Estimating pressure drop of air in pipes 2

[handee]

Anyone have a good source to estimate pressure drop of air (atmospheric pressure) flowing through pipes (3" to 6" diam). My flowrates are only about 10-15 SCFM. TIA

 

[tombmech]

The Moody Diagram is still the fundamental source for fluid flow in pipes - including air in ductwork. An online calculator may be found here:

http://www.lmnoeng.com/moody.htm

As a nice resource, The Engineering Toolbox has an entire page on fluid mechanics. The selection above was from one of the links on this page:

http://www.engineeringtoolbox.com/fluid-mechanics-t_21.html

Specific charts and tables have been developed for HVAC ductwork, and can be found in ASHRAE or SMACNA literature.

 

[handee]

Thanks for the references. Apparently, my pressure drop must be pretty small, since the Moody calculator says my friction factor is out of range.

Do you think the ASHRAE or SMACNA literature (where can I find it?) has a simple chart I can get a ballpark number from?

 

[katmar]

Although you are working with air, which is a compressible fluid, from a practical point of view the compressibility of gases can be ignored if the pressure drop is less than 10 to 15% of the supply pressure in absolute terms. As this seems to be the regime in which you are working it means that you can use one of the many pipeflow calculators which have been designed for liquids.

One of the nicest free pressure drop calculators I know of is simply called "dP" and is available from

http://www.xs4all.nl/~kostermw/dP/index.htm

The only drawback is that it works only in metric units. But I am sure that if you Google for "free liquid pressure drop software" you will get dozens of hits.

In the example you give, the velocities are very low (15 SCFM in a 3" pipe = 5 ft/sec) which is probably why the online calculator you tried choked on the data.

Using my own "home rolled" software and checked with "dP" I get a pressure drop of around 0.0025 PSI per 100 ft of 3" pipe with a flow of 15 SCFM. This is VERY low, and confirms that it was OK to treat the air as an incompressible liquid.

regards
Harvey

 

[walkes]

Some of the charts in the ASHRAE handbook drop off at 50 CFM. Loren Cook's Engineering Cookbook has a chart that goes to 10 CFM and small ductwork.

They also have an electronic version available.

http://www.lorencook.com/About/destools.shtml

 

[tombmech]

Actually, Walkes is exactly correct. 50cfm is the limit of the ASHRAE friction chart, but the Loren Cook Cookbook has it down to 10. At 3 inch diameter and 15 cfm, that corresponds to a pressure drop of 0.08" H2O per 100 feet. If you convert to psi, that is fairly close to Katmar's calculated value of 0.0025 psi.

Interestingly, that pressure drop is a general midrange value along the line of 500 fpm, a very common HVAC velocity. 15 cfm in a 3" duct is actually 305 fpm, which is not that unusual - just that the flow rates are so small. (Katmar is still right about that one: 5 ft/s x 60 sec/min ~ 300 fpm)

Unfortunately, I have seen real applications where this airflow and duct size is critical.

I apologize for not actually running the numbers (another lesson) when I gave you the first links. However, it is still true that the ASHRAE and Loren Cook charts are magnified versions of the laminar section of the Moody chart. It is linear when plotted on a log-log scale.

Thanks to Walkes for the references.

 

[handee]

Wow! You folks are great. 0.08"H2O certainly is low. That's about as accurate a number as I need at this point. I thank you all. And thanks for the links and references.