Compressed air pressure drop

[jfaucher]

My problem is going as follow: an existing equipment is supplied with compressed air through a 24ft long flexible hose with an I.D. of 0.31". 20 SCFM at a minimum of 15 PSIG is currently required. Now, a modification to this equipment will increase the flow requirement to 28 SCFM. I want to calculate what will be the pressure drop through the flexible hose with the new increased flow, in order to establish if my shop air at 90 PSIG will still supply the minimum of 15 PSIG at the equipment inlet, or if I may have to increase the flexible hose diameter.

Since the flow is quite high in regard to the hose I.D., I get relatively high velocity, which means the pressure drop is not negligeable (certainly over 20 PSIG). So, how should I evaluate the actual flow velocity along the hose, in order to calculate the pressure drop, knowing that the actual CFM (not the standard CFM) will vary along the hose because of the pressure drop? Is it an iterative calculation? Or integration?

Looking at the web, I found a few online calculators to evaluate the pressure drop in a compressed air line. However, they all give me significantly different results!

Thanks for your help!

Jean-Pierre Faucher, ing.

http://www.cel-aerospace.ca

 

[BigInch]

Break the line up into smaller segments where the pressure drop is not more than 10% of the total line. You might have to iterate a bit to do that.

If you just want to make a quick estimate of the pressure drop as the ratio of the squares of velocity, more or less, its probably about 2x the pressure drop you have at 20 cfm. How's that fit with the answers you have now?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[crjones]

remember that a good amount of the hose pressure drop comes from the end fittings. They are smaller than the id of the hose. Hose as a rule should be one size larger than the port it is going into to keep the same flow area and reduce the drop.

 

[jfaucher]

BigInch,

I tried, as you recommanded, to divide my hose into small segments, doing iterative calculation in excel spreadsheet. The results looks good as long as I divide my hose in segments of 2 ft or less, giving a local pressure drop of less than 10% of total line as predicted. I assumed constant temperature.

However, I was initially surprised to observe that my Re keeps constant all the hose long, even if my actual CFM (so my effective velocity) is changing. Actually, while the density is proportional to the local air pressure, the real CFM are inversely proportional to the local air pressure. This also means that friction factor becomes a constant.

Jean-Pierre Faucher, ing.

http://www.cel-aerospace.ca

 

[25362]

For gases with Re in the range 100,000 to 500,000 the friction drop can be (roughly) estimated as:

[Δ]P_f = (L/50D_H) [ρ]V²/2​

Where

L = length of duct
D_H = hydraulic diameter
[ρ] = density
V = linear bulk velocity

The drop in fittings should be added.