A common error we see in compressed air systems, in addition to poor piping practice, is line sizes too small for the desired air flow. This isn’t limited to the interconnecting piping from compressor discharge to dryer to header. It also applies to the distribution lines conveying air to production areas and within the equipment found there. Undersized piping restricts the flow and reduces the discharge pressure, thereby robbing the user of expensive compressed air power. Small piping exacerbates poor piping practices by increasing velocity- and turbulence-induced backpressure.
Pipe size and layout design are the most important variables in moving air from the compressor to the point of use. Poor systems not only consume significant energy dollars, but also degrade productivity and quality. How does one properly size compressed air piping for the job at hand? You could ask the pipefitter, but the answer probably will be, “What we always do,” and often that’s way off base.
Another approach is matching the discharge connection of the upstream piece of equipment (filter, dryer, regulator or compressor). Well, a 150-hp, two-stage, reciprocating, double-acting, water-cooled compressor delivers about 750 cfm at 100 psig through a 6-in. port. But most 150-hp rotary screw compressors, on the other hand, deliver the same volume and pressure through a 2-in. or 3-in. connection. So, which one is right? It’s obvious which is cheaper, but port size isn’t a good guide to pipe size.
Charts and graphs
Many people use charts that show the so-called standard pressure drop as a function of pipe size and fittings, which sizes the line for the what is referred to as an acceptable pressure drop. This practice, too, can be misleading because the charts can’t accommodate velocity- and flow-induced turbulence. Think about it. According to the charts, a short run of small-bore pipe exhibits a low total frictional pressure drop, but the high velocity causes an extremely large, turbulence-driven pressure drop. Then, there’s the question of the meaning of acceptable pressure drop. The answer to this question often isn’t supported by data, such as the plant’s electric power cost to produce an additional psig.
We’ve audited many plants during the past 20 years and found the unit cost of air for positive displacement compressors runs from several hundred dollars per psig per year to several thousand dollars per psig per year. At current energy costs, you don’t want the pipe to be a source of pressure drop.
Not knowing the energy cost of lost pressure as a function of line size can lead to a blind decision. Unfortunately, this is what we find in most of the air piping systems installed during the past 30 years. Older systems that were designed with care are often right on the mark, except if they’ve been modified after the original installation.
Some might call pipe sizing a lost art, but we see the issue as a lack of attention to detail, basic piping principles and guidelines. Read on to learn how to size air piping using velocity, which, when combined with appropriate piping practice, ensures an efficient compressed air distribution system. As compressed air system consultants and troubleshooters, we use these guidelines to design new piping systems and to analyze existing system performance and opportunities for improvement.