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a/c air flow rate 3
- Thread starter sam42
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A rule of thumb is about 400 CFM/ton! but it is highly recommended that a heal load calculation is performed to get good performencefrom the A.C. unit there are CD's you can buy for these calculations if you are in California and it's new construction then it's mandatory acording to CEC regulations.
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This thread has been discussed somewhat recently, and the "rule of thumb" varies widely depending upon cooling application. I agree that a load calc should be used, however for residential applications i think 300-350 cfm per ton is more in the ballpark. 400-450 cfm per ton is for very high sensible heat ratio cooling applications such as computer rooms, and electrical room, where there is little latent cooling load. Residences usually have more latent cooling load (SHR in the 0.75 range)therby lowering the cfm per ton of installed cooling capacity.
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400 cfm/ton is standard ASHRAE design criteria. For residential construction,depending on how much glass the house has, 400 to 500 cfm/sq. ft will do--you can extrapolate the tonage from that. I've actually designed closer to 600cfm/sq.ft. before, but there was hardly any envelope load in that instance. If you have a lot of windows facing south or west, better go with around 400cfm/sq.ft.
Imok2 and Cubsfan - as far as reading different material, I try not to read too much... rather I'm going more from memory, and my memory gets foggy sometimes. Sounds like we have 2 votes for 400+ cfm/ton and 2 votes for 300-350 cfm/ton for residential applications. We need some tiebreakers to sound off with their opinions. Now if we were talking square feet of living space per ton of installed cooling capacity, i might be more in agreement, with your numbers... Let's hear from some other opinionated Engineers on this, and tally up the votes. Cheers
Yeldud: I feel bad about asking you to read this next thing especially at turkey time but I have MR. who will varify my story (I think!)
Imok2 - Okay I'm taking the turkey off the table till this is resolved! If you are looking at rules of thumb for sizing tonnage for residential ac units, then the guidelines in the link you provided are applicable. The link recommends the typical range of cfm per square foot for residential ac applications, which, when multiplied by square footage, and divided by 400 will give you a nominal unit tonnage. Just because your numbers work out to a nominal 5 ton unit, rated at 400 cfm/ton, this does not mean you will be circulating 2000 cfm through your residence to satisfy the load. Why? Because the circulating air flow rate required is based upon the sensible cooling loads you are trying to satisfy, which is not represented by the total unit tonnage. For instance you may have a 3.5 ton sensible load, plus assuming 1 ton latent load for moisture removal, gives you 4.5 tons and you select a 5 ton unit. But you only need to circulate enough air for the 3.5 ton sensible load occurring in the spaces, because the latent load removal occurs at the evaporator. Your 5 ton unit does not need to circulate 2000 cfm to do 5 tons of cooling, and in fact after you add the pressure drop of the cooling coil and ductwork, you will probably find the unit incapable of delivering 2000 cfm, even on it's highest blower speed setting. Rather it may be able and only needs to circulate in range of 1500-1800 cfm, depending upon cooling load and system resistance, and if you do the math this works out to an air circulation rate of 300-360 cfm per ton of installed capacity. I think there are two issues being confused here; rules of thumb for sizing equipment tonnage versus rules of thumb for actual air flow circulating rates for various applications. Hope this helps to clarify the issue. Hope all who celebrate it, have a nice Thanksgiving. cheers
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