commpressed air tips

7/26/2019 Commpressed Air Tips

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COMPRESSED AIR

A COMMON SENSE APPROACH TO ITS USEAND ABUSE

Common Myths about Compressed Air

Compressed air is free as a power source it is

nine times more expensive to use than electricity.

More pressure is better raising pressuresystem-wide will require more power on-line.Pressure problems are best solved at the point ofuse where they exist, not with more power in thecompressor room.

Our system operates at 100 psiG not likelysince most compressed air systems havepressure variations of 10% before the air leavesthe compressor room.

Production needs 100 psiG maybe one or twoapplications are thought to require 100 psiG, butfor the most part the true minimum requirementfor the majority of the plant is much less.

Reducing compressor-operating pressure willsave energy how far pressure is reduced willdetermine the savings, the further it is reducedthe less stable and reliable the system willbecome. The first time production is interruptedoperating pressure will be returned to previouslevels and savings will disappear.

Increasing compressor-operating pressureincreases operating costs while it is true thatcompressing a cubic foot of air to a higherpressure requires more energy, overall operatingcosts will increase only if the entire system isallowed to operate at the elevated pressure.

Receiver sizing rule of thumb: 1 gallon per cubic

foot of compressor rating receivers are sized tomanage events in the system, this type of rule ofthumb pays no attention to the needs of thesystem.

Fixing air leaks is an easy energy savingmeasure reductions in on-line power areseldom seen until a majority of the system airleaks are repaired. Repairing some leaks in acompressed air system without a DemandExpander System or similar pressure controldevice, can actually cost more money byincreasing the volume through the other leaks.

Dryer is better some facilities install -40 Fpressure dewpoint dryers in hopes of fixing their

moisture carry over problems. Often the problemshave little to do with dryer type. Installing this typeof dryer can pose a whole new set of systemproblems to be dealt with like adding 15% morecompressor power to accommodate the dryerspurge requirements!

A filter removes 99.9999% of 0.01 micronparticles filter performance is tested per theDOP (dioctylphthalate) test that measures theamount of carry over downstream of a filter beingtested. DOP is used because it consistentlygenerates particles that range in size between 0.3

0.6 micron with little variance. A claim that afilter can remove smaller particles is based on theresults of this test and variances in filtering media,not actual results with 0.01 micron particles.

Many of the assumptions listed above are realbarriers to operating compressed air systemsefficiently. Education is the best first-step

measure that can be taken in improvingcompressed air system operating efficiency.

Common Inefficiencies in Compressed AirSystems

Many compressed air systems waste as much as 40%of their total operating cost.One reason for that is that compressed air is oftenviewed as a free utility by the people that consumethe air.

Common inefficiencies include:

Lack of integrated system control of multiplecompressors

Failure to store compressed air energy for useduring peak demand periods

Leaks at both point-of-use and supply-sideequipment

Severe fluctuation in pressure

Indiscriminate use of open blowing

Inappropriate production use of compressed air

Simple lack of maintenance, including neglect ofdirty filter cartridges

Non-existent system-wide control and monitoring

By optimising your compressed air system, you havethe ability to increase production, and improve quality.

Common Sense of Compressor Air SystemMaintenance

Compressed air is probably one of the mostuniversal operations within a plant environment. Ittranscends industries, operations and applications.Compressed air is used to power tools, moveconveyers, transport products and make processapplications possible. Considered a power source,compressed air systems are increasingly more reliable

and predictable. Simply, it is considered the fourthutility. Therefore, similar to electricity, disruption ofthe compressed air supply can cause costlyproduction delays. With more and more companieseliminating capital budgets to purchase back-upcompressed air systems, compressor downtime forrepair, adjustments and maintenance becomes acritical issue.

In a recent industry study, 20 percent of calls loggedinto equipment manufacturers help desks could havebeen avoided by proper installation and or


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maintenance procedure, which is why so many people-- from compressor manufacturers to consultants --take time out to preach about compressed airreliability and efficiency. However, before you attend aseminar, sign that predictive / preventive maintenancecontract or call your compressor manufacturer, readthis article. This guide will provide you with what couldbe considered common sense advice for compressedair system placement and maintenance that could

reduce the amount of downtime you experience withyour compressed air system. We will reviewcompressor location, power source, ventilation, piping,filtration, cooling systems, and preventivemaintenance.

Compressor Placement

While proper maintenance can help preventcomplaints from compressor users, there are severalissues that can be addressed before the compressedair system is actually in use. Proper compressorlocation, power sources, and ventilation can helpprevent unscheduled downtime and environment

issues.

Location Selection: One important considerationwhen utilising a compressed air system within anoperation is where to physically locate the unit. Whilethere isnt just one way to install a compressor youshould be aware of all the advantages anddisadvantages to each system. For most plantenvironments, compressed air systems are designedto fit in a centralized area, adjacent or near the actualapplications in which it serves or, in certain scenarios,in an outdoor installation (see sidebar for guidelinesfor outdoor installations).

Again, each scenario will offer different sets ofadvantages and problems. Regardless if yourcompany chooses a centralized, decentralized oroutdoor installation, you should consider theadvantages and shortcomings and prepare forpotential problems. For instance, if a compressor islocated indoors in a centralised compressor room, thecompressed air system is protected from the weather,allows for easy access for maintenance andmaximizes plant floor space. However, centralizedcompressed air systems usually requires additionalspace to provide adequate ventilation and additionalpiping to reach the actual operation, which canincrease the potential of system pressure drops.

A decentralised compressed air layout allows forcompressors to be located closest to the largest airusers and reduces pressure drop through air lines.However, this configuration can also result in thehighest probability of incorrect filtration as well asnoise and heat complaints.

Another issue surrounding compressor location is theambient temperature of the area. Compressed airsystems subjected to low temperatures may deal withslow starting, possible control line freeze problems, a

condensate freeze problem and/or a possible fluidmisapplication. To remedy these issues, maintenancepersonnel can specify heaters and heat tracing keyelements to minimize the freezing or simply relocatethe unit to a warmer area of the plant. On the otherend of the spectrum, compressed air systemsexposed to extremely high temperatures canexperience unscheduled shutdowns, increasedmaintenance, and decreased lubricant life. These

factors can be reduced by adjusting ventilation,utilizing a higher performance lubricant or again,relocate the compressor to a better location.

Power Ratings: The quality of the incoming powerfrom your utility company will greatly affect thereliability of the electrical components of yourcompressor.

For obvious reasons, the power supply should befree of any phase variation and voltage droops. Forthis reason many manufacturers offer phase andvoltage monitors on their air compressors in order tohelp extend the life of the motor and any other

electric/electronic components.

A simple rule to keep in mind when selecting a powersource is matching voltages -- the voltage emitted bythe power source and the voltage needed to run thecompressed air system. The closer the voltages, thelonger your motor will last. During relocation "voltagematching" can be solved by having the original motorrewound or simply buying a new one.

Ventilation: One of the leading complaints by plantworkers and causes of unscheduled shutdowns isheat. Because compressed air systems generate suchlarge amounts of heat, require extensive ventilation is

required. Contrary to popular belief, ventilation isequally important for all compressors, regardless if it iswater-cooled or air-cooled.

When there is insufficient ventilation, heated air fromthe compressor exhaust remains around the unit andis then ingested by the compressor increasing theoperating temperature of the unit. This will cause theunit temperature to spiral upward and eventuallyshutdown. It is important to plan for ventilation andaccess when deciding compressor placement. Plantdesigners need to be aware that they need to allow forthree feet around the entire compressor package formaintenance and approximately 42-inches at the

motor starter access panel. In addition, you need toavoid areas that are extremely humid or whoseambient temperatures exceed 115F.

In addition to the actual ventilation area around thecompressor, it is helpful to duct the cooling air exhaustof a compressed air system to either an outdoor areaor an energy recovery system. Regardless on howyou decide to duct the exhaust heat, addressing thisissue at installation time can help extend the life ofyour compressor. Specifically, by exhausting the heat,


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you can increase the life of your coolant, heatexchanger, bearings and hoses.

Filtration

Poor air filtration is the leading cause of early deathfor air compressors. Here are a few guidelines to helpensure that your compressor will continue to produce

clean, dry air:

Know Your Environment: One common mistake thatcompressor users make is when they neglect toevaluate the quality of the air that they will be usingwithin the compressor. To get to know yourenvironment, evaluate the size and make-up of air-borne particulates and ask yourself some questionsregarding your surroundings:

Is the compressor near a chemical process?

Is chemical cleaning being done in the area?

Are noxious fumes present?

Most environments fall into one of three categories --dusty, hostile and clean. Here is a brief descriptionand the potential problems:

Clean: A clean environment is defined as having lowdust and debris. This type of environment does notrequire much more beyond what would be consideredstandard maintenance. A common problem, however,is that many people think that because they aresituated in a "cleanroom" environment theircompressed air system is safe from air quality issues.However, cleanroom environments often containgases that are incompatible with the cooler lubricant.One solution to this problem may be to add additional

ducting that will bring in ambient air from outside thefacility.

D u s t y : Dusty conditions, on the other hand, maycontain dust as well as dirt, casting sand, and otherairborne particulates. The hazards created by theseconditions can be reduced by using a high dust inletfilter. While it may not remove any additional particles,it can reduce frequency of replacement.

H o s t i l e : A hostile environment is defined as havingcaustic gases/chemicals, chlorine, ammonia, acids, inthe air. With a hostile environment, one solution maybe to remove the problem by relocating the

compressed air system or the caustic materials.Another option is replacing standard materials ofconstruction with more tolerant materials; for instancestainless steel coolers vs. copper coolers. In addition,to save money, evaluate the compressor fluid life inthe hostile environment. A smart move may bepossible conversion to a more cost effective fluidgiven the shortened life.

A proper evaluation of air quality at the time ofinstallation and at least once a year could help preventa premature failure of your compressor.

Confirm Inlet Filter Size: When inlet filters are notsized properly, it allows micron size dust to enter thecompression system, which can decrease the life ofthe coolant and separator filters. A basic guideline for

maintenance personnel is to monitor the pressuredrop of filters and replace elements before the cost ofincreasing pressure drop, due to dirt or dust build up,exceeds the cost of a replacement element. Inlet andoil filters left too long before changing can literallychoke a compressor, reducing its flow. This will alsoaccelerate the wear rate of rotating elements, such asbearings, in rotary screw compressors.

In addition, you should remember that the air filter thatcame with the compressor originally may no longer beadequate for your changing facility. Systematicallyevaluate your air filtration needs to fit your application.

Evaluate Your Compressed Air Dryer Needs: Liquidwater occurs naturally in air lines as a result ofcompression. Additional condensation occursdownstream as the compressed air continues to cool.Moisture in compressed air is responsible for costlyproblems in almost every application that relies oncompressed air. Some common problems caused bymoisture are rusting and scaling in pipelines, cloggingof instruments, sticking of control valves, and freezingof outdoor compressed air lines. Any of the se couldlead to downtime of your compressed air system.

Compressed air dryers help to reduce the watervapour concentration and prevent liquid waterformation in compressed air lines. Dryers are a

necessary companion to filters, aftercoolers, andautomatic drains for improving the productivity ofcompressed air systems.

Refrigerated and desiccant dryers are the mostcommonly specified for correcting moisture relatedproblems in a compressed air system. Refrigerateddryers are normally specified where compressed airpressure dew points of 330F. to 390F. are adequate.Desiccant dryers are required where pressure dewpoints dip below 330F.

Evaluate Your Cooling Water: Aftercoolers areessential elements of air compressors. These

aftercoolers are heat exchangers that utilize eitherwater or ambient air to cool the compressed air. Thecompressed air is typically cooled to within 15- 25 Fof the cooling media. In addition, aftercoolers typicallyremove 60 percent of moisture content in the air andhelp insure that the temperature of the air within thepiping system is not considered a safety hazard.

Just as clean cool air is important to everycompressor, clean cool water is critical to units fittedwith water-cooled heat exchangers. At a minimum,water conditions should meet the manufacturers


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requirements for flow, pressure and temperature;however, one item that is often overlooked is therelevant "hardness" of the water. Hard water depositslead quickly to clogging and fouling of coolers causingtemperature shutdowns. Water quality test kits arereadily available from hardware or even swimmingpool supply stores. Once a "bad" condition isidentified, the cure could be as simple as scheduledchemical treatments of your cooling tower or the

addition of an electro static or magnetic treatmentsystem.

Piping

Regardless of what you do to maintain yourcompressor, if you are not maintaining your pipingsystem your efforts have been wasted. All air/waterinlet and discharge pipeworks are affected byvibration, pulsations, temperature, pressure, corrosionand chemical resistance. In addition, lubricatedcompressors will discharge small amounts of oil intothe air stream; therefore, you need to assurecompatibility between discharge piping, system

accessories and software.

Nearly all of the compressed air systemmanufacturers recommend that customers do not useplastic piping, soldered copper fittings and rubberhose as discharge piping for compressed air systems.Plastic piping is not recommended because sometypes might react with compressor fluids, soften due toheat or shatter due to pressure or pulsation of thecompressor. Soldered, copper fittings will eventuallywork loose due to pulsating caused by thecompressed air system. Rubber hose piping isunacceptable because it is easily attacked by todayslubricants. In addition, flexible joints and/or flex lines

can only be considered for such purposes if theirspecifications fit the operating parameters of thesystem.

Condensate Removal: After compressed air leavesthe compression chamber the compressorsaftercooler reduces the discharge air temperature wellbelow the dew point (for most ambient conditions),therefore, considerable water vapor is condensed. Toremove this condensation, most compressors with abuilt-in aftercoolers are furnished with a combinationcondensate separator/trap. One concern when dealingwith condensate is the Clean Water Act, which forbidsthe routing of condensate to floor and storm drainsand to the ground outside even after condensateseparation.

In situations such as this, a drip leg assembly andisolation valve should be mounted near thecompressor discharge. A drain line should beconnected to the condensate drain in the base. Keepin mind that it is important that the drain line mustslope downward from the base to work properly. It ispossible that additional condensation can occur if thedownstream piping cools the air even further and lowpoints in the piping systems should be provided with

drip legs and traps. It is also important that thedischarge piping is as large as the dischargeconnection at the compressor enclosure. All pipingand fittings must be suitably rated for the dischargepressure.

Careful review of piping size from the compressorconnection point is essential. Length of pipe, size of

pipe, number and type of fittings and valves must beconsidered for optimum efficiency of your compressor.

Preventive Maintenance

If someone asked, "what is the key to maintaining anefficient compressed air system," my answer wouldhave to be -- preventive maintenance. This is the oneway the operator can actively help preventunbudgeted maintenance expenses from cropping up.One way to execute a preventive maintenanceprogram is by data trending.

Data trending is the recording of basic operation

parameters including pressures, temperatures, andelectrical data. For example, slowly increasingtemperature indicates a variety of maintenancerequirements including cooler core cleaning,overloading of system and possible mechanicalproblems. Another example might include slowlydecreasing pressure, indicating increased system flowrequirements, reduced compressor performance orincreased system leakage. Keep in mind, once apreventive maintenance program has beenimplemented, a key element often overlooked is dataanalysis. If the data is never reviewed, looking fortrends, the benefit is lost.

Finally, the operator should understand that the sameinformation used to evaluate and establishrequirements for buying a new compressor should beused to re-evaluated periodically to ensure yourcompressor is still capable of doing the job. If not,there is a good chance you may be asking it to domore than it can, which will inevitably lead to a shortlife.

Many times a compressor must be installed outsidedue to jobsite conditions or limited space within amanufacturing facility. When this occurs there arecertain items that should be incorporated into theinstallation to help ensure trouble free operation,including:

The compressor must be purchased with the OutdoorModification Option to provide NEMA 4 electrics anda cabinet exhaust on the end of the unit rather thanthe top to prevent re-circulation of cooling air;

The compressor should be installed on a concrete paddesigned to drain water away. If the concrete pad issloped, the compressor must be leveled. In order toproperly pull cooling air through the aftercooler, thebase/skid must be sealed to the concrete pad;


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The roof of the shelter should extend a minimum of 4feet around all sides of the compressor to preventdirect rain and snow from falling on the unit. Air-cooledmachines must be arranged in a way that prevents airre-circulation. (i.e. hot exhaust back to the packageinlet).

If the installation includes more than one compressor,

the hot air exhaust should not be directed towards thefresh air intake of the second unit or an air dryer.

Arrange the machine with controller/starter enclosurefacing away from the sun as radiant heat can affectstarter performance. In addition, direct sunlight andUV rays will degrade the membrane touch panel.

Power disconnect switch should be within line of sightand in close proximity to the unit operating panel.

Incoming power connections must use suitableconnectors for outdoor weather tight service.

A minimum of three feet clearance must be allowed onall four sides of the unit for service access. If possible,access by a forklift and or an overhead beam hoistshould be kept in mind (for eventual service to airendor motor).

Some type of protection such as a fence or securitysystem should be provided to prevent unauthorizedaccess.

Compressed Air Systems Approach

Traditionally, the phrase compressed air system isused to refer to compressors, dryers, coolers, filters,etc. The problem with this very narrow systemdefinition is that it overlooks the interrelationshipbetween supply side components and their demandside counterparts. The supply and demand sides of amanufacturing facility do not work independently ofeach other. They work (or often dont work) togetheras a system. The entire compressed air systemshould be analysed, monitored and controlled.

Both sides must be coordinated by suitable controls inorder to work together.

Pre-Comp Air Diagram

The traditional compressed air systems definitionignores the demand side and its point-of-useapplication needs.

Optimisation and Prevention MaintenanceThrough Advanced Control Systems

This section was originally written about the textile

industry but the same basic principals apply to allusers of compressed air.

Why Worry About Compressed Air?

During the last quarter the textile industry hasembraced new technologies, which have increasedproductivity and improved quality. Many of these newtechnologies have brought with them a new focus onan utility that has become as important as electricityand water - compressed air. Compressed air makestoday's air jet spinning, air jet weaving, air jettexturising and air splicing possible.

Once thought of as a powerhouse utility, with nothought to the cost of producing compressed air or onmethods of optimising these costs, today's textilemanagers have realized that an efficient, reliablecompressed air system is a necessity.

Compressed Air Systems

The make-up of compressed air systems vary fromplant to plant. The different types of air compressorswhich make up these systems are either positivedisplacement; reciprocating and rotary, or dynamic;centrifugal. In plant air needs such as blow down,controls and operation of pneumatic cylinders, the

compressed air does not come into direct contact withthe textile product. Therefore, reciprocating or rotaryair compressors have been commonly utilized.

For air jet weaving, spinning and texturising,compressed air is in direct contact with he product,which mandates the use of 100% oil-free aircompressors to insure product quality. For theseapplications and any applications with large plant airneeds, centrifugal air compressors are commonlyutilized.


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The requirement of air jet technology for oil-freecompressed air created separate compressed airsystems in many plants. These plants utilize older,lubricated air compressors for plant air needs whileusing oil-free air compressors for their air jet needs. Alarge number of plants, however, have takenadvantage of the higher efficiency of newercentrifugal, oil-free air compressors to provide air forboth plant and air jet needs. For this reason the focus

of this discussion will be on centrifugal compressors. Itshould be noted, however, that the ideas put forth canbe carried over to other types of compressors.

Cost of Compressed Air

Before we can investigate methods of conservingcompressed air we should review the factors whichcontribute to the cost of compressed air.

Generally, these factors can be grouped into thefollowing categories:

Fixed Charges and Repairs -- Usually about 15% oftotal costOperation - Usually about 20% of total costUtilities - Usually about 65% of total cost

While fixed charges, such as depreciation, insuranceand taxes, cannot typically be reduced, repairs canprovide an area of possible cost reductions. Majorrepairs can be often be avoided with properpreventative maintenance. Advanced control systemscan provide the tools to utilise preventativemaintenance to reduce repair costs. Vibration analysiscan also be utilised, either alone or in conjunction withan advanced control systems, for further reduce majorrepairs.

Operational costs include the monitoring of thecompressed air systems and the parts and labornecessary for regular maintenance. These costs arenecessary and, typically, are kept to a minimum. Thecost of power to compress air is the area in whichmost cost savings solutions exist. Most compressedair systems can be made more efficient by simplyoperating at the lowest pressure the systems canhandle. Since it takes power to compress air to ahigher pressure, maintaining the lowest possiblepressure uses the least power.

In order to keep the pressure low, air leaks becomemore important. Not only do air leaks cause pressuredrops, but they also cost money. Table 1 indicatesthat the cost from a small leak in term of dollars isconsiderable. Identification and repair of leaks canprovide another method of power savings.

T A B L E 1

Diameter ofOpening

Cubic Feet ofWaste Per

Month

Cost

1/32" (0.75mm) 45,400 6.81

1/16" (1.5mm) 182,000 27.30

1/8" (3mm) 729,000 109.35

1/4" (6mm) 2,920,000 438.00

Based on sharp-edged orifice continuously at 100psig with air costs at 15 pence per 1000 cubic feet.

To accurately determine the costs of compressing air,measurements of power and compressed air flow areessential. System efficiency, decay of that efficiencyand incorrect usage of compressed air can all bedetermined through these measurements. Thesemeasurements make it possible to investigate costsavings through various methods of conservingcompressed air.

Advanced control systems can provide variousmethods of power conservation, particularly in multi-unit installations. Before we go in-depth to thesemethods we should first review the standard controlsystems in use in many textile plants today.

Standard Controls

All compressors are supplied with some type ofcontrol system. These systems will typically monitor

the compressed air system and automatically adjustfor demand. Additionally, the primary health functionsof the compressors are monitored to provideprotection against breakdown.

For centrifugal compressors an inlet throttling deviceis utilized to throttle inlet flow to the compressor tomaintain a constant discharge air pressure. Inlet flowcan be throttled to a minimum point at which point airis bypassed to maintain the constant discharge airpressure. The typical control package will control thethrottling and bypassing of air. It can even provideunloading of the compressor of low system demandwith reloading on falling system air pressure.

The control system will monitor compressortemperatures, pressures and vibrations and comparethese actual values against alarm and shutdownsettings. Additionally, most systems are capable ofproviding alarms for basic preventative maintenancesuch as dirty inlet air or oil filters. While thesestandard control systems provide efficient control forsingle compressor installations, they may not meet thenew needs of the modern textile manufacturer withmulti-unit installations.


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Modern Textile Control Needs

The textile mill of today has become very flexible inorder to operate under a wide variety of marketconditions. For this reason, most compressed airsystems are made up of multiple compressors in orderto allow efficient operation at less than full plantproduction. Multiple compressor installations also

allow for effective planning for future plant needs.These multiple compressor installations have createda new set of control needs for the modern textile plant:

Optimisation of power usageSystem dependability - avoiding unplanned downtimeSystem reliability - planning maintenance

These special control needs are not typically providedin the standard controls provided with eachcompressor.

Optimising System Power Consumption

Multiple compressor installations, when left theirstandard controls, will typically see the strongestcompressor taking the lead by operating at full load.While the weaker compressors make up the remainingsystem needs by operating at partial loads. Theproblem with this configuration is that one compressoris operating at a much less efficient point.

A central energy management system should becapable of forcing all of the compressors to share theload equally. This can be accomplished by manymethods, for example controlling all inlet valves to thesame throttle point. Systems, which do this, haveshown savings of up to 8-10%.

Checks can also be made to determine if the optimumnumber of compressors for a certain load areoperating. Basis these checks, compressors can bestarted and stopped, within the motor startingcapabilities, to insure a minimum number ofcompressors are operating at any point in time.Savings from these checks are dependent on the loadvariations of a specific system.

System Dependability

Loss of compressed air pressure in today's textile millscan result in hours of lost production and damage to

product in process. For these reasons, systemdependability must be optimised to provide a system,which can protect against unplanned outages. Whileeach compressor's standard control panel providescompressor protection, no system protection isprovided. A central energy management system cannot only supply system protection, it can also optimiseit.

A central energy management system should becapable of monitoring the health of each compressorin order to determine an alert or shutdown status as

soon a sit occurs. This will allow the system to bringanother compressor on-line before the air pressurereaches problem levels. The system can the alert thecompressor operator that a compressor hasencountered trouble so that maintenance can becompleted.

An automatic system such as this allows unmanned

operation of a compressed air system. Thus,maintenance personnel that have been required foryears to monitor the compressed air system can nowspend time optimising system performance byrepairing system air leaks, providing preventativemaintenance, and providing for other plantmaintenance needs.

Preventative Maintenance

The key to system reliability is a strong preventativemaintenance program. In past years this has meanttaking data by hand and then reviewing the datalooking for specific trends. This method oftenoverlooked problems until it was too late to planpreventative maintenance on a schedule that wasacceptable to production. Therefore, an automaticform of data collection with trending and indication ofpreventative maintenance requirements wasnecessary.

To accomplish this task a method of data collectionmust first be developed. Since digital data is best usedfor this process, this means that temperatures,pressures and vibrations must be collected viaelectronic devices such as RTD's, pressuretransmitters and vibration probes. This data is alreadycollected on many modern compressors for use ontheir standard control panels. On older compressors it

may be necessary to update the standard controls orprovide direct signals to the central energymanagement system. Once the data is collected andtransmitted to the central energy managementsystem, it must be analyzed. Through data trending,potential problems can be detected far before theywould cause a compressor to fail. This will allowmaintenance to be planned in conjunction withproduction needs.

A central energy management system could alsoprovide an accounting system for routinely scheduledpreventative maintenance such as oil and filterchanges. The system could simply schedule the

routine maintenance items and indicate tomaintenance a daily schedule of items to becompleted. After each item is completed it is recordedinto the system thus updating the maintenance log foreach compressor.

While an advanced control system does reduce theworkload on the personnel responsible forcompressors, these personnel are still necessary. Avisual check of the compressor is still the best methodof identifying leaks, faulty condensate traps and manyother indications of problems.


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Where To Start

Once the need for advances control systems isrecognized, there are several questions that must beconsidered. These questions will help to defineseveral questions that must be considered. Thesequestions will help to define the type of system thatshould be further investigated. At this point it may be

in your best interest to consult your compressormanufacturer for assistance on adaptability of theircompressors to advanced control systems. This willimpact your decision on the following questions.

First, does your facility currently use a distributedcontrol system? A distributed control system, or DCS,is used to control more than one system within afacility. For instance, it may control compressors,pumps, lightning and air conditioning. If your facilitydoes use a DCS it may be beneficial to utilize it foradvanced compressor control. The benefit of this typeof system is that is designed to exactly fit your uniqueneeds. Often though, this type of system is tooexpensive due to the cost of programming of the DCS

and the cost of transmitting the data to the DCS.Additionally, much time must be spent to develop thealgorithms necessary for system optimisation.

Second, does your compressor manufacturer have anadvanced control system which fits your needs? Manycompressor manufacturers have developed advancedcontrol systems for their compressors. These varyfrom simple sequences, which simply start and stopcompressors to elaborate computer-based systems,which provide for modern textile needs. Some of thesesystems can even be linked to an existing DCS topass on compressor data. In this way, the compressorvendor supplies the programming and algorithms for

compressor control while preventative maintenanceand compressor logs can be maintained on the DCS.

Finally, how sophisticated do you want to get? It isimportant to define your unique system needs beforeyou purchase a central energy management system.The level of system sophistication varies with the costof the systems. These systems can cost any wherefrom 5,000 to 500,000. Without defining yourspecific systems needs it is very easy to end up withthe wrong system for the wrong price.

Summary

There are many effective methods of identifying waysto reduce the costs of compressing air. Among theseare compressed air surveys, compressed air leakdetection, vibration analysis, maintenance contractsand advanced control systems.

Modern textile plants can utilize advanced aircompressor control systems for:

Efficient Energy UsageControlling System Dependability

Controlling System Reliability

These systems can be as simple or complex as anindividual plants needs. Determination of your uniqueneeds can lead to finding an advanced control systemthat will allow your facility to operate without worry ofloss of compressed air.

A Guide To Using Compressed Air ControlSystems To Improve Efficiency For MultipleCompressor Installations

Compressed air is considered a utility used in avariety of plant functions from transporting material, tooperating production machinery and power tools.Because most facilities use multiple compressors, anexcellent opportunity for energy savings exists in theefficient control of these multiple unit systems. Sinceplant air demand is constantly fluctuating there existsseveral operating options that can provide significantsavings during part load conditions. This article willreview several options for compressor control systemsand help plant personnel address varying compressedair demands.

The Basics

To understand the logic behind system controls, agood place to starts is by reviewing some of the basicprinciples associated with compressed air usage:

A compressor that is running at idle will usuallyconsume over 30% of its full load power. This is due inpart to degrading motor efficiency coupled withrelatively high unloaded motor horsepower.

Air flow in CFM is dependent on pressure. Aspressure decreases, air flow through an orifice,regulator, etc. will also decrease. A 0.25 inch orificewill discharge 126 CFM at 125 PSIG and only 95 CFMat 90 PSIG -- a reduction of 25%.

A single-stage rotary screw compressor consumes0.5% of its input power to produce each pound (PSI)of discharge pressure. A two-stage compressor willconsume 0.4% input power per pound per pressure.

To help reduce the wasted costs of unusedcompressed air, it is important to design a system thatlimits compressor operations to meet plant demand. In

addition, it is also important to reduce dischargepressure since it will decrease both power and airconsumption.

For our discussion, we will identify a "typical"manufacturing facility that we can use to illustrate thebenefits and limitations to various control systems.Our typical manufacturing facility will include:

Three air compressors (usually different sizes andpossibly different designs),


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Varying compressed air demands, andA poorly defined control hierarchy amongcompressors.

The most critical step required for any control schemeis an understanding of system demand.

The two key elements of system demand are pressure

and capacity. Any facility interested in improving itsproductivity and efficiency must have anunderstanding of the amount of air pressure andcapacity that is required by their air-operatedequipment.

Most air system audits reveal that plant airrequirements are typically lower in pressure thancurrent compressor discharge pressure. In addition,plant air capacity requirements vary significantly overthe course of a "typical" production day. A typicaldemand profile is illustrated in Figure A, with two shiftoperation and capacity requirements lower during thesecond shift.

Using this information, now we can look atcompressed air control scheme options. In order toremain brief, this article will group all of the variouscontrol options into four categories:

Category 1 -- No Control SchemeCategory 2 -- Local Control SchemeCategory 3 -- Central Control SchemeCategory 4 -- Global Control Scheme

Category 1 -- No Control Scheme

Over 80% of facilities have no true control scheme for

their compressed air systems. Each compressorsimply runs constantly at its initial pressure setting.This can result in compressors idling needlessly,sometimes for multiple hours each day.

Example: A single 100 hp air compressor idling onlythree hours per day, 300 days-per-year, with a powercost of 0.06 /kW hr. equals an electrical cost of1,400 per year.

Another result is that compressors may operate athigher than required discharge pressures.

E x a m p l e : The same 100 hp air compressoroperating at 125 PSIG may be capable of fullymeeting system demand operating only at 110 PSIG.This additional 15 PSIG pressure translates into apotential power cost of 3,200 per year. In addition,the same 15 PSIG increases air consumption by 11%due to increased air flow at the higher pressure.

Category 2 - Local Control Scheme

The simplest of control schemes, Category 2 isdefined when the individual controls of each

compressor are adjusted to operate in concert.Without a control scheme (Figure B) the pressures arenot complementary, nor do they support each other.

By adjusting the local controls (Figure C), a morelogical system is provided while at the same timeoverall system pressure is reduced. The addition ofautomatic stop/start controls to each unit allows those

machines that are idling needlessly to stop, increasingthe system efficiency further. This type of systemusually yields a 10-20% improvement in efficiency.

Note: Most compressed air system manufacturers dooffer some version of the stop/start control. Thesimplest version consists of a timer and a relay. Thetimer initiates as soon as the compressor unloads. Ifthe compressor continues to run unloaded until thetimer runs out, the relay is tripped, stopping thecompressor. Should plant pressure decrease to thelow pressure set point, the compressor automaticallycomes back on line.

Category 3 - Central Control Scheme

Category 3is the first option utilizing a true systemcontroller. When utilizing a local control system, eachcompressor is operating in concert, but independently.

A central control scheme replaces the local controls ofthe individual compressors with a master or "central"controller.

The first advantage provided by a central controlscheme is an overall reduction in operating pressure.Figure "D"illustrates the savings possible by replacingseveral pressure switches with a programmablecontroller and a single pressure transducer. All threecompressors are now controlled using a 2 PSIGdifferential. This provides substantial energy savingssince it reduces total pressure by 15 PSIG or more. Inaddition, this pressure signal can be locateddownstream of the air clean up equipment, furtherincreasing system efficiency. Due to the variablepressure differentials of in line filter elements, systempressure would automatically adjust to optimiseefficiency.

Second, with a 2 PSIG differential, a virtually steadysystem pressure can now be maintained. This canoffer increased production savings by reduction inscrap rate due to fluctuating pressure.

Category 4 - Global Control Scheme

In a Category 3 system, compressor control istypically centralized in the powerhouse. In a Category4, compressor control now becomes part of the overallplant programmable logic controller (PLC) system.

There is a considerable initial cost when installing aglobal control scheme. However, the level of controlprovided by this investment yields an even higherdegree of energy efficiency as well as a considerable


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reduction in operating costs. Some examples of thesesavings include:

Load sharing ability utilising "smart" system selectionof on line equipment based on demand characteristicsand equipment specifications (i.e., selection of a 200CFM unit over an 800 CFM unit to provide air during a150 CFM demand period).

Remote monitoring and notification of equipmentalarm and shutdown setpoints.

Automatic data trending and low level analysis.

Full integration with existing facility controls.

In conclusion, there is no doubt that for most facilitiesupgrading controls to a Category 4 will providesubstantial energy savings. However, the true

justification must be tested on the incremental savingsover lower level, less expensive options. For manyfacilities simply investing the time and effort in a

Category 2 upgrade may generate the majority ofenergy savings. Keep in mind, each facility is uniqueand should be evaluated based on its current situationand specific requirements.

Piping Rules Of Thumb

Use large enough carbon steel pipe so as not toexceed 3 PSIG pressure loss through the entire line.(The target is a maximum 10% pressure drop throughthe entire system, i.e. from air compressor to farthestdrop.)

Discharge pipe is to be the same size as aircompressor outlet.

Install a pipe tee in the discharge pipe to blow toatmosphere if necessary for control and adjustment.This will also serve as a convenient connection for arental compressor if required.

Install pressure gauges throughout the system fortroubleshooting. Locations should include the receiver,headers, tools, production equipment and the end ofplant piping system.

Use long radius elbows. Try to use flow resistant

fittings and valves. Use ball or butterfly valves.

Slope main lines approximately 100mm per metre ofpipe away from air compressor. Install drop legs forcondensate removal.

Locate headers and sub-headers near air uses andmanufacturing equipment. A loop system is ideal,providing two way flow distribution.

Slope piping so that condensate travels with the flowof air and away from the compressor.

Take all drop lines from the top of main pipe lines andlocate them near main points of air use. Do notconnect multiple air users to the same drop. Use onedrop for each air user.

Use carbon steel pipe as discharge pipe material.Never use PVC or ABS. Consider using Schedule40 black iron, galvanized, copper, stainless steel, oranodized aluminum.

Size the pipe for maximum CFM required. This willequal full load production plus future expansion plans.

Install an air receiver at intermittent high demandpoints such as occasional sandblasting, air motors,etc.

Air receiver size should be one gallon of storage per 1CFM of air compressor output as a minimum in order

to permit the compressor controls to operatecorrectly..

Always consider leakage and future expansion inorder to eliminate compressed air systemobsolescence. A 10% per year growth rate iscommon.

Be sure to read and understand equipmentinstruction and installation manuals and discuss thelayout and piping requirements before installation.

The How Tos of Compressed Air

Compressor sizing, Air treatment sizing, Receiversizing,Distribution piping,Point of use components

Compressor sizing The easiest, least costly andmost common method for sizing a compressor is todetermine existing peak demand, add 20-30% forgrowth and add one compressor that matches theresulting cfm calculation. Typically, peak demandperiods are of shorter duration than say, second, thirdor weekend shifts. Having one compressor sized forpeak demand will mean it is operating inefficiently anaverage of 85% of the time. Installing multiplecompressors to match this peak while incrementallymatching other lower compressed air demand periodswill pay for the additional procurement costs in energysavings.

Air treatment sizing Central heat exchangers,filters and dryers must be sized for compressordisplacement at extreme ambient conditions. Mostcompressors are capable of running in a high ambientbut little consideration is given to the air treatmentcomponents in this regard. If a 1,000 Acfm air cooledcompressor operates in a 110 F ambient, the effect of


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ambient pressure, temperature and relative humidityrates the compressor at 900 Scfm with an inlettemperature to an air dryer of at least 120 F. Thismeans the dryer must be capable of handling nearly1,600 Scfm or dewpoint will not be maintained.

The other end of the scale must be considered aswell. If the temperature is very low, ambient air

contains very little moisture for removal. The air dryermust not be so large that it freezes up from lack ofheat loading.

Receiver sizing One gallon per rated cubic foot?

Never! This rule of thumb, though popular and easy toremember, does not take system events into account.Receivers are sized to accommodate largeintermittent system air demand, a stand-bycompressor start, local high rate of flow requirementsand insulation of critical pressure users. The object ofeach case is: to prevent a compressor start, maintainpressure in the system while a compressor starts,servicing demand from storage separate from themain system, and to operate the system at lower

pressures, respectively.

In the central compressor configuration, avoid placingany receiver capacity upstream of dryers or filters.Receiver capacity should always be installeddownstream of air treatment to avoid surges acrossthis equipment that might result in carry-over to thesystem.

Distribution piping A good rule of thumb that iscommonly used is to limit pressure drop to less than 1psiG per 100 linear feet. This applies to the rate offlow through any particular section of piping, and haslittle to do with total compressor capacity. Certain

point of use applications may take compressed air at arate of flow greater than the capacity of availablecompressors for a short duration. If the piping is sizedper the capacity of the available compressor(s) ratherthan the rate of flow, it might represent a restrictionthat causes pressure to drop system-wide.

Point of use components Rate of flow

considerations at the point of use is much moreimportant than in sizing distribution piping. When endusers complain of low pressure the first thing blamedis the piping because the user is at the other end ofthe system or the piping system has been expandedhaphazardly over the years (or whatever the excuse),

the real problem is almost never the piping. The realsource of problems described as low pressureusually resides in the choice of installed point of usecomponents.

Pipe drops, filters, regulators, lubricators, quickdisconnects, and hose must all be sized for the rate offlow at the point of use. A common mistake is to buy atool that uses 100 Scfm, apply a 10% utilization factorto it and size all of the in-line components for 10 Scfm.The components need to be sized for the 100 Scfm

rate of flow, not some averaged demand level used tosize compressors!

commpressed air tips

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