It often seems that compressed-air system performance is evaluated only from the perspective of the supply equipment. If pressure anywhere in the system is below whatever is believed to be the minimum acceptable level, the common diagnosis is "insufficient supply." Little more is done to evaluate what is going on in the system. In existing systems, demand usually is calculated by adding up the rated capacity of all the compressors that are on, regardless of how much power they are pulling. Users do not realize that an on compressor is only an indication of cost — not an indication of need!

Without demand, there is no requirement for supply. Most compressed-air systems have little or no storage and an uncontrolled approach towards expanding the air to the various pressures at which it will be used. Compressor manufacturers have developed formulas and perceptions based on the assumption that all of the demand is managed. In reality, less than half of the air (by volume), which is consumed is regulated, and half of the regulators in use are adjusted to their wide-open position. Typically, 80% of total demand is unregulated. As real demand increases, supply pressure drops and 80% of the total use volume diminishes proportionally to the reduced density of the supply air. The inverse is also true.

Demand in compressed air systems can be viewed as many holes through which air flows and expands to do work. The number of holes, whether they are open or closed, how fast they open and close, the coincidence of these occurrences, and the various operating pressures determine the demand in the system. Following are the categories of usage:

Appropriate production usage — This term can be applied to usage for which the compressed-air system was installed in the first place. Some examples of appropriate usage would be valves, cylinders, instruments, air motors, pneumatic hand tools, and, in some cases, blowing applications. A portion of the total appropriate uses necessary to production will be regulated, while the balance will be unregulated. These applications are appropriate for compressed-air usage even if not properly controlled.

Inappropriate production usage — These are applications that could be accomplished better with electricity, hydraulics, or mechanical power instead of compressed air. Examples include use of plant air for aspiration of a flue gas, agitation or oxygenation of liquids, or aeration. These applications should be serviced with a single-stage, low pressure blower. When plant air is used instead, there is seldom an understanding of cost or consequences. Sometimes it is simply an effort to avoid the purchase of alternative non-air-using equipment to produce the same functional result. You certainly would not install a ⅜-in. air hose to blow air with an annual operating cost of $18,000 when a 1-hp blower could do the same thing with an installed cost of $400 and an operating cost of $850/yr.

Leaks — Leaks represent waste, which is internal to the production equipment as well as in the general piping system from the compressors to the points of use. Leakage noise can range from inaudible to extremely irritating. Most leaks start small and then grow. It is not unusual for the sum of all leaks to equal up to one-third of the total air usage if they are not brought under control. The best way to evaluate leakage problems is to monitor the demand flow, corrected for pressure and temperature.

Artificial demand — This is the excess volume of air that is created on unregulated users as a result of supplying higher line pressure than necessary for the applications. It includes leaks, drain valves, and blowoff. When the supply pressure fluctuates, artificial demand increases and decreases from a minimum to a maximum waste level. As real production demand decreases and the pressure rises, artificial demand increases. Repairing leaks in the system causes pressure to rise and all unregulated demand (including the balance of the leaks) increases proportionately to the pressure rise.

Because little care is used in selecting regulators and filters, they frequently have high pressure drops. Operators will increase the pilot pressure to improve the workability of their equipment to solve application problems. When operators no longer can elevate the pressure, they run into the supply pressure of the system. At this point the application will track the supply pressure. The increased volume created is artificial demand, which can represent 10 to 25% of the total air used.

Expander offers solution — A demand expander can correct this problem when adjusted to the system's minimum required pressure. An expander is a main line control valve (or valves) that controls the maximum pressure at which demand air can be removed from the system. Unlike a regulator, which restricts flow to control pressure, an expander increases the volume from the higher upstream pressure to the control pressure. Because expanders are sized for the expanded flow at the lowest operating pressure, they impose an almost immeasurable resistance to flow on the system. They require very little supply energy to function properly. Compare this to a regulator which can require 5 to 10% of the system's input energy to overcome resistance to flow.

Expanders also are very precise control devices, normally using a programmable controller platform centered, proportional-integral-derivative (PID) control format. The expander has a control and response sensitivity within tenths of a psi. The use of an expander allows storage to be maintained in the upstream supply system to handle variations in demand rather than using "on board" power.

Another problem in the system provokes operating at elevated pressures. If the system is operating correctly, and demand is stable, a neutral (or 0 cfm) rate of change occurs. This implies that supply energy and demand energy are equal. When more air-using equipment comes online, this is referred to as a demand event. The excess demand over the supply energy is expressed as a negative rate of change. Until the supply system responds to the event, the air required is taken from the demand piping system, causing the pressure to drop. This pressure decay will be greater at the point of use and diminish closer to the supply. The decay will continue systemwide until supply adjusts; then the system will assume a positive rate of change until the air removed from the system is replaced, pressure is brought back to the original control point, and rate of change returns to neutral.

Open blowing — Open blowing is plant air used for moving product, drying, wiping, cooling, and parts and scrap ejection. These applications typically are little more than copper tubes or pipe nipples attached to rubber hose or polyvinyl tubing. Although regulation should occur, these applications seldom have regulators installed. Depending on the shape and configuration, a ¼-in. copper tube can pass 48 to 108 cfm at 70 to 100 psig. This represents 13 to 30 bhp of supply energy. At $0.06/kW-hr, plus maintenance and depreciation, compressed air costs about $2.00 per 100 cfm per hr of usage. That means the ¼-in. copper tube used for open blowing could cost between $4,037 and $18,922/yr on a 3- shift basis. The people who randomly apply these nozzles do not know the financial implications of their action, or what alternatives are available.

Open drainage — This occurs when plant air is released through open valves, notched ball valves, and motorized or solenoid-operated drain valves to dispose of compressed air effluent, such as water and/or lubricant. Although these seem like a positive means of effluent removal, the consequences can be expensive. The usage is not usually significant by volume, but the high rates of flow for short periods of time can depress the supply pressure enough to keep any compressor from unloading or turning off. Let's investigate the use of five timer-operated, motorized, ½-in. ball valves to drain effluent from a small system. If left open, each valve will exhaust 477 ft3 of air at 100 psig in one minute. If the timers are set for 5-sec drain cycles, each valve will consume: 477 x (5/60) = 39.75 ft3/cycle.

Assume that the supply system has 60 ft3 of storage per psig, or 2169 gal of capacity for all tanks and piping. This implies that for every 20 ft3 of air removed from the system (above the amount that is being put into the system), the pressure will drop 1 psig. Every time one drain valve opens and dumps 39.75 ft3 of air for five seconds, the system pressure drops 2 psig.

If all of the timers are set for 5 sec of draining every five minutes, the statistical probability of coincidental drain events would be quite high, at least for up to three valves. If three valves actuated simultaneously, the supply pressure would drop 6 psig. Because one or more drains are open at least 8.33% of the time, pressure could not be kept high enough to time out the motor on an off compressor before the pressure dropped to reload the compressor. If all five units function simultaneously, which will happen statistically, the 5-sec flow seen by the compressor room would be 198.75 ft3, which is a rate of flow of 2,385 scfm. This would be more than enough to load the next available compressor, regardless of its size.

If you feel compelled to use solenoid or motorized valves for drainage, adjust the timer to the shortest possible duration and increase the frequency. This not only will reduce the air flow per cycle but also the potential for coincidental drainage events. The objective is to remove liquid, not air.

Centrifugal compressor bleed bypass or blow-off — This is part of the normal control functions of a centrifugal compressor. A substantial portion of the cooling of the compressor is assigned to air being compressed. There is a minimum flow required to prevent overheating. When the demand for air in the system is below the minimum stable mass flow for the type of compressor, the control system will blow off the difference between the minimum stable flow and the actual demand requirement to atmosphere.

Blowing off compressed air to the atmosphere is an intentional waste of energy if the total minimum stable flow capacity of the on-line centrifugal compressors is more than the actual requirement. It is not uncommon for all or some of the centrifugal compressors, which are on, to be blowing off. This is not necessarily because the controls are not working properly. It is common to oversize compressors.

Blow-off or bleed bypass is real demand that requires energy, whether it is productive or not. The objective in operating a centrifugal should be to keep each base load unit fully loaded and operating on its natural curve on a year-round basis. You can configure an arrangement of centrifugal-only compressors that do not blow-off if the following occurs:
Demand is determined by correcting for mass flow at density to the anticipated operating pressure, including the full range from maximum to minimum and off production
Supply capabilities are determined from actual curves at various inlet conditions and operating control approaches to determine the best sizes and fits for the range of demand required
  Actual limited throttle capabilities including field adjustments are evaluated based on performance curves for the range of inlet conditions at the anticipated operating pressure, and
  A backup compressor to support a unit failure is properly designed and integrated into the configuration. This implies that the off compressor is evaluated for the permissive start requirements from a hot start. Control storage must be provided to limit the minimum acceptable pressure drop that occurs while the compressor motor is being turned on and the compressor goes through its permissives. It will then have to close the blow-off valves and open the inlet throttle valve allowing the capacity of the unit to stop the decay of pressure and replace the air lost in control storage.

Attrition — This is the additional air consumption that occurs on applications that result from unmanaged wear. Attrition typically is a normal function of sand or grit blast nozzles, textile machinery nozzles, etc. Solid particulate in the air stream will cause nozzle wear.

Unattended attrition can increase a particular volumetric consumption by 50% and frequently provokes the increase of pressure at both the point of use and the supply. A ½-in. nozzle with 1/16-in. wear, which has had supply pressure increased from 80 to 90 psig (to compensate for the wear), will increase the volume by 50%. Monitoring attrition is essential. Blast nozzle operators have calipers that can slide into the nozzle and indicate an acceptable or unacceptable level of wear. Blast operators know that excess wear spreads the pattern, reduces force per square inch, inhibits desired quality, and impedes labor efficiency. On stationary applications such as air jet looms or spinning machines, mass flow at pressure should be measured regularly to monitor wear. The need for a few more cubic feet of air on each of a few hundred production machines can indicate the need for another compressor.

The logic behind any attrition management program is benchmarking the mass flow at pressure or measuring the actual wear on the nozzle. The nozzle or insert should be changed when the cost of energy to maintain the wear exceeds the cost of changing the nozzle or insert.

Purge air from desiccant dryers — This air is consumed in the process of stripping air dryers of moisture. The process can range from 3% to 14.7% of the total air systems capacity from one method of purging to another. There are specialty categories of air, such as CDA 100, which is used for the microelectronics industry where purge can approach 25% of total capacity for the system. This is primarily used where the desired pressure dew point can be as low as -100° F.

The purge rate of flow is a function of the capacity of the dryer and its purge pressure, which normally is adjustable. An air reactivated or heatless dryer rated at 3,000 cfm at 100 psig has a purge flow of 441 cfm at 100 psig. If the air flow through the dryer is 1,000 cfm, the purge rate of flow will not change. With dew point control, the total cycle time increases, but the rate of flow will remain at 441 cfm for the preset purge time duration. The time between purges will lengthen as the flow through the dryer drops.

The rate of flow, not the cycle time, affects the system and loads compressors. It may seem that if the length of the cycle is doubled, the amount of purge will be cut in half. The effect of loading or the peak compressor requirement will not change; the same purge cycle will just occur less frequently. This will reduce the power rate consumed, but not the power of demand.

Bleed air or control bypass — This a point-of-use consumption where air is bled off the system or bypasses an application as a means of improving the accuracy of pressure and/or flow control. Where accuracy of pressure is important, and there is considerably more power or higher pressure than needed on line, the pressure can fluctuate erratically or perturbate. There is normally a control or storage-associated problem that is compensated for, with bleed or bypass control.

Constituents of demand — In general, the previously discussed issues represent the constituents of demand encountered in audited systems. The last four categories — bleed air, purge air, attrition, and bleed bypass — only represent 23% of all systems, while the others are typical constituents.