Results 1 to 13 of 13
  1. #1
    Join Date
    Nov 2007
    Location
    singapore
    Age
    45
    Posts
    24
    Rep Power
    0

    Thumbs up capacity difference between CU and FCU



    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



  2. #2
    Join Date
    Jan 2009
    Location
    United Kingdom
    Age
    46
    Posts
    242
    Rep Power
    0

    Re: capacity difference between CU and FCU

    Quote Originally Posted by balaji View Post
    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!

  3. #3
    Join Date
    Sep 2007
    Location
    Split Croatia
    Age
    57
    Posts
    6,151
    Blog Entries
    6
    Rep Power
    36

    Re: capacity difference between CU and FCU

    Quote Originally Posted by AbsoluteWDJ View Post
    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.

  4. #4
    Join Date
    Sep 2005
    Location
    UK
    Posts
    33
    Rep Power
    0

    Re: capacity difference between CU and FCU

    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

  5. #5
    Join Date
    Nov 2006
    Location
    At home
    Posts
    181
    Rep Power
    18

    Re: capacity difference between CU and FCU

    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!

  6. #6
    Join Date
    Jan 2009
    Location
    United Kingdom
    Age
    46
    Posts
    242
    Rep Power
    0

    Re: capacity difference between CU and FCU

    Quote Originally Posted by Frostycold View Post
    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.

  7. #7
    Join Date
    Jan 2009
    Location
    United Kingdom
    Age
    46
    Posts
    242
    Rep Power
    0

    Re: capacity difference between CU and FCU

    Quote Originally Posted by nike123 View Post
    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!

  8. #8
    Join Date
    Sep 2007
    Location
    Split Croatia
    Age
    57
    Posts
    6,151
    Blog Entries
    6
    Rep Power
    36

    Re: capacity difference between CU and FCU

    Quote Originally Posted by AbsoluteWDJ View Post
    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.

  9. #9
    Join Date
    Aug 2008
    Location
    Baltics
    Age
    62
    Posts
    786
    Rep Power
    16

    Re: capacity difference between CU and FCU

    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.

  10. #10
    Join Date
    Jan 2007
    Location
    UK
    Age
    53
    Posts
    82
    Rep Power
    18

    Re: capacity difference between CU and FCU

    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!



    PS- tonight I am well into Italics!!
    "I'm anaspeptic, frasmotic, even compunctuous to have caused you such pericombobulation."

  11. #11
    Join Date
    May 2007
    Location
    Auckland
    Age
    74
    Posts
    3,357
    Rep Power
    37

    Re: capacity difference between CU and FCU

    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

  12. #12
    Join Date
    Nov 2007
    Location
    singapore
    Age
    45
    Posts
    24
    Rep Power
    0

    Thumbs up Re: capacity difference between CU and FCU

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

  13. #13
    Join Date
    Sep 2007
    Location
    Split Croatia
    Age
    57
    Posts
    6,151
    Blog Entries
    6
    Rep Power
    36

    Re: capacity difference between CU and FCU

    Quote Originally Posted by balaji View Post
    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!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •