Although you may see it as a free resource, compressed air is anything but free. In fact, in many industrial plants, air compressors consume more energy than any other single end use. Moreover, once the air is compressed to the desired pressure, it often has to be dried and cooled before it’s sent through the distribution system to the end use, requiring even more energy (figure 1). According to the US Department of Energy (DOE), compressed air accounts for 10% of industrial electricity consumption.
Fortunately, most industrial facilities can improve compressed air system efficiency through measures that have rapid paybacks. According to the DOE industry sourcebook Improving Compressed Air System Performance: A Sourcebook for Industry (PDF), compressed air often uses more electricity than any other type of equipment. Improving inefficiencies in compressed air can save you up to hundreds of thousands of dollars in annual energy savings, depending on facility use. A comprehensive optimization of a compressed air system usually requires a trained professional. However, you can take some cost-effective steps today to make your system more efficient, including reducing system pressure to the minimum necessary, identifying and fixing leaks, eliminating misapplied uses of compressed air, improving system control, and implementing a regular maintenance plan.
What are the options?
Rotary screw compressors
If yours is a small to midsize facility, you’re better off using a rotary compressor (figure 2). Although they’re less efficient, rotary compressors cost less to maintain, and, depending on the size of your facility, a reciprocating compressor (which is more efficient but more expensive to maintain) may not be worth the investment.
If you operate a larger facility, you’ll benefit from a reciprocating compressor (figure 3). Although reciprocating compressors cost more to maintain, they’re more efficient; for big facilities, the extra 5% to 10% efficiency gain over a rotary compressor makes the compressor cost-effective.
You can use compressed air hoses to quickly clean or dust off components in between production cycles. Depending on your application (such as drying or debris removal), there may be other forms of compressed air suited to your uses, such as air knives. Air knives create a blade of high-pressure air to shear away moisture or particulates without touching the components.
How to make the best choice
Reduce system pressure
Reducing system pressure to the necessary minimum is frequently the most cost-effective and quickest payback opportunity for energy savings in a compressed air system. It should be your first step in system optimization.
A compressed air system should deliver air at the lowest appropriate pressure for system needs while also supporting spikes in demand with stored compressed air. However, some compressed air systems operate at higher pressures than necessary because they’re overworking to meet faulty end-use requirements or outdated production demands, or they have suboptimal air-storage capacity.
Operating your compressed air system above the minimum necessary pressure is wasteful for three reasons.
Increased compressor energy
The higher the air pressure needs to be, the more compressor energy it takes to reach appropriate levels. One rule of thumb is that for systems operating at about 100 pounds per square inch (psi), every increase of 2 psi raises input power to the compressor by 1% at full flow. The opposite is also true—one DOE case study found that a pressure drop of 2 psi resulted in a 1% reduction in power.
Increased air consumption
For unregulated compressed air end uses, the volume of air consumed depends on the air pressure—as pressure gets higher, more air is consumed. You can often reduce compressed air requirements and energy costs without affecting performance by reducing system pressure.
Waste through leaks
The higher the system pressure, the more air is driven through the leaks that are common to compressed air systems. For example, at 80 psi, about 21.4 cubic feet per minute (cfm) of air will flow through a 1/8-inch diameter leak. At 100 psi, the flow would increase by over 20% to 26 cfm—wasting thousands of dollars annually.
Reducing pressure without affecting production processes requires you to be aware of the minimum pressure at which each compressed air end use can operate. If you find that none of the compressed air end uses in your plant require the pressure being delivered, you can save energy at almost no cost by dialing back the compressor discharge pressure in small increments to the minimum that maintains satisfactory equipment performance.
Sometimes you need elevated pressures to compensate for unacceptable pressure drops that occur due to large, intermittent compressed air consumers on the same distribution system. In such cases, adding secondary storage capacity at or near the point of use is an inexpensive solution to smooth out pressure fluctuations throughout the system. Fact Sheet 6 of the DOE’s sourcebook describes how to calculate the volume of secondary storage needed for a particular application.
Another reason compressed air systems operate at unnecessarily high pressure is that one or more end uses require it. In such cases, it can be profitable to install either a booster compressor with local storage or a separate compressor and air-distribution system dedicated to high-pressure end uses. Doing so allows the rest of the plant to operate at lower pressure and can result in dramatic energy savings.
Finally, if there are drops in pressure throughout the air treatment and distribution system, you will need to raise the initial discharge pressure to make sure that it’s enough to fulfill the end use after traveling through the system. In a compressed air system that’s properly designed and well maintained, the pressure at the end use should be at least 90% of the initial discharge pressure.
Almost any component of the compressed air system after the compressor can be a source of pressure drop, from dryers and filters on the supply side to undersized facility components such as distribution piping, equipment hoses, disconnect couplings, filters, regulators, or lubricators on the demand side. If pressure at the end use is significantly below 90% of compressor discharge, work through the system one component at a time to identify where the major pressure drops are. When specifying or replacing equipment, always look for information on pressure drop at the maximum anticipated flow rate and select equipment that minimizes it. Also, be sure to clean or replace filter elements regularly.
Finding the optimum pressure can be an iterative process. Some of the suggestions we provide here, such as eliminating leaks, can result in higher air pressures at the end use. However, it’s important to measure and record air pressure at each end use, before and after you make any improvements to the system, because you may be able to gain even greater energy savings by further reducing system pressure.
Find and eliminate air leaks
According to the DOE, it’s common for leaks to consume 20% to 30% of compressor output, which can add up to thousands of dollars per year in unnecessary electricity costs. Moreover, leaks reduce system pressure, which can cause air tools to operate inefficiently, and, in turn, affect production. Finally, compressors must replace all the air that leaks out, causing them to run for longer periods and reducing their lifetime.
Option A: Addressing leaks yourself
Eliminating leaks can be cost-effective and is often manageable without hiring contractors, especially if you have facility personnel with engineering knowledge. The first step is estimating the amount of leakage. There are two straightforward ways to do this, but you have to perform them while production is shut down.
For systems that have on/off or load/unload controls, allow the compressor to bring the system up to the pressure setpoint. Then, allow the compressed air system to run through several cycles (more cycles will give you greater accuracy) as the pressure drops due to leakage, and the compressor kicks on or loads up to bring pressure back to the setpoint. During each cycle, record the amount of time that the compressor is on or loaded. The ratio of the on or loaded time to the total time of the test is the leakage fraction.
For systems that have other types of capacity control, you can estimate leakage by noting the time it takes for system pressure to drop from its setpoint to one-half of the setpoint pressure with the compressor off and no production activity. The leakage rate (L), measured in cfm, is determined by the equation:
L = [(PS x V) ÷ (2 x T x 14.7)] x 1.25
where PS is setpoint pressure in pounds per square inch gauge (psig); V is the total system volume, including all storage and distribution piping, in cubic feet; T is the time in minutes it takes for the system to drop to one-half the setpoint pressure; 14.7 is the conversion value from psig to atmospheric pressure; and 1.25 is a correction factor. By comparing this leakage rate to the total volume of compressed air delivered, you can estimate the fraction of compressed air costs that are caused by leaks. Systems with leakage rates of 10% or more can likely be improved; your goal should be to bring the leakage rate down well below 10%.
The next step is to find and eliminate the leaks. Many leaks are audible and easily located, especially during nonproduction periods. For others, an ultrasonic leak detector (available from many manufacturers) is an effective tool. Once you’ve found a leak, eliminating it is often just a matter of tightening the connection, but sometimes it will be necessary to open a joint, clean the threads, and apply the proper thread sealant. In some cases, you may find that you need to remove and replace faulty equipment, such as damaged hoses and drain valves. Finally, ensure that the air supply to all equipment is shut off when it’s not in use. You can find solenoid valves (coils that create magnetic fields to open or close a valve) that automate the process.
Once you’ve found and eliminated as many leaks as possible, reevaluate the leakage rate to determine the effect you’ve had on the system and estimate the resulting savings. Also, look at the system pressure during normal plant operation—you may find that you’re able to further reduce the compressor discharge setpoint and gain additional savings.
Option B: Hiring out contractors
Contractors may be a better option if your personnel isn’t well equipped to perform in-house assessments and upgrades. If your facility is older, contractors are often more thorough than in-house staff and can find and eliminate more leaks, justifying their higher costs.
Have an air leakage contractor perform a fan pressurization test. This test depressurizes a space with either a large external fan or the building’s air-handling equipment. The contractor can determine the total building air leakage using measurements of both the inside and the outside air pressures and the airflow rate. In addition, while they’re performing the test, your contractors can locate where the leaks are occurring within the building either by feel or by using artificial smoke to trace the airflow. They may use an infrared scanner in conjunction with a blower-door test as another way of identifying leaks.
Once contractors find the air leaks, they use caulking, foam, or other sealants to repair them. Contractors should repeat a fan pressurization test after they’ve completed all the sealing to verify the work and quantify the air leakage reduction.
Also, have an HVAC contractor verify the performance of the ventilation system. You should do this initially to mitigate air leakage and then periodically as part of recommissioning efforts, after major renovations, or in response to occupant comfort complaints. Be aware of humidity, as a tighter building envelope may cause humidity to increase, which can result in moisture problems. Make sure to check the supply and return airflow rates to ensure balanced operation, and verify outdoor airflow rates to ensure that adequate fresh air is being delivered.
Mitigate building pressure
HVAC systems can create positive and negative pressures within buildings through duct air leakage or unbalanced airflow between supply and return air. Mitigating building pressure is just as important as sealing air leaks, especially when the pressure is negative. Negative pressure can cause backdrafting in natural-draft chimneys, or it can suck moisture and pollutants into a building.
Identify and eliminate inappropriate uses of compressed air
Compressed air is used in applications that could be performed more efficiently and more effectively using different methods. Figure 4 lists several common misapplications of compressed air, along with better alternatives. You’ll find a more comprehensive discussion in the DOE’s sourcebook.
Figure 4: Common inappropriate uses for compressed air
Although you’ll need up-front capital to fund the solutions, such as buying alternative equipment, the payback will be quick because using compressed air for these processes is so inefficient. Eliminating inappropriate uses will reduce compressed air consumption and may allow you to shut down one or more compressors entirely. Shutting a compressor down can save capital in the future as well—if you need additional compressor capacity in the future, you’ll have it ready and waiting.
Improve system control
All air compressors operate most efficiently when running at full capacity, but it’s rare for compressed air demand to precisely match the full load output of its air compressor. Frequently, some type of control system operates one or more compressors to match compressed air supply with demand. You can find many types of control technologies and strategies, some of which are specific to the type of compressor you have.
Regardless of compressor type, you can optimize system efficiency by operating as many compressors as necessary at full load and operating only one “trim” compressor with good partial load efficiency to match supply with demand. Choose the most-efficient compressors for full load operation. If you find that you’ve got two or more compressors simultaneously running under partial load that are feeding into the same compressed air system, it’s probably time to revamp your control strategy.
Perform regular maintenance
Like all other building systems, compressed air systems require regular maintenance to maximize operational efficiency and minimize unscheduled shutdowns due to equipment failure. Implementing a preventive maintenance plan that includes actions like replacing filters and fluids, adjusting belts, inspecting components, and identifying and repairing leaks will help you avoid increased energy consumption. Manufacturers of compressed air systems provide maintenance procedures and schedules for each component.
You should also consider benchmarking your compressed air system. Recording baseline values for system parameters such as power consumption, pressure, airflow, and temperature will provide data that maintenance staff can use to identify problems early on—for example, a steady increase in power use without increasing pressure or airflow indicates a drop in system efficiency.
Resources for further information
Because most compressed air systems have opportunities for cost-effective savings, hiring a trained professional to perform an audit of your compressed air system is a good idea. Whether you’re looking to hire an expert or planning to work on your compressed air system yourself, you’ll find the following resources to be helpful:
- Compressed Air Systems. This website from the DOE’s Department of Energy Efficiency provides many resources, including tip sheets, guidelines for selecting a compressed air service provider, free software for assessing your existing compressed air system and estimating potential savings from efficiency measures, and access to the industry sourcebook.
- Compressed Air Challenge (CAC). The CAC is a collaboration of compressed air manufacturers, distributors, and their associations; industrial users; consultants; state research and development agencies; energy-efficiency organizations; and utilities. On its website, you can learn about upcoming training opportunities and download documents covering a variety of compressed air concepts as well as case studies that demonstrate them.
What’s on the horizon?
For decades, facilities have used compressed air systems as the go-to method to provide pneumatic pressure. For now, compressed air appears to be a mature technology with no immediate developments in sight.
Who are the manufacturers?
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