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.
Obviously, the lower the pressure drop in transporting air, the lower the system’s energy input. Lower header pressure also reduces unregulated air flow (including leaks) by about 1% per psi of pressure reduction.
Eliminate the drop
Most charts show frictional pressure drop for a given flow at constant pressure. Wall friction causes most of this loss, which is usually denominated as pressure drop per 100 ft. of pipe. Similar charts express the estimated pressure loss for fittings in terms of additional length of pipe. When added to the length of straight pipe, the sum is called total equivalent length. These charts reflect the basic calculations for pressure loss, which include:
- Air density at a given pressure and temperature.
- Flow rate.
- Velocity at pipeline conditions.
- The Reynolds number.
- Other factors, including a friction factor based on the size and type of pipe.
The calculations and chart data help to identify only the probable minimum pressure drop. Internal roughness and scaling dramatically affect the pipe’s resistance to flow (friction loss). Resistance increases with time as the inner wall rusts, scales and collects more dirt. This is particularly true of black iron pipe.