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Can someone explain why a PVVT is usually chosen to model VRV/VRF?

asked 2014-09-26 23:27:23 -0500

yongqingzhao gravatar image

updated 2014-09-30 09:42:58 -0500

As we all know, Packaged Variable-volume Variable-Temperature (PVVT) systems can be used to model VRV/VRF system in eQUEST software, but what I am confused is why a PVVT system can be used.

Load adjustment in pvvt system is implemented by varying supply air flow and supply temperature. But in VRv system,load adjustment is implemented by varying refrigerant flow in each coil. There is a significant difference between the vrv and pvvt system. And l always can not get reasonable result using this way, the heating energy consumption is always too small.

Can someone explain why a PVVT is usually chosen to model VRV/VRF?

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Should the question be expanded to ask for tips as to how you can properly set up a PVVT system to most representatively model a VRV system (such as getting the heating consumption about right, etc)?

Or whether PVVT is more appropriate than PSZ? (LG recommends using a PVVT and give you curves for it, Daikin recommends PSZ and give you curves for it).

Or should both be a separate question?

Julien Marrec gravatar image Julien Marrec  ( 2014-09-27 06:31:22 -0500 )edit

Tank you,Julien.I guess I don't express clearly. Yeah,PSZ is also chosen to model the VRV/VRF system.But this is also what I am confused.PSZ is significant different from PVVT.Why the both system can be chosen to model VRV/VRF. And I had ever modeled VRV system using PSZ.It seems that the simulation result using PSZ is more reasonable.Finally I want to know the following two points: 1.Why PVVT and PSZ can be chosen to model VRV system. 2.Which system is more appropriate.

yongqingzhao gravatar image yongqingzhao  ( 2014-09-27 20:04:18 -0500 )edit

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answered 2014-09-29 20:17:40 -0500

yongqingzhao gravatar image

I get the answer from eQUEST-USER list as following:

For VRF systems, I’ve seen direction to use PSZ, PVVT, and PTAC. PVVT is the approach endorsed/outlined by the Oregon energy trust in a powerpoint PDF that has made the rounds here on the lists a few times, and as such is probably “most suggested.” PVVT by distinction can model variable/multispeed fan control at the terminal level, which may or may not reflect your actual equipment (depends on make/model). Fan control can also be made “constant” in that regard if need be, as such this may be the most flexible solution. PSZ stands out from this trio as a zonal system type – this fact may make it an easier system to consider in relation to a model or baseline you’ve already substantially developed with a zonal system type.

There is more than one right answer, and it’s worth noting/acknowledging VRF is fundamentally different in operation than any system type currently within eQuest.

Review of LG and Mitsubishi’s suggested practices / guidance shows some slight contradictions. Both notably prescribe system type PSZ, to start, but Mitsubishi explicitly mentions PVVT and PTAC as viable alternatives as well. Somewhere along the way I recall reading that energy recovery elements may be more easily accounted for with PSZ over PVVT… can’t recall the specific source. LG by has produced a library of (air-cooled) curves specifically for use with modeling variable speed compressors. Mitsubishi in contrast (perhaps, in response… also for air-cooled VRF) asserts it isn’t critical to explicitly model variable speed compressors to approximate VRF within eQuest, rather that the PLR/EIR/Capacity curves furnished by the manufacturer can, together, adequately address performance.

As I understand it, any chosen approach to VRF in eQuest involves “post-processing” hourly reports to quantify and sum “free” heating/cooling energies. There is some variance in guidance (between the energy trust of OR and Mitsubishi) on how to sum those free heating/cooling energies... both methods appear conservative to my understanding of how VRF works (summing each hour’s energies in isolation of the preceding hourly results), but then I am certainly no expert.

~Nick

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Daikin is a manufacturer that recommends PSZ as a system and gives you curves for it. See here There is a problem in the official daikin curves which is easily fixable (80-character limit)

Julien Marrec gravatar image Julien Marrec  ( 2014-09-30 00:59:15 -0500 )edit

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Asked: 2014-09-26 23:27:23 -0500

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Last updated: Sep 30 '14