RE: [EnergyPlus_Support] Deriving Bi-Cubic performance curves

[Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Thanks, Niraj!
I want to predict Capacity and EIR for these compressors using Saturated Suction Temperature and Saturated Discharge Temperature as inputs.  Those two variables apparently interplay in a bi-cubic form.  Bi-cubic curves use 10 (!) coefficients that are combinations of the two (e.g., X, X^2, X^3, Y, Y^2, .... X*Y^2, Y*X^2, etc.)
I have been using the Excel LINEST function for all curves including linear, quadratic, cubic and bi-quadratic - and it works wonderfully for those.  Not so for bi-cubic... That's my problem; I have a nice solution for other curve types that isn't working for bi-cubic curves.  It could be a limitation of the LINEST function.... but I'm not sure.

The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653
________________________________________
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Niraj Poudel [nirajpdl@xxxxxxxxx]
Sent: Wednesday, September 18, 2013 16:24
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Deriving Bi-Cubic performance curves

Jim,

are you trying to explain some dependent values (say, y) with tabular data of the different independent values (say, x and z)? If so you might want to perform a regression analysis to come up with a curve (model) that explains the dependent variable. I am not familiar with compression objects performance curves, so if I am missing something please do ignore my message.

Niraj


On Wed, Sep 18, 2013 at 3:19 PM, Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx<mailto:jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx>> wrote:
Dear forum,
I am involved with a modeling project for a refrigerated warehouse.  The compressor object expects a bi-cubic performance curve, so I am busy making an Excel template in which to place tabular data and derive curve coefficients.  I've never used b-cubic curves before, but it appears that my normal method of curve derivation in Excel does not work for these curve types. Since I'm no expert on this topic, it is very likely that I'm missing something!  Have any of you derived bi-cubic curves?  Are you willing to share some insights?

The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653<tel:616%20450%208653>
________________________________________
From: EnergyPlus_Support@xxxxxxxxxxxxxxx<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx> [EnergyPlus_Support@xxxxxxxxxxxxxxx<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>] On Behalf Of jeannieboef@xxxxxxxxx<mailto:jeannieboef@xxxxxxxxx> [jeannieboef@xxxxxxxxx<mailto:jeannieboef@xxxxxxxxx>]
Sent: Wednesday, September 18, 2013 13:05
To: EnergyPlus_Support@xxxxxxxxxxxxxxx<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>
Cc: EnergyPlus_Support@xxxxxxxxxxxxxxx<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>
Subject: Re: [EnergyPlus_Support] Merging zones having FCUs (Merging FCUs)

Or work backwards... Total real FCU Rated powers = Total Rated FCU Volume Flow * Pressure Equivalent / (flow weighted average efficiency)

...just not sure if the efficiency should be flow or power weighted.

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On 18.09.2013, at 17:32, Richard Raustad <RRaustad@xxxxxxxxxxxx<mailto:RRaustad@xxxxxxxxxxxx><mailto:RRaustad@xxxxxxxxxxxx<mailto:RRaustad@xxxxxxxxxxxx>>> wrote:



Obviously there is a flaw in the DB approach. Try averaging the fan
pressure rise instead.

Fan:ConstantVolume,
75.0, !- Pressure Rise {Pa}

On 9/18/2013 6:38 AM, hisham.rashrash wrote:
> Hi,
> For one case, I tried to merge actual zones into larger thermal zones. Now, I have to model FCUs, using TemplateHVAC:Zone:FanCoil objects, in such a way that I put one FCU in each zone that must represent 4, 5 or even 11 real FCUs for example. I used the following approach:
> 1. I used the formula from DesignBuilder website (http://www.designbuilder.co.uk/programhelp/fans.htm) to find Fan Delta Pressure:
> Delta P = 1000 * SFP * Fan total efficiency (SFP: Specific Fan Power [W/(L/s)]
> I calculated each real FCU's Fan Delta P and added them up to find the equivalent FCU's fan Delta P.
> For example:
> I have 4 FCU's one for each of 4 guest rooms that I merged into one large room for simplification, the actual HVAC schedules give the following:
>
> FCU No. Power [kW] Air Flow [L/s]
> ==================================================
> 1 0.3 160
> 2 0.3 145
> 3 0.3 145
> 4 0.3 145
>
>
> Eqv. Delta P= 1000*(300[W]/160[L/s]+3*300[W]/145[L/s])*0.7=5657.33[Pa]
>
>
> 2. I found the equivalent FCU's airflow rate by adding the real FCUs' flow rates:
> Eqv. Flow Rate=160+3*145=595[L/s]
>
>
> I used the above values in the HVACTemplate:Zone:FanCoil object for the corresponding zone. The results were that the equivalent FCU's power calculated is 5 times larger (about 6[kW]) than actual component-FCUs' power when added up:
> >From table above: 0.3[kW/FCU]*4[FCUs]=1.2[kW]<6[kW]
>
>
> Please advise.
>
>
> Thank you very much.
>
>
> Regards,
> Hisham
>
>
>
>
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--
Richard A. Raustad
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Florida Solar Energy Center
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Niraj Poudel, Architectural Engineer.
PhD student, PDBE Program.
Clemson University, Clemson, SC.
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