding power drawn

by the chiller.  However, in the EDR guidelines for modeling energy

efficient chillers, there is an example Manufacturer Data Request Form

which has a note in both the capacity and power columns that says “Note to

Chiller Manufacturer Rep., Please allow your program to size (don’t input

capacity and kW into your program)”.  This makes sense for the full load

capacity, but does it apply to the part load?  If so, what is the

difference between %load and part load capacity?

 

2)    

Does there need to be a part load data point for every full load data

point?

a.    

I would say yes based on page 26 of the EDR guidelines, which says “each

part-load point must have a corresponding full-load data point with

matching evaporator and temperatures.”  This seems to answer my question. 

But, on page 28 of the same document in the example Manufacturer’s Data

Request Form, they do not ask for part-load points that correspond to

full-load points.  This is a bit confusing, so I would just like to

confirm.

 

3)    

When asking for part load data from a manufacturer, does it matter which

%load I ask for at each data point corresponding to a CHWS & ECT?

 

4)    

In the attached spreadsheet, the difference between curves on the sheet

named Curve 1 and the ones on sheet Curve 2 is the divisor in the PLR

equation (starting in cell W15).  The given PLR equation is PLR =

Qi/(Qref*CAPFT).  Curve 2 represents my interpretation of this equation

where Qref is a constant representing the capacity at the point of unity. 

However, when I used this curve in a model, the chiller energy came out

negative (magically created energy instead of consuming it).  A colleague

of mine read a document that suggested that Qref in the PLR equation is a

different Qref than in the other equations and it could be interpreted as a

variable reference (which I would call Qi, not Qref).  So, we tried it and

it worked, or at least it caused the chiller to consume energy instead of

creating it.  I have a difficult time believing that Qref is a variable and

not a constant, but the fact that the curve worked with Qref as a variable

and didn’t work as a constant forces me question it.  Since then, I tried

the same curves in another model and they both work, which confused the

situation more.  Is my interpretation that Qref is a constant representing

the capacity at the point of unity correct?

 

For ref, the EDR guidelines I am referring to can be found at the following

link:

 

http://www.energydesignresources.com/Resources/Publications/DesignGuidelines/tabid/73/articleType/ArticleView/articleId/138/Design-Guidelines-HVAC-Simulation-Guidelines.aspx

 

 

Reply to this post at:

 

http://www.esimforums.com/equest/viewtopic.php?f=13&t=105&view=unread#unread <http://www.esimforums.com/equest/viewtopic.php?f=13&t=105&view=unread%23unread> 

 

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