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. (See “There’s a Gremlin in your air system. Its name is turbulence,” page 37, Plant Services, July 2002).
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-inch or 3-inch 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.
Interconnects and headers
The interconnecting piping is a critical element that must deliver air to the distribution headers with little pressure loss, if any. This isn’t only an energy question. Also ensures the capacity controls will have sufficient effective storage to allow them to react to real demand and translate less air usage to a comparable reduction in input electrical energy.
The main distribution headers not only move air throughout the plant, they also supply the appropriate local storage that ensures the process feeds have adequate entry pressure and flow. The main header system represents storage that supports the operating pressure band for capacity control. You want the pressure drop between compressor discharge and point of use to be significantly less than the normal operating control band (10 psig maximum).
The objective in sizing interconnecting piping is to transport the maximum expected volumetric flow from the compressor discharge, through the dryers, filters and receivers, to the main distribution header with minimum pressure drop. Contemporary designs that consider the true cost of compressed air target a total pressure drop of less than 3 psi.
Beyond this point, the objective for the main header is to transport the maximum anticipated flow to the production area and provide an acceptable supply volume for drops or feeder lines. Again, modern designs consider an acceptable header pressure drop to be 0 psi.
Finally, for the drops or feeder lines, the objective is to deliver the maximum anticipated flow to the work station or process with minimum or no pressure loss. Again, the line size should be sized for near zero loss. Of course, the controls, regulators, actuators and air motors at the work station or process have requirements for minimum inlet pressure to be able to perform their functions. In many plants, the size of the line feeding a work station often is selected by people who don’t know the flow demand and aren’t aware of how to size piping.
In our opinion, new air-system piping should be sized according to these guidelines. For a system that doesn’t meet the criteria, the cost of modification must be weighed against the energy savings and any improvements in productivity and quality.