COMPRESSED AIR: A FACILITIES PERSPECTIVE
Compressed air represents one of the most critical power resources in most modern manufacturing and process environments. It ranges between seven to forty percent of the total electrical usage in the most plants. If the pressure drops beyond an acceptable level, production is interrupted. If the contaminant level of the compressed air varies significantly in terms of moisture, lubricant, or dirt, production quality is effected. In terms of wire to work, it represents the most inefficient means of transmitting power in the plant. A relatively well designed compressed air system with little waste will produce approximately eleven percent of the input energy in the form of work at the point of use. At $.06 per kwh, three shift a day, seven day a week production, every 100 cubic feet per minute costs approximately $15,000 per year to operate. 1000 cfm can cost over $150,000 per year to operate. If the your plant produces 10% pretax profit, you must generate $1,500,000 in production revenue to support 1000 cfm of average usage per year. Despite this information, well intended production personnel give little thought to the use of compressed air and actually think that more is better. More pressure, .....more volume, ....more dryness, .....more whatever is good.
In one textile manufacturing plant, the production manager was interviewed during an audit regarding his use of compressed air. His expressed attitude was We take it. You make it. What more needs to be understood. The following illustration was developed specifically for his benefit showing the cost value relationship of compressed air and his investment.
Since production paid for compressed air, but it was presented only as part of a large bucket of cost called overhead, they had no idea what if any impact it had. They also had no idea whether it was a good investment. In this case, the received 75% of what was produced, but only directly benefited from 35% of his total investment. With actual financials to support the constituents of demand, the production and maintenance managers came to an agreement as to what each of them had to do to get the best return on investment. They also both committed to accountability and education for their personnel.
Most manufacturing facilities have no idea how much compressed air they actually use or need. At best, they may know the minimum acceptable pressure and air quality required experientially. This information has probably been handed down like a legacy from previous operating personnel. There is probably some significant fudge factor between the perception and the reality. It is highly unlikely that anyone knows specifically what the compressed air costs, and there are no rules for the use of it on the production side of the system. Production installs new compressed air usage on a regular basis with no discussion with facilities personnel and no idea what impact this change may have on other production applications, systems reliability, or the systems operating cost. In the average facility, this expensive and critical utility is used as though it is a limitless resource. If this approach were taken with any other utility, such as electricity, the voltage, amperage, and power quality would be so influenced that production could not and would not tolerate it for a day.
Operating Approach - Are you operating this utility consistent with good business practices?
With no one in facilities, production, or management understanding these previously mentioned issues, the rules of engagement in the operation of the compressed air system are normally as follows:
The telephone becomes the instrument of choice to validate performance. If they dont call, life is good. No reasonable manager can look at this typical operating situation and believe that this makes any sense at all. Part of the problem is that plant management has never seen this perspective in its entirety. In every instance, where production has been exposed to this information quantitatively, appropriate assignment of responsibility is corrected. Even production management cannot condone this approach, when faced with the financial and qualitative results.
Constituents of Demand - Is your use of compressed air getting the best value for our investment?
Five years ago, a concerned consortium of users and utilities asked that a number of quantitative systems audits be combined to see what the typical situation would look like. In an analysis of forty two systems which were audited, on the average, less than 50% of the total compressed air produced in these facilities represented reasonable usage which contributed to productivity. The following represents the constituents of demand which were found in these forty two plant compressed air systems on the average:
Financials - Do you understand the impact that compressed air has on our bottom line?
The systems that were audited in the above analysis tended to be larger systems averaging 1760 kw of on board power including compressors, dryers, pumps, and fans. The average compressed air usage was 8130 cfm at 103 psig. The average cost of electricity was $.048 per kwh. The average hourly usage per year was 7760 hours. The average annual cost for electricity was $655,564.80. The cost of make up water, water treatment, operator labor, maintenance, outside labor, parts inventory cost, depreciation, insurance, property tax, administration, and supervisory cost added an average of $375,825.40 per year to the electrical cost. The total annual operating costs averaged $1,031,390.20. Consistently, the individual plants did not know what their compressed air costs were. Those that speculated on their costs underestimated it typically by more than 50%. You cant make reasonable business decisions, when you cant accurately determine the financial consequences. If we divide the total cost by the operating hours per year, we have $132.91 per hour. When we divide the cost per hour by the units of 100 cfm, we have $132.91 ÷ 81.30 units or $1.63 per 100 cfm per hour of operation. In doing this exercise, we have quantified a unit value for compressed air that production can work with in estimating the operating cost of an application and evaluating the best alternatives. If we were to take the average quantity of leaks of 18% and multiplied it times the total average volume of 8130 cfm, we would have 1463 cfm. If we multiply the unit cost of $1.63 times 14.63 units, we would have an hourly cost of $23.85 for leaks. Multiply this times the hours of service and we have an annual cost of $185,052 for yearly leaks. If someone in production is going to apply a 1/4 open nozzle at 90 psig to dry or wipe a wet article somewhere in the production process, it would consume 94 cfm. .94 units of air times the hourly cost of $1.63 per hour times 7760 hours per year generates an estimated annual operating cost of $11,890. You could perform the same function with 1/2 hp positive displacement blower. The open blow nozzle costs nothing to apply compared to perhaps as much as $750 for the blower and installation. The annual cost of operation for the blower would be $150.20 per year to operate. The question is whether a little extra effort and up to $750 in expense is worth more than $10,000 in operating cost. The answer should be obvious, but it is not unless there is a clear understanding of unit cost, accountability for operating cost, and a mandate from management to treat the use of compressed as a business decision. The use and installation of all other utilities is carefully applied and reviewed. This is primarily because of code and operating personnel who understand both the financial and operational consequences of poor applications. I think that its interesting that a $10,000 business decision in most plants requires several signatures, yet anyone in production a make such a decision with no discussion at all. Please see Attachment A for an example of a detailed breakdown of operating cost for an actual compressed air system. Attachment B is a blank worksheet which you can use to work out your annual compressed air operating cost. You have to use sound accounting principles, if you want management to show an interest in opportunities and issues.
Quality - Do you measure compressed air as assigned or unassigned cause as it impacts production quality?
There are a number of things that can cause off quality relative to compressed air. The most prevalent is the least obvious. This is when one intermittent application at a high rate of flow causes a critical pressure user to drop in pressure. One seldom determines the cause of the problem. Instead we tend to treat the effect. The problem doesnt report itself, only the result. The normal diagnosis is insufficient supply. Chances are you operate the system at a high enough supply pressure that when the event occurs, the pressure does not drop into the critical user. The size of the additional compressor that is operating to service this approach will determine the degree of pressure fluctuations which will occur. The result is a lack of repeatability at the point of use at an unnecessarily high operating cost. The most significant problem of forcing the system to work with power is inconsistency. You are far better off being consistently wrong than inconsistently right. At least you can figure out what the problem is. The best operating approach is to control the demand air density at variable mass independently of the supply system. When you do this, you can store clean air on the upstream side of the demand control to support the transient events instantaneously. This will allow you to regulate demand at the lowest pressure all of the time. By controlling demand independently, supply can be operated at the best independent pressure where we can optimize compressor performance. You get accuracy at the lowest required pressure and optimum supply performance at or near the isothermal design of the compressors. Point of use quality at the best operating efficiency.
* Control storage allows for 14 psi of pressure drop with no change in demand psig. The capacitance of control
storage should be equivalent to the largest event for the allowable pressure drop for the Ž the time required to get the next available compressor to support the transient change in demand. Supply changes to adapt to demand without
any change in demand pressure. We have generated transparency, the ideal utility configuration. In most systems, supply energy is relatively constant with demand changing in pressure and air quality constantly.
Leaks, dirty point of use filters, and increased air flow across installation components all cause the article pressure to drop on production using equipment. Since differential pressure increases as a square function of
Another problem that causes off quality is contamination in the system. There are all kinds of perceptions for this. To make things simple, you need to provide the following in order to maintain your clean up system:
The most critical component of air quality is the temperature of the air at various control points. A 10×F rise in temperature can alter the clean up equipment performance by 26%.
Reliability - Do you have a risk management plan to prevent production downtime with compressed air?
Most concerned buyers of compressed air equipment attempt to differentiate equipment based on how it may influence the reliability of the system, ie, better stuff reduces the risk of interruption or minimizes down time. There isnt any perfect, if that is what we are looking for. This is rotating equipment. It will fail. The premature failure will likely be as a result of how you operate the system and the equipment in it, rather than what you chose. What would be more important to find out about is the short comings of the equipment
With compressed air being the most poorly designed and managed of all of the industrial utilities, there is certainly a great deal of opportunity to improve productivity while reducing operating cost. We have reviewed the highlights of a reasonable perspective on managing the compressed air system. When we say system, we mean the supply, the demand, and the in between. If you treat production usage as a black hole which must be satisfied at any cost, you have stepped outside the bounds of reason. Your cost will sky rocket, while performance declines. With the current demands from management for more effective use of our assets, this area of the compressed air certainly can be categorized as low hanging fruit.
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