Quick Check for Sizing Units & Ductwork for Adequate Airflow
Friday, July 3rd, 2009This page does not explain everything you need to know about proper duct sizing a system for optimal comfort, but provides some general guidelines and concepts.
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*You could ballpark the CFM using the static test & the air handler’s graph. You could measure the CFM delivered to each room with a hood Alnor Balometer, it’s usually the best instrument to use, but not cheap. Anemometers are reasonably priced & should be used for checking velocity from diffusers.
Measuring the air velocity from diffuser’s is a bit tricky because you should use the diffuser mfg’ers data which you should always have with you.You can usually get the diffuser mfrs data, say its a 1.5-Ton system that already has 6″rd branch duct runs, to achieve enough CFM airflow, you need close to 700-FPM velocity in the ducts.
I would want to use a diffuser with a little more free open sq.ft area than the 6″ duct which area is 0.19635-sq.ft., say middle of the room in the ceiling; Hart & Cooley 2-way curved blade 12×6 has Ak .235-sq.ft. Free-Air-Area delivering 140-CFM at 600-FPM - diffuser face velocity.
This would help lower the velocity of the duct through the diffuser & reduce air noise.Throw is 7.5-ft toward each wall. Terminal velocity at the occupant level is 75-FPM.
If one diffuser face-velocity were to yield 135-CFM times 5 outlet runs - yields 675-CFM *X’s 26.666-BTUH per CFM = <18,000-BTUH, - that’s 450-CFM per-ton of cooling means less BTUH per one CFM.
Whatever CFM you need for that room or area, divide the Sq.Ft. free-air-area into the requires CFM to get the FPM velocity.Say we’re using 450-cfm/ton of airflow; 12,000-BTUH / 450= 26.6666-BTUH per CFM.
You need 3600-BTUH for that room, 3600 / 26.666 that’s 135-CFM / by .235-sq.ft. diffuser area is only 574-fpm face velocity. Using 400-cfm per/ton / 12000-BTUH is 30-BTUH per CFM.
Taking the manifold gage head pressure & gage condensing temperature is very important data. Coupled with a condenser air discharge temp-reading, if the condenser gage pres/temp is too high compared to the TH reading, there may be non-condensibles in the system.
Also, there is a legitimate formula I use to determine the operating BTUH it is delivering at all the data taken.
All the mfg’ers ought to list the condenser temp-split (it varies with EER & SEER) just like they list the indoor split, it is valuable trouble shooting info.
You can also use the condenser temp-split (it contains both Latent & sensible heat) combined with the indoor data to plot the indoor CFM. I was never good at math, but those equations have to balance, & they do work!
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Quick Check for Sizing Units & Ductwork to Achieve Adequate Airflow Actually, even on service calls where there are cooling problems the ductwork should have a quick Manual D performed.
Then take the ESP static pressure & compare to blower graph or chart, also take the FPM duct velocity.Then do a quick estimate of airflow per equipment tonnage.
To find area of a round duct; Duct diam is 7″; 7″X7″= 49-sq.ins., X’s .7854 = 38.04845-sq.ins divided/ by 144= 0.2672541-sq.ft. area X’s FPM Velocity 600-FPM = 160.35246-CFM X30 = 4,810.5738 each 7″ run X’s 6 branch runs = 28,863-BTUH, or airflow for 2.4-ton.That would also be good for 2-ton; at 550-FPM velocity X’s 0.2672541= 147-CFM X 30 = 4,410-BTUH each run X 6-runs = airflow for 26,460-BTUH.Never sell units requiring more airflow than the duct system & Blower will support! - udarrell