Tag: pipe sizing

How to Size Pipes for Your Compressed Air System

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Most facility’s compressed air systems have evolved over time. A spur added here a spur added there. Eventually pressure drop issues develop. Common practice is to increase the air pressure at the compressor. While it may address the symptom it does not address the problem and is very costly. For every 2 PSI increase in pressure requires 1% more energy.

A properly designed system will be a loop with spurs. This will ensure all airsystem

drops will share the air equally. The header loop should be able to carry all the air the compressor is capable of producing.  Best practices suggest the distribution header should be sized to allow an air velocity not to exceed 30 ft/second. The formula to calculate this is:

A =    144 * Q * Pa
       V *60 x (Pd +Pa)

Pipe Diameter = √ (A*4/3.14)

Where:

A = cross sectional area if the pipe bore in square inches or ∏ x diameter squared / 4
Q = Flow rate SCFM
Pa = Prevailing absolute pressure. Sea level is 14.7
Pd = compressor gauge pressure or psig.
V = Design pipe velocity ft/sec

Example: Size a header for 500 SCFM at 100 PSI at an elevation at sea level

A = 144 x 500 x 14.7 / 30 x 60 (100 + 14.7) = 5.13  square inches

Pipe diameter then is square root of  (5.13 * 4) / 3.14 = 2.56″

So an 2.56″  internal diameter pipe would be the proper size header.

The same formula can be used to calculate the sizes of the drops. In this case you would use the demand flow rate for Q.

Joe Panfalone
Application Engineer
Phone (513) 671-3322
Fax (513) 671-3363
Web: http://www.exair.com
Twitter: http://www.twitter.com/exair_jp
Facebook: http://www.facebook.com/exair

Air Pressure Loss

Published on by Kirk EdwardsLeave a comment

Loss of air pressure scenario:

  1. Customer calls in because she is not pleased with the performance of her new 12″ Super Air Knife. Customer states something like “this thing couldn’t spread the seeds of a dandelion”.
  2. EXAIR does not take umbrage to the statement, we begin to troubleshoot the situation. We generally begin by trying to determine the line pressure of her compressed air system; “What is your line pressure?”
  3. Customer states 95 PSIG line pressure, and potentially utters something else about being a monkeys uncle if this Super Air Knife is not broken.
  4. EXAIR reassures the customer that the Super Air Knife is most likely functioning properly – BUT – being starved for air. We typically ask if she knows what pressure is running through the knife?
  5. Customer thinks since there is 95 PSIG on the main supply, the Super Air Knife must also be at 95 PSIG.
  6. We ask what size air line is feeding the knife, because we know there can be a significant pressure loss due to improperly (small) sized air lines.
  7. Customer states 1/4″ air line and push to lock fittings.
  8. We state that the air line is much too small for delivering the proper amount of compressed air and the result is a pressure loss, the knife is fine but not geting enough air volume to do any work. We recommend the proper sized air lines, further assure the customer this will take care of the problem and let them know we are available if they still cannot get the Super Air Knife to work.
  9. More times than not, customer does not call back.
  10. Sometimes they call back and let us know that we could spread the seeds of a dandelion, it’s working great now, and thanks again.

If air lines or fittings are too small they cannot deliver the volume necessary to maintain pressure. Compressors sitting on one end of the plant and feeding equipment on the other end will experience pressure losses due to friction in the air lines, resulting in lower pressure in the far end of the plant compared to the closer end.

It is important to measure pressure at various places and points in your compressed air system in order gage your pressure and even diagnose some equipment.

  1. Measure your pressure right at the compressor discharge. This will give you a good baseline of what you should expect.
  2. If you have any additional dryers or treatment equipment you should also measure pressure after this equipment.
  3. Measure your pressure after each filter in the system. A large pressure loss after a filter will indicate a clogged filter and that it is time to change it out.
  4. If you have installed a primary receiver tank, measure the pressure at the outlet. Since the plant will use its compressed air from out of the receiver, a pressure loss here will indicate high demands periods or the need for a larger receiver.
  5. Measure pressure at the end of long headers. This reading will reveal any pressure losses due to friction loss over long distances. It can also indicate the need for larger diameter headers.
  6. Lastly, measure pressure before a filter/regulator at an end use application, also measure pressure right at the inlet of the end use product. This is the example for the Super Air Knife above. A pipe tee on the Super Air Knife inlet with a pressure gauge in the open leg of the tee would have indicated a large pressure loss when compared to the pressure at the filter/regulator. This is a clear sign of undersized compressed air feed lines.

If you have any questions for us along the way, please let us know.

Kirk Edwards
Application Engineer
kirkedwards@exair.com