VRVII index related questions

[Peter_1]

Let’s take a REYQ28M with a nominal cooling capacity of 78.5 kW (Indoor temp. : 27°CDB, 19.0°CWB / outdoor temp. : 35°CDB / Equivalent piping length : 7.5m according to catalogue)
We connect indoor units with a total index of 100 to this outdoor. Machine will deliver 78,5 kW.
Machine will evaporate at let’s say 0°C (TE). Ambient is 32°C.

We then connect indoor units for an index 130 at same ambient. What TE will we see then and moreover, what will then be the new TC at same ambient conditions?

I had a discussion yesterday where I said that with an index of 130, compression ratio will decrease, COP will increase because evaporating pressure will increase and the influence of an increase of TE is much larger than the same increase in TC.
In the hypothesis that a 5K TE increase should result in a 5K TC increase and all other parameters remains, even then COP will rise and compression ratio will decrease.

If there will be anyhow an increase in TC with an index 130! The condenser is made for an ambient of 39°C, so it’s possible that with an ambient of 32°C and an index of 130 TC will stay stable.
So if TC stays stable at 32°C with an index of 130, then the savings are not that small.

Is TC kept stable with the M series and if so, at what level? To a fix TC or floating with ambient?

May these machines run for longer time full load at an index 130?
If they run for a longer time on an index 130, will the total efficiency then be smaller or larger then running at constant 100?

Can this be checked or simulated with the VRV Express?

all this is not only related to a VRV but also to any VRV-like system.

Any comment please?

[Rob White]

.

I thought the whole idea of a VRV was to allow for
the flexibility of running multiple evaporators?

So therefore you will never be running at 100% capacity
due to the nature of evaporators ramping up and down?

Indexing to 130 does not mean that the system will be
running at 130% of it capacity because all the evaporators
will not be running at the same time?

The idea of a VRV is to maintain a Back pressure (suction pressure)
and Head pressure (discharge pressure) to a point where they
are at the correct point as set in the parameters. Compression ratio
is kept as low as possible because running a compressor is much more
expensive than running either the condenser or evaporator fans.

So discharge temperature needs to be the bare minimum above ambient
(normally about 15°K) and the evaporating temp needs to be the bare
minimum under air off evaporator temps (normally about 10°K).

Floating the head pressure and the suction pressure is what allows the over
sizing of the whole system to about 130%, the evaporators and the condensing
unit don't actually need to talk to each other in real terms (they do I know).

The whole purpose of the system is for the outside unit to run on just maintaining
pressures and the evaporators ramp up and down only to control the temp of
the room they are in.

So in my opinion indexing to 90, 100, 130, or even 150 will have
no real effect on the COP rating because the system will never
run at 90%, 100%, 130% or even 150% all the time, hence the
term Variable.

Regards

Rob

.

[install monkey]

well stated rob, the term "diversity", although your indoor capacity is greater than the outdoor capacity some units will lose performance when the system is at full demand, only as when they get nearer to the setpoint they perform as per indoor capacity states.

[Peter_1]

I need to clarify: in an important case I'm following closely, one said that you never may attach more than index 100 to an outdoor unit. If you do so, then there's a possibilty you overload the outdoor unit because...compression ratio will increase, COP will drop and compressor will wear more due to the increased massflow.

I also said these machines reduce themselves to protect them from overloading or overheating.

The original installer calculated 130 kW needed indoor capacity and installed 100 outdoor capacity. This was a big fault and the main reason why several compressor failed. I disagreed with him.
The only thing which will happen is that the outdoor machines will run longer but they will not run at 130% in some sort of an overdrive as most non-technicians (lawyers) thought on that meeting. They only can run at 100% and that's it.

What we will see in case all indoor should ask at once for cooling is an increased suction pressure resulting in a reduced compression ratio and an increased COP. He disagreed with me.

Therefore also my question: if you run at 100, ambient 30°C, condensing temeprature 40°C and you increase to 130 indoor, is there enough condensor surface available to maitain 40°C? Anyhow, an increase of 5K at suction side will only result in +/- 2K on condnsor side.

[Peter_1]

Narkom, this is not valid for all systems and especially not for a VRV where the condensor is designed for full load. Your schematic is too general but thanks for your input.

[Rob White]

Therefore also my question: if you run at 100, ambient 30°C, condensing temeprature 40°C and you increase to 130 indoor, is there enough condensor surface available to maitain 40°C? Anyhow, an increase of 5K at suction side will only result in +/- 2K on condnsor side.

I think I understand what you are explaining peter.

With a correctly sized VRV the condenser is sized for the total
load. When the capacity varies the fan speed increases
and decreases to allow for this, so if the load increases then
the only way to increase the size of the condenser is to increase
the air flow over the condenser by speeding up the fan. If this is
within the design specification then it should work OK?

Regards

Rob

.

[install monkey]

http://www.mediafire.com/view/?cc302n8gdcr1xyt install manual for a vrv system stating index limits per system. no one installs a vrv with indoor capacity matching the outdoor, its normal to go 130% over on indoor capacity, but if you've added an extra 30% to an existing system then pipe sizes may need to be recalculated as well as additional charge