capacity difference between outdoor and indoor in VRV

[balaji]

why the capacity of outdoor unit should be more than indoor unit in VRV, if it is how much it should be, how to calculate this anybody pls

[Brian_UK]

Read the design book for the VRV systems.

[balaji]

which book you want me to refer there is no direct answer for this question if anybody knows pls reply me immediately

[ebathiya]

first u need to make a load calculation for required space then select the outdoor units and indoor units it is simple both capacities must be same doesn't nessary to higher outdoor capacity than indoor.if u r using a extra capacity u hv 2 think about ur power consumption.

[ebathiya]

wt is the brand ur vrv, all the selection depend on their brand and engineering data books r differ from brand 2 brand.

[balaji]

first u need to make a load calculation for required space then select the outdoor units and indoor units it is simple both capacities must be same doesn't nessary to higher outdoor capacity than indoor.if u r using a extra capacity u hv 2 think about ur power consumption.
thanks for your reply but as per plan the outdoor units should be installed in roof top so we have to consider the pipeline distance if both outdoor and indoor are same capacity it wont be woking efficiently
similar as a pump size will vary based on head if iam wrong correct me pls

[balaji]

thanks for your reply but as per plan the outdoor units should be installed in roof top so we have to consider the pipeline distance if both outdoor and indoor are same capacity it wont be woking efficiently
similar as a pump size will vary based on head if iam wrong correct me pls
what happen to the experienced persons nobody clarifying my doubts
why iam asking silly ?

[Philip]

what do you mean of VRV?sorry not familiar to this Abreav..regarding your concern,capacity bet.indoor and outdoor vary defends on the Relative Humidity required for your aplication.the higher the difference the lower the rh.about your concern on pipe length and pressure draft,just have right design on pipe size and you will be worry free.(the longer pipe run the bigger suction line)

hope this is acceptable to you

[Thermatech]

The manufacturers technical data book should have a page with a graph showing the system overall performance reduction for long pipe lengths.
For example
When the system is operating in cooling with long pipe length there is a pressure drop from the indoor units to the outdoor unit on the suction pipe.
The outdoor unit suction pressure sensor tells the outdoor unit circuit board what the suction presure is at the outdoor unit & the outdoor unit trims compressor speed to achieve its target suction pressure.
But
because of the long pipe length to the indoor units the actual suction pressure back at the indoor units is much higher pressure than at the outdoor unit. The higher suction pressure at indoor units also means higher suction temperature & so reduced cooling performance.

For example
For many years Daikin & Hitachi had an important dip switch on the outdoor circuit board. Pipe length dip switch. The commissioning engineer set this dip to match the installed pipe lenght to furthest indoor unit.
For longer pipe lenghts the outdoor unit sets a lower target suction pressure / temperature which makes a lower suction / temperature at the indoor units which makes better cooling performance at the indoor units.
But
This really only works at part load conditions.
When the outdoor unit compressor gets to maximum speed it cannot ramp up to higher speed to maintain target suction pressure / temperature.

Typically maximum pipe length installation will have reduction of about 10 to 15 % cooling performance due to pipe pressure drop.

So you might want to slightly oversize the outdoor unit so that it can cope with the long pipe lenght & maintain correct cooling performance at the indoor units for situations when all indoor units are at maximum cooling.

Stongly suggest you get advice from the manufacturer.

BTW
Daikin VRV3 seems to do some clever stuff during initial test run & the outdoor unit calculates its ideal tearget pressure / temperature so the commissioning engineer does not have to set any pipe length dip switch any more.
Perhaps the resident Daikin chaps can provide a detailed explanation of how the outdoor unit does this?

[BritCit_Juve]

Basically all manufacturers will have different rules but the important point to remember is that the system will only ever acheive the condensing units maximum ACTUAL output.
For most of the manufacturers the rule has always been that the outdoor unit can have upto 135% of its rated capacity attached on the indoor end (MHI have now brought that upto 150%). So for example they say you can attach indoor units with a total capacity of upto 37.8kW to a 28kW machine however if all the units required cooling or all units required heating then they will only do a percentage of their rated capacity (ie 10kW rated indoor unit would only be capable of 7.4kW (10x(28/37.8))).
Also the manufacturers rate both their indoor units and outdoor units for cooling at 27°Cdb 19°Cwb in room, full fan speed with an external ambient of 35°C and no piperun. The indoor unit capacity will therefore need correcting for actual fan speed, actual room and ambient conditions, total piperun, total height differance lowest to heighest indood and lowest or heighest indoor to outdoor as well as the capacity of the indoor units attached.
The all use different factors for these so you would have to use the manufacturers design guide or selection software.
For three pipe these rules also work but there are usually small adjustments.
Never put a unit on a VRV system if it is in a server room as they are going to need a relatively stable cooling capacity not one that is going to be changing as units chop in and out of the system as the outdoor unit may need to shut itself down to adjust outdoor coil capacity as the load changes
Brit

[mkardehi]

in common VRV systems usually indoor capacity divided by outdoor capacity could be between 90% to 135% but in reallity as Britcit Juve said outdoor unit cannot give more than 100% of it's capacity so in cases of large indoor units capacity reduces.
And about your question , you should first select your indoor units then adding them together and perform some corrections because of extra pipe length or extra ambient teperature according to your VRV technical ,then you can select a proper outdoor which may be a little bigger than your indoor units but not more than 10%

[mekaniko5]

I think these guys are correct. You can't assume just any number because this is manufacturer specific. You also have to consider the pipe lenghts which will surely add to the constraints of choosing the correct balance between your indoor and outdoor. I came across the Digital Scroll system being used by different OEMs and is also a VRF system. You may want to take a look at how they tackle capacity ratios at their website.