VRV Outdoor Unit - Standby

[engineer052]

Hi everybody,
i want ask if there is any way to make standby unit to duty VRV outdoor unit, is it possible?
what i need as piping accessories?
what i need for outdoor data connection? for example interface module?
please i need advice.

The Picture below showing outdoor connection

thanks
Mohammed
Mechanical Engineer

 

[Drazen]

what do you need it for? every vrv manufacturer offers their own hydraulic setup, but that has to be included in design.

vrv units of most manufacturers can sustain about 130% of nominal load without trouble, so you already have 30% standby if you design for nominal capacity. if you specify unit with at least two compressors, if one compressor burns out you still have 65% capacity, and for commercial applications usually you can temporarily rearrange priorities until repair is done.

 

[engineer052]

Dear Drazen thanks for quick response
the problem that the owner want 100% backup, so i have to make my design with 100% back up.
this backup required for maintenance and emergency not for drop in cooling load.
another question. if one of compressor burns out from group or module shall we stop all group and clean out refrigerant pipes?

Mohammed Sulieman
Mechanical Engineer

 

[Drazen]

these pipes are not cleaned in regular maintenance.

if you are in design phase, design outdoor unit with 100% provision and you are well, you need to check whether unit is large enough to contain at least two compressors, and to check minimum acceptable load, comparing it with occupancy schedule.

of course, you will run at efficiency bellow optimal almost all the time, but your client likely knows that when posting such demands.

 

[engineer052]

i already considered all that points you mentioned for the load calculation and ambient temperature for outdoor ... etc
for example , if you have one module 36 HP (10+10+16) outdoor, if there is damage in one of three units, what will happen??
we lose this group until make the maintenance , so my question i want to add one more 16HP as stand by unit with this group
it will be (10+10+16)duty + 16HP standby connected to same pipe with isolating valve normally close and open if there is one unit is blackout.

 

[engineer052]

Any new ideas ???

 

[EnergyProfessional]

the refrigerant systems typically are not connected. So your 10,10, and 16 ton units can't be backup for each other. Talk to manufacturer.....
even if you connected all the units to one circuit, one leak will take them all out and you have difficulty meeting ASHRAE 15 and 34 with that much refrigerant.

 

[Drazen]

you can work regularly with all parts while one is in maintenance, but if you insist on 100% reserve, than, as mentioned, design for it, there is no any reason to make improvisations on manufacture proprietary system, you just need to check whether operation with lowest possible building load is acceptable, but range for vrf is very wide and most can work with say 10% of nominal capacity.

 

[EnergyProfessional]

Drazen: The way I understand the three units he needs will have three independent refrigerant circuits and would normally serve three parts of the building (each unit having multiple zones). If one unit is down, the part of the building is down. I'm not aware they offer the option to connect multiple outdoor units to one single refrigerant loop for the indoor units. My information may be outdated and based on a Mitsubishi design a few years ago, though. Another option for redundancy is for the critical spaces to use two indoor units from different outdoor units. but control would be a hassle and having each small room having two units seems costly.

Maybe other manufacturers have different options. but really the next person to talk to should be manufacturers.

 

[Drazen]

herrkaleun, most of larger vrf/vrv systems have more than one outdoor module connected to common refrigeration system. 16 HP mentioned is top capacity for one-module outdoor unit.

It is exactly redundancy why it should be designed as one system within installation limits - if some of outdoor modules fail, others are still working and in most of commercial applications you can easily reset cooling priorities during repair so that most of customers don't even feel lack of capacity.

it is solution offered by manufacturers, their refrigerant piping system are proprietary as a rule, they offer/impose all branch fittings together with calculation/specification software.

everything op wants can be specified within proprietary manufacturer's system - there is no other approach available currently - i would like if it would exist, but it doesn't - dy piping hydraulics is much related to compressor oil which is circulating within system together with refrigerant, and each manufacturer uses own oil.

 

[Ziggypump]

With this being a VRV system it sounds like you would want to size the heat pumps(?) for 100% redundancy and the controls will be a pain. This is something that will need to be discussed with the vendor, as I picture quite the controls issue. There will need to be changeover, failure alarms, run times, god knows how much. Not to mention the refrigerant piping connection sizes and fittings.

I don't know why the system wouldn't be able to do something like this, but close coordination with the manufacturer will be needed. Probably be a very heavy and expensive controls system. Hopefully there's an option.