×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Compressor, Piping and Flow Rates

Compressor, Piping and Flow Rates

Compressor, Piping and Flow Rates

(OP)
Hi, first post here, I hope someone can help me.

I have a machine that requires 900 L/min of compressed air at 125 psi. For this machine, I purchased a 10 HP compressor that should deliver 1250 L/min of compressed air at about 150 psi. We installed 1/2 inch piping (about 150 feet of it) and our compressor struggles to deliver the volumetric flow rate needed by the machine at about 75 psi.

I believe that this has something to do with the 1/2 pipe we installed and the way to fix the problem is to put in a 1 inch pipe.

So my questions are the following,

1. Am I right that the 1/2 inch pipe is limiting the flow rate and causing the pressure to drop to 75 psi and increasing the pipe diameter will fix this problem?

2. If so, the entrance pipe to my machine is 1/2", so will it do any good to install a 1 inch pipe when the last 20 feet (of the 150) will still be 1/2 inch?

3. Since I already havethe 1/2 inch pipe in place, instead of remocing this pipe to install a 1 inch pipe, can i just install another 1/2 inch pipe that goes parallel to this one and save some money?

4. Finally, is there any drawback to installing a pipe with a diameter much greater than what I need?

Thank you.

RE: Compressor, Piping and Flow Rates

I don't know where to start.  When you say "900 L/min of compressed air" you have basically said NOTHING.  Are you talking about a volume at suction conditions?  Standard conditions?  If standard, what "standard" are you using?  If suction, what is your elevation?  When you are talking to salesmen or management you can be imprecise in your language, but when you are asking for Engineering assistance you have to provide information in a format that lends itself to Engineering evaluation.

Also "psi" said pretty close to nothing.  Are your values in "psig" or "psia", the difference is huge at these pressures.

Mixing metric and imperial units leaves me confused about what units you want to do the analysis in.  I'm in the U.S. so I'll do it in imperial units.

OK, lets just make up the values you left out.  Lets say that your volume flow rate is at suction conditions.  Lets also say that your pressures are in psig and that you are at sea level and your local atmospheric pressure is 14.7 psig and ambient is 60F.

This means that your 900 L/min (46 MCF/day) at suction conditions is also 46 MCF/day at standard conditions.  Using nominal values for pipe roughness of 1/2" tubing, I get your pressure drop with 46 MCF/day at 42% pipe efficiency using the AGA equation (59% at the compressor rated capacity).  That implies that the line is longer or you have standing water in the line from condensation.  At typical efficiency for new clean pipe (95%), your pipe should deliver 136 psig at 64 MCF/day and everything should be fine.  

But everything is not fine and you want to loop your line to "solve" the problem.  If you don't know why you have a problem, throwing money at it is rarely effective.  If your 1/2" line is full of water, two of them would also fill up, in fact would fill up faster (the existing line has a velocity around 90 ft/sec which will sweep standing water pretty well, two 1/2 inch lines would drop the velocity to 25 ft/sec and won't move liquid quite as well).

Now lets talk about your loop assumptions.  Gas does not flow in a pipe diameter, it flows in a volume (area times length).  So it is common to think that two 1/2" lines is twice the capacity on of 1/2" line.  Not so, 1/2" diameter + 1/2" diameter = the flow area of 0.707 inch diameter pipe (but the friction is higher, so it is closer to 5/8").  In fact, if you looped your 1/2" pipe with a 7/8" pipe (if such a thing were available) you would have approximately achieved the flow area of a single 1" pipe.

In conclusion, you should be looking at why you have 75 psig dP in 150 ft of pipe.  The "solution" to your problem could easily be to install dehydration equipment at the compressor discharge instead of a new line.

David

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources