Posted by Don on July 29, 05 at 13:34:52:
Still looking at 12v compressors, and comparing CFM ratings, and trying to decide if a portable model would do the trick or whether I should go with the tankk mounted in the Sub. The portable I am looking at has a 33% duty cycle, so time to fill a tire is important (well, I want a fast-fill anyway..)
OK, here goes:
Tire is LT265/75-16E. Diameter is 31.6", width 10.5". Volume of tire is pi x 15.8 radius squared x 10.5 width = 8,234.79 cu in. Subtract volume of 16x8 rim, and net volume for the tire is 6626.3 cu in. Since i cu ft = 1728 cu in, volume of tire is 3.83 cu ft.
The compressor mfr says the portable model will fill a 3-gal tank from 0-105 psi in 1 min 40 sec (1.66 min). Since there are 7.48 gals in a cu ft, the volume of a 3 gal tank is .4 cu ft. However, you are pressurizing to 105 psi, or 7.5 times atmospheric pressure ( I used 14 psi = 1 bar, I couldn't remember the exact #). So, 7.5 x .4 = 3 cu ft of air that you pump into the 3 gal tank to pressurize it to 105 psi.
If the compressor can pump 3 cu ft of air in 1.66 mins, that's 1.81 CFM.
Now a tire has 3.83 cu ft, but only pressurizing to 80 psi, or 5.7 bar. 3.83 x 5.7 = 21.88 cu ft of air to fill the tire. At 1.81 CFM it would take 12.08 mins to fill the tire from 0-80 psi. However, the tire is not flat, it's only aired down to 20 psi for sand travel. Assuming this means it's 1/4 full (20/80), it should only take 75% as long, or just over 9 mins to air up the tire.
If anyone has any knowledge of volumetrics, please jump in. I am assuming a linear p/v relationship here, and for some reason I don't think that's entirely accurate.
The above calcs are for the portable unit. How would infation time change if I introduced a 3 gal tank pressurized at 150 psi into the equation?
*whew*
Don