[Equest-users] Chiller Curves (oh boy!)

Wed Nov 3 11:39:27 PDT 2010

Michael:

Thank you very much for the offer.  Next time I am auditing a site with a York, 
I will be sure to contact you.

Paul Diglio

________________________________
From: "Michael.Hupel at jci.com" <Michael.Hupel at jci.com>
To: Carol Gardner <cmg750 at gmail.com>
Cc: "equest-users at lists.onebuilding.org" <equest-users at lists.onebuilding.org>; 
equest-users-bounces at lists.onebuilding.org
Sent: Wed, November 3, 2010 2:07:37 PM
Subject: Re: [Equest-users] Chiller Curves (oh boy!)

As the OEM rep for YORK chillers I am more than willing to provide NPLV/IPLV and 
other part loading efficiency data.  Can even provide a comparison to other 
chiller manufacturers or some of the options such as VSDs or hot gas by-pass 
options.   

Chillers are normally specified to supply cooling for the hottest day of the 
year and yes add a factor for potential future load such as increased occupancy, 
equipment etc.   

Michael Hupel, B.Tech., LEED AP
Account Executive
Project & Business Development
Johnson Controls L. P.
3070 Mainway Drive 
Burlington, ON L7N 3X1
(905) 335-3325 ext. 237
(905) 730-9642 cell 

From:  Carol Gardner <cmg750 at gmail.com>  
To:  "John T. Forester" <JohnTF at bvhis.com>  
Cc:  "equest-users at lists.onebuilding.org" <equest-users at lists.onebuilding.org>  
Date:  11/03/2010 01:54 PM  
Subject:  Re: [Equest-users] Chiller Curves (oh boy!) 
________________________________

I agree with you, John, but have a couple of other thoughts to add. The first is 
I have not met a mechanical engineer that does not select a chiller that is 
capable of supplying all loads. In fact, they generally oversize a bit to 
accommodate possible future loads. The only piece of equipment I have ever seen 
selected at over 100% was a VRV system and that is because they actually work 
better at 120% loading. The second is that you can generally get a catalog from 
a manufacturer to have on your shelf that has technical information like how the 
chiller unloads based on temperatures and what the efficiency is at part load so 
you can look up the chiller you need info for and not need to bother the ME or 
the vendor. If you can't get/don't want a hard copy, most of this info is on 
line, just look for technical specifications.

Best,

Carol

On Wed, Nov 3, 2010 at 9:48 AM, John T. Forester <JohnTF at bvhis.com> wrote: 
Nick, 

I think you’re on the right track.  Below are some of my thoughts on your 
conclusions.  I hope the modeling community will set me straight if I’m wrong 
here. 

1.        Getting multiple performance runs from vendors that show part-load 
performance independent of the CW and CHW temps can often be challenging.  
Adding the “maximum capability” task to that for each of the conditions requires 
a pretty detailed understanding of the selection software.  I’d say if you can 
get PLR data for 3-4 different CHW temps while holding the CW temps constant at 
85, 75, 65 (and sometimes lower) – you’re ahead of most modelers.  Working with 
the Mechanical Design Engineer and the vendor together has been successful for 
me in the past. 

2.        Defining the chiller capabilities at the “maximum” may only come into 
play if you expect your model to overload the chiller above the specified design 
capacity (I’m thinking building additions or process loads). At this point, this 
data (or knowing what the default eQuest curves do in that range) would be 
useful.  Depending on your project, the time spent on developing curves for PLR 
>1.0 may not be justified. 

3.        If you don’t have “max” data and don’t want eQuest to assume 
performance at a part-load ratio >1.0, you can set the DESIGN-PLR to 1.0. 

4.        Either way, you want your curves to be normalized at whatever 
condition you specify (Design or Rated) and you want to enter those values on 
the Basic Specifications tab. 

John 

John T. Forester, P.E., LEED AP, Mechanical Design Engineer I BVH Integrated 
Services I617.658.9008 tel I617.244.3753 fax IOne Gateway Center Suite 506, 
Newton MA 02458 I www.bvhis.com I Hartford ● New Haven ● Boston 

size=2 width="100%" align=center tabindex=-1> 
From: Nick Caton [mailto:ncaton at smithboucher.com] 
Sent: Wednesday, November 03, 2010 12:06 PM
To: John T. Forester; equest-users at lists.onebuilding.org 

Subject: RE: [Equest-users] Chiller Curves (oh boy!) 

John, 

The design/max ratio is exactly what I’m discussing below when I say “DESIGN-PLR 
ratio,” so we’re definitely in the same ballpark =). 

I’d like to apply/paraphrase your advice to a few conditions to be sure I’m 
getting it correctly:   

-          If we create all 3 custom curves, and normalize each to a point at 
maximum (not design) capacity, then the design-to-max ratio (DESIGN-PLR) should 
be set to 1.00. 

-          If we create only the part load efficiency curve (EIR-FPLR or 
EIR-FPLR&dT), and wish to use the library defaults for EIR-FT and CAP-FT, then 
we should normalize this curve’s data points to ARI conditions (as that’s what 
the library curves are normalized to, per James’s email – I think I’ve read this 
somewhere also), specify an ARI  capacity, EIR and conditions on the basic 
specifications tab, and enter a DESIGN-PLR of [ARI capacity/maximum capacity 
(for the same conditions)]. 

-          If we create all 3 custom curves, and try to normalize each to either 
ARI or design conditions, then we should specify capacity, EIR, CHWT, CWT and 
condenser GPM corresponding to either the ARI or design conditions of that 
normalizing point.  In that case, we also specify a DESIGN-PLR using either the 
ARI or design capacity divided by the maximum capacity for the same conditions. 

Profound (to me) Conclusion:  In No instance should we Ever attempt creating 
custom curves and NOT have at least one run from our manufacturer telling us 
what the maximum (not design) capacity is for the normalizing point.  This 
conclusion would only apply to centrifugal chillers only. 

Does this all sound right? 

~Nick 

James,  

Yeah, if all the part load data you received held the same CHWT and CWT equal, 
you might be able to make your part-load curve if it could have been a quadratic 
EIR-FPLR curve (like a reciprocating chiller), but not a bi-quadratic 
EIR-FPLR&dT (as with my centrifugal VSD chiller).  You definitely could not 
approach generating custom EIR-FT or CAP-FT curves without varying condenser and 
chilled water temps.  That exact issue happened to me the first few times I 
tried to reign my chiller reps in =).   

This time, I convinced my rep to give me multiple part load runs holding the 
CHWT constant and varying the CWT incrementally.  This let me build the 
bi-quadratic EIR-FPLR&dT curve as I had at least three different dT’s 
represented in my part load data points.  I plotted the 3D curve in excel to 
check my work and darned if the generated coefficients seem to be really 
accurate =)!   It’s currently looking like a bittersweet revelation however – 
the library curve for a water-cooled centrifugal VSD chiller (see attached 
visualization) seems a LOT more generous (more efficient) at low part loads than 
the one I’ve generated which matches my rep’s data…  I might share a visual of 
my custom curve for comparison once I’m dead-sure it’s accurate – I’m trying to 
clarify a few things with my rep right now. 

~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: John T. Forester [mailto:JohnTF at BVHis.com] 
Sent: Wednesday, November 03, 2010 10:29 AM
To: Nick Caton; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] Chiller Curves (oh boy!) 

Nick, 

When defining a centrifugal chiller in eQuest, one of the items on the Basic 
Specifications tab under the Design vs. Rated Conditions is a Design/Max Cap 
ratio.  By default, this is 92% for a water cooled unit.  I believe this gets at 
the discussion in the help pages that talks about maximum capacity versus design 
capacity and how the chiller vendor will spec a piece of equipment.  Typically, 
vendors don’t often get asked (or provide) what the “Maximum” capacity of a 
spec’d unit is.  Therefore the performance data that they provide are at “design 
conditions.” 

If you change the chiller type to a reciprocating chiller, this “Design/Max Cap” 
ratio is disabled and the default specified condition changes from “Design 
Conditions” to “Rated Conditions.”  This suggests that there is little “extra” 
capacity when a selection is done for that type of chiller. 

If you do get “maximum capacity” data and create curves from that data, you will 
want to change the Design/Max Cap ratio to 1.0 so eQuest knows that there isn’t 
any spare capacity at the chiller.  Also if the data points you are using to 
normalize your curves are different than the design conditions for your energy 
model, you will want to change the “Chiller Specified at” value to “Rated 
Conditions” and enter the rated conditions for CHW temp, CW temp and CW gpm/ton 
to match your normalized curves. 

Hope this helps, 

John 

John T. Forester, P.E., LEED AP, Mechanical Design Engineer I BVH Integrated 
Services I617.658.9008 tel I617.244.3753 fax IOne Gateway Center Suite 506, 
Newton MA 02458 I www.bvhis.com I Hartford ● New Haven ● Boston 

<hr size=2 width="100%" align=center tabindex=-1> 
From: 
equest-users-bounces at lists.onebuilding.org[mailto:equest-users-bounces at lists.onebuilding.org]
 On Behalf Of Nick Caton
Sent: Wednesday, November 03, 2010 11:03 AM
To: Carol Gardner; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Chiller Curves (oh boy!) 

Thanks for the response Carol! 

That 120% load case is what I’m getting at – let me try to explain a little 
further: 

In the DOE2 help files, the vocabulary for centrifugal chillers is “design 
capacity” and “maximum capacity,” where “design” means the capacity at the rated 
or designed conditions (at which you define / specify your chiller), and 
“maximum” means the capacity the chiller is really capable of under the same 
conditions if it runs balls-out (maximum power to the refrigerant drive). 

The help file excerpt I copied below with the red line is pretty explicitly 
telling us to normalize the part load values to the maximum capacity for 
centrifugal chillers.  I’ve highlighted a second line for clarity.  The EDR 
guidelines I linked below are saying you can instead normalize to the design 
capacity for the EIR-PLR curve if that’s all your field measurements or 
manufacturer rep can provide. 

I’m asking – are both approaches right?   

My first and second questions are kinda tied together… How would choosing to 
normalize to either the maximum or design conditions affect how we should handle 
the DESIGN-PLR ratio, if at all? 

~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: Carol Gardner [mailto:cmg750 at gmail.com] 
Sent: Tuesday, November 02, 2010 10:04 PM
To: Nick Caton
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Chiller Curves (oh boy!) 

Let me take a crack at this. If by design capacity you mean the chiller running 
at 100% load, you would create the curve(s) by normalizing around your ARI 
design conditions i.e. the PLR curve would be 1.0 at this point, call it ARI Cap 
and the other points would be 90% Cap/ARI Cap, 80% Cap/ARI Cap, etc. The same 
would go for your temp curves. If, however, your chiller is operating at 120%, 
or some such other level, I would normalize the curve around the ARI design 
conditions of the chiller at 120%. I had to do this for a VRV hp that was 
selected at the 120% design condition.

I find this from the DOE2 manual the most helpful: 

Volume 2: Dictionary> HVAC Components> CURVE-FIT> INPUT-TYPE = DATA 
INDEPENDENT-2 
Used for all curveshaving two independent variables. A list of up to twenty 
values of the second independent variable. The number of values should be the 
same as for DEPENDENT. 

Example 1: defining a curveby inputting a set of data points. 
A packaged system (PZS) has cooling performance significantly different from 
that used in the default  model. The manufacturer lists the data shown in Table 
46, for cooling capacity, at 2000 cfm design air flow rate, as a function of 
outside dry-bulb temperature and entering wet-bulb temperature. 

Table 46  Cooling capacity (kBtu/hr) vs. temperature 
Outside
Dry-bulb Entering Wet-bulb 
72F 67F 62F 
85F 69 65 60 
95F 68 63 (ARI) 57 
105F 65 60 53 
115F 62 55 49 

In this example the independent variables are the entering wet-bulb temperature 
and the outside dry-bulb temperature. Because there are two independent 
variables and they have units of temperature, we input a curveof TYPE 
BI-QUADRATIC-T using the given data points. The dependent variable is not the 
cooling capacity listed in the table but rather the cooling capacity divided by 
the cooling capacity at the ARI rating point (95 F outside dry-bulb and 67 F 
entering wet-bulb). In other words, the capacities should be normalized to the 
ARI rating point., as shown in Table 47 

Table 47  Normalized capacity vs. temperature 
Outside
Dry-bulb Entering Wet-bulb 
72F 67F 62F 
85F 1.095 1.032 0.952 
95F 1.079 1.0 (ARI) 0.905 
105F 1.032 0.952 0.841 
115F 0.984 0.873 0.778 

The CURVE-FITinput will look like the following: 
CAP-CURVE-1 = CURVE-FIT
TYPE               = BI-QUADRATIC-T
INPUT-TYPE         = DATA
DEPENDENT          = (1.000,1.079,0.905,1.032,0.952,0.841,
                      0.984,0.873,0.778,1.095,1.032,0.952) ..
IN-TEMP1           = (   67,   72,   62,   72,   67,   62,
                         72,   67,   62,   72,   67,   62) ..
IN-TEMP2           = (   95,   95,   95,  105,  105,  105,
                        115,  115,  115,   85    85,   85) .. 
Example 2:Defining a curveby inputting coefficients 
We want a furnace to have a constant efficiency as a function of part load. To 
do this we must replace the default FURNACE-HIR-FPLR with a curvethat will give 
a constant efficiency. The curveTYPE is QUADRATIC in the part load ratio (PLR). 
PLR correction curvesare always multiplied by the unit capacity, not the load, 
to obtain the energy (fuel or electricity) use. Thus the curvewe want is: 0.0 + 
1.0*PLR + 0.0*PLR*PLR. The input will look like: 

New-Furnace-HIR-fPLR = CURVE-FIT
TYPE               = QUADRATIC
INPUT-TYPE         = COEFFICIENTS
COEFFICIENTS       = (0.0,1.0,0.0) .. 
Then in the SYSTEM command we include: 
   FURNACE-HIR-FPLR = New-Furnace-HIR-fPLR 

On Tue, Nov 2, 2010 at 3:21 PM, Nick Caton <ncaton at smithboucher.com> wrote: 
Hi everyone! 

I think I have finally wrapped my mind completely around custom chiller 
performance curves for a centrifugal VSD chiller.  I’ve got a few specific 
questions now that I’m on the other side of the fence: 

1.       Is it necessary for the data points of a part load efficiency curve 
(EIR-FPLR&dT in my case) to originate from data with a 1.0 (100%) PLR ratio 
corresponding to a maximum vs. a design load capacity?  From what I gather in 
the EDR reference(re: “Method 2” on PDF page 32/65), this curve can be generated 
using part-load readings assuming a design capacity at the 100% loading mark… 
but the DOE2 help entry for “EIR-FPLR” seems to suggest otherwise (copied below 
– see highlighted line). 

2.       If the above part load efficiency curve is created based on data where 
the 100% loading point corresponds to the maximum (not design) capacity, should 
“DESIGN-PLR” (the ratio of design to maximum capacity) be set to 1.00 and the 
capacity of the chiller be specified at its maximum (not design) for the 
design/rated conditions?  As I write this question it sounds like I’m chasing my 
tail – someone straighten me out =)! 

3.       When you veterans finish a project with sets of custom performance 
curves, do you have any suggestions for a naming scheme for future 
reference/re-use?  I’m currently thinking to keep the curves grouped in an .inp 
snippet I for importing along with an equipment cutsheet… but I’m certain I’ll 
forget the all the details as quickly as humanly possible when this project is 
behind me… 

 ~Nick 

NICK CATON, E.I.T. 
PROJECT ENGINEER 
25501 west valley parkway 
olatheks 66061 
direct 913 344.0036 
fax 913 345.0617 
Check out our new web-site @ www.smithboucher.com 
EIR-FPLR 
Takes the U-name of a curve that adjusts the electric input ratio as a function 
of 

· The part load ratio (PLR) –  The PLR is defined as the ratio of the hourly 
load to the hourly capacity;  Load / Caphour 

· The evaporator/condenser dT -  The temperature differential between the 
condenser and leaving chilled-water. The  meaning of the condenser temperature 
varies according to condenser type.  

For most chillers, the dT has a relatively small effect on part-load 
performance. However, for variable-speed centrifugal chillers, the effect of dT 
is as important as the PLR.  This is because the pressure rise across the 
impeller is proportional to the square of the impeller’s speed. Unless some form 
on condenser temperature relief is employed to reduce the temperature (and 
pressure) differential across the chiller at part load, the performance of a 
variable-speed chiller may not be significantly different than that of a 
constant-speed chiller. 

To model power consumption as a function of the PLR only, use a CURVE-FIT of 
TYPE = QUADRATIC or CUBIC.  To model as a function of both PLR and dT, use a 
BI-QUADRATIC-RATIO&DT curve. The curve must be normalized to 1.0 at full load 
and the rated temperature differential.  

Note that, for centrifugal chillers, ‘full load’ is defined as the ‘maximum 
capacity’, not the ‘design capacity’.Refer to the DESIGN-PLR keyword for more 
information. 

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Carol Gardner PE 

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