By Ron Marshall
Most compressed air piping systems are radial feeds, with one-way piping branches feeding out from the compressor room in all directions. Often, the piping starts out in larger size and, if the design engineer wanted to save costs, the diameter of the piping might be reduced near the end of the lines. Some compressed air savings can often be gained by installing a looped compressed air system rather than radial feed, with the savings in pressure drop having much to do with diversity.
It is fairly common to find the radial compressed air lines all sized to be the same—however, the flow in each branch may not be equal. Therefore, the pressure loss in one section of the piping may be very different than the other. The pressure loss in the section with the heaviest flow becomes the line with the lowest pressure at the end.
With compressed air systems, having proper pressure is important. A critical tool or machine receiving low pressure due to excessive pressure loss in compressed air piping will cause problems, and a common way to solve this is to boost the compressor discharge pressure to compensate. Boosting the pressure raises the pressure at the critical end use, but also all the other unregulated uses in the plant.
Increasing the compressor discharge pressure causes the compressor to consume more power, about 1% per 2 psi increase. It also causes higher flow in any unregulated uses by about 1% per every 1 psi increase. This also increases the compressor power.
Looping the piping has two effects, it equalizes the pressure drop because now the critical end use has at least two more diverse parallel paths to receive compressed air. And having a loop typically means the piping is the same size throughout the loop, rather than reducing the size near the end.
Having a parallel path is very effective in reducing the pressure loss. Because the pressure loss in piping varies with the square of the internal velocity, best case would have one half the flow in each line, which would result in one quarter the pressure loss.
Further to this, if pipe size is increased, there is a significant increase in internal area of the pipe, which also reduces internal velocity. For example, use of 3-in. pipe rather than 2-in. would reduce the pressure loss by a factor of eight. Similar pipe size comparisons exist for other size upgrades.
For this reason, in terms of energy efficiency, where practical, it is always good to consider looping your piping—and using multiple loops of well sized piping is the best of all.
Jack Hagerty says
How does this differ from just doubling the cross-sectional flow area? Going from 2″ pipe to 3″ gives you just over twice the flow area and a single run of 3″ would cost no more than a double run of 2″, maybe less.