Compressed Air: A Facilities Perspective

A sensible viewpoint on managing plant compressed air systems.

Compressed air is a critical power resource in most manufacturing and process environments. It constitutes from 7-40 percent of the total electrical use in most plants. If the pressure drops below 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 affected.

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 11 percent of the input energy in the form of work at the point of use. At 6 cents per kWh, 3 shifts a day, 7 days a week, every 100 cfm costs approximately $15,000 per year; 1000 cfm can cost more than $150,000 per year. If a plant produces 10 percent pretax profit, it must generate $1.5 million in production revenue to support 1000 cfm of average use per year. Despite this information, well-intentioned production personnel give little thought to the use of compressed air.

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 through experience. This information probably has been handed down from previous operating personnel.

There is probably a significant fudge factor between perception and reality. It is highly unlikely that anyone knows specifically what the compressed air costs, and there are no rules for its use 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, system reliability, or system operating cost. In the average facility, this expensive and critical utility is used as though it is a limitless resource.

Fig. 1. Operating Approach: Good business practices should be followed when operating a compressed air system. By investigating the source of a problem, companies can avoid buying a $100,000 compressor to solve a problem that can be corrected by replacing a $10 filter.

• Production can use air any way it chooses with no communications or accountability. All problems on the production side of the system will be corrected on the supply side of the system with no problem definition. All problems should be interpreted as insufficient supply or treatment. If leaks or inappropriate use become excessive, without definition or investigation, facilities personnel are expected to increase the supply of compressed air to more than correct the results of the situation (see Fig. 1). One would liken this approach to jacking up the taps on the substation to correct ground faults in the electric distribution system.
• If production is unhappy and wants more compressors or cleanup equipment to correct any poorly defined symptom that shows up in production, money will be appropriated immediately with no consideration for the impact on operating costs and no justification required. On the other hand, if the same problem can be corrected by applying production needs differently, or the system can be retrofitted to lower operating costs to correct the problem, stringent return on investment requirements must be met for dollars spent. Even if these financial hurdles can be met, there will be a competition with production for capital. In most facilities, competing with production for anything is typically a losing exercise. All of this occurs while management is demanding a reduction in the facility's operating budget.
• After surviving these unwritten rules for any period of time, facilities personnel and maintenance management simplify the operating protocol as follows: Do whatever it takes in operating the compressed air system so that they won't call. The telephone becomes the instrument of choice to validate performance. If they don't call, life is good. No reasonable manager can look at this typical operating situation and believe that it makes any sense. 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.

Fig. 2. Generating Transparancy: 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 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. In most systems, supply energy is relatively constant with demand changing in pressure and air quality constantly.

Five years ago, a concerned consortium of users and utilities asked that a number of quantitative system audits be combined to determine a typical situation. In an analysis of 42 systems which were audited, on average less than 50 percent of the total compressed air produced in the facilities contributed to productivity. The Average constituents of demand for compressed air systems in 42 surveyed plants table represents the constituents of demand that were found in these compressed air systems on the average.

There were many uses that fell into the inappropriate user category other than those listed, including vacuum generators, sparging, aspirating, vibrating, and atomizing liquid. All of these uses could be accomplished more effectively using an alternative form of power such as vacuum, mechanical pumps, or blowers.

Artificial demand is the volume of air that is generated by operating users at higher pressures than necessary to achieve the desired results. It also would be described as the volumetric difference between the volume at the actual pressure and the volume that would be consumed at the lowest acceptable operating pressure.

The open blowing applications were primarily applications that could have been better applied with blowers. At the least they should have been applied with high-efficiency nozzles or amplifiers. The bulk of these applications were for wiping, item cooling, personnel cooling, and parts or scrap ejection.

Drainage was represented primarily by solenoid or motorized valves that discharged more air than effluent. There were also many cracked bypass valves and direct open blowing. Despite the fact that the percentage of volume was relatively small, the impact these drains had on the systems was quite significant. In a majority of cases, although the volume was small, the rate of flow coupled with the systems' capacitance caused sufficient momentary pressure drops that prevented at least one compressor from unloading or timing out in the systems.

Dryer purge represented such a low average percentage because few of the systems had desiccant dryers. Of those that had these dryers, only a small percentage were air-reactivated or heatless. Where heatless dryers were in use, the impact on the system was significant not only relative to volume, but also to event pressure drops that occur on the tower switchover.

There were only five systems where nozzle wear or attrition applied. The applications included wear on air jet looms in textile plants and nozzle inserts on sandblasting equipment. Slight increases in nozzle size can increase the air consumption appreciably.

Financial considerations
What impact does compressed air have on the bottom line?

The systems that were audited in the analysis were larger systems averaging 1760 kW of onboard power including compressors, dryers, pumps, and fans. The average compressed air use was 8130 cfm at 103 psig. The average cost of electricity was 4.8 cents per kWh. The average hourly use per year was 7760 hours. The average annual cost for electricity was $655,564.80.

The cost of makeup 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 underestimated typically by more than 50 percent. Reasonable business decisions cannot be made when financial consequences cannot be accurately determined.

If the total cost is divided by the operating hours per year, the result is $132.91 per hour. When the cost per hour is divided by the units of 100 cfm, the result is $132.91 divided by 81.30 units or $1.63 per 100 cfm per hour of operation. This unit value for compressed air allows production to estimate the operating cost of an application and evaluate the best alternatives.

The average quantity of leaks (18 percent) multiplied by the total average volume of 8130 cfm is 1463 cfm. Multiplying the unit cost of $1.63 by 14.63 units results in an hourly cost of $23.85 for leaks. Multiplying this figure by the hours of service results in an annual cost of $185,052 for leaks.

If production applies a 1/4 in. open nozzle at 90 psig to dry or wipe a wet article somewhere in the production process, it would consume 94 cfm. These units of air times the hourly cost of $1.63 times 7760 hours per year generates an estimated annual operating cost of $11,890.

The same function can be performed with a 1/2 hp positive displacement blower. The open blow nozzle costs nothing to apply compared to perhaps $750 for the blower and installation. The annual cost of operation for the blower would be $150.20. 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 air 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. It is interesting that a $10,000 business decision in most plants requires several signatures, yet anyone in production can make such a decision with no discussion at all. Sound accounting principles need to be used to get management to show an interest in opportunities and issues.

Quality
Is compressed air measured as an assigned or unassigned cause as it impacts production quality?

There are a number of causes for poor quality compressed air. The most prevalent is the least obvious; when one intermittent application with a high rate of flow causes a critical use to experience a drop in pressure. The cause of the problem is seldom determined. Instead, the effect is treated. The normal diagnosis is insufficient supply.

Chances are the system is operated at a sufficiently high supply pressure that when the event occurs, the pressure does not drop for the critical use. The size of the additional compressor operating to compensate will determine the degree of pressure fluctuations that 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. The best operating approach is to control the demand air density at variable mass independently of the supply system. This allows clean air to be stored on the upstream side of the demand control to support the transient events instantaneously and demand at the lowest pressure can be regulated all of the time. By controlling demand independently, supply can be operated at the best independent pressure so compressor performance can be optimized. The result is 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 (see Fig. 2).

Leaks, dirty point-of-use filters, and increased air flow across installation components all cause the article pressure to drop on production equipment. Since differential pressure increases as a square function of flow increase, even a small leak can cause the pressure to drop and affect quality. Point-of-use filters are seldom monitored for dirt loading or cartridge change. In fact, most plants have no point-of-use filter cartridges in inventory.

In the absence of measurement, erratic operation of equipment would imply that the filter might need service or that a leak test should be performed. Unfortunately, the problem is normally diagnosed as insufficient supply energy. When the point-of-use regulator can no longer be increased, a telephone call is made to the compressor room operator.

Another problem occurs when applications increase the cycles per minute or the rate of flow is increased. Both situations require resizing some or all of the installation components so there will not be a decrease in point-of-use pressure. If production anticipates increasing cycles, rate of production, or air consumption, the installation needs to be reevaluated.

Contamination in the system is another problem that causes poor quality. To maintain the cleanup system:

• Size the filtration and drying equipment for the heat load and mass flow at density.
• Maintain a consistent temperature into the equipment within the design parameters.
• Design and maintain a superior drainage system. Do not cut corners.
• Store enough clean, dry air on the downstream side of the cleanup equipment to support transient events in demand without generating a velocity across the cleanup equipment.
• Control the water flow and temperature across all heat exchangers.
• Provide adequate monitoring equipment to observe process results. Benchmark and trend approach temperatures relative to ambient and cooling temperatures. The equipment will eventually foul and fail; the system does not have to if a predictive maintenance approach is taken.

The most critical component of air quality is the temperature of the air at various control points. A 10 deg rise in temperature can alter the cleanup equipment performance by 26 percent.

Reliability
Is a risk management plan in place 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. There is no perfect piece of equipment. This is rotating equipment; it will fail. The premature failure will likely be a result of how the system is operated and the equipment in it. It is more important to find out the shortcomings of the equipment and how it fails. Armed with this information, equipment can be selected and the system designed to control risk and minimize downtime. Other basic actions that will improve the reliability of the system include:

• Select compressors that are not so large in relation to the total demand that the failure of one of them can cause the system to fail.
• Write a failure scenario in a supply, demand, pressure, storage, and time algorithm. Request and test the permissive response time to start all compressors from a cold start to full load with the motor and the compressor off. Measure the capacitive storage of the system expressed in cubic feet per psig. This information is essential to automatically back up the failure of a compressor or an unusually large demand event without achieving an unacceptably low pressure.
• Choose smaller, faster, more automation-friendly compressors that will support risk more effectively than larger, slower units that back up other large units.
• Provide as much parallel individual compression and cleanup equipment as possible, so that if a compressor or piece of cleanup equipment fails, the entire train of equipment is not lost. Too many systems are designed with a number of parallel trains with a compressor, aftercooler, filter, and dryer. You can have one compressor from one train and a dryer from another train down for service and lose both trains from service.
• Trend and benchmark all system variables and deltas against design performance. Maintenance can be anticipated in advance of failure. If the system is large or critical, a central management information system may be necessary.
• Develop a failure plan for the demand side of the system. In the event of a supply side equipment failure, manually or automatically limit the least important use sector so that the required pressure holds in the balance of the system. If the demand usage by sector is prioritized, the least important to the most important can be automatically limited or adjusted until the system is stable. The greatest risk of interruption will be when three or fewer compressors are on line and there is no demand side risk management program.

Compressed air is the most poorly designed and managed of all industrial utilities. It provides a great opportunity to improve productivity while reducing operating cost. Compressed air systems include the supply, the demand, and the in between. If production use is treated as a black hole that must be satisfied at any cost, the bounds of reason have been violated. Costs will skyrocket while performance declines. With the current demands from management for more effective use of assets, compressed air certainly can be categorized as low-hanging fruit. MT

This article is based on a paper presented during National Manufacturing Week, March 18, 1999, in Chicago, IL.

R. Scot Foss is president, Plant Air Technology, P.O. Box 470467, Charlotte, NC 28277; telephone (704) 844-6666; e-mail This e-mail address is being protected from spambots. You need JavaScript enabled to view it ; Internet www.plantair.com