VRF performance curve question

asked 2015-11-30 12:12:57 -0500

ngkhanh
2156 ●1 ●3 ●12

updated 2015-11-30 12:49:36 -0500

I am trying to create and import VRF system performance curve into Openstudio. There are some great articles from FSEC about VRF model in EP. I have several questions about:

Cooling/heating combination ratio correction factor curve : There are 2 separated zones for that curve : one for PLR <= 1 and one for PLR >1 but there is only one input for curve (different with Cooling energy input ratio modifier function of Low/high part-load ratio curve which are individually curves input for PLR <=1 and PLR >1). I have question that if combination ratio installed capacity (ID cooling capacity / OD cooling capacity) >1 but most of time , the system has never reached PLR >1 (because of load profile and sizing factors) so will simulation get right result with PLR >1 curve or we must change Cooling/heating combination ratio correction factor curve inside HVAC iteration loops by EMS for annual period (weather file) simulation?
Cooling capacity ratio boundary curve and cooling boundary ratio boundary curve: what's difference between 2 curves? I see that they have same coefficient values in default OS VRF setup
Indoor VRF terminal DX cooling coil model has much differences with normal DX coil (PTHP, unitary,...), is it right?
Input for new VRF model added in EP 8.4 : May we keep the old performance curve or need to renew them all? is there any guide for configuration new VRF model in 8.4?

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answered 2015-11-30 13:00:27 -0500

rraustad

13599 ●72 ●12 http://www.fsec.ucf.edu/

updated 2015-11-30 13:53:03 -0500

The combination ratio curve is used to calculate a multiplier when the installed total indoor unit capacity is greater than the outdoor unit capacity. If the installed indoor unit capacity is less than or equal to the outdoor unit capacity, a combination curve is not required and the assumed multiplier is 1. EMS cannot be used to adjust this performance curve result. It's usefulness comes when you decide to add another indoor terminal unit and perform a different simulation. If the new indoor unit capacity is greater than the outdoor unit capacity, the multiplier is automatically calculated for you.
As this document explains, only 1 Cooling Capacity/Energy Ratio Function of Temperatures curve is all that is required. However, most if not all VRF systems have very distinctly different performance at low and high outdoor temperatures. Thus users will typically model the VRF system using 2 performance curves. The boundary curve tells the program which performance curve to use based on outdoor and indoor temperatures.
Yes, the VRF coil model is different from the typical DX coil and actually requires fewer inputs.
No, the the new VRF model in V8.4 does not use the capacity and energy input ratio performance curves. It is instead a component model that relies more on physics to determine performance. Do not use the old curves.

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Since this question was tagged with OpenStudio, I'll mention that the new VRF model is not yet supported in the API. It will show up in a minor release between 1.10.0 (due in a couple weeks) and 1.11.0 in late March.

ljbrackney ( 2015-11-30 14:34:03 -0500 )edit

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answered 2015-12-01 02:49:59 -0500

ngkhanh
2156 ●1 ●3 ●12

updated 2015-12-01 02:52:50 -0500

Thank for your explanations. I would like to confirm some outline as follow:

"The combination ratio curve is used to calculate a multiplier when the installed total indoor unit capacity is greater than the outdoor unit capacity. If the installed indoor unit capacity is less than or equal to the outdoor unit capacity, a combination curve is not required and the assumed multiplier is 1" it means that regardless the total ID capacities is less than or even with OD unit, we still need to add curve of PLR >1 for the combination ratio curve. Simulation program would not use it when check current combination ratio less or even 1, is it right ? .

2.from this [www.osti.gov/scitech/servlets/purl/10...], there are some sample table and I want to check whether I rightly understand : table A4: Cooling Capacity Ratio Modifier Function of Low/High Temperature Curves and Cooling Energy Input Ratio Modifier Function of Low/High Temperature Curve Name ; table A5: Cooling Capacity/Energy Input Ratio Boundary Curve ; table A6: Cooling Energy Input Ratio Modifier Function of Low/High Part-Load Ratio Curve Name; table A7: Cooling Combination Ratio Correction Factor Curve Name (low for PLR <=1; high for PLR >1).

Thanks

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Table 19 in the document link in answer 2) above explains that combination ratio is a function of the cooling capacity ratio, not PLR. I deleted the CR curve in the example file VariableRefrigerantFlow_5Zone and the simulation runs. So the answer is no, you only need the CR curve if you plan to model a system with indoor unit capacity > outdoor unit capacity.

You can use the curves in the verification document, however, you will be modeling the performance of the unit chosen to perform the validation. If your unit performs differently, new performance curves should be created.