Compressed Air System

The 14 Rs of compressed air efficiency

Install, operate, and maintain a system that makes the best use of resources.

By David M. McCulloch, Frank Moskowitz, and Ron Marshall, Compressed Air Challenge

Educational institutions have realized the need for emphasizing the basics, sometimes referred to as the three Rs — reading, ’riting and ’rithmetic. There are also several basic Rs to be kept in mind if you want to install, operate, and maintain an efficient compressed air system:

  1. reduce leakage losses
  2. reduce pressure at points of use
  3. reduce pressure at source (compressors)
  4. reduce system pressure fluctuations using adequately sized and located air receivers and controls
  5. reduce number of partially loaded compressors to only one
  6. remove inappropriate applications
  7. reduce system pressure drop losses with properly sized piping and valves
  8. remove moisture content of compressed air with the proper type and size of dryers
  9. remove condensate without loss of compressed air
  10. reduce downtime through preventive maintenance
  11. record system data and maintenance
  12. review air usage patterns regularly
  13. recover heat
  14. reduce energy costs (return on investment and cost of operation).

R #1 — Reduce leakage losses

Compressed Air Challenge

The Compressed Air Challenge is a voluntary collaboration of manufacturers, distributors and their associations; industrial users; facility operating personnel and their associations; consultants; state research and development agencies; energy efficiency organizations; and utilities. The mission of the CAC is to be the leading source of product-neutral compressed air system information and education, enabling end users to take a systems approach, leading to improved efficiency and production and increased net profits.

In a typical plant compressed air leaks amount to 20-30% of the total of all the compressed air produced. In worst case scenarios, where no detection and repair programs exist, leakage levels can be more than 50%.

A ¼-in. leak in a 100 psi system having a pressure of 100 psig, will allow more than 3 million ft3 of free air to escape in one month. At an average specific power of 18 kW/100 cfm, this amounts to 107,000 kWh of lost energy or $10,700 in energy cost per year at $0.10/kW. This problem is worsened in systems operating at even higher pressures.

Leakage rates drop with lower operating pressures. If the system pressure could be reduced to 80 psig, for example, the leakage flow, and energy use in a well controlled system, would drop by 17% not including additional savings due to compressing to a lower pressure, which could amount to additional savings of 10%.

Leaks can be both intentional and unintentional.

  • Intentional leaks include open condensate drain cocks and valves.
  • Unintentional leaks include leaking pipe joints and valves, damaged hoses, and inexpensive poor-fitting quick-disconnect couplings.
  • Equipment not in use may also be using some compressed air. Such equipment should be isolated from the distribution system by a valve.

One way to determine the leakage rate in a system is to do special testing when all of the production equipment in the plant is shut down. If the compressors can be run in load/unload mode, the time loaded as a percentage of total running time will represent the percentage of total capacity going to leaks. Alternatively, for compressors with other than load/unload controls, you can do a volume bleed down test. This method requires the use of a pressure gauge downstream of the receiver and an estimate of total system volume, including any downstream secondary air receivers, air mains, and piping (V, in cubic feet). The system is then started and brought to the normal operating pressure (P1), and the compressor is turned off. Measurements should then be taken of the time (T) it takes for the system to drop to a lower pressure (P2), which should be a point equal to about one-half of the operating pressure. Leakage can be calculated as follows:

Leakage (cfm free air) = [V x (P1-P2)/(T x 14.7)] x 1.25
Where:
V is volume in cubic feet
P1 and P2 are starting and ending pressures in psig
T is time in minutes

The 1.25 multiplier corrects leakage to normal system pressure, allowing for reduced leakage with falling system pressure to 50% of the initial reading. Again, leakage of greater than 10% indicates that the system can likely be improved. These tests should be carried out once a month as part of a regular leak detection and repair program. Click here to view a description of these tests.

Unfortunately, leaks are not a problem with a one-time cure. Maintaining a lower leak level requires ongoing vigilance and a mindset that will not allow leaks to be tolerated. Recognized leaks must be tagged and repaired as soon as possible.

Case study

At a large automotive manufacturing plant an energy team consisting of volunteers, mostly union labor from the shop floor, was led by an energy coordinator. The first step in initial mission was to reduce energy waste by targeting air leaks. Baseline data was gathered during normal production and during a Christmas shutdown. From this information, a leak reduction program was developed and approved by management, based upon estimated potential savings.

In the next step, leaks were identified and tagged for repair. The results of the efforts were published each weekend, and news of the success spread through the plant. Each team member was given a red “Energy Team” jacket. They developed a procedure for all employees to report leaks and be rewarded for their efforts. Bulletin "leak" boards were installed, and progress in fixing leaks was posted. Messages were displayed on TV monitors throughout the plant. Soon, everyone was aware and involved in the program, which produced a cultural change.

The initial baseline showed a compressed air usage rate averaging almost 12,260 cfm. Within four years, this had dropped to 6,250 cfm, saving approximately $2,000 per day. The reduced rate remained relatively constant with the increased awareness.

R #2 — Reduce pressure at points of use

Many plants use a single common compressed air distribution system to supply a variety of end-use applications. When this is the case the total system must maintain a pressure that is high enough to satisfy the equipment having the highest pressure requirement, even though the majority of the equipment might require a much lower pressure. This higher pressure causes all the unregulated compressed air equipment in the plant to use more air and also increases the power required by the air compressor by 1% for every 2 psig in higher pressure.

When specifying new equipment requiring compressed air, it’s often possible to specify a lower required operating pressure, such as 70 psi, to minimize the system pressure requirements. It also may be possible to retrofit existing equipment for lower pressure operation by replacing less-than-optimal components. On existing equipment, once retrofits are done, it is often possible to progressively reduce the main air supply pressure to determine the minimum pressure at which the equipment will operate efficiently. For equipment that can’t be optimized, it may then be possible to segregate equipment onto a separate system, so the majority of the compressed air system can be operated at a lower pressure. The portion requiring a higher pressure could then be supplied by a dedicated-compressor air system, or by a booster compressor drawing air from the lower pressure system.

The pressure drop across air treatment equipment at the end use must also be taken into account and should be monitored to prevent a forced increase in compressor discharge pressure or an unintended decrease in pressure at the points of use. Filters, in particular, should have element pressure drop monitored and changed regularly.

A major consideration is how accurately the minimum desired pressure at the point of use can be maintained. Fluctuating system pressure can cause production quality problems, including torque variations of tools and inconsistent paint spray. Pressures that are higher than necessary can be caused by compressor control problems and, when they occur, can boost end-use air flows by causing artificial demand. Artificial demand occurs because unregulated end uses will use more air at higher pressure.

Case study

A lumber mill sorting machine had various kicker and lifter cylinders installed to move the lumber into position and perform additional lifting operations. It was found that most of the machine actuation cylinders required a power stroke in one direction only and the unloaded return stroke needed much less power. Pneumatic circuitry was installed that supplied 100 psi air to the cylinders on the power stroke, but only 40 psi air was used for the retract stroke. This operation was found to use 60% less air on each retract stroke and 30% less compressed air overall.

R #3 — Reduce pressure at source (compressors)

Real savings will not be realized unless the discharge pressure at the compressors can be reduced. A rule of thumb commonly used for a typical 100 psi compressed air system is that the energy requirement of the compressors is reduced by 1% for every 2 psi decrease in system pressure. In some cases, due to undersized piping, some of these savings may be lost due to increased velocity at the lower pressure, through dryers, filters, and piping.

Some air compressors are purchased with a pressure rating substantially higher than required at the points of use. Running the compressors at an elevated pressure may compensate for pressure drop across filters and dryers and negate any restriction in the distribution piping and valves, but to save energy the control pressure set points and their operating band should be set as low as is practicable, not to the maximum allowable.

The pressure drop across individual components and sections of the distribution system should be measured to determine if they are within acceptable limits. These pressure drops force compressor discharge pressures higher to compensate. Corrective action should be taken where indicated. This may include changing types or size of pipes, valves, dryers or filters. The pressure drop from the compressor discharge to the points of use should not exceed 10% of the compressor discharge pressure.

Changing a main filter element at a pressure differential of 6 psid instead of the typical 10 psid will save energy costs during the time the drop would have been above 6 psid. This change would save about 1% if it results in lower compressor discharge pressure. Or better still, the use of mist-eliminator-style filters designed to have a pressure differential not exceeding 3 psid at end of filter life can save substantially more energy.

Case study

A U.S. Postal Service processing and distribution center, operating 24 hours per day and 365 days per year, had a peak demand of 620 scfm. All equipment, with the exception of flat sorting machines (FSMs), required an air supply of 100 psi. The FSM required 118 psi, and the whole system was operating at a nominal compressor discharge pressure of 128 psi. The compressor discharge pressure was lowered to 100 psi and a 3 kW air-operated booster installed to provide 118 psi for the FSMs, resulting in a total system energy savings of approximately 12%.

Rules of Thumb for Relating Discharge Pressure to Energy Consumption: For systems in the 100 psig range, for every 2 psi increase in discharge pressure, energy consumption will increase by approximately 1% at full output flow (check performance curves for centrifugal and two-stage lubricant-injected rotary screw compressors). There is also another penalty for higher-than-needed pressure.  Raising the compressor discharge pressure increases the demand of every unregulated usage, including leaks and open blowing.  Although it varies by plant, unregulated usage is commonly as high as 30-50% of air demand.  For systems in the 100 psig range with 30%-50% unregulated usage, a 2 psi increase in header pressure will increase energy consumption by about another 0.6-1.0% because of the additional unregulated air being consumed (in the worst-case scenario, the extra flow could cause another compressor to start). The combined effect results in a total increase in energy consumption of about 1.6-2% for every 2 psi increase in discharge pressure for a system in the 100 psig range with 30-50 unregulated usage.

R #4 — Reduce system pressure fluctuations using adequately sized and located air receivers and controls

Systems with inadequately sized receivers and distribution piping can experience significant fluctuations in system pressures and can have problems with compressor cycling that waste considerable energy.

Reciprocating air compressor pressure pulsations can be dampened by an air receiver close to its discharge. The receiver also shields the compressor and the system from fluctuations in demand for compressed air that momentarily exceeds the capacity of the compressor. Other compressor types also benefit from a receiver close to the discharge.

An air receiver placed before a compressed air dryer can provide effective radiant cooling and promote the fallout of condensate and entrained lubricant. However, since the dryer is normally sized to match the output from the compressor, a demand for compressed air in excess of the compressor and dryer rating will result in the dryer being overloaded by air flow from the fully loaded compressor and the air stored in the receiver. An air receiver placed after the dryer will contain already dried air; during peak demands the dryer sees only the output from the compressor, so is never overloaded.

Sizing of the primary air receiver is extremely important and can affect the choice of compressor capacity control system. Figure 1 shows the effect of different receiver sizes on an installation with a lubricated rotary screw compressor having a load/unload capacity control system. The calculations include the time necessary to blow down the sump/separator vessel to prevent foaming of the lubricant (40 s) and the time to re-pressurize the vessel (3 s). A fully unloaded bhp of 25% is used, but this can vary depending on the compressor cooling method and the condition of the compressor unloading controls. Receiver capacity is shown per unit of compressor capacity.

compressor
Figure 1. With multiple fully loaded compressors, and only one part loaded unit, the required receiver capacity relates to the capacity of the partly loaded compressor.

One can see on the graph that a compressor with only 1 gal of storage per cfm output consumes more than 80% of its full load power when loaded at only 40% capacity. The same unit with 10 gal of storage per cfm would consume a much lower 60% of full load at the same flow. Unless there is ample receiver capacity and/or very light load demand, load/unload may not be the most efficient means of capacity control. With multiple fully loaded compressors, and only one part loaded unit, the required receiver capacity relates to the capacity of the partly loaded (trim) compressor, not the total capacity of all of the compressors.

A properly sized receiver close to the compressors is essential but may not be sufficient to prevent erratic system pressures. Intermittent demands for relatively large volumes of compressed air can draw down the pressure of the whole distribution system in a short period of time, causing problems for other applications requiring stable pressure. An air receiver located close to these points of use can provide the required demand with stored air, which will prevent the large demand from significantly affecting the overall system pressure. This allows more stable system pressure and more response time to replenish the air receivers more efficiently.

Pressure/flow controls can be placed after the primary receiver to maintain a stable downstream system pressure within +/- 1 psi. Due to the accurate pressure, the effect on product quality alone is well worth the initial investment. The constant system pressure also allows lowering of pressure at points of use and at the compressors with considerable reduction in energy requirements. In some cases, a compressor can be shut down.

Case study

A mineral processing facility was experiencing inefficient compressor operation due to a poor compressor control strategy. The facility had one large base 350 hp and two trim 150 hp compressors installed but only 400 gal of main system storage. Because the storage was inadequate the trim compressors could not be run in the more efficient load/unload mode. Larger storage of 4,000 gal was installed, which resulted in better compressor control; however, when the plant maintenance personnel tried to lower the pressure, some problems were experienced at a baghouse that needed a large pulse of air at a high pressure to work properly. Investigation revealed low pressure at the baghouse manifold was caused by a high flow of air requirement passing through small baghouse feed lines after each cleaning pulse. Local storage of 60 gal was added at the baghouse manifold which was protected with a check valve and restricted through a needle valve. This restriction reduced the flow so the new storage tank charged slowly, but still allowed a large pulse of air to flow to each cleaning pulse. The local storage provided enough air that the baghouse could operate at 60 psi allowing the main system pressure to be turned down.

R #5 — Reduce number of part-loaded compressors to only one

The specific power (kW/100 cfm) of a compressor increases at partial loads, regardless of the type of capacity control system used. In the past, water-cooled, double-acting reciprocating compressors were readily available and generally had discrete steps of capacity output that achieved very good energy turndown at partial loads. Inlet valve unloading allows steps of 100%, 50% and 0%. The addition of clearance pockets can provide additional steps of 75% and 25%. This offered the best mode of trimming overall capacity with other compressor types operating at full capacity. Newer, more modern variable-speed-drive (VSD) compressors are now available that have turndown ranges that are equal to or better than multistage reciprocating compressors making these compressor a good choice for trim duty.

Where oil-injected rotary screw compressors are operating in parallel, each with inlet valve modulation, it is very inefficient to have all of them modulating at the same time. Controls should be set to have all but one compressor on load/unload control, with the set points arranged so that only the one compressor at a time will trim. This provides the most efficient control mode. Care must be taken to ensure that the compressors on load/unload control are capable of full capacity up to the unload set point. The compressors on load/unload control also should have a timer to stop the compressor when it has been running unloaded for a period of time, usually 10 min to avoid too-frequent starts, but keep the compressor armed for automatic start if the compressor is needed.

Centrifugal air compressors are best used as base load compressors, with another type to accommodate the load swings. The use of inlet guide vanes is more efficient than an inlet butterfly valve on centrifugal compressors, but provision is still needed to start blowing off air at partial loads to avoid surge. Centrifugal compressors operating in discharge bypass control, blowing excess capacity to atmosphere to avoid surge, waste a significant amount of energy. Where possible, unloading the compressor is preferred.

The majority of new compressors of all types are equipped with microprocessor controls, which can be arranged for more precise monitoring and control. Most modern controls readily allow sequencing and can be easily tied in with centralized plant control systems. They also allow better tracking of required maintenance and more energy savings.

Case study

A large aerospace manufacturer had a system of three 350 hp, 1,500 cfm centrifugal compressors feeding its large aircraft parts plant. The plant had typical loads averaging 750 cfm during the day and 350 cfm during night and weekend operation. Due to a very high flow when the plant filled its autoclaves for parts, curing 3,000 cfm of compressed air for 10 to 15 min was required. Fill operations happened less than 10% of the time during the main shift. Unfortunately, due to the characteristics of the compressors, this fill required two running compressors all the time because compressor failures occurred when they tried shutting down one of the compressors on automatic start. The compressors would not run reliably in load/unload mode so the units ran in inefficient modulation mode with blow-off. Operation in this control mode made the system specific power 55 kW/100 cfm, an extremely inefficient level.

The plant replaced these compressors with a system of four rotary screw compressors, two of which used VSD technology. The system used an efficient master compressor controller that matches the compressors to the load. The peak loads are now being supplied by a high-pressure storage system. The new specific power for the system is 21 kW/100 cfm. The project is saving 2,380,000 kWh/year.

R #6 — Remove inappropriate applications

In general it takes the equivalent of 7-8 hp at the air compressor to produce 1 hp of shaft output at a compressed-air-powered tool. At best this means the use of compressed air for end uses is about 10-15% efficient, and much less if leakage is included. Due to this inefficient conversion of energy, it’s expensive to use compressed air inappropriately for uses that could better be powered by some other energy source.

This applies to end uses that use high-pressure air for a low-pressure requirement. The energy used to compress air is not recovered when passed through a pressure regulator for outputting a lower pressure. Where the pressure has to be reduced below 80% of the compressor discharge pressure for a specific application, the application should be reviewed for an alternative air supply at a reduced pressure.

Equipment requiring air at 22 psi or less should be supplied by a blower rather than from a compressed air line. This includes air lances, agitation, blow guns, mixing, and pneumatic conveying. Blowers also may be used for regeneration of desiccant type dryers.
Fans, rather than vortex tubes, should be used for cooling electrical cabinets.

A vacuum pump should be used rather than a compressed air venturi tube.

Click here to view an extensive list of potentially inappropriate applications.

Case study

A large cabinetry plant was using compressed-air-powered vortex coolers on various electrical control panels to prevent overheating. Each cooler consumed 20 cfm continuously 24 x 7 at about 100 psi. An industrial engineer studied the coolers and found they were consuming the equivalent of 5 kW/cooler costing $3,140/year to operate. The engineer replaced the compressed-air-powered coolers with thermostatically controlled refrigerant-style cabinet coolers that had the equivalent Btu capacity cooling, yet consumed only 0.5 kW, costing $135/year to run. Simple payback on the conversion was 1.3 years.

R #7 — Reduce system pressure drop losses with properly sized piping, valves

Many existing compressed air systems weren’t designed for their present state but simply grew to meet plant expansion needs, resulting in systems that aren’t adequately sized for current demand. Many distribution piping systems are based upon the size of the discharge connection at the compressor and may be totally inadequate for the flow rate and length of pipe.

Air velocity at any point in the distribution piping should not exceed 50 ft/s (fps). To avoid moisture being carried beyond drainage drop legs in main distribution lines, branch lines having an air velocity of 50 fps shouldn’t exceed 50 ft in length. Hoses and their connections often are inadequately sized, causing excessive pressure drop.

The operating pressure drop between the air compressor discharge and the points of use shouldn’t exceed 10% of the compressor's discharge pressure. A loop-type distribution system is recommended. Gate valves are preferred for their minimal pressure drop.

Aftercoolers, dryers, and filters should be sized for the full capacity of the compressors, and pressure drop across each item should be minimal. Particulate and coalescing-type filter pressure drop should be monitored regularly, and elements should be replaced before the pressure drop becomes excessive with substantial energy loss. Early element replacement costs can be recovered quickly by energy savings.

Case study

A fiberglass parts manufacturer was having production issues with some air-powered cutters used to free the fiberglass parts from their molds. Tool performance was adequate at the start of the cut but the production rate fell steadily in a short period of time, especially if more than one cutter was used at the same time.

An air auditor studied how the tools were connected to the system. He found that the plant designers preferred to use long 50-ft hose reels to provide compressed air to the tools. These reels were connected to the main distribution system using quick-connect couplings at the input to the reel and at the tool.

The tools were rated to provide full performance at a pressure of 90 psi at the tool. To test the actual pressure, the auditor made up a pressure test gauge so the tool could be connected in series. The pressure at the tool with no air flowing measured about 110 psi. When the trigger of one of the cutters was pulled, the pressure fell to 55 psi, much lower than the tools needed. The pressure at the tool was improved to 90 psi through optimization of the connectors and hoses feeding the tool by removing component, shortening hoses, and increasing the size of the components.

R #8 — Remove moisture content of compressed air with the proper type and size of dryers

Different applications require different levels of pressure dew point. Air should be dried only to the level required by a specific application. Systems often have a dryer immediately after the compressor, drying all of the compressed air to a level not needed at many of the points of use. This is an unnecessary use of energy. Each application should be reviewed to determine the amount of drying necessary. Very often a refrigerant-type dryer having a pressure dew point of 38 °F is adequate for the majority of applications, although consideration must be given to distribution piping and drains, which may be exposed to temperatures below freezing. Only the air going to an application requiring a lower pressure dew point should receive further treatment.

The location of a dryer relative to an air receiver is debatable. An air receiver between the compressor and the dryer may provide some radiant cooling and separation of condensate and lubricant. However, an intermittent demand for compressed air in excess of the compressor and dryer rating, will result in the dryer being overloaded and an increase in the pressure dew point. Location of the air receiver after the dryer ensures that the air flow through the dryer doesn’t exceed its rating and dry air is stored in the receiver to meet any intermittent demand. In some systems, a receiver at both locations can be worth the investment.

The pressure drop through the dryer also should be determined and monitored as the resulting back pressure can cause the air compressors to cycle more frequently, resulting in less efficient operation.

A refrigerant-type dryer may not require any filtration before or after it, whereas a desiccant-type dryer requires a coalescing filter before it to protect the filter bed and a particulate filter after it to stop carryover of desiccant fines. These filters cause additional pressure drop and must be maintained. Desiccant-type dryers also require the use of purge air for regeneration, and the quantity of purge air must be considered in sizing of the air compressors. Dew point control systems and other strategies are available to minimize the amount of purge air needed. In some cases, a blower or a vacuum purge system can be used more economically.

Case study

A tire shop had a small compressed air load made up of the tools and equipment required to service automobiles, large trucks, and tractors. One compressed air line went outdoors to feed a tire-filling station. A main desiccant air dryer removed moisture from all the compressed air in the shop to a level of -40, so the line would not freeze in winter months. An air audit at the shop showed that, while the average compressed air load in the facility was only 9 cfm, the compressor actually produced an average of 28 cfm. An investigation showed that the uncontrolled heatless air dryer installed in the shop was consuming an average of 19 cfm or 68% of the total output of the compressor. A refrigerant air dryer was purchased and a small point-of-use zero purge air dryer was installed for the outdoor supply line. The reduction in compressed air demand saved 70% in compressed air electrical costs.

R #9 — Remove condensate without loss of compressed air

Various means are used to drain off condensate from dryers, air receivers, filters, and header drop legs. The amount of condensate will vary with geographic location and atmospheric conditions of temperature and relative humidity. Drain traps should be sized for the anticipated rate of accumulated condensate and chosen for the specific location and anticipated contamination by lubricants being used.

The relatively common practice of leaving a manual drain valve cracked open shouldn’t be tolerated as it wastes compressed air. For all types of drain traps, bypass piping is recommended to facilitate proper maintenance.

Float-type drain traps. The mechanical nature of float-type devices combined with the contaminants present in condensate make these devices an ongoing maintenance item, often neglected. The float is connected by linkage to a drain valve, which opens when an upper-level setting is reached and closes when the drain is emptied. The float device varies from a simple ball to an inverted bucket, but the basic principle is the same. An adequately sized drain valve is essential for satisfactory operation and to prevent blockage. A float which sticks in the closed position won’t allow condensate to be drained, while a float which sticks in the open position will allow the costly loss of compressed air.

Electrically operated solenoid valves (“time cycle blowdown”). A solenoid-operated drain valve has a timing device that can be set to open for a specified time and at specified intervals. Again, the size of the valve and any associated orifices must be adequate to prevent blockage. The valve is set to operate without reference to the presence of condensate or lack of it. The period during which the valve is open may not be long enough for adequate drainage of the amount of accumulated condensate. On the other hand, the valve can operate even when little or no condensate is present, resulting in the expensive loss of compressed air.

No air loss or zero air loss drain valves. These use a magnetic reed switch or a capacitance device to detect the level of condensate present and operate only when drainage is called for. When an upper-level inductance sensor detects liquid, the microprocessor opens a solenoid. A lower-level inductance sensor signals for the drain to be closed.

It’s vital to maintain traps and drains in good operating condition. If the drains and traps are clogged, condensate will fill vessels and pipes in a short period of time and be carried over into the system in the form of liquid water, and may:

  • cause corrosion and deposits in the air receiver
  • prematurely exhaust the capacities of pre-filters and desiccant dryers
  • overload refrigerant-type dryers
  • cause moisture accumulation in the system piping, resulting in corrosion
  • cause malfunction of air-operated valves, making operation sluggish or erratic
  • wash away lubricants from operating cylinders of air-operated valves or other similar equipment
  • cause some of the lubricants used on solenoid valve O-rings to become sticky or gummed up, causing the solenoid valve to become inoperable.

Also, some of the system piping may be installed outdoors and exposed to varying ambient temperatures. Accumulated water may freeze during winter and cause damage to piping and instruments.

Case study

A timer drain at a pharmaceutical company was set to drain the air dryer water separator at regular intervals to avoid water carryover. The backup compressor feeding the air dryer had a sophisticated control designed to sense rapid changes in pressure and start the compressor in response to ensure the compressor could rapidly load before the pressure fell below its load set point. The timer drain had a large drain orifice that consumed a significant amount of air, enough to cause a change in pressure when it drained. Due to this pressure fluctuation, the compressor would start but not load. The timer drain frequency was such that the compressor constantly ran unloaded consuming 90,000 kWh/year.

R #10 — Reduce downtime through preventive maintenance

Prevention is better than cure. Neglect can lead to costly downtime of production equipment and more extensive repairs. Manufacturers’ recommended maintenance items should be required, documented, and reviewed regularly for the development of any trends.

The use of compressor synthetic lubricants, stated to be good for 8,000 hours of operation, doesn’t mean the associated lubricant filter and air-lubricant separator also are good for the same period. The pressure drop across lubricant filters and separators should be monitored regularly.

Records may indicate a normal interval between changes, which may then be planned. Some compressor microprocessors will signal required maintenance, and this should not be ignored.

Automatic condensate drains must be checked regularly to ensure satisfactory operation.

Case study

A foundry making railway wheels couldn’t keep the pressure up in the plant, even with all four of the compressors running. The liquid coolers in their compressors had become so dirty that the compressors couldn’t run at full load so had to be modulated down to lower output. The overly hot air produced by the compressors and dirty dryer coolers caused the air dryers to constantly trip off. The plant loading had increased to a point where low pressure was a constant problem. The site was forced to rent and operate three expensive diesel compressors just to keep up.

Investigation revealed that inadequate management had allowed the leak level to increase to a point where 1,100 cfm of compressed air was being used on the weekend, even with no production. Poor maintenance practices had allowed the compressor set points to drift so badly that one compressor wasn’t even loading when pressure was low. Another had developed a problem that kept its inlet valve closed, greatly reducing its output.

Repairs, replacements, and adjustments were done to coolers, cooling water quality, compressor controls, and leakage levels, including finding and fixing a 550 cfm leak in a baghouse. Annual savings were measured at $90,000/year, not including diesel compressor fuel and rental costs. The plant is now running on three compressors.

R #11 — Record system data and maintenance

All maintenance items should be recorded and the records analyzed, so that timely preventive measures can be established. In addition, records should be kept of all operating pressures before and after major components and at strategic points in the system. These will indicate potential problem areas requiring corrective action.

R #12 — Review air usage patterns regularly

Recording operating pressures at strategic points throughout the system can reveal changes in usage but may not adequately indicate the rate of change of pressure due to changes in demand. Data logging can help in this area.

Over time, new production machines may be added, while others may be eliminated. The person responsible for the compressed air supply needs to be kept informed of such changes, which may require upgrades to the compressed air system, including plant expansion, rather than waiting until a problem develops. Low pressure at a point of use may not require additional compressor capacity; the problem may be due to fluctuating demand at another point of use, and the problem could be solved by the addition of a secondary air receiver close to that application.

R #13 — Recover heat

The majority of rotary air compressor packages are air-cooled, and it’s estimated that, of the total power, 80% results in heat to the oil cooler and an additional 13% to the air aftercooler. This provides a substantial potential for heat recovery.

Frank Moskowitz is a Compressed Air Challenge instructor for the Fundamentals and Advanced levels of training, an AIRMaster+ instructor and a Department of Energy (energy savings) expert on compressed air systems. He’s also vice-chair for ASME Standard EA-4-2010 "Energy Assessment for Compressed Air Systems" and is a member of International Standards Organization (ISO) technical committee for air compressors and compressed air systems energy management. Contact him at fmoskowitz@drawproservices.com.

Ron Marshall is a certified engineering technologist in the province of Manitoba and has received certification as an energy manager, demand side management and measurement and verification professional through the Association of Energy Engineers. He was the first Canadian participant to qualify as a DOE AIRMaster+ specialist and is involved as a member of Compressed Air Challenge's Project Development and Marketing Committee. Marshall has worked in the industrial compressed air field for 16 years as an industrial systems officer for Manitoba Hydro's Customer Engineering Services Department, and he is Manitoba Hydro's industrial compressed air systems expert whose efforts support the utility's Power Smart Performance Optimization program. Contact him at rcmarshall@hydro.mb.ca.

The atmospheric air blown across radiator-type coolers can be used for space heating of plants in cold weather conditions. An additional fan may be needed to supply the necessary pressure head for ducting and distribution of this air.

In some water-cooled compressor applications, water heating has been accomplished for use in the plant.

Case study

A small company reconditions propane bottles for resale at various depots throughout its territory. The plant uses a 25-hp air compressor that produces the equivalent of about 15 kW of heat in average conditions. During reconditioning, the propane bottles are painted and dry while hanging on an overhead conveyor. The compressor heat is captured and redirected to the propane bottles to assist in heating the make-up air and drying the paint. The remainder of the heat is redirected to the facility production areas to help to displace building heat. The building has all electric heating so the compressor heat displaces the equivalent kW loading. Estimated savings are $2,500/year in electric heating costs.

R #14 — Reduce energy costs (return on investment and cost of operation)

The whole objective of The Compressed Air Challenge is to reduce the energy consumed by compressed air systems. Implementation of the recommendations can result in some compressors being shut down and used for standby. The resulting savings in the cost of operation go right to the bottom line of a company's financial statement.

In addition, the right amount of air at the right pressure and of the right quality will enhance product quality, reducing defects, scrap, and warranty costs. Customer satisfaction will be improved.

Following the 14 Rs results in an efficient compressed air system that pays dividends.

This article is based on a paper by David M. McCulloch, retired, and modified by Frank Moskowitz, Draw Professional Services, and Ron Marshall, Manitoba Hydro.