The Compressed Air Challenge® (CAC) is a voluntary collaboration of industrial users; manufacturers, distributors and their associations; consultants; state research and development agencies; energy efficiency organizations; and utilities. This group has one purpose in mind - helping you enjoy the benefits of improved performance of your compressed air system. The mission of the Compressed Air Challenge (CAC) is to provide resources that educate industrial users about optimizing their compressed air systems.
One of the many issues that can affect system efficiency and pressure stability in compressed air systems is pressure drop. “The first and foremost complaint I normally hear from an operator or production area is, ‘I don’t have enough pressure’”, says Frank Moskowitz, and one of CAC’s Advanced Management instructors., “The air compressor operator usually gets the blame, but often the problem is actually a flow restriction that manifests itself as low pressure.”
“Pressure drop problems can stem from undersized distribution piping, this leads air system operators to spend significant time and money in optimizing their distribution systems”, says Tom Taranto, another CAC Advanced Management instructor, “but often most of the problem is between the header and machine in the last 30 feet of piping, what I call the ‘last dirty thirty’. Students of our Fundamentals and Advanced seminars will learn about these issues and some strategies needed to pinpoint these problems.”
This is an excerpt from CAC’s “Improving Compressed Air System Performance: A Sourcebook for Industry”
Pressure drop is a term used to characterize the reduction in air pressure from the compressor discharge to the actual point-of-use. Pressure drop occurs as the compressed air travels through the treatment and distribution system. A properly designed system should have a pressure loss of much less than 10 percent of the compressor’s discharge pressure, measured from the receiver tank output to the point-of-use.
Excessive pressure drop will result in poor system performance and excessive energy consumption. Flow restrictions of any type in a system require higher operating pressures than are needed, resulting in higher energy consumption. Minimizing differentials in all parts of the system is an important part of efficient operation. Pressure drop upstream of the compressor signal requires higher compression pressures to achieve the control settings on the compressor. The most typical problem areas include the aftercooler, lubricant separators, and check valves. A rule of thumb for systems in the 100 psig range is: for every 2 psi increase in discharge pressure, energy consumption will increase by approximately 1 percent at full output flow (check performance curves for centrifugal and two-stage, lubricant-injected, rotary screw compressors).
There is also another penalty for higher-than-needed pressure. Raising the compressor discharge pressure increases the demand of every unregulated usage, including leaks, open blowing, etc. Although it varies by plant, unregulated usage is commonly as high as 30 to 50 percent of air demand. For systems in the 100 psig range with 30 to 50 percent unregulated usage, a 2 psi increase in header pressure will increase energy consumption by about another 0.6 to 1.0 percent because of the additional unregulated air being consumed. The combined effect results in a total increase in energy consumption of about 1.6 to 2 percent for every 2 psi increase in discharge pressure for a system in the 100 psig range with 30 to 50 percent unregulated usage.
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An air compressor capacity control pressure signal is normally located at the discharge of the compressor package. When the signal location is moved downstream of the compressed air dryers and filters to achieve a common signal for all compressors, some dangers must be recognized and precautionary measures taken. The control range pressure setting must be reduced to allow for actual and potentially increasing pressure drop across the dryers and filters. Provision also must be made to prevent exceeding the maximum allowable discharge pressure and drive motor amps of each compressor in the system.
Pressure drop in the distribution system and in hoses and flexible connections at points-of-use results in lower operating pressure at the points-of-use. If the point-of use operating pressure has to be increased, try reducing the pressure drops in the system before adding capacity or increasing the system pressure. Increasing the compressor discharge pressure or adding compressor capacity results in significant increases in energy consumption.
Elevating system pressure increases unregulated uses, such as leaks, open blowing, and production applications, without regulators or with wide open regulators. The added demand at elevated pressure is termed “artificial demand,” and substantially increases energy consumption. Instead of increasing the compressor discharge pressure or adding additional compressor capacity, alternative solutions should be sought, such as reduced pressure drop and strategic compressed air storage. Equipment should be specified and operated at the lowest efficient operating pressure.
What Causes Pressure Drop?
Any type of obstruction, restriction, or roughness in the system will cause resistance to air flow and cause pressure drop. In the distribution system, the highest pressure drops usually are found at the points-of- use, including undersized or leaking hoses, tubes, disconnects, filters, regulators and lubricators (FRLs). On the supply side of the system, air/lubricant separators, after coolers, moisture separators, dryers and filters can be the main items causing significant pressure drops.
The maximum pressure drop from the supply side to the points-of-use will occur when the compressed air flow rate and temperature are highest. System components should be selected based upon these conditions and the manufacturer of each component should be requested to supply pressure drop information under these conditions. When selecting filters, remember that they will get dirty. Dirt loading characteristics are also important selection criteria. Large end users who purchase substantial quantities of components should work with their suppliers to ensure that products meet the desired specifications for differential pressure and other characteristics.
The distribution piping system often is diagnosed as having excess pressure drop because a point-of-use pressure regulator cannot sustain the required downstream pressure. If such a regulator is set at 85 psig and the regulator and/or the upstream filter has a pressure drop of 20 psi, the system upstream of the filter and regulator would have to maintain at least 105 psig. The 20 psi pressure drop may be blamed on the system piping rather than on the components at fault. The correct diagnosis requires pressure measurements at different points in the system to identify the component(s) causing the excess pressure drop. In this case, the filter element should be replaced or the filter regulator size needs to be increased, not the piping.
Minimizing Pressure Drop
Minimizing pressure drop requires a systems approach in design and maintenance of the system. Air treatment components, such as after coolers, moisture separators, dryers, and filters, should be selected with the lowest possible pressure drop at specified maximum operating conditions. When installed, the recommended maintenance procedures should be followed and documented. Additional ways to minimize pressure drop are as follows:
- Properly design the distribution system.
- Operate and maintain air filtering and drying equipment to reduce the effects of moisture, such as pipe corrosion.
- Select after coolers, separators, dryers and filters having the lowest possible pressure drop for the rated conditions.
- Reduce the distance the air travels through the distribution system.
Specify pressure regulators, lubricators, hoses, and connections having the best performance characteristics at the lowest pressure differential. These components must be sized based upon the actual rate of flow and not the average rate of flow.
Best Practices and Tips for Compressed Air Piping Systems
A brief synopsis of “Section 3, Distribution System” from “Best Practices for Compressed Air Systems”. This book 325 page book is available at our bookstore.
Pressure losses due to inadequate piping will result in increased energy costs and variations in the system pressure, with adverse effects on the production process.
A. How to select pipe sizes
The compressor room header into which the air compressor(s) discharge(s), should be sized so that the air velocity within the header does not exceed 20 ft/sec, allowing for future expansion. Distribution header piping leaving the compressor room should be sized to allow an air velocity not to exceed 30 ft/sec, to minimize pressure drop.
It also is recommended that the air from each compressor not enter the header at 90 degrees to the header axis, but at a 45 degree angle in the direction of flow and always using wide radius elbows.
Iron and carbon steel piping generally is sized by the nominal bore diameter. Copper and steel tubing normally is sized by outside diameter.
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B. How about the future?
The main header and distribution piping should be sized to take into account anticipated future expansions. If the initial piping is sized only for present flow requirements, then any additions will cause increased pressure losses in the entire system.
The next size larger pipe will add to materials costs, but may add little to installation labor costs and reduce the pressure drop substantially, with corresponding savings in operating costs.
C. How about materials?
Many industrial plants use schedule 40 steel piping, with or without galvanizing, for 100 – 125 psig service. Many food, pharmaceutical, textile and other plants which use non-lubricated compressors, install stainless steel piping to avoid potential corrosion problems and resulting downstream contamination.
For special applications, Federal, State and Local Codes should be consulted before deciding on the type of piping to be used. The usual Standard to be applied is ANSI B31.1.
For Health Care Facilities, consult Standard NFPA 99 of The National Fire Protection Association.
CAC Training can help you discover your plant’s pressure profile
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