History of 400 CFM standard

jedieasy

Why is 400 CFM per Ton a standard air flow figure for HVAC? I think it has its roots in the early refrigeration industry (like the fact that it takes 12,000 BTUs of energy to convert 1 Ton of water into ice), but not sure exactly how. Is it the rate at which air needed to be introduced to the 1 Ton of water to freeze the water in 1 day?

Yuri

1 ton of refrig is 12,000 btu's/hr. Not sure where the 400 cfm came from. Can go lower if a TX valve is used for better control and lower cfm gets you better dehumidification.

gene2

I am still searching for the exact beginnings or source, I think it is a basic rule of thumb that is easier to use in the field than 320 cfm as shown on the physcometric chart. Willis Carrier & his teams of engineers developed these charts & concepts that we study in HVAC schools. Unless we were pursuing a degree in mechanical engineering, most of our eyes just glassed over untill the next chapter.

The 12,000 BTU's per ton is based on physics. The nomenclature can change accordingly but it will always take the certain amount of energy to change as represented in the physcometric charts.

beenthere

The 12,000 BTU's to convert 1 ton of ice into water, is a common misconception. 12,000 BTUs per hour for 24 hours equals 1 ton of cooling.
Its 288,000 BTUs.

Heat of fusion for ice is 144 BTUs per pound. 144X2000=288,000
288,000/24=12,000

400 CFM per ton is a safe average purpose air flow standard. A compromise on performance and efficiency. Probably left over from the early days of 6 and 8 fins per inch evap coils.

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surenuff

We can get different capacity from different equipment (a/c vs. refrigeration) by different Temperature differences and airflows, types and sizes of coils etc. Way back when, in a world far far away, the Govt. stepped in and said we will all rate our equipment the same way. Not every a/c will be exactly 400 cfm per ton of airflow, in fact they are all different especially with high effecient systems. But if you are trying to get in the ball park of knowing what a unit should be doing, it will be 400 cfm per ton for regular units, and high effecient units on high.(cooling of course)

hvaclover

had a 3 1/2 ton 14 SEER Westinghouse when paired wit a VS blower only needed 300 cfm per ton.

But Yeah, Carrier came up with the cfm standard when he invented the psychometric chart.

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nicktech

didnt you and willis go to HVAC camp together as kids?

hvaclover

yeah, and made him my (use your imagination)

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nicktech

ooooh, lol

skwint

From what I've learned, the "ton" wording dates back to the days of using ice on trains for cold storage. They would calculate how many tons of ice they needed to keep their fruits and veggies cold during a long trip. The wording just got adopted into our industry.

As far as the 400 CFM rule, I always shoot for 450-480 CFM per ton when sizing duct work. Heat Pumps require 450 CFM anyway. The more CFM per ton, the lower your static pressure will be and that means less air noise and more comfort for the homeowner.

beenthere

Huh???

Inorder to have a lower static at a higher CFM. Your duct work needs to be that much larger yet.

A system moving 1200 CFM at a ESP of .7"(say 24X8 duct), would need the duct work enlarged to 30X8 to have a lower ESP moving 450CFM.

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skwint

Not sure I understand the point that you're trying to make... explain?

beenthere

You said more CFM per ton is lower static.

How can it be lower static. Unless you make the duct work that much bigger?

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nicktech

the ton term is a scientific comparison of heat tranfer into 2000lbs of ice to melt it.
a ton of ice requires 288,000 btu's in a 24hr period to completely go from solid to liquid. 288,000/24=12,000 btu/hr, hence the ton terminology.

hvaclover

Yer all nuts.

Static is what your old lady gives for for going to the booby bar.
400 is the number of words in a ten second burst the wife spurts.

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Rusty999

400 CFM/ton is the standard based on the amount of "heat content" in a given sample of air. Taking a wet bulb reading of the return air and the supply air, you can refer to a psychometric cart and get the heat content of each reading per CFM. The difference between the two is the Delta h.
Using this value,
Delta h X CFM X 4.5 (constant value) =Q (Total heat removed)

This provides for the total heat, both sensible and latent. The value Delta T only provides for sensible heat.

When you decrease the CFM, especially with a TXV system, it doesn't always change the total heat or Q value as the equation suggests. In turn the constant (4.5) will shift, changing the ratio of sensible to latent heat, resulting in better dehumidification.
Notice on days when the humidity is low you can get a 20 degree delta T, but when the humidity hits you're lucky to get 15 degrees? This is the result of shifting the constant (4.5). The system is working to deal with high latent loads and results in less sensible capacity.

beenthere

Your formula is correct.

But it is not based on 400 CFM per ton. It is based on 13.3 cubic foot of standard air weighing .075 pounds. The .075 pounds is part of the 4.5 constant. And only accurate at sea level.

13.3 cubic foot X .075 pounds = 1 pound of standard air.

.075 pounds X 60 minutes = 4.5 pounds per hour.

Specific heat of air = .24 BTUs per pound

4.5 pounds per hour X .24 specific heat = 1.08

The 4.5 doesn't change if the CFM is changed. Since a pound of standard air, is a pound of standard air.

It is changed for altitude though. Since it takes more cubic feet of air to weigh 1 pound.

At 5,000 foot elevation/altitude. A pound of standard air would have a volume of 15.56 cubic foot, instead of 13.3 cf. This is why high altitude systems often run 450 to over 500 CFM per ton of A/C.

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Rusty999

I did say that the 400 CFM resulted (or was the product of) from the standard sample. The standard being a pound of air, at 70 degrees under constand pressure & volume. Also this assumes a set humidity. Changing the temperature, pressure, volume or water content changes the specific heat, weight and characteristics of the sample. With the sample no longer being "standard" the "constant" is no longer.
Many assumptions are made in the design process to enable syatems to operate under a variety of conditions.

jomon george

hai


in residential

cooling space load will be 1 cfm / person
and when considering the ventilation load 10 cfm per person ,so the total cfm /person will be ;
11cfm/person/square feet
and in visv supose if it is a office then the value will be changed as 1.2 and 20 so the total cfm will be 21.2cfm/psn/sf
and in visv supose if it is a theatre or auditorium
the value will be 2.5 and 15 ;
so the total will be 2.5+15=17.5cfm/psn/sf
and in visv supose if it is a school
the value will be 1.5+15
the total will be 16.5cffm/prsn/sf
(where the numbers 1.5and2.5 and 1 are the ventilatrion factors;'
eg:- in a class room there are 30 pupils were tere.supose the total area will be 25sf*30sf=750sf
if we r determine the cfm req;-floor area *1.5cfm (for school)
750*105=1125cfm
when considerin g the ventilation factor

15cfm /prsn
that is number of pupil * ventilation factor = required cfm

so the ventil.ation required is 30*15=450
therefore the total cfm required inthat class room will be 450+1125=1575cfm
so for that room an ac required having 1575 cfm

beenthere

That doesn't match up to code requirements.

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