[Equest-users] Air-cooled Chiller EIRFPLR curve in eQUEST

Tue Oct 20 09:13:55 PDT 2015

Hi,

Bill made great points on the rated v design as well as "Capacity" being
mislabeled.

I'll try to get slightly more directive regarding how to create the curve
from the data you have (per my understanding at least... correct me if I'm
wrong) as well as trying to highlight that one needs to be real careful
when fitting a biquadratic curve...

*Normalized curves*
First, you have to remember that this curve is "normalized" at the
rated/design conditions, meaning the dependent variable Z is 1.0 when
independent variables (X, Y) are at rated conditions.

In your case, you need two independent variables: the PLR, the
evaporated/condenser dT (in your case air-cooled chiller it's dT between
outdoor air temperature and leaving chilled water) and one dependent
variable EIRFPLR.

For the EIRFPLR value, my understanding is that you should calculate as
InputkW_i / InputkW_n (where 'i' is at the specific operating point and 'n'
is the rated value).

*See attached spreadsheet for how I understand you should fit this curve
(red columns)*

*It's critical that you set the RATED conditions properly because of this
normalization.*

*Curve fitting with 10 points...*
*Please also note that fitting a biquadratic curve (essentially a 3D
surface) with only 10 points will result in not so great results... *
If you want to get any accurate, you need to get a broader range of
conditions, when outside temperature is 90F but the PLR is 50% for example
(which can happen quite often if you have temperature setbacks for example,
or simply an oversized cooling plant for that matter...).

*If you try fitting with those 10 points, by entering them as a curve fit
with "raw data points" in eQuest *(or doing the curve fit by
yourself...) *you'll
get some coefficients, no problems here, but do they actually represent
something physical?* The mathematical fit is actually really good, but
that's not quite enough. (a biquadratic curve has 6 coefficients, for 10
points, overfitting is bound to occur...)

Try and plot that and you'll see that:

1. It is SIGNIFICANTLY different than any curve already in eQuest
2. Some quite non-physical things can actually happen. Like if you have
80°F outside, and a PLR of ~30%, you get a resulting EIR that's negative...
(3. In your case this isn't such a problem because "thankfully" you have
three datapoints at the same dT but with different PLR, which makes it a
little bit more robust, but the rest of them can almost be described as a
line rather than a surface. Often you can find radically different surfaces
that will produce the same almost perfect R²...)
4. I'd bet that if you get 2 more datapoints at different conditions you'll
get a completely different curve (overfitting)

Here is an interactive 3d graph
<https://plot.ly/~jmarrec/214/zfx-y-biquadratic-curve-fitting/> (orbit
holding left click, and zoom with your mouse wheel) on which you will find:

- A 3d surface for the fitted curve, based on your data entered in eQuest
as a curve fit with "raw data points" and getting the coefficients from
there
- A 3d surface (grey colorscale) that is one of the curve from eQuest's
library ('RecipAir-EIR-fPLR-3Comp/Ckt') for reference
- A 3d scatter plot (the crosshairs are the datapoints from your
manufacturers)
- A translucent neutral plane (EIRFPLR=0): anything below that plane is
unphysical

See what I mean?

Hope this was somewhat informative and not just too long to read...

Best,
Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr
<http://www.linkedin.com/in/julienmarrec>

2015-10-19 20:58 GMT+02:00 Bishop, Bill <bbishop at pathfinder-ea.com>:

> Kusum,
>
>
>
> The entry for “Chiller Energy Consumption” in the DOE-2 help files is a
> good place to start. It explains how hourly chiller energy is determined.
>
> The three curves that determine hourly chiller energy are EIR-FT, EIR-FPLR
> and CAPACITY-FT. I believe the data in your attachment could help you
> create a custom curve for any one of these inputs, given the other two. I
> would create a custom EIR-FT curve based on the default eQUEST curves for
> EIR-FPLR and CAPACITY-FT for an air-cooled chiller, but you could create a
> custom EIR-FPLR curve using default curves for the other two as well.
>
>
>
> The independent variables (knowns) that you have based on your
> manufacturer’s data are CHW-T, DBT, (or t1, t2 per the help menu
> description), Load, rated Capacity and EIR. (More correctly, you have *design
> capacity* at 105F since the eQUEST *rated capacity* is based on 95F
> outdoor drybulb.)
>
> The dependent variables are hourly capacity as a function of temperature
> Caphour, and PLR which is a function of load and Caphour.
>
> The manufacturer’s data column for “Capacity” is mislabeled. It is
> actually load, as seen by multiplying the “Percent Loading” column by the
> design capacity. Actual chiller capacity should *increase* slightly with
> decreasing ambient temperature. So the table doesn’t actually show you the
> part load ratio PLR, which is the load divided by the hourly capacity.
>
>
>
> Regards,
>
> ~Bill
>
>
>
> *William Bishop, PE, BEMP, BEAP, CEM, LEED AP **|** Pathfinder Engineers
> & Architects LLP*
>
> *Senior Energy Engineer*
>
>
>
>
>
> 134 South Fitzhugh Street                 Rochester, NY 14608
>
> *T: (585) 698-1956*                        F: (585) 325-6005
>
> bbishop at pathfinder-ea.com <wbishop at pathfinder-ea.com>
> www.pathfinder-ea.com
>
> [image: http://png-5.findicons.com/files/icons/977/rrze/720/globe.png]Carbon
> Fee and Dividend - simple, effective, and market-based.
>
>
>
> *From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] *On
> Behalf Of *John Aulbach
> *Sent:* Monday, October 19, 2015 12:26 PM
> *To:* Kusum Sekhalia <skusum3 at gmail.com>;
> equest-users at lists.onebuilding.org
> *Subject:* Re: [Equest-users] Air-cooled Chiller EIRFPLR curve in eQUEST
>
>
>
> Hi Kusem:
>
>
>
> I don't have too much time, but if you are holding a 44 DegF CHW temp and
> varying the outdoor temp (essentially your condenser), aren't you simply
> changing the capacity of the chiller based on outdoor temp. I believe you
> need to leaving the outdoor temp at a fixed temp, then apply your part load
>  and the equivalent kW.
>
>
>
> eQuest will vary the chiller peak based on outdoor loadwith onwe of the
> other chiller curves (guys, help me).
>
>
>
> John Aulbach
>
>
>
>
>
> On Monday, October 19, 2015 9:05 AM, Kusum Sekhalia <skusum3 at gmail.com>
> wrote:
>
>
>
> Hi all,
>
>
>
> I am trying to create an air-cooled chiller EIRFPLR curve but am getting
> errors. I am not able to figure out what am I doing wrong? I have attached
> the part load data from the manufacturer. Can someone help me or provide me
> some information which shows step by step on how to create the curve. I am
> using the Design guidelines HVAC simulation pdf to create the curve but not
> successful so far. Please help.
>
>
>
> Thank you in advance.
>
>
>
> Kusum Shekhlia
>
>
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