The compressed air systems in industrial plants are one of the most important utilities powering process machinery and tools. When trouble strikes, things shut down and production stops, then the phone starts to ring. Nothing gets the attention of facility operating personnel more than a very loud conversation with the boss. There are very often hidden troubles that don’t get any attention, but significantly drive up the cost of compressed air and cause annoying transient production outages.
This article discusses five very common signs of compressed air problems. They generally challenge the operation of all but the very best industrial facilities.
1. Variable pressure
Compressed air systems exist to create pressure. The potential energy contained within the air pressure is transmitted through piping and then changed to mechanical energy to run process machinery and tools. Any large variation in pressure will change the characteristics of the driven system. Figure 1 shows a typical data chart with compressor discharge pressure varying from a low of 95 psi to a high of 134 psi due to poor compressor control.
Too low a pressure will starve machines, cause them to be sluggish and lead to production outages. Too high a pressure will increase the forces created by any compressed air powered device, speeding up the operation, sometimes causing production problems. High pressure will also increase the electrical cost of producing compressed air by about 5% for every 10 psi-increase in compressor discharge pressure. Further to this, any air consuming device that sees higher pressure will consume more compressed air, about 1% for every 1 psi in increased pressure, causing the air compressors to demand even more power.
The causes of varying pressure are many, including:
- not enough compressor capacity
- improper compressor setpoints
- lack of adequate storage receiver capacity
- compressors shutting down unexpectedly due to overtemperature
- pressure drops across compressor room components (dryers, filters, piping)
- pressure differential across undersized system piping or at the end uses (undersized filters, regulators, hoses, fittings)
- large transient demands that exceed the local system capacity.
By far the most common condition is to have plant pressures set to excessively high levels to compensate for all these pressure losses, plus set a bit higher as a margin of safety. When this happens, the average pressure is too high during light loading, but the pressure will sag low during peak plant demands. Sometimes the variation is 20 to 30 psi, in extreme cases much more. In general, if your compressed air system is running at pressure levels above 100 psi, some of these pressure losses are likely the cause.
Figure 1. This typical operating profile, captured by data loggers shows highly variable pressure caused by poor compressor control. The pressure is higher than normal due to compensation for high pressure differential across compressor room piping, filters, and air dryers.
Best practice plants strive to minimize pressure losses through good design practices and the selection of well sized compressed air components. Target less than 5% pressure loss across the dryers and filters in the compressor room. Seek to keep pressure loss at less than 2% across the main distribution system. At the end uses, choose oversized filters, regulators, hoses, and connectors that minimize pressure loss during peak conditions, targeting under 5% pressure reduction.
2. Undersized storage receivers
In many ways the lack adequately sized and correctly located storage receivers can allow problems to occur within a compressed air system. Here’s a test: if a new visitor to your compressor room doesn’t do a double-take when they see your main air receivers, the tanks are likely too small. Notice well that storage tanks do not have any power cables running to them, but they can have a big effect on your electrical bill.
In the past, the rule of thumb in sizing receivers was to have a total capacity equal to one gallon per cfm times output of the largest compressor in the system. So, if the compressor is a 100 hp compressor with 400 cfm output, a 400-gallon tank was installed. But this sizing rule has proven to be inadequate. Good practice is to have total storage between 3 and 5 gallons per cfm, making the receivers for the example compressor between 1,200 and 2,000 gallons. Best practice would be to install 10 times the capacity or 4,000 gallons.
And if you were paying attention, you would have noticed we mentioned more than one receiver; best practice is to have about 30% of the total capacity as wet storage before the air dryer, and the remainder after the dryer as dry storage.
And you shouldn’t stop at the compressor room, having correctly sized local dedicated storage receivers within the plant at the end-uses can help protect critical pressure applications from low pressure and also filter out large transient compressed air demands that might affect compressor control, keeping an extra compressor running for no reason.
If adequate storage exists, and excessive pressure drops have been addressed, this will allow compressor discharge pressures to be reduced, lowering operating costs. Figure 2 shows an example of the power reduction in choosing larger storage.
3. Poor compressor control
Having variable pressure, point number one, is usually a symptom of having a poor compressor control strategy. Another symptom is having poor energy performance; this condition is often hidden because it is rare to have any energy meters on air compressors.
The primary reason compressors are controlled is to maintain the pressure to within certain upper and lower limits. As the flow demanded by plant processes changes, the control strategy must change the output of the compressors to match the demand. If too much air goes into the system, the pressure rises, too little, the pressure falls. The goal is to maintain an equilibrium where the pressure remains constant at an adequate level to maintain consistent production. There will be some point, called the minimum pressure level, where machines start to fail due to inadequate pressure, so it is important to design the system to ensure this condition never happens, even if the largest compressor in the system fails unexpectedly.
Ask the Experts: Compressed Air Challenge members answer your questions
Individual lubricated screw compressors are typically controlled in these ways:
- Start/Stop – The compressors turn off when the pressure gets too high and turn on when the pressure drops to a lower limit. This type of control is very energy efficient but cannot be used on compressors more than 30 hp to avoid motor burnout.
- Modulation – An inlet valve closes off the flow of air to the compression element reducing the mass flow through the compressor. This type of control is very inefficient but is sometimes the only way compressors can be controlled if there is inadequate effective storage capacity, or the need to run the compressors at higher than rated pressures.
- Load/Unload – The compressor motor remains running, but the flow of air stops when the compressor is unloaded. In unloaded condition, zero air flow, the compressor still consumes about 30% of its full load power, which is inefficient. This mode of operation is moderately efficient compared to modulation. Use of this mode on systems with limited effective receiver capacity causes rapid cycling, which reduces the potential for energy savings substantially. If you check the operating hours of your compressors and notice a lot of unloaded run time, it is a sign of inefficiency. If your trim compressor (the one loading and unloading) takes less than 2 minutes per load/unload cycle, this is a sign you don’t have enough effective storage capacity.
- Variable Displacement – Some manufacturers add displacement controls as an option, which bypass part of the screw element, efficiently reducing the power consumption at part loads of between 50 and 100% capacity. Below this the compressor will load and unload. Very often this type of control has been inadvertently disabled by some well meaning service person; sometimes the setting is just a screwdriver adjustment (and a lot of people have screwdrivers). Ask your service provider if you have this type of control, and if confirmed, make sure some competent service provider does the coordination of the pressure switches, modulation control, and displacement control (yes that’s right, some compressor models require three separate adjustments each time the pressure setting is changed).
- Variable Speed – Variable speed control is the most efficient way to operate compressors in part load. Where appropriate, VSD compressors should not be used in extreme environmental conditions. Having a well-sized and properly set up VSD compressor in a system saves the most energy, but there are best practices to consider. You should avoid operation of the VSD compressor in light load conditions where the flow is below the compressor minimum speed, lest water build up in the lubricant. And it is important to ensure, in a system of multiple compressors, that the VSD variable range (the capacity between minimum speed and full speed) is equal to or larger than the largest fixed speed compressor with which the compressor must work. If this sizing rule is not followed there will be control gap issues, which cause multiple compressors to fight for control. If your VSD is the same size as your other compressors, this is a sign of trouble.
The choice of individual compressor controls has a lot to do with the stability of the produced pressure and the energy efficiency of the system. In multiple systems the pressure settings of the compressors need to be well selected, so they come on and turn off in an orderly fashion. If you have more than three compressors and don’t have a compressor sequencer, this is a possible sign of inefficiency. Best practices plants have constant pressure produced within a single band controlled by a central controller. Typically, the most efficient systems have one or more VSD compressors installed.
Figure 3. Operation of a VSD compressor with fixed speed compressors requires the VSD target point to be nested within the pressure bands of all the fixed speed units. (Source: Compressed Air Challenge Advanced Management of Compressed Air Systems Training).
4. Contaminated compressed air
Poor compressed air quality is a sign of trouble. The most common contaminants are water and lubricant, but rust and dirt can mix with these liquids forming an ugly mess that can clog up pneumatic equipment and contaminate your final product. To test your air, take a white paper towel and spray some air through it. Did it come out clean?
As strange as it may seem, high temperatures are usually to blame for water and lubricant contamination. Compressed air produced by a compressor contains about double the amount of water for each 20°F increase in discharge temperature. When compressors overheat this extra water can overwhelm air dryers and cause moisture to pass though. It is also more difficult to filter out lubricants and free water if the air is at high temperature. High temperatures also age compressor lubricant faster, reducing the quality and lubrication properties.
If you hit a wall of heat each time you enter your compressor room, this is a sign you need to look at your cooling system. Maintaining cooling air (and liquid coolants) as cool as possible goes a long way.
Having well-sized air drying and filtering can prevent contamination from occurring. Size for worst case peak flows, ambient temperatures, and operating pressures. Choose energy efficient dryers, use cycling refrigerant style rather than non-cycling, use desiccant style dryers only if required, these cost 3 to 5 times more to operate. Choose energy controller options on desiccant dryers. Oversize filters to reduce pressure differential and use low differential “mist-eliminator” filters where appropriate. The selection of the type of dryer and filters can have a great effect on the control of your compressors and the effectiveness of the storage capacity.
5. Uncontrolled waste
As systems age, more and more things get connected, it is just the way things go. A blower here, an air motor there, all adding up to an expensive increase in the flow, sometimes requiring extra compressors to be purchased and fed with electricity. And, as time goes on, leakage starts to spring up in anything flexing, bending, vibrating, and rotating. If your plant sounds like a pit of vipers and does not have a leakage detection and repair program, it is a sign of trouble.
In extreme cases as much as 80% of the compressed air produced by the plant compressors can go to waste in supplying unrepaired leakage, flow due to misuse, and increased flow due to higher than needed pressure (called artificial demand). Typical values average 50% of the total flow. This represents a high potential for savings through optimization.
Best practice plants have a good maintenance program to limit these waste areas and have policies in place to prevent the misuse of compressed air. These plants train their people to be on watch for leaks and inappropriate uses. The best plants have a good system of permanent monitoring of compressed air system pressure, flows, power, and dew point in place to help them monitor compressed air costs and detect waste. Target leakage levels under 15% of the average flow. The best plants achieve under 10%.
If one or more of these signs of trouble seem familiar to you, then it may be time to act. There is no reason you need to struggle with contaminated compressed air produced by an unreliable inefficient, wasteful, and expensive system. These conditions are all preventable.
Times have changed in the compressed air industry, and the focus is now on efficiency and reliability. It may be time to contact your compressed air service provider to have your system analyzed to find solutions to your more obvious and some hidden problems.
If you are interested in learning more about compressed air system optimization, go to the Compressed Air Challenge calendar and sign up for the many available training sessions. You will be glad you did, as are the many thousands of training participants before you.
This story originally appeared in the August 2022 issue of Plant Services. Subscribe to Plant Services here.
