capacity difference between CU and FCU

[balaji]

Hi,
I saw some of the consultant design in my office most of the consultant designed the CU capacity lower than the FCU.is it correct
for example

A 90 kw VRV CU is connected with 14 kw x 10 nos of FCU that means 140 kw
Is it correct will the system perform efficiently i cant understand what should be the standard

from my point of view i feel that both should be same or its better to slightly upsize the CU bcoz of pipe run

anybody pls
How to calculate the CU capacity based on FCU

[AbsoluteWDJ]

Hi,
I saw some of the consultant design in my office most of the consultant designed the CU capacity lower than the FCU.is it correct
for example

A 90 kw VRV CU is connected with 14 kw x 10 nos of FCU that means 140 kw
Is it correct will the system perform efficiently i cant understand what should be the standard

from my point of view i feel that both should be same or its better to slightly upsize the CU bcoz of pipe run

anybody pls
How to calculate the CU capacity based on FCU

If the office space requires 90kw I don't see a problem. If the office space requires 140kw there would be an issue when system is in full demand. It will depend on design conditions and heat load of office space. I believe they should match condenser size with fan coil total size but sometimes it comes down to cutting costs to win the contract!

[nike123]

If the office space requires 90kw I don't see a problem. If the office space requires 140kw there would be an issue when system is in full demand. It will depend on design conditions and heat load of office space. I believe they should match condenser size with fan coil total size but sometimes it comes down to cutting costs to win the contract!

Maybe consultant has in mind factor of simultaneity which is lower than 1. Common practice with large number of indoor units.

[Frostycold]

This is quite common in VRV/VRF. The outdoor unit is rated at 28kw cooling & 31kw heating but when one adds up the capacities of the indoor units they exceed by 40% eg 10 indoor units adding up to say 40kw. This is known as Building Divergence. this is based on the assumption that all the areas served by the units do not require the full capacity of the outdoor unit at the same time. The system will not deliver more than the outdoor unit is capable of delivering ie in this example 28kw cooling & 31kw heating however the unit in the Boardroom is only required when the room is occupied but the comms room is allways on. The 28kw/31kw figures used in this example are from a Mitsubishi who work on a building divergence of 50%. The only time one experiences problems is either in winter during a cold snap when all units require heating simultaneously or in summer when all units are calling for cooling. The units central control system takes this into consideration & priorotises the units in its circuit. This normally shows as a callout because someone complains that the A/C isnt working but when one checks the systems faulty codes no codes are there because the system has no faults.
Hope this answers your question.
regards
Frostycold

[SkyWalker]

spot on frosty, the only thing i would add is steer clear of putting Fan coils which are part of the VRV in a server room as this area is to critical and requires its own systems.

i have seen this done on a two pipe system and well...... come winter whole floor want heating but the server still needs cooling! big DOH!

[AbsoluteWDJ]

This is quite common in VRV/VRF. The outdoor unit is rated at 28kw cooling & 31kw heating but when one adds up the capacities of the indoor units they exceed by 40% eg 10 indoor units adding up to say 40kw. This is known as Building Divergence. this is based on the assumption that all the areas served by the units do not require the full capacity of the outdoor unit at the same time. The system will not deliver more than the outdoor unit is capable of delivering ie in this example 28kw cooling & 31kw heating however the unit in the Boardroom is only required when the room is occupied but the comms room is allways on. The 28kw/31kw figures used in this example are from a Mitsubishi who work on a building divergence of 50%. The only time one experiences problems is either in winter during a cold snap when all units require heating simultaneously or in summer when all units are calling for cooling. The units central control system takes this into consideration & priorotises the units in its circuit. This normally shows as a callout because someone complains that the A/C isnt working but when one checks the systems faulty codes no codes are there because the system has no faults.
Hope this answers your question.
regards
Frostycold


Exactly. Thats why I'm not a fan of undersizing outdoor systems.

[AbsoluteWDJ]

Maybe consultant has in mind factor of simultaneity which is lower than 1. Common practice with large number of indoor units.

Maybe...Do you agree with this though Nike? At the start of the year a friend of mine installed a City multi, he was the installer and not the main contractor. When we had a cold spell (around -10c ambient) the customer complained the newly installed system wasn't heating and had to send their staff home. My friend was called in and he had to reclaim refrigerant. The refrigerant amount was correct! So then Mitsubishi was called in. The outcome was that the contractor/design had designed a system with a lower outdoor capacity than indoor capacity. Thus when it mattered the most the system struggled to achieve a reasonable heating load. This is why I don't agree with it. I'm aware that all manufactured VRV/VRF has the potenial to do this and I'm sure the consultants use this in their favour to reduce costs!

[nike123]

Maybe...Do you agree with this though Nike? At the start of the year a friend of mine installed a City multi, he was the installer and not the main contractor. When we had a cold spell (around -10c ambient) the customer complained the newly installed system wasn't heating and had to send their staff home. My friend was called in and he had to reclaim refrigerant. The refrigerant amount was correct! So then Mitsubishi was called in. The outcome was that the contractor/design had designed a system with a lower outdoor capacity than indoor capacity. Thus when it mattered the most the system struggled to achieve a reasonable heating load. This is why I don't agree with it. I'm aware that all manufactured VRV/VRF has the potenial to do this and I'm sure the consultants use this in their favour to reduce costs!

That could mean two things:
Bad design
Conditions out of range predicted by design

When selecting and designing equipment it is first to know what is your task.
If your task is to ensure that in all predictable conditions you have available full capacity, then you need to design outdoor capacity accordingly.
If your task is to ensure that in most time you have covered most demands than design could be very different.

That all depend on usage.

[Yuri B.]

The unhappy inhabitants of the premices (already) served by VRF with undersized outdoor unit now have to have in winter at hand a back-up heating system - the conventional electrical radiators.

[laf100]

With VRV/VRF it is essential that if you rely on diversity (ie greater indoor capacity, than outdoor) that the indoor fan coils are sized up correctly. If they are, they will have enough off cycle time for the outdoor duty to be "shared" between them.

If, however the sales engineer has tried to win the job by sizing slightly smaller fan coils than actually needed, when there is very high heating/cooling demand, none of the fan coils will ever hit setpoint, and will all be thermostat on, all the time, leaving the outdoor NO CHANCE!

:cool:

PS- tonight I am well into Italics!!

[Magoo]

Generally speaking not all FCU are going to at full capacity at the same time, in a building north face versus south face, east face versus west face. The sun does not shine in all faces at the same time. Load varies at each quarter. Apart from server rooms as mentioned earlier post, where load is constant.
So consultants design .. say 75% loadings. Lowers capital cost of install equiment, also design conditions., inside versus outside probably 1.5 months of the year.
magoo

[balaji]

Thank you guys it will be more interesting if you discuss with proven technical terms and examples

[nike123]

Thank you guys it will be more interesting if you discuss with proven technical terms and examples

We, in the mean time, gladly awaiting for your examples and proven technical terms!