What is in this article?:
- Compressed air efficiency
- Three concepts to remember
Carefully engineered responses to compressed-air demand will result in more efficient systems and savings in operating costs.
A compressed air system is one of the most misunderstood of all industrial utilities. Even the best engineering courses have little more than a few hours of education regarding air. If you have trouble getting the correct information from salesmen, your only source of information must be experience.
As a result, here are some things that regularly occur in industrial settings:
• If there is a complaint about low pressure in the production area, another compressor is turned on. If that doesn’t help, another is turned on. If maintenance runs out of compressors, they buy more until the pressure rises enough to keep production people from complaining.
• If production people complain about contamination and the compressor appears to be working based on supplier service inspection, maintenance will replace the equipment with more of the same or deem it inappropriate and buy different clean-up equipment.
• If problems persist on a piece of equipment, maintenance will change service companies and never buy that brand or type of equipment again.
What should be painfully obvious is that none of these decisions had anything to do with business sense, systems jeopardies, or any problem definition. Keep in mind that many shop mechanics make their operational and maintenance decisions based on the following commandments:
• keep it running,
• control the maintenance cost,
• anticipate and prevent failure, and
• keep the telephone from ringing.
Real-world example #1
At one of our plants (I was working for a different company then), we had a chance to reconfigure the air system to adapt to an improved manufacturing process. Until then, the role of utilities was so submissive to production that manufacturing didn’t even see the point of a briefing. I asked what the rules were for the use of compressed air at the point of use. A production manager clarified things quickly. He said, “We take it...you make it! If you do a good job, we can keep the costs down.” That seems pretty clear, doesn’t it?
Everyone wanted to discuss what new supply-side equipment should be put into the budget. I thought it made more sense to figure out what we needed. History had shown that no matter how much air was available, we quickly needed all of it. Instead of discussing how many compressors, what brand, type, and size, I got them to agree that demand should drive the supply. The spokesman for production told us they were going to need about 4000 cfm more than what they had now. I asked how they determined the 4000 cfm, and at what pressure they needed it?
The pressure they requested was 90 psig — higher than any previous accepted minimum pressure. Utilities had taken their numbers and inflated them by a 20% “fudge factor” on volume and 5% on pressure. When done, we had arrived at 4800 cfm at 115 psig.
The preliminary guess at compressor and auxiliary requirements put this expansion at around 1250 bhp. Correcting the fudge factor put it closer to 800 bhp. This was 322 kW less and at $0.08 per kW per hour, meant $25.81 less per hour just for electricity or $266,074 per year difference in operating cost. The reduction in capital was more than $76,000 including installation.
We next refined the loading and determined that we had sized the units too large. Their sizes were based on the peak of 4800 cfm and convenient to sizes available from vendors. We also realized that the base load changed dramatically, not only within any shift, but from shift to shift. With this information, we realized that we only had enough blow off capability. This led us to look at two different compressors.
We therefore increased the number of compressors and substantially reduced their size. Although we didn’t realize it, we had also reduced the risk of interruptions because of there were less consequences of a unit failure. Our capital requirements were reduced further, as well as the annual operating cost. We were entering the reconfiguration process with a $302,074 reduction in first-year capital and operating cost just by reducing the fudge factor.
Next, I asked what was the minimum acceptable air pressure in production and if it was needed. Production said they needed 90 psig, but it would be nice if they had more. I asked how much more. They said 100 psig. Why? They said more would be “nice.” This would have required 2008 hp to elevate the pressure 15 psig for the entire system above the current lowest pressure of 85 psig. Doing this would cost $1.14 million. I asked what they would get for the elevated pressure. The spokesman said, “I don’t know...probably feel a little better about the reliability of the air system, but I didn’t consider how much that would cost.” You can buy a lot of psychiatric couch time to improve your feelings for $1.3 million!
Real-world example #2
A second example involved an automotive assembly plant that had 73,000 scfm, but more than 19,000 cfm of leaks. This plant was a 24-hr, 7-day operation; they never shut down unless they absolutely had to. Under these circumstances, no one had noticed how much air was being serviced in the system in shut-down mode. This was based on the number of compressors they had to run to maintain pressure when they did nothing. Everyone agreed that there were many leaks.
No one knew how much the leaks amounted to or what was the energy required to support them. One person mentioned they had tried to get management to support a leak-fixing exercise. But management didn’t want to spend an unspecified amount of money for an unmeasurable result.
We determined the volume of what we could not shut off and subtracted it from total supply at our lowest load. That was 19,000 scfm at full line pressure — air that was being used with the plant shut down! (I’ve seen hundreds of plants with this percentage and a few plants with more leaks by volume). We used more than 4800 bhp of compression equipment to maintain the extraordinary waste. That’s more than $318 per hour, or $2.79 million per year.