View Full Version : CFM per ton range
I'd like to ask a question to our pro's: What is the highest and lowest CFM/ton airflow you would consider reasonable in a given climate? I am well aware Trane documents 450 cfm/ton for dry climates and 350 for humid climates. Yet have heard other professionals talk about installations which are outside this envelope.
This is not a practical question for me, I'm content to stay within manufacturer's recommendations for at least as long as my (extended) warranty lasts. Just want to hear some professional discussion and learn from you if you have the time. It's fascinating to learn what others do, how they analyze things.
Thank you -- Pstu
[Edited by pstu on 10-24-2006 at 05:08 PM]
10-24-2006, 08:37 PM
For almost any standard system that's installed on the face of the earth, and here's the key part, is sized correctly, that 350-450 cfm per ton will work perfect. Granted you can throw in some other cfm ranges with variable speed blowers, but they don't consistantly stay at a lower speed, they eventually ramp back up to (or very close to) that 350-450 cfm mark.
If you have to go much outside of those specs (which is one of the only rules of thumb you can really use in HVAC) there's something else that's the problem.
Now on high velocity systems I'm not 100% sure what CFM ranges they use... but they are a different breed of HVAC.
I had read a couple of pros post on this board, that in high-humidity applications they chose to slow down airflow to perhaps 250-300 cfm. Of course they paid careful attention to coil temperature and refrigerant metering -- I know freezing the coil would be an obvious failure, but I am not familiar with all the things that could go wrong. The late Airman1 was one such pro as I remember, and there were others.
This assumes a "normal" AC system, not a high velocity one.
What I have heard much less about, is the practice of possibly raising airflow in order to get greater sensible heat removal. Obviously that would blow latent removal all to hell, but that might be acceptable in some climates. Has anyone on this board had experience with over 450 cfm/ton?
I suppose duct problems would quickly arise if this was not well designed from top to bottom. With more air pumping, ESP would rise dramatically, and total energy consumption might go up not down. Am I right?
Thanks for any opinions on this rather esoteric subject! -- Pstu
10-25-2006, 01:49 PM
when you go over that 450 per ton, you will probably experience some water blow off from the coil unless there is absolutely no moisture to be had. Also, the coil a pressure drop, if you try to cram too much air through it, you will probably deliver less air at the registers and have a noisy unit. Any duct leaks would be intensified.
Low airflow can be controlled with an ECM motor. If a tech sets it to 300 per ton, thats the lowest it will deliver. If he tries to do that with a PSC motor, the slightest increase in static will drop him below the 300 and the unit will likely fail.
Also... in order to get full capacity out of the machine you need the right airflow.
10-25-2006, 02:26 PM
The only time that I've exceeded 450 per ton is computer room applications. As Doc mentioned, no moisture in the room. We were pushing 900 to 1000 CFM per ton on a job. However, 600 CFM per ton is the norm that I see in computer rooms.
10-25-2006, 06:14 PM
Are these flow rates based on old "A" coils or the new "N" coils with greater total surface area per ton and a TXV?
10-27-2006, 08:22 PM
The amount of air flowing is all about the sensible heat gain.
Places with high air flow 'per ton' are big sensible loads like the computer rooms mentioned. Office spaces with lots of glass can be a big sensible load as well.
If you have a lot of outside air especially in a humid climate the airflow per tonn will be quite low, I have three systems here under 200 "CFM per Ton", these are not houses these are commerical and industrial applications. Two are 100% outside air. They all have deep row coils.
Pretty much a standard response of mine if I get called somewhere and they are complaining of high humidity is to slow the fan down, insist the thermostat be put in Auto Mode, and even go as far as taking the circuit boards out, and adding a relay transformer to stop the damn blower timning off after the compressor stops.
A prime candidate to take all these measures is an over sized system compounded by the "3.5 ton coil on the 3 ton condenser".
I have also come across some airman scenarios where the 5 ton condenser is on the '4 ton air handler'. In this case it was overcharged to keep the suction temperature above freezing.
Lennox used to list how their various air handlers match up against the various condensing units, including how the 3 ton air handlers would match up against the 3.5 ton condensers. Slight reduction in total capacity at the expense of sensible cooling. Increase in latent cooling. More dehumidifcation as it ran, and would run longer as it had less sensible capacity.
Bottom line the air flow is all about how much sensible cooling you need. 400 CFM per ton is the same thing as saying a SHR of 0.72 based on air being supplied 20 degrees cooler to the space than what the space is being manitained at or
If you are having a problem dehumidifying, you are not having a problem controlling temperature, you have too much sensible cooling.
The CFM per ton is a load check figure and a rule of thumb.
The CFM required is what it is, I am not worried f it calculates out under "300 CFM per Ton"
10-27-2006, 08:31 PM
About 165 CFM per ton, ended up on the roof of a nght club. Maybe 25% outside air but a lot of sweaty people shaken there money makers and sweating up a storm.
Close up of the main coil, brown stuff is heresite, a corrosion protection, they even painted it on the distributers.
10-29-2006, 01:45 PM
A hotel here had some 20 ton Carrier packages being used as 100% outside air.
A total missaplication. Things were going out on high head pressure as they were not built for the load, the techs were pulling gas out of them.
Prettty much humidity pumps, they could not remove enough moisture.
Then the manager figures the supplies in the corridor are too noisy, so a tech slows the fans down again. Started going out on head pressure as was removing even more moisture.
I wish I knew how slow a speed he had them going at. They are long gone now, replaced by some Aaons
Is CFM/ton calculated by using the equipment's total capacity tonnage,
is it calculated by using the Manual J total heat gain capacity?
BTW, these are not always the same!
Just a thought to ponder.
10-29-2006, 05:27 PM
Well in the photo I posted they were pretty much one and the same, it was custom built. A bit of a luxury actually.
But you could calculate 2.1 tons and you can install a 2.5 ton system.
Commercial system you are picking out VAV boxes targetted on what you calculate for a peak. Based on the differrence of supply and room temp, that is how much CFM you need to control the temperature in the room.
That sets the airflow right there. How much air you need to control temperature. You ideally find a system that can supply dry enough air, at that supply temperature, to control the humidity generated in that room. This can increase tonnage a bit.
BUT, you have loads on the coil, like outside air, and that jacks up the tonnage and lowers the "CFM per ton". The air is hotter, more humid than return air, and you have to cool it to the condition needed to control the temperature and humidity in the room.
It is all about the pyschrometric chart, just something to think about before you proportion out the air flow of a air handler on high speed sometimes and wonder why it is too humid. You need air moving to be cool, but can be too much air sometimes.
I am pretty much amazed that the #1 value is humidity removal, that sensible heat removal can coexist with very high or very low airflows. Believe I have heard this stated before, but with something so strange to my ears, it's needed to hear it again.
As I hear things, with very high airflows there is a risk of blowing condensate off the coil unless the air is so dry there *is* no condensate. Just repeating what I heard, so someone can correct me if I am wrong. Do I have any need to know the terminology of "A-coil" and "N-coil"? At my present level of education it is Greek to me, but I'm wondering how much that matters to a homeowner.
I am really grateful for the experts on this board to hear my questions and try to answer them on a level I can understand. Thanks guys!
Best wishes -- Pstu
10-29-2006, 10:06 PM
it doesnt matter if you have an A coil, N coil, M coil or even an MMMMMM coil. They are all designed to use about 400 cfm per ton.(I am speaking residential) If someone wants to make it more or less, they get to be the responsible engineer. A real responsible engineer will take ownership.
10-30-2006, 12:23 AM
probably 400 CFM per ton at 500 FPM doc.
Is it the case that a coil needs air *speed* within a certain range, otherwise some ill effects would happen? I am guessing the condensate being blown off the coil could happen if air speed is too high. Could that be a reason not to get airflow too high, or at least to re-design the system with larger coil area so that air speed is kept within a design range?
You might wonder why I even ask such a question. A DIY acquaintance has told me about using radically high airflows in house A/C, on the order of 700+ cfm/ton, it just seemed too weird to me. Some intuition told me something bad might happen and now I am guessing this is one of them.
Thanks for helping me learn about this.
Best wishes -- Pstu
[Edited by pstu on 10-30-2006 at 08:54 AM]
10-30-2006, 08:28 AM
2000 CFM - 5 ton air handler on a 3 ton condenser 666 CFM per ton and not blowing water off of the coil becuase one, the face velocity through that coil is not excessive, the coil is bigger and two that much CFM per ton and chances are the coil is running dry, all sensible cooling.
I would be most of the residential airhandler coils sized for 400 CFM per ton, at a face velocity of 500 FPM. You start creeping above 500 FPM and mositure can carry over.
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