The amount of energy required to operate the system not only is based on how much air is consumed in demand, but also how it is used. The relationship between the supply arrangement and the way demand is used, will also determine the energy consumed. In examining demand the question must be asked, "Why do we operate the system the way we do?" Breaking down the issue provides the information necessary to manage the system most efficiently.

Minimum load is the condition with the least amount of energy requirement, but it usually represents the most hours of operation per year in most systems. In manufacturing, a daylight or on-production mentality is often developed. From Friday night at 11:00 until Monday morning at 7:00 equals 56 hr of weekend compressed air service. With three shifts, 8-hr/shift, 5 days/week, the weekend represents the longest shift or 31.8% of the total time (2,812 hr/year). If this low load condition also includes the third shift, the condition of usage can represent as much as 4962 hr a year out of a possible 8,760 hr. Low or minimum load is usually not evaluated and winds up being the stepchild when sizing the system and its equipment. During the minimum load condition, there is usually a significant amount of partial load on a larger-than-necessary compressor or compressors that are on and were sized for the peak demand.

Low-load requirements should be evaluated on their own for the best operating mode. In many cases, this operating condition supports only auxiliary requirements, such as heating, ventilating, and air conditioning controls in the system; a dry sprinkler system; mixing motors that may be operating around the clock; diaphragm pumps; instrument air; etc. Although these may be legitimate usage, small isolated support might make more sense rather than supporting the entire system. In many cases, some users also could be better applied with electrical drive equipment.

Another poor use of air during the minimum load condition is abandoned production air usage. Operators turn off their electrical controls but do not close the air valve on the machines when they leave their work stations. Up to one-third of the low load condition has been found to be representative of this usage. There needs to be specific management direction regarding air usage shut off when a work station is abandoned. This can account for as much as 5% of the annual operating cost of compressed air in the plant.

If demand is managed with a demand expander, pressure could be reduced considerably during low load to control operating costs. The percentage of unregulated air consumers' volume, including leaks, usually increases as the demand diminishes and system pressure rises. This is particularly true when the supply is poorly controlled and sized much larger than the low load needs. If normal production is operated at 90 psig, the demand control pressure could be reduced to between 55 and 70 psig on the low load condition, depending on the equipment needs.

Even the most diligent maintenance professional can easily overlook the opportunities of minimum load. This is the place to begin auditing the air system. It represents the start of determining the constituents of demand as well as a significant opportunity for operating cost management.

Some facts about air leaks

They are insidious and will grow in time. Typical air line contaminants are water vapor and oxides, which make an excellent lapping compound. Passing these contaminants through normal leak annulars ensures wear. If the system is controlled by pressure only, leaks will grow at a faster rate than in a demand-limited or controlled system. If some leaks are fixed — and the demand pressure rises as a result — the remaining leakage volume will increase in direct proportion to the relative increase in pressure. With this elevated velocity, the leaks will increase in physical size until the increased volume causes the pressure to drop to the original level of waste.
If system waste is allowed to grow unattended, the demand will eventually accommodate the supply that is on line until all compressors that are running become fully loaded. As leaks rob work energy from the system, the mass flow lost must be replaced if the pressure is going to be managed. The replacement air brings in water vapor, acid gas, hydrocarbon vapor, and other industrial contaminants that must be processed and removed. Most systems with contamination problems can be fixed by controlling leaks and other waste in the system.
Vapor seeks the lowest vapor pressure. This engineering anomaly can be helped along when we have a combination of a high percentage of leaks combined with desiccant or low dew point drying. If the ambient relative humidity is also high, water vapor will diffuse into the system from the atmosphere using the leaks as a vehicle. The higher the vapor pressure differential, the more effective the molecular diffusion or jet pump effect. Because leaks are neither planned nor managed, they increase flow through components in the system. The increase in flow causes an exponential increase in differential pressure across the components, resulting in a drop in downstream pressure. At the point of use, components are selected with little regard to differential. It is commonplace to elevate the regulated pressure to correct application workability. With this sloppy approach, leaks at the point of use have a most profound effect on the system. Imagine the capital and operating cost for installing another compressor at the supply end because of nagging complaints of continuously dropping pressure at one or more use-points. Sadly, leaks and plugged filters are usually the cause.
Leaks are the primary cause of problems with compressor control systems. Unfortunately, service providers neither use ultrasonics or regularly soap control lines to check for leaks. A few inaudible leaks can false-load a compressor as though there is large downstream demand. The result of this type of problem is severe cycling or hunting in the modulating control mode, and
It is nearly impossible and impractical to eliminate all leaks from a system. Twenty percent of all leaks, by volume, are inaudible and very small. By unit count, 70-80% of leaks fall into this inaudible category. It is relatively easy to find and eliminate 75% of a system's total leak volume. Beyond this level, it is difficult to justify the return on labor invested even on a benchmarking basis. Most maintenance personnel only fix audible leaks. Keep in mind that a leak would have to be very large in order to be heard over typical industrial background noise.

How compressors are oversized

When the initial sizing of a system is calculated, volumes at various pressures are added with no correction for mass density. There also are generous fudge factors for pressure and volume used at the various assessment stages of sizing. As an example, a manufacturer of equipment measures his demand at 100 cfm at 70 psig and then increases the pressure to 90 psig as a fudge factor. He expresses the demand as 100 cfm at 90 psig. This overstates the required mass flow at density. Percentages are arbitrarily added to volume. Pressures are elevated to accommodate the compressor specifications. Most of this is done to offset the unknowns or the fact that last time this exercise was done, mistakes were made. It is assumed generous oversizing will take care of the previous errors, whatever they were. If we had to pick a percentage relative to common oversizing, it would exceed one-third of the actual demand.

The turn-down or throttling capabilities of a centrifugal compressor can range from 20 to 40% of the total capabilities at the lowest operating inlet temperature. As the inlet temperature rises, the throttling capacity reduces, because the curve drops without the minimum stable flow changing. This may be evaluated on a unit- by-unit basis when the engineering evaluation occurs. Unfortunately, the effect on system operation is not evaluated considering the total number of compressors which will be operated versus the range of demand required. Most systems are evaluated based on peak demand. They are seldom evaluated for minimum demand or turn-down requirements.

All centrifugal compressors have protective controls to ensure the compressor does not operate at or below its minimum stable flow point. These are either electrical minimum current (current limit low) or pneumatic blocks, which cause the compressor to blow off. The correct means of adjusting these limits or blocks is to perform a throttle surge test of the compressor at a specific inlet temperature and relative humidity, and operating pressure. By determining the input power at which the pressure begins to rise on the throttle surge curve or line, you can compare this to the rated performance of the compressor and interpolate what the minimum stable mass flow is at operating pressure.

Once you perform the test, you can adjust the blow-off controls to activate slightly before this point on the throttle line. Curves are seldom supplied (or requested) for compressors. The method that is used by the factory service technicians is to set the limits generously enough so that none of this needs to be done. The result is significant limits imposed on the throttling capacity of the compressor. We commonly find the compressor fields adjusted to blow off at 15 to 20% throttled off the full load capacity of the machine at the coldest condition and 5 to 10% of the full load capacity on the hottest summer day.

Because these units are very permissive, they are relatively slow to load up from a motor off and ready to start condition. The result is that most systems with centrifugal compressors have one extra compressor on all the time. When you evaluate the demand turndown and add another compressor to the online supply to protect the system against a unit failure, this forces all compressors to throttle. It is not uncommon to size a system to operate with two or three centrifugal compressors to support the demand in the system plus the extra on compressor to cover for a unit failure. If the demand requirement is less than 2/3 of the total capacity of the three compressors plus the extra unit online, you will have to blow off one or more compressors. Unfortunately most evaluations attempt to determine the type of compressor to acquire, when more than one type may be the prudent choice. Mixed types of compressors seldom are applied to any specific system. Other types of compressors, like positive-displacement units, have less limited turndown capabilities and can start from a cold off position within seconds. There is no need to operate another base load compressor in the event of a unit failure. Most evaluations begin by asking what type, size, and number of compressors are needed for the system. The question might be better asked: What types, sizes, and number of compressors will best suit the range of demand and ambient conditions that will be seen?

R. Scot Foss is president of Plant Air Technology, Charlotte, N.C., which specializes in air system auditing and design. This series of articles is based on his book, Compressed Air System Solution Series. To order a copy, click here.