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## temp rise formula

Hi, does anyone have a temp rise formula for 3 phase to calculate CFM on an AHU. Thanks for any help, John

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cfm= heater btu's / delta t X 1.08 you can use the long formula with 1.73, but this will get you in the ballpark

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Thanks for the formula, is there one to get the BTU's using the amp draw from the heaters and motor and actual voltage? I used the one for single phase and it can not be correct, is there one for 3 phase? Any help is greatly appreciated, John

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Dear John E,

The correct formula for what you are looking for is as follows:

[(V1 + V2 + V3) /3 x (A1 + A2 + A3)/3] x 1.73 x 3.41 = BTU/Hr.

Then plug it into the following formula for your calculated CFM:
BTU/Hr. / 1.08 x Delta T = CFM

Sabre11134, your formula would not only NOT get you in the ballpark, you might miss the entire city to boot!

Although not asked for, here is the formula for single phase resistant heat:

[(V1 + V2) /2 x (A1 + A2)/2] x 3.41 = BTU/Hr.

Then plug it into the following formula for your calculated CFM:
BTU/Hr. / 1.08 x Delta T = CFM

Hope this helps……..

Respectfully Submitted,
John J. Dalton

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That's perfect and exactly what I need. Thank you very much. thank you both for your time, John

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Dear John E,

My pleasure my friend…………………

Respectfully Submitted,
John J. Dalton CM

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Originally Posted by john dalton
Dear John E,

The correct formula for what you are looking for is as follows:

[(V1 + V2 + V3) /3 x (A1 + A2 + A3)/3] x 1.73 x 3.41 = BTU/Hr.

Then plug it into the following formula for your calculated CFM:
BTU/Hr. / 1.08 x Delta T = CFM

Sabre11134, your formula would not only NOT get you in the ballpark, you might miss the entire city to boot!

Although not asked for, here is the formula for single phase resistant heat:

[(V1 + V2) /2 x (A1 + A2)/2] x 3.41 = BTU/Hr.

Then plug it into the following formula for your calculated CFM:
BTU/Hr. / 1.08 x Delta T = CFM

Hope this helps……..

Respectfully Submitted,
John J. Dalton
sounds like you are going to get the same figures with out going three miles to get the fact 1 kw equals 3413 btu's. looks like a big ego here

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Dear Sabre11134,

“…you can use the long formula with 1.73, but this will get you in the ballpark…”

Without using the three phase conversion constant (1.73) that you said was not needed to get you in the ballpark, your formula would only give you an answer that was 57.8% of the correct one……..I ask you this question sir:

Is an answer off by over 42% in the ballpark to you?

By the way, if I had an ego, then I would have listed the three phase constant as 1.7320508 +/- 1,000,000th.

Respectfully Submitted,
John J. Dalton

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Here is a quick easy question for your ego's. What's is the easist way to get your delta T, is it just simply taking the difference from return and supply

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Dear Northwestmechanic,

Sabre11134 was the one that brought up egos, not me. As far as egos are concerned, based on the complexity of this industry and the growing lack of knowledge I find I don’t poses, I’m not surprised by the mistakes I make in my job, but rather shocked that I don’t more of them.

As for your “quick and easy question”……………the answer is no, it is not as simple as“…taking the difference from return and supply…” You have to get the air entering and leaving the electric heater as close as you can to the heater, while at the same time making sure you are not in a line of sight between the heating elements and your temperature sensor, least radiant heat would give you a false reading (higher reading).

Respectfully Submitted,
John J. Dalton CM

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CFMx1.08x Delta -T air =BTU/ Hr. So what is the required airflow for a 10kw duct heater to keep it from riding limits? If I use this formula based on figures obtained from the duct heaters temp rise and and CFM ? Will I get 10 kw regardless of air volume? I know I'm not the sharpest tack in the box. Thanks, Ken

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Dear Joken,

“…So what is the required airflow for a 10kw duct heater to keep it from riding limits?..”

Although the range of temperature rise through a normal duct heater will differ drastically based on the application, the type of heating element used in the heater, the type, location, and range of high limit controls, and the heater manufacturer, an acceptable range is between 20 and 40 degrees F for most conventional HVAC uses. If we were to take a mid-range of 30 degree F as our target point we could then perform the following calculation to determine our target CFM value:

(10,000 w x 3.41) / 30 degrees F x 1.08 = 1,052 CFM

Hope this helps…………………

Respectfully Submitted,
John J. Dalton CM

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John, If answering these questions is your job, you are doing it well and I thank you. I actually only have 500cfm which equates to around a 60 degree rise. I need that much for reheat when the RTU is cooling. I'm trying to make a poor design work. Do you have an opinion as to wheather the reheats will hold up. 145 limit on the bottom 240 center and top. I guess time will tell since they need to be that large. Thanks again, Ken

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