Compressed air experts answer your questions

Jan. 26, 2012
Our panel of industry experts answer your questions about compressed air piping, energy efficiency, total cost of ownership and more.

Tom Taranto, owner of Data Power Systems, and a technical committee member and qualified instructor at Compressed Air Challenge (; Wayne Perry, technical director at Kaeser Compressors (; and Paul Johnson, global sales manager at Parker Legris (, participated in a webinar on compressed air systems on Jan. 25. Click here to access the archived webcast.

At the conclusion, participants in the webinar posed an assortment of questions to our panelists. The questions and their answers are below.

Question: What is the best method to keep closed loops draining to drip legs?

Wayne Perry: If this is a looped piping system, make one or more corners low points in the system and install a drip leg at the corner. At the bottom of the drip leg, install a zero-loss drain trap and service it regularly. However, if you have water in your compressed air distribution system, something else is wrong with your air treatment. I would look that direction first.

Tom Taranto: In any main header piping system, it is recommended to slope the pipe so that liquid water if present will drain away from the compressor supply point. Recommendations are typically 1 in. per 50 to 100 ft of run. After traveling some distance a tee fitting can be placed vertically allowing installation of a riser to regain elevation of the pipeline. At the bottom of the tee is an opportunity to install a drip leg with a zero-air-loss automatic drain valve. In large systems where thermal expansion of the piping is a concern, expansion loops are often designed to allow for pipe movement. These expansion loops are usually installed in a horizontal plane in the ceiling area. If the expansion loop is instead installed vertically, a natural drip leg is formed.

Question: In a typical refinery, say 150 kbpd, what type of savings are associated with air systems and what is the paypack period?

Wayne Perry: Refineries are difficult to quantify. Much of the air is process air, and that is unlikely to change much. For air that is used for general plant use and instrument air, savings of 30-35% are common. If you have a separate compressed air system for those uses, you can see the savings pretty clearly. If you are tapping off the process air for general use and instruments, it may be such a small portion of the capacity that optimizing the uses may not result in much visible change.

Tom Taranto: I am not aware of a statistically significant body of data relating compressed air savings by plant size and industry type. There was a publication done by the U.S. Department of Energy in 2004, Evaluation of the Compressed Air Challenge Training Program, which has some interesting information.

Below are some excerpts from the report:

“Annual energy savings. Among those who implemented improvements, average energy savings was estimated at 148,563 kWh per year or 7.5% of system energy use. At the facility level, savings ranged from a few thousand kWh per year up to 1.2 million kWh per year. At an average cost of $0.05/kWh, the mean value of annual savings from compressed air system efficiency projects implemented by the respondents was $7,428, with a range of $1,000 to $58,465.

“Costs for compressed air system efficiency measures. Expenditures on compressed air system improvement projects (including engineering and project management) ranged from $500 to $4,000,000, with an average of $150,000. The most expensive projects included replacing compressors (17% of respondents).

“Project payback periods. Payback periods on projects for which estimates of cost and savings were available ranged from 3 months to more than 10 years, with a median of 5 years. Projects with the longest paybacks were compressor replacements. This suggests that respondents had reasons other than energy savings to undertake compressor replacements. The median payback period for measures other than compressor replacements was 1.8 years, which is in keeping with the experience of members of the Ad Hoc Evaluation Committee.”

Question: Should copper pipe in the compressor room just be soldered, or should it be brazed?

Wayne Perry: Copper pipe used in compressed air systems should never be soldered (sweated); it should always be brazed. Brazing is the only way to make a joint that is strong enough for compressed air service. Brazing requires more of a craft than soldering, and a copper installation should be done only by someone who is experienced in brazing copper. When we tested people who were installing copper systems, we found that about 90% of them were not doing the work properly. We also know of injuries that resulted from improperly brazed joints in compressed air systems.

Question: So which material is the most cost effective (lowest total cost of ownership)?

Wayne Perry: If you are going to install it and never, ever change it, I think stainless steel is a good bet. If you may reconfigure in the future, aluminum can’t be beat. As a matter of opinion, aluminum is best for most applications. It does not require a craftsman to install, and it is leak-free.

Question: Do you find plants eager to switch compressed air intensive end uses with other, less energy intensive ways?

Wayne Perry: I don’t see plants wanting to change. Even when you point out that there may be less than a one-year payback, people tend to think that this is the way it has always been done, and, if I change something and it doesn’t work, I’m in trouble. Corporate engineering offices are a different story. A few of them are getting the message and are making changes to the way they design plants. A couple of years ago, an auto maker was working on an assembly plant design that used zero compressed air. Fortunately, I should retire before everyone switches to direct electric.

Tom Taranto: Plants can be very reluctant to change end-use methods that may impact the production process. That being said, the motivation is cost; it really comes down to dollars and cents.

There is a perceived (and perhaps real) risk in changing the production process. If a production manager is going to take a risk and make a change, what is his reward? If changing the use of compressed air has the potential to improve the production process by increasing production rates, improving quality, or reducing scrap and rework cost; there is a good incentive. In this case plant personnel may be very eager to implement changes to energy intensive compressed air end-use applications.

If on the other hand the savings are purely energy cost reduction, the production manager may be very reluctant to make any change to the compressed air end use. Since most plants do not directly bill production departments for the amount of compressed air that they use, there is little direct benefit to the production manager. When the guy using compressed air is not the one paying the bill for compressed air, there is little if any reward. So why take a risk and switch compressed air intensive methods with a less energy-intensive approach?

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