What is in this article?:
- Improving Compressed Air System Efficiency: Part 1
- Three concepts to remember
- Compressed air storage capacity
Carefully engineered responses to demand will result in more efficient systems and compressed air savings.
Three concepts to remember
Several years later, I worked in the field for a major pneumatic tool and hoist manufacturer. This experience provided me with the next steps in my thinking process. During nine weeks of training, we were taught how to optimize the air tools as they were applied to industrial applications. Three major ideas came from this study.
One was that the efficiency of the equipment is a function of how it was installed. Whenever we demonstrated or tried a tool for a user, we should always try to use our own company-supplied hose and fittings. There was no explanation or technology supplied, other than the fact that the tool would work better. It would achieve higher torque, grind more metal, etc. We found that most manufacturers, including my employer, did not go out of their way to specifically state the required volumes or pressures required for proper operation.
The second idea is that you can get applications to work more efficiently at the same pressure or get the same efficiency with more volume at lower pressure. The volume of air required by the tool is a function of the supply pressure, so the higher the supply pressure, the higher the exhaust flow. You can achieve the same amount of work energy by increasing the mass inversely proportionate to the reduction of the absolute pressure.
The third idea is that because of compressed air supply limitations, a different style of tool or size of air motor is sometimes necessary.
To further complicate everything, users often break the compressed air decision-making up to two or three different levels of management, based on the capital cost of the components. All too often, compressor decisions are made by an executive who doesn't know an air compressor from a milling machine. When you sort all this out — depending on the number of required bidders from each group, coupled with the levels of management — 20 or 30 different opinions pop up about what should be done. All these groups have different interests and priorities. It's little wonder that very few air systems work. One of the outcomes is that if the system works — even remotely as well as the designer intended — everyone is delighted. When you begin to look at all of this systematically, you have to change to a new pair of glasses to understand what's going on.
Measuring compressed air
For many years, people have been trying unsuccessfully to measure the performance of compressed air systems. This, of course, has been done with every other utility with no problem. The old "compressed air is free" thinking stems from a lack of knowledge of quantities and cost. You can't determine cost without usage. Then you have to compare demand volume at pressure against supply energy in real time.
Most plants have nothing other than the minimum acceptable pressure gauges on the equipment. A few alwo measure flow from the compressors, but this does not measure energy. I'm not sure what value this has other than perhaps holding suppliers accountable or noticing change in ambient or maintenance conditions.
Most people who measure flow are interested in demand accountability. They accumulate volume without pressure over a long time, such as a month. This gives a very distorted view of usage and energy because it does not detail peaks, valleys, energy, or efficiencies — which drive cost. Flow and pressure must be measured with respect to time if the information is to be meaningful — usage vs required energy. Until you have information supporting the quantitative problem, it is difficult to get any support for solutions from management or utilities. The air system has a lot of problems, and if we spend a lot of resources on problems, things will get better. Not many managers will invest in that one.
The few systems where I was able to collect data gave me an opportunity to measure constituents of demand — such as leaks and artificial demand. I had always been comfortable with a 10% to 20% value for these two items. What I was realizing was that it was more like 25% to 40% on the average, and that didn't include any supply inefficiencies or inappropriate production usage. Combined with leaks, it is common to see more than 75% of all usage unregulated. Once you know the size of the problem, and you translate that into monetary terms, you can get management's attention. As you dig into the solutions, you need information in order to benchmark progress and build discipline to maintain standards.
As users began including information gathering into retrofits, we were able to see that the estimates of usage when originally designing the system missed the actual usage by a mile. It took a long time to figure this one out. You might start believing that the manufacturers of air equipment don't want people to know what the relationship can be between air usage and power required to support it.
It takes 30 scfm at 90 psig or an equivalent weight flow in real time to generate 1 hp of compressed air work energy at the point of use. It takes about 8 to 9 hp of compressor and auxiliary equipment to generate that power. A typical ¼-in. air drill requires 8.5 scfm or 0.2 hp of electricity. A 7-in. disc grinder requires 60 scfm — about 15 to 18 hp of electrical support. Rather than judging or guessing why the industry does what they do, I'll show you. They provide use factors, which supposedly are based on the typical volumes. As an example, they would estimate that the ¼-in. drill would operate for 20% of a minute and the grinder may have a 50% use factor, or 30 seconds per minute. They then estimate the volume per minute from:
8.5 scfm × 0.20 = 1.7 scfm, or 60 scfm × 0.50 = 30 scfm
Because all installation components must be sized based on rate of flow — not average flow per time period or cumulative cycle flow over a minute — most systems and components are sized improperly. You cannot take 2 ft3 per 5-sec cycle four times per minute and call it 8 cfm. The rate of flow is 2 scfm × (60 sec / 5 sec), or 24 scfm. The results are high differentials and an inability to handle peaks. The systems approach to these problems results in high operating pressures and extra compressors that remain on all of the time, to manage large or coincidental demand events.