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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.

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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yeah, and made him my (use your imagination)didnt you and willis go to HVAC camp together as kids?

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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.

Huh???The more CFM per ton, the lower your static pressure will be and that means less air noise and more comfort for the homeowner.

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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Not sure I understand the point that you're trying to make... explain?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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the ton term is a scientific comparison of heat tranfer into 2000lbs of ice to melt it.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.

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.

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.

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.

Many assumptions are made in the design process to enable syatems to operate under a variety of conditions.

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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

:thumbsup::thumbsup::thumbsup::thumbsup:

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;'

:thumbsup::thumbsup::thumbsup::thumbsup:

That doesn't match up to code requirements.

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