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mirfgtech
04-27-2010, 12:34 PM
I am working on a project that a condensing unit needs to be ventilated for proper operation. I plan on using a powered ventilator.

I have run across two conflicting ventilation formulas to determine the CFM needed

(A) Copelands formula is: CFM = BTU/HR
°TD

(B) Mech Engineer Forumla of: 400CFM / Ton

http://i20.photobucket.com/albums/b244/rbedell/Misc/1-CUVent.jpg

Using an example of 12,000 BTU...
(A) If you use Copeland's formula, one comes up with 1200 CFM
(B) If you use the Mech Eng's formula, one comes up with 400 CFM

Quite a disparity.

Can anyone shed some insight on this situation...???

icemeister
04-27-2010, 01:09 PM
That 400 CFM/Ton is a rule of thumb for comfort cooling...not applicable here. The Copeland formula is closer to being correct.

For ventilation of air-cooled refrigeration condensing units I have always used a rule-of-thumb of 1000 CFM per compressor horsepower. This has been an accepted standard for many years and I believe it's stated in the ASHRAE handbook (somewhere).

If you want to be more specific, use the basic formula for sensible heat rise:

BTUH = 1.08 x CFM x TD

Solving for CFM:

CFM = BTUH / (1.08 x TD)

To calculate the BTUH, which is the amount of heat rejected by the unit and is also referred to as the Total heat of Rejection (THR), you will need the design capcity of the compressor and its power input KW. This is available from the compressor manufacturer's website.

THR = Capacity (Btuh) + (3412 x KW)

Let's say you have a 5 HP compressor with a capacity of 30,000 Btuh, it draws 5.0 KW and you want to ventilate over a 10ºF rise:

THR = 30,000 x (3412 x 5.0) = 47.060 Btuh

CFM = THR / (1.08 x TD) = 47,060 / (1.08 x 10) = 4357 CFM

mirfgtech
04-28-2010, 08:36 AM
icemeister:

THANK YOU.....

I recognized that formula and went looking to confirm. It is the same formula used in calculating CFM for a furnace.

I never gave it any thought that this would apply to venting a Mechanical Space.