Advertisment

Stop Blowing So Much on Compressed Air

When it comes to compressed air, people sometimes focus most of their attention on the least important costs. “The costs associated with energy usage and the costs associated with maintenance are significantly higher over the life of a compressor than is the cost of the compressor itself,” notes Paul Humphreys, vice president, communications and branding, for Atlas Copco Compressors.

And, when it comes to energy management and maintenance of compressors, most plants can do better. When Wayne Perry, technical director for Kaeser Compressors, was in Vietnam a couple of years ago doing a compressed air analysis, the managing director of the plant said he was glad Perry was there. “He said they wanted their compressed air system to run just as efficiently as they do in the United States,” recalls Perry. His response to the manager: “If that is all you’re looking for, you’re already there. You’re wasting about half your power, and that’s about what they do in the U.S.”

Why is compressed air efficiency so difficult to grasp for so many organizations? Ownership, or rather lack of ownership, may be the primary cause, according to Randy Finck, director, global advanced services, for Ingersoll Rand Industrial Technologies. The question is always asked: Why doesn’t someone do something about compressed air inefficiencies? “Most of the solutions are well-known,” states Finck. “However, year over year, there only seem to be incremental improvements, rather than dramatic improvements. So, if the problems are known and the solutions are known, why wouldn’t we fix them?”

Who is responsible for compressed air system efficiency in an industrial facility? There are many stakeholders, according to Finck, but who is ultimately responsible? In the grand scheme of things, he believes, there are a lot of people responsible for production efficiencies, and a lot of people responsible for other processes in the facility. However, when it comes to compressed air system efficiency, rarely do you find an owner. “If everyone owns it, then maybe no one owns it, and this may be why we don’t see dramatic improvements,” he adds.

Start at the End of the Line

Most plants have opportunities to save energy in the compressor room, with more efficient compressors, filters, dryers, piping, etc. But before you look at ways to generate compressed air more efficiently, it pays to look hard at how you’re using it – compressed air demand – and whether you really need as much as you’re consuming.

There are two significant opportunities to reduce demand for compressed air. One relates to inappropriate use. The other relates to leaks. Both can be discovered with a compressed air system audit. In fact, paying for an outside audit can end up resulting in a cost savings. The outside experts can provide you with a list of where and why inefficiencies are occurring, as well as what to do about them. An audit should include an evaluation of current demand and distribution before recommending changes to the supply side (such as introducing more or larger compressors). You may discover that a lot of demand is unnecessary, such as demand that results from inappropriate use and/or leaks.

Compressed Air Energy Rules of Thumb
  • It takes about 8 horsepower of electrical energy to produce 1 horsepower of work with compressed air.
  • 1 cubic foot per minute (cfm) costs about $100 per year in energy.
  • Half of compressed air is wasted by excessive pressures, inappropriate uses and leaks.
  • Each 2 psig excess pressure or pressure drop costs about $400 per year per 100 compressor horsepower.

Eliminate Inappropriate Use

Many employees believe and act as though compressed air is free. Examples involve using it as a personal cooling device, blowing oil off machined parts, blowing sawdust off woodworking devices, and using it as a “broom” to blow dirt off the floor.
“Inappropriate uses are anything that could be done with direct electric, such as a fan, hoist, or motor,” Perry says. (It takes 7 hp of compressor to run a 1 hp air motor.)

At one plant meeting, Perry asked the plant manager how much money each worker was allowed to spend without getting approval. The manager said they were not allowed to spend anything. Perry responded by explaining to the manager that workers were each spending about $10,000 a year by using compressed air as a way to keep cool. “I showed him that he could insulate and air condition the plant for less money than he was spending on compressed air,” he states. “The manager went out and purchased floor fans for even less money, and the workers love it because it moves a lot more air.”

As Humphreys sees it, inappropriate use is a management issue. “It is a job for management to make sure it is being used for what it should be used for,” he says.

Manage Leaks

According to the U.S. Department of Energy, the average facility has 30% to 35% leakage if it hasn’t taken any recent action to identify and repair leaks. And, a survey conducted by the Office of Industrial Technologies found that 57% of facilities have taken no action during the past two years.
“Leaks are a big issue,” emphasizes Perry. Kaeser has done about 2,000 air demand analyses, where it goes into facilities, puts sensors on everything, and logs what the systems are doing. Of that 2,000, it found about 1,200 where no production was going on, so it could get a good look at what the leak load was. The average leak load was about 34%. “Some of them were as high as 75% and 78%,” he reports. “Some were as low as 8%, which is a sign of a really good system.”

Five Things to Check before a Compressed Air Audit
Compressed air system auditor Andrew Sheaffer says attention to these five apoints can significantly improve the quality of an audit – and may let you avoid one altogether. “As independent auditors, we seek to develop cost-saving recommendations that reduce air system operating cost by 25% to 50% while improving system performance. So clearly, air audits are a sound investment,” Shaeffer says. “Just complete this checklist before we arrive and make us really work for our fee.” See the complete checklist

Another reason people don’t pay much attention to leaks is that they’re not “glamorous,” according to Ingersoll Rand’s Finck. “They aren’t as interesting as increasing efficiency by utilizing a high-efficiency motor or variable-frequency drive,” he states. “However, it is important to realize that the potential energy efficiency results are often more dramatic as a result of fixing leaks than they are for some of the other more ‘glamorous’ solutions.”

Managing leaks should be an ongoing process, according to Trey Ragsdale, product marketing manager for Atlas Copco Compressors. “Leaks reduce pressure on the system, so when you repair leaks, pressure will increase, causing other leaks to get larger,” he explains. That is, repairing a leak will increase pressure in a system, making other smaller leaks lose more air. As a result, repairing large leaks is not a complete solution.

Often, when it comes to leaks, people fix them once, and then ignore them after that, according to Finck. In fact, this is often a reason that leaks aren’t addressed on a regular basis. “People realize that leaks are only going to return, so why fix them in the first place?” he points out. To ensure continuous demand reduction, or even just to sustain a hard-won reduction, you need to create a leak management program. The program should continually identify the leaks, repair them, validate the repairs, and then maintain those reductions every six to 12 months.
In specific, you should have a leak inspection program such that every part of the plant is inspected at least once a quarter to identify and repair leaks.

The best area to start looking for leaks is at the point of use. Pay special attention to areas such as couplings, hoses, tubes, hose and tube fittings, pipe joints, quick disconnects, condensate traps, valves, flanges and packings.

There are several ways to detect leaks: listening (ideally when the plant is quiet), running your hands over the pipes and equipment, using soap bubbles, and using an ultrasonic leak detector (USD). Most experts agree that the USD is the least familiar but the most effective. Leak inspections should be done with a high-quality USD during times of production and non-production.

When performing leak detection and repair, it is important to prioritize. Start with large leaks (10 cfm or larger), then medium leaks (5 to 10 cfm), then smaller leaks (5 cfm and smaller). Over time, watch the leaks that continue to grow. For example, the smaller leaks that were not repaired the first time will become medium, since fixing the large leaks has led to a pressure increase. At this time, you can repair the smaller leaks.
In terms of repair, Teflon tape alone is not effective. Tape will seal the contact points in threads, but it won’t fill the voids. It is important to use thick liquids or pastes to fill the voids in the threads. Apply the sealants to the pipes and fittings. Once they are cured, they can also add mechanical strength and vibration resistance to the joint.

After repairing leaks, it is then important to adjust and modify the compressor control system (discussed later) in order to capture the new energy cost reductions, directly proportional to the reduction in compressed air demand.
In fact, as you repair leaks, you may be able to eliminate one or more compressors and thus reduce energy usage, or maybe eliminate a large compressor and be able to replace it with a smaller compressor.

Take a Systems Approach to Compression

There are several ways to increase the efficiency of compressors and compressor systems. These include introducing compressor controls and paying attention to dryers and filters.

First, though, it is important view the compressors and their components for what they actually are - a system. The reason is that, when you change one thing to a system, that change affects many other things in the system.

“It is important to look at a compressed air system with a systems approach, rather than a component approach,” emphasizes Kaeser’s Perry. “Most people buy a compressor, a filter and a dryer. They don’t think about how all of these are going to work together.” For example, they may buy a 1,000 cfm compressor and a 1,000 cfm dryer, without realizing that the dryer is probably too small for the room it will be in. It may be hot, and the compressor may not be able to feed the dryer with 100 °F or cooler air, and they wonder why they have water in the system. Start with the demand, and then work your way back to the compressors to determine the best way to set up the system.

  Leaking Systems Tight Systems Tight plus Controls
Process Demand 1,500 cfm 1,200 cfm  
Demand Reduction   300 cfm (20%)  
Power 259 kW 243.5 kW 206.9 kW
Dynamic Efficiency 5.8 cfm/kW 4.9 cfm/kW 5.8 cfm/kW
Annual Energy Cost $108,780 $102,270 $86,898
Net Savings   $6,510 (6%) $21,882 (20%)
*Example system at 90 spi and $0.05/kWh

Another benefit of using the systems approach is that it can help you save money elsewhere. “The more a compressor compresses air, the hotter it gets, and you can calculate how much heat you generate in the compression process,” continues Perry. “So, basically, a compressor is a big heater.” If you have a 100 hp compressor, you also have, by default, a 75 kw heater. In most systems, though, a heat exchanger then dumps this heat away. If you’re paying to heat water, though, why not use the compressor as a source of that heat? When you do this, you end up with a large water heater that also happens to put out compressed air. “What happens in most cases is that the water being run through the compressor that ends up being heated is sent to a cooling tower to cool it and then bring it back to cool the compressor,” notes Perry. “Just eliminate the cooling tower. In Europe, this type of heat recovery is very common.”

Controls Are Key to Efficiency

Most plants end up running too many compressors too much of the time. When production demand goes up, it asks maintenance to turn on a compressor. However, production rarely ever then calls maintenance back to have the compressor turned off. A control system can automate all of this.

Multi-compressor control systems automate compressor shut-down according to demand. In addition, timers can shut down unneeded equipment at night or on weekends, and then turn it back on when the plant reopens.
An effective control system will prevent problems such as too many compressors running at the same time, compressors running “just in case,” and fluctuating plant pressures.

“The goal with compressor controls is to match supply to demand as closely as possible,” states Ingersoll Rand’s Finck. You need to ask: Are you running the right compressors? And are you running any compressors unnecessarily? Are you running the correct sets of compressors? Are you running one large compressor when you should be running two smaller compressors? “You can turn off compressors that aren’t required,” he continues. “It would surprise you how many compressors are running unloaded. The rationale is that, just in case the demand arises, people want them to be available to supply the demand.”

According to Kaeser’s Perry, installing a master controller, just by itself, can probably save about 20% on energy. “When compressors run on individual controls, a lot of them run when not necessary,” he explains. “I am doing a study in a plant now that is currently running two 600 hp centrifugal compressors and four rotary screw compressors.” The plant could actually run with one 600 hp centrifugal and three rotary screws. However, they don’t have a master controller that would let the machines know which ones should be running, and which ones don’t need to be running. Perry says turning those extra compressors off would save them about $250,000 a year.

A lot of master controllers today also have the ability to collect data, so you can get up to a year’s worth of readings telling you exactly what is going on with the compressed air system, how efficient it is, whether the leak load is growing over time, and so on.

Dryers Are Not Created Equal

“Dryers are important, because wet air will significantly reduce the life of tools,” notes Atlas Copco’s Ragsdale.
According to Ingersoll Rand’s Finck, the horsepower requirements for refrigerant-type dryers are relatively small. However, the opportunities for pressure drops across those devices, if they aren’t sized correctly, probably outweigh any efficiencies that can be gained from horsepower savings on the dryers.

According to Kaeser’s Perry, the least expensive dryer is a refrigerated dryer. This works as long as the pipe is not exposed to temperatures near or below freezing. So, in most cases, there is no reason to run anything other than a refrigerated dryer. “I see people who have put in dessicant dryers because they had refrigerated dryers and still had water in the line,” he notes. “However, it may have been that there was water in the line because the refrigerated dryer wasn’t properly sized for the application.” For example, they might have purchased a 1,000 cfm dryer for a 1,000 cfm compressor, but really needed a 1,200 cfm dryer. The problem with dessicant dryers, according to Perry, is that they require more energy to run and more expense to change the dessicant over time.

Filters Can Choke Efficiency

Clogged filters can cause significant pressure drops. Filters need to be replaced at appropriate intervals. When they are clogged, pressure drops increase significantly. When new, most filters create a pressure drop of 1-5 psi. However, when they reach the end of their service life, pressure drop can exceed 15 psi.

It is a good idea to add pressure gauges to filters, which should be available as add-ons to most filters, to keep track of what the pressure drops are.

“Some people think they need a finer filter to keep the particulate out at the point of use,” points out Kaeser’s Perry. For example, instrument air quality might require no particulate larger than about 40 microns. However, some people want filtration down to less than 1 micron. This creates a resistance to flow, which leads to pressure drop, which means that you have to compress to a higher pressure - a waste of energy.

One particular problem for filters is black iron pipe. If this type of piping gets any moisture in it at all, rust will form, and that will end up becoming particulate. “I get asked a lot what the best kind of piping is,” states Perry. His response: The most expensive is stainless steel, but it is also the best. Next in order are aluminum, copper, galvanized, polyethylene, and then black iron. “Polyethylene is not my favorite, because it sags when it gets hot, but it is definitely better than black iron pipe,” he adds.

Measure, Monitor, Repeat

So, ultimately, what is an efficient compressed air system? That is something you need to determine for your own facility. “You need to create a metric,” suggests Ingersoll Rand’s Finck. “Once you have a metric, then you measure against the metric.” This means continuously monitoring against various key performance indicators (KPIs) that are tied to the metric so you can make sure you are within your tolerances. You can then decide what you are going to do if you fall outside of the tolerances, and then what steps you are going to take to maintain the improvements.
It is very important to identify an owner to do all of this. It is also important to have a control plan or sustainability plan in place, so that these improvements don’t become a single event, and so that you maintain the improved efficiencies.

Add a comment

You cannot post comments until you have logged in, and have an appropriate permission level. Login here or register for a new account.