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## 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. The metric units confuse me :/

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4. 2040m3/H equals 1200 CFM
1500m3/H equals 882 CFM

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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. 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.

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Originally Posted by motoguy128
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?

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Originally Posted by ACPM
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.

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

500 feet is equal to 152.40 meters

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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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