new AC duct size advice
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  1. #1
    Join Date
    Apr 2013
    Posts
    27

    new AC duct size advice

    I would appreciate some help evaluating a proposal for renovating an existing central air conditioning system.

    The new units provide 3 airflow rates of 17-21-25 m3/min (618-742-883 CFM) and 24-29-34 (847-1024-1201). As I understand it, I have to allocate a proportion of the max airflow based on the ratio of the cooling requirement for each room. The larger unit has a max airflow of 2040m3/h and the room loads are 2kW, 1,5kW, 3kW and 3kW or proportionate flows of 430, 325, 640 and 640m3/h. One room is a bedroom, the rest are used during the day.

    The smaller unit has a max airflow of 1500 m3/h and the room loads are 2kW, 3kW, 2kW, 1kW and 1,5kW or proportionate flows of 315, 475, 315, 157 and 236m3/h. The last two rooms are bedrooms.

    The old duct is visible and the new proposal varies thus (sizes in metres):

    Larger unit 2040m3/h:
    0,60x0,20 (actual 0,60x0,20), room 1: 0,20x0,20 (actual 0,30x0,15)
    0,50x0,20 (actual 0,54x0,20), room 2: 0,15x0,20 (actual 0,30x0,15)
    0,40x0,20 (actual 0,50x0,20), room 3: 0,25x0,20 (actual 0,30x0,15)
    Room 4: 0,25x0,20 (actual 0,50x0,15)

    Smaller unit 1500m3/h:
    0,45x0,20 (actual 0,65x0,20), room 1 0,20x0,15 (actual 0,50x0,15)
    0,35x0,20 (actual 0,45x0,20), room 2 0,25x0,15 (actual 0,30x0,15)
    0,25x0,20(actual 0,45x0,20), room 3 0,15x0,15 (actual 0,30x0,15)
    0,15x0,20 (actual 0,35x0,20), room 4 0,15x0,15 (actual 0,30x0,15)
    Room 5 0,20x0,10 (actual 0,30x0,10)

    I don’t have airflow numbers for the actual units but I guess we can assume that the newer units are better (faster flow, lower power consumption)

    How can I evaluate the new numbers? I'm worried that the smaller duct might be cheaper but with more noise.

    Also, we are thinking about creating zones the bedrooms, two of which we don’t use much (either automatic or manually grills)- should we consider the max airflow for day/ night use and size accordingly? Or as per above, the max airflow if all rooms were to be cooled (ie bedrooms during the day and lounge during the night)

    Thanks in advance for any input.

  2. #2
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,336
    The metric units confuse me :/

  3. #3
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,233
    Brits or Canadians will need to reply, IF any.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,699
    2040m3/H equals 1200 CFM
    1500m3/H equals 882 CFM
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    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Apr 2013
    Posts
    27
    Hi there, here are US units in decimal (sorry dont know how to make EXCEL format ft/inch). The CFM values are shown in the original post:

    Section Load (kW) CFM Actual width (feet) Actual height (feet) Actual area (feet2) Proposed width (feet) Proposed height (feet) Proposed area (feet2)
    Unit 1 head 1,97 0,66 1,29 1,97 0,66 1,29
    Room 1 (night) 2,0 253 1,15 0,66 0,75 0,82 0,49 0,40
    Trunk 1,77 0,66 1,16 1,64 0,66 1,08
    Room 2 1,5 190 1,15 0,66 0,75 0,66 0,49 0,32
    Trunk 1,64 0,66 1,08 1,31 0,66 0,86
    Room 3 3,0 379 1,15 0,66 0,75 1,15 0,49 0,57
    Room 4 3,0 379 1,15 0,66 0,75 0,98 0,49 0,48
    Unit 2 head 2,13 0,66 1,40 1,48 0,66 0,97
    Room 1 3,5 258 0,66 0,66 0,43 0,82 0,49 0,40
    Trunk 1,48 0,66 0,97 1,15 0,66 0,75
    Room 2 3,0 221 1,15 0,66 0,75 0,82 0,49 0,40
    Trunk 1,48 0,66 0,97 0,98 0,49 0,48
    Room 3 3,0 221 1,15 0,66 0,75 0,66 0,49 0,32
    Trunk 1,15 0,66 0,75 0,66 0,49 0,32
    Room 4 (night) 1,0 74 1,15 0,66 0,75 0,49 0,33 0,16
    Room 5 (night) 1,5 110 1,15 0,49 0,57 0,66 0,33 0,22

    Hope someone can help me evaluate the proposal in the US units!!

  6. #6
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    You seem good at math. Supplies should be under 900feet per minute velocity. Ideally closer to 700fpm. Return 700fpm max, 500fpm ideal. Divide the CFM by the corss sectional area of the duct in square feet and you get velocity.

  7. #7
    Join Date
    Apr 2013
    Posts
    27
    Quote Originally Posted by motoguy128 View Post
    You seem good at math. Supplies should be under 900feet per minute velocity. Ideally closer to 700fpm. Return 700fpm max, 500fpm ideal. Divide the CFM by the corss sectional area of the duct in square feet and you get velocity.
    Thanks for the reference figures, which in SI are:
    900 fpm= 4,6 m/s
    700 fpm= 3,6 m/s
    500 fpm= 2,5 m/s

    I guess these figures are the limits to avoid noise? Should the design slow airflow in the branches to the grills?

    These limits are not being observed at branch ends, I guess distance travelled and friction have to be factored in. Are there tables for typical aluminium covered fibre-glass duct?

  8. #8
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,233
    Quote Originally Posted by ACPM View Post
    Thanks for the reference figures, which in SI are:
    900 fpm= 4,6 m/s
    700 fpm= 3,6 m/s
    500 fpm= 2,5 m/s

    I guess these figures are the limits to avoid noise? Should the design slow airflow in the branches to the grills?

    These limits are not being observed at branch ends, I guess distance traveled and friction have to be factored in.
    Are there tables for typical aluminium covered fibre-glass duct?
    1. No, dP limits.
    2. Yes.
    3. ?. use ACCA Manual D

    https://www.acca.org/industry/system-design/speedsheets
    Good Luck.

    ...............________

    http://www.sciencemadesimple.com/length_conversion.php

    500 feet is equal to 152.40 meters
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  9. #9
    Join Date
    Apr 2013
    Posts
    27
    the duct manufacturer passed me another spreadsheet to cross-check sizing with their specific material and this assumes airflow at the internal unit as 5-6m/s (1230-1476 fpm) and is reducing the duct sizes I have to create an end airflow of 3-4m/s (738-984fpm)

    That seems to go against these recommendations:
    http://www.engineeringtoolbox.com/du...ity-d_928.html

    The ducts will terminate on rectangular plenum facing motorized grills, should I adjust sizing taking into account pressure drops to end in 2,5m/s (500fpm) as suggested earlier?

    Any ideas as to why these sheets are suggesting end airflows of 3-4m/s (590-787fpm)?

    thanks!

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