The compressed air system is the most misunderstood of all industrial utilities; the best engineering course has little more than a few hours of education regarding air. You can't get the necessary training through vocational technical institutions, colleges, or universities. If you have trouble getting the correct information from the salespeople who call on you, then your primary source of information is experience. As a result, here are the kinds of things that regularly occur in industrial settings:
• If a complaint arises about low pressure out in the production area, another compressor typically is turned on. If that doesn't help, another one 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 either replace the equipment with more of the same or deem it as being inappropriate and buy a different kind of cleanup equipment.
• If problems persist on a machine, maintenance will change service companies and never buy that brand or type of equipment again.
What should be painfully obvious here is that none of the decisions made had anything to do with business sense, systems jeopardies, or any kind of problem definition. 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 a plant where I used to work, we were asked to reconfigure the air system to adapt to an improved manufacturing process. Up to this point, the role of utilities was so submissive to production that manufacturing didn't even see the point of a briefing. I made the mistake of asking what the rules were for the use of compressed air at the point of use. One of the production managers 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 we should put into the budget. I thought it may make more sense to figure out what we needed. However, history had shown that no matter how much air was available, we'd need all of it in a relatively short period of time. They were all set to get into the heated subject of how many compressors, what brand, type, and size, but I got them to agree that demand should drive the supply. The spokesman for production told us that they were going to need about 4,000 cfm more than what they were using now. I asked how they determined the 4,000 cfm, and at what pressure they needed it at?
The pressure they requested was 90 psig. This number was 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 all this was done, we had arrived at 4,800 cfm at 115 psig.
The preliminary guess at the compressor and auxiliary requirements put this expansion at around 1250 bhp. Correcting their 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 (not including water, maintenance, labor, depreciation, etc.). This translated to $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 individual units too large. The sizes were based on the peak of 4,800 cfm and at sizes convenient for our vendors. We also realized that the base load changed dramatically along with the trim, not only within any shift, but from shift to shift. With this information, we realized we did not have any trim capability, only blowoff capability. This led us to look at two different types of compressors: one was for the base and the other type was for trim.
Therefore, we increased the number of compressors and substantially reduced the size of each. Although we didn't realize it at the time, we had also reduced the risk of interruptions because of the reduced consequences of a unit failure. At the end of this exercise, our capital requirements and annual operating cost were reduced further. We were entering the reconfiguration process with an $302,074 reduction in first-year capital and operating cost by reducing the fudge factor alone.
Next, I asked what the minimum acceptable air pressure was in production and if it wasneeded. Production said they needed 90 psig, but it would be nice if they could have more than that. I asked how much more. They said 100 psig. Again, why? They said more would be "nice." This would have required 2,008 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 24/7 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 a shut-down mode. This would be 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 the energy required to support them. One person mentioned that they had tried to get management to support a leak-fixing exercise. They learned that management didn't want to spend an unspecified amount of money for an unmeasureable result.
We determined the volume of what we could not shut off and subtracted it from the 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 have 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.