energy efficiency best practice guide to compressed air ...energy efficiency best practice guide...

Energy Efficiency Best Practice GuideCompressed Air Systems

Table of Contents i

1 Introduction 4

2 What are the business benefits of compressed air efficiency? 5

3 What is your opportunity? 6

4 Solution 1 – Improve the efficiency of your existing system 7 4.1 Step 1: Review air demand 7 4.2 Step 2: Reduce leakage 104.3 Step 3: Fix pressure drop 11 4.4 Step 4: Review air receivers 12 4.5 Step 5: Maintain separators, filters, dryers and valves 13 4.6 Step 6: Select a compressor 13 4.7 Step 7: Measure the improvement 17

5 Solution 2 – Design a new system 18 5.1 Establish air demand (quantity and quality) 18 5.2 Design your piping and fittings 185.3 Select and locate air receivers 18 5.4 Select dryers, separators and filters 19 5.5 Locate inlet and discharge air outlet 205.6 Select a compressor and control system 20

6 Selecting a service provider 21 6.1 Questions to ask service providers 21 6.2 Database of compressed air service providers 21

Appendix A Compressed air system overview 22

Appendix B Methods for estimating compressor energy consumption9 23

Appendix C Measuring leaks 24

Appendix D Cost savings from the installation of a DDS system 25

Appendix E Glossary 26

Appendix F Further reading/references 27

Contents

Table of Contents 3

Figure 1: Compressor costs over a ten-year lifecycle 5 Figure 2: Compressed air usage and potential savings for the typical compressed air user 5 Figure 3: Compressed air usage and potential savings for the typical compressed air user 9 Figure 4: Single main pipe layout with branch lines 11 Figure 5: Example ring main with take-off points 11 Figure 6: Power losses in various diameter pipes 12Figure 7: Graph of a load profile suitable for a fixed speed and variable compressor setup 15 Figure 8: Typical power curve of a compressor using a load/unload control system 15 Figure 9: Typical performance of rotary oil lubricated compressors with different control systems 16 Figure 10: Typical compressor performance graph 23

List of Figures

Table 1: Compressed air use and substitution 7 Table 2: Air quality classifications 8 Table 3: Air leakage, wasted energy and cost for equivalent hole diameter 10 Table 4: Advantages and disadvantages of air compressor types 14 Table 5: Annual energy and cost savings with reduced compressor inlet temperature 20 Table 6: Cost savings from the installation of a DDS system. 25

List of Tables

Introduction 4

This document is a step-by-step guide to improvingenergy efficiency in compressed air systems andachieving best practice. By following this guide, you willbe able to determine what changes can be made inorder to reduce operating costs, improve the operationand performance of equipment and improveenvironmental outcomes.

The guide has been developed to lead decision makersand service providers through system changes; it is notintended to be a thorough technical guide. Referencesfor more detailed technical information are provided.

1 Introduction

What are the business benefits of compressed air efficiency? 5

Despite the fact that they are essential for manybusinesses, compressed air systems are often ignoreduntil something goes wrong with them, or thecompressors fail to keep up with rising air demand.Compressed air systems use up to 10% of totalindustrial electricity use in Australia, so it makes senseto look at their energy cost. Figure 1 illustrates the costof a typical compressor system over a 10-year lifecycle,showing how important energy costs are to the overallcost of a system.

Figure 1: Compressor costs over a ten-year lifecycle1

With 73% of the cost of a compressor due to energyuse, significant cost savings will be made by improvingenergy efficiency, as well as the added benefits ofimproving the performance of your system and reducingyour organisation’s ‘carbon footprint’.

Energy efficiency will also increase the proportion ofcompressed air that is used for production andminimise unnecessary wastage, again resulting insignificant cost savings. An example of just how muchdemand on the compressed air system can be wastedis shown in Figure 2 below.

Figure 2: Compressed Air Usage and Potential Savingsfor the Typical Compressed Air User.2

2 What are the business benefits of compressed air efficiency?

Energy cost73%

Capital cost18%

Maintenancecost 7%

Installationcost 2%

Productiondemand 73%

Leaks 19%

Artificialdemand 16%

Artificialdemand 6%

Systeminefficiencies 8%

What is your opportunity? 6

Delivering the best outcome for your business requiresa whole systems approach to the design, installation,operation and maintenance of your compressed airsystem.(Refer to Appendix A for a compressed air systemoverview).

Defining the limitations of your current compressed airsystem is the key to finding the best solution toachieving energy efficiency for your business:

• Can I make my system more efficient?• Do I need a new compressor?• How do I expand my existing system?• What do I need to know to install a new system?

This guide offers step-by-step solutions to help youidentify opportunities to implement best practice toachieve energy efficiency of your compressed airsystem.

• Solution 1: Improve the efficiency of yourexisting systemDo you have a compressed air system that is fulfillingneeds but could run more efficiently? This processmay only involve a small investment but can providesignificant energy savings and costs. Is your existingsystem struggling with the demand? Do you need toupgrade your air compressor? This process will alsohelp in selecting the appropriate compressor for yourneeds, which could well be smaller and cost less thanyou originally thought.

• Solution 2: Design a new systemAre you planning a brand new compressed airsystem? This process outlines the steps required toensure you achieve excellent design and to help youunderstand where to spend your valuable capital.

Are you expanding your plant, workshop or factoryand need to ensure that your compressed air systemwill work effectively? This will involve elements of bothsolutions. Firstly, ensure your existing system isrunning efficiently (Solution 1) and secondly, if yoursystem needs to be expanded, design the newcomponents (Solution 3). Following this process willensure that you are not wasting money purchasingmore than you actually need. Additionally, informationgained from reviewing efficiency may guide theselection and design of the new components of thesystem.

3 What is your opportunity?

Solution 1 – Improve the efficiency of your existing system 7

This solution firstly focuses on improving the efficiencyof usage, distribution, storage and treatment ofcompressed air before considering the compressoritself.

4.1 Step 1: Review air demand

Before any improvements to your compressed airsystem can be made, you should first determine how itwill be used and which aspects need improving byreviewing the air demand on the system.

4.1.1 Inappropriate usesCompressed air is often used for jobs because of itsavailability and because an alternative would requirehigher capital cost. Compressed air is a very expensiveform of energy and the associated running costs oftenmean the overall cost is more expensive thanalternatives. Consider using alternative equipment withlower running costs. However, there are often very goodreasons for using compressed air powered equipment.Carefully consider the advantages and disadvantageswhen making these substitutions, such as the weight oftools or the safety risks of electric tools. Table 1illustrates some examples of inappropriate uses ofcompressed air, along with some solutions.

Table 1: Compressed Air Use and Substitution3

Compressed Equipment SolutionsAir Use Used

Blowing Nozzle/gun Air knife, Inductionor Cleaning nozzle, low

pressure blower,broom/brush

Cooling Cooling Air conditioning induction systems,system chilled water, fresh

air ventilation, fans

Drying of water Nozzle/gun Solenoid control, on product air knife, induction

nozzle

4.1.2 Current and future usesCompile a list of (i) all the equipment that currently usescompressed air and (ii) equipment that is planned to beinstalled that will use compressed air. Include the totalnumber of all tools or equipment of the same type.Determine the requirements for each pieceof equipment:

• Maximum air pressure (in kPa)Identify the highest maximum pressure required byyour system. This is the pressure that should beavailable as your supply pressure. Any less and yourequipment may not function properly; any more andthe system is running at a higher pressure thanrequired and costing you money.

• Average flow (in L/s)Add the average flows of all equipment on yoursystem. This value indicates the average flow requiredof your air compressor. Although the actual demandfluctuates above and below this value, any rapidincreases in this demand can be met by the storedcapacity in the air receivers. Therefore, this is the flowthat your compressor should be capable of producingcontinuously.

• Air quality (pressure dewpoint for moisture,concentration limits for dirt and oil)Depending on your business, you may have allequipment needing the same air quality or have arange of air-quality requirements. A simpleclassification of air-quality levels is provided in Table 2.

4 Solution 1 – Improve the efficiency of your existing system

Table 2: Air Quality Classifications4

Air Quality Applications

Plant Air Air tools, general plant air.

Instrument Air Laboratories, paint spraying, powdercoating, climate control

Process Air Food and pharmaceutical processair, electronics

Breathing Air Hospital air systems, diving tank refillstations, respirators for cleaningand/or grit blasting

If most of your equipment uses low-quality air while youhave a few pieces of equipment that require high-qualityair, such as breathing air, then consider moving thatequipment on to a point of use system with a muchsmaller compressor and dryer. This will save on energycosts, as you will be using less energy in treating the airfor your entire system.

4.1.3 How much energy is your compressor using?Estimating how much energy your compressor is usingis relatively easy. Two methods are described inAppendix B.

4.1.4 System load profileA system load profile shows the demand on thecompressed air system over time (load is another termfor demand). It is important to measure and analyse thesystem load profile if you are to effectively improve theperformance and efficiency of any compressed airsystem.

Measuring the profileTo obtain the load profile, the airflow from thecompressor must be measured at various points over aperiod of time. It is also possible to measure the systempressure and the power drawn by the compressor anddryer at the same points in time in order to see how theflow, pressure and power consumed by the systemchanges over time. This profile should be obtained overa typical production cycle so that demand on thecompressed air system can be seen at all stagesof the production cycle. For example, a seven-dayperiod that is not part of a holiday season might be atypical production cycle.

Ideally, the load profile should be measured by usingflow meters on the main compressed air branch linesand the flow recorded at regular intervals by a datalogger. Electronic pressure metering on the main systemlines and power metering for the compressor and dryerwill enable you to look at all aspects of your system’sperformance and better diagnose any problems.However, setting up this extent of metering is a time-consuming process that requires technical knowhow. Ifyou do not have these resources, you could use acompressed air service provider to assist by conductingan air audit (refer to Section 6 Selecting a serviceprovider) or you could use a less vigorous method ofdetermining the load profile.

There are a number of ways to determine how yoursystem is using compressed air. Power metering for thecompressor will indicate the demand over time. Thecontrol system for your compressor may be start/stop,load/unload or it might run on a variable speed drive. Inall cases, by looking at the power usage over time andidentifying when the compressor loads and unloads,you can get a rough idea of when the peak demandtimes are.

Additionally, if you can obtain a pressure-flow curve foryour compressor (these are usually available from themanufacturer) and your compressor has an outletpressure gauge, you can determine the flow. Recordingthe pressure at various times will enable you to developa plot of flow over time, just as flow metering wouldhave done. You can also gain an understanding ofdemand by looking at the demand side rather than thesupply side. If the demand cycles of the equipment thataccount for the largest use of compressed air arerelatively short, it might even be possible to observethem yourself, noting how long and at what times theyare using compressed air. In this way, you can build arough plot of the largest uses of compressedair in the load profile.

Analysing the profileOnce you have obtained some information for a loadprofile, you can identify how your compressed airsystem is being used. Figure 3 illustrates a typical airdemand profile, showing the air demand in m3/hr(normal temperature and pressure) over time. The redlines indicate that, while the peak demand is 980 m3/hr,the average demand is only 190 m3/hr. The fact that theaverage demand is only a small fraction of the peakdemand is a sign that the compressor is oversized, notrunning at its highest efficiency point and most likelywasting energy. A smaller compressor could be usedwith a larger air receiver and the peak demand couldstill be supplied to the system. For added benefit, try toimprove the demand spikes as well.

With a profile such as this, you can identify whichequipment is responsible for various features of theprofile. You can also determine the maximum demandfor each piece of equipment and how this compares tothe maximum total load as a percentage. This is aneffective method of ranking your equipment in terms oftheir compressed air consumption, or significance tothe compressor system.

You may also be able to obtain a graph of the powerconsumption of your compressor and/or dryer over thesame period of time as the load profile above. Bycombining these two graphs, you could see how theefficiency of the system changes over time.

A key performance indicator that can be used tomonitor the ongoing efficiency of the system is kW perL/s. The lower the kW/L/s, the more efficient the systemis. This profile is valuable in identifying the times atwhich your compressor may be running at part loadand so increasing your costs per unit of air demand.Taking an average value of this graph will give you youraverage efficiency in kW/L/s.

Figure 3: Compressed Air Usage and Potential Savingsfor the Typical Compressed Air User. 2

Optimising the profileThe compressed air system may be more effective if theprofile is flattened or, in other words, if the largeswitching loads can be operated at different times sothat the demand is more stable. If, for example, apurging function can be run while a major line or pieceof plant is not running, the load profile will be moreconstant.

In some cases, it may be more efficient to place certainloads on a separate system. For example, if there is oneload that only operates after hours while all othersoperate during the day, it could be placed on a separate(point of use) compressor so that the main compressorsystem can be shut down and not have to runinefficiently at part-load. Although there are now twocompressors, the overall efficiency of your two systemswill be greatly improved.

Solution 1 Improve the efficiency of your existing system 10

4.2 Step 2: Reduce leakage

Leaks can waste up to 50% of the compressed airproduced by your compressor. Reducing leakage is akey measure that can be used to improve energyefficiency. Table 3 provides an indication of the cost ofleaks.

4.2.1 Measuring leakageIf there is a flow meter installed immediately after yourair compressor or receiver, measuring leakage is assimple as reading this flow meter when all compressedair equipment is turned off and all outlet valves areclosed. Similarly, if you obtained data for the load profileas outlined in the previous step, then you simply needto ensure that you record data for the system load whileall equipment uses of compressed air are turned off.If flow metering is not available to you, there are twomethods that may be used to determine systemleakage. These methods are detailed in Appendix C.

4.2.2 Finding leaksLeaks can occur in any number of places, such as:• hoses and couplings• pipes and pipe joints• pressure regulators• valves left open• equipment left running or not isolated• threaded fittings not properly sealed with thread

sealant or dirty.

Apart from listening for leaks, which can be deceptivelyunreliable in a noisy environment, there are two keyways to find compressed air leaks. The simplest is tobrush soapy water over areas suspected of leaking andlook for bubbling. Although cheap and simple, this canbe a very time-consuming process. The second isultrasonic leak detection. Ultrasonic detectors canpinpoint leaks very accurately and quickly by detectingthe signature ultrasound signals of high-pressure leaks.They can operate in noisy plant environments, soequipment does not have to be turned off. While sometraining is required in their use, operators can becomecompetent after less than an hour.

Some compressed air service providers will be able toperform ultrasonic leak detection programs for you ifyou do not have the resources yourself. Once leaks arefound, it is very important that you keep track of themeffectively. The best way is to ‘tag’ the leaks with brightcolours as soon as you find them. Use a site map torecord the location of the leak. Give each leak a gradingfor the amount of air loss through it, perhaps betweenone and ten. Record this grading on the tag itself andon the site map. The leaks with the highest prioritygrading should be fixed first.

One Australian company successfully implementeda leak management program with a compressed airparts supplier.

The supplier attended site on a monthly basis duringregular shut downs, reviewing and repairing simpleleaks on the spot. Larger leaks were tagged andrepaired at a later date.

Table 3: Air Leakage, Wasted Energy and Cost for Hole Diameter5

Equivalent Hole Quantity of air Annual energy Annual costDiameter (mm) lost in leaks (L/s) waste (kWh) of leaks ($)

0.4 0.2 133 13

0.8 0.8 532 53

1.6 3.2 2128 213

3.2 12.8 8512 851

6.4 51.2 34040 3404

12.7 204.8 136192 13619

4.2.3 Fixing leaksFixing leaks often involves tightening or replacingconnections, fixing holes in pipes or repairing damagedequipment such as pressure regulators. Often, simplycleaning and applying thread sealant to fittings will help.Replacing equipment will be necessary in somesituations.

4.2.4 Leak management programOnce leaks are regularly being repaired, theimplementation of practices to avoid leaks getting out ofcontrol will ensure that the compressed air systemremains efficient. These practices can include:• regular inspection and maintenance of compressedair equipment• regular inspection of air pipes, bends and valves• ensuring that all air lines are properly supported so asnot to cause leaks through excess stress• removing or properly isolating any unused parts of thepipe distribution network or unused pressure regulators• implementing a leak reporting program among staff.Making staff aware of the cost to the business fromleaks and encouraging them to actively report leaks isan important step, as they are always present on theshop floor and are best placed to notice any changes.

4.3 Step 3: Fix pressure drop

Pressure drop is another major cause of inefficiency incompressed air systems. Pressure drop is the decreasein pressure between the compressor and the point ofuse of the system. It occurs due to the loss of energyfrom the compressed air as it flows through the system.A certain amount of loss is inevitable from somecomponents such as filters, dryers and separators.However, there are ways to ensure a minimal pressuredrop is obtained. A compressed air system that isperforming well should have a pressure drop of lessthan 10% between the compressor outlet and all pointsof use.

4.3.1 Measuring pressure dropUsing a calibrated pressure gauge, measure thepressure at the compressor outlet and at each pressureregulator, with the regulators set to maximum pressure.Determine the lowest pressure recorded aftermeasuring at all the regulators.

Pressure drop = compressor discharge – lowestavailable pressure at regulators

The larger your pressure drop, the more inefficient yoursystem is, as the compressor must work harder toobtain a higher pressure while only a lower pressure isbeing utilised.

4.3.2 ValvesThe two common types of valves that are used incompressed air systems are ball valves and gate valves.Both types have their place depending on theirapplication. Ball valves are useful for isolating flowand have the lowest pressure drop of the two, whilegate valves are useful for controlling flow.

4.3.3 Piping layoutThere are two common air distribution layouts – a singlemain system as illustrated in Figure 4 and a ring systemas illustrated in Figure 5. A ring main setup isconsidered best practice, however, a single main setupmay also be useful in many applications. Try to restrictyour compressed air system to these layouts to ensurethat they maintain efficiency. The more bends you have inyour piping network, the higher the pressure drop will be.

Figure 4: Single Main Pipe layout with Branch Lines.6

Figure 5: Example ring main with take-off points.7

Compressedair source

Additional take off points

Additional take off points Main pointof use

Compressorhouse

Processring main

Workshopring main

Paint shopring main

4.3.4 Pipe diameterThe diameter of the piping in the distribution networkhas a large impact on the pressure drop of the system.The smaller the pipe, the more energy is lost by thesystem. Figure 6 illustrates how one such power lossfor a given diameter pipe may look.

While the pipe diameter may have been large enoughfor the capacity required at the time of installation, ifdemand has since increased, the pipe diameter maynow be too small for the system’s requirements.

4.3.5 Pressure settingOnce you have ensured that you have minimisedpressure drop, repeat the measurement of the pressuredrop in your system. Using the information collectedabout the pressure requirements of the equipment inStep 1, you can determine the required pressure.

Pressure required at compressor = maximum pressurerequired by equipment + minimised pressure drop

4.4 Step 4: Review air receivers

Receivers store compressed air in order to meet rapidincreases in demand. In this way, air compressors canbe turned off for certain periods or run at a lower load.Compressed air systems often make use of a main airreceiver to provide this storage. Although the size of thereceiver may have been large enough when it wasinstalled, equipment additions may now mean that thereis not enough storage to supply each air demand event,putting greater demands on the compressor.

The typical compressor control system will measure thesystem pressure and, once it reaches an upper limit, willdisengage or stop the compressor. Any demand forcompressed air is then provided solely by the receiver.

Once the pressure drops below a set lower limit, thecontrol system will start the compressor in order torepressurize the system.

If your receiver is too small for your air demand needs,the compressor will be running longer than it needs to.By improving the ability of your storage to meetdemand, use of the compressor is minimised, reducingenergy usage and wear and tear. In an existing system,there are two things you can do to improve air storage:

• Install a larger main receiver for the entire system.• Install secondary receivers near equipment that will

rapidly increase its demand for compressed air.

If you have added a large number of smaller tools toyour system over the years, upgrading the size of yourmain receiver may be most beneficial. If, instead, youhave added equipment that has high, intermittentdemand, it may be more beneficial to install secondaryreceivers for that equipment.

Pipe bore (mm)

45 50 55 60 65 70 75 80

4.5 Step 5: Maintain separators,filters, dryers and valves

Nearly all compressed air systems have some level ofair treatment to remove oil, water vapour and particulatematter from the compressed air stream.

A compressed air system will usually contain:• A separator to extract lubricant oil from the air stream.

A separator is often part of a packaged compressor.• A dryer to condense and extract water vapour from

the air.• A filter to remove contaminants. Often, there is a filter

fitted to the inlet of the air compressor.

The level of quality required by your system dependsupon the application. For example, food-processingapplications will require a very high air quality, while amechanics workshop will require a lower standard.Treating your compressed air to a higher quality than isrequired wastes energy and may also result in highermaintenance costs.

4.5.1 FiltersRegularly check and replace filters. Blocked filters willcause a significant pressure drop and so waste energy.Gauges can be installed that measure the pressuredrop across the filter and indicate when a new filter isrequired.

4.5.2 DryersDryers are often an essential part of the system. Thereare three main types of dryers: refrigerant, membraneand desiccant. Fitting refrigerant and desiccant dryerswith control systems makes it possible for the dryer tomatch its energy use to the air demand at that time,therefore reducing energy usage. Whilst desiccantsystems use no direct electricity, unlike refrigerantsystems, they use compressed air to purge thedesiccant, thus still using energy to operate. This needsto be included in any comparison of energy costsbetween dryer systems. Dessicant dryers will oftenpurge a set volume of air on a regular frequency,despite the actual flow through the dryer at that time. Itis possible to retrofit dessicant dryers with dependentdewpoint switching (DDS) controls.

This system will measure the dewpoint within the dryerand only purge when it is required, therefore reducingthe amount of wasted compressed air. For a typicalsystem using a 55kW screw compressor and adessicant dryer, the installation of a DDS system couldsave up to 60% of annual operating costs of the dryer.Refer to Appendix D for a breakdown of this example.

4.5.3 Drain valvesReplace manual, disc and timed drains with newelectronic level sensing drains to reduce the amount ofwaste. Drain valves are fitted on equipment in whichwater condenses, such as after-coolers, receivers,dryers and filters. There are various types, but they allrelease condensed water from the equipment. Thelonger these drains remain open, even partly open, themore compressed air is lost from the system. Electroniclevel sensing drains help to optimise the time that drainsare open for.

4.6 Step 6: Select a compressor

Optimising and upgrading your compressor should bethe last consideration when looking at improving theefficiency of your existing compressed air system. Makesure that you have followed Steps 1–5 first to maximiseusage, distribution, storage and treatment ofcompressed air.

4.6.1 Compressor typesReciprocating compressorsReciprocating compressors work through the action ofa piston in a cylinder. Pressure can be developed onone or both sides of the piston. They are usually themost expensive to buy, install and maintain, and requirelarge foundations due to their size and the vibrationsthey cause. Despite these disadvantages, reciprocatingcompressors have their uses. They are good for high-pressure applications (13 bar pressure and above) andfor high air quality applications. They are also the mostefficient for quite small applications (in the 1–4 kW range).

Screw compressorsScrew, or rotary, compressors use two meshing helicalscrews, rotating in opposite directions at high speed, tocompress air. These compressors are usually the lowestcost to purchase and install. They lose efficiency rapidlyat part load unless variable output compressorsare used.

Vane compressorsVane compressors have a rotor with steel sliding vaneswithin an eccentric housing. The vanes form pockets ofair that are compressed as the rotor turns until anexhaust port is exposed. Vane compressors havesimilar energy efficiency to screw compressors, butoften have better air quality.

Centrifugal compressorsCentrifugal compressors use high-speed rotatingimpellors to accelerate air. To reach operatingpressures, several impellor stages are required. Theyhave relatively low installation costs, but are expensiveto buy because they are precision machines, however,they are generally economical in large sizes, in the 200kW and above range. They are efficient down to around60% of their design output, below which they have littleturndown in their energy consumption.

Table 4: Advantages and Disadvantages of Air Compressor Types2

Compressor Advantages Disadvantages

Reciprocating Suitable for high pressures High noise levelsEfficiency: Can be relatively small size High maintenance costs7.8 – 8.5 and weight Suitable for smaller systemskW/m3/min Smaller initial cost Requires strong foundation

Simple maintenance High oil carry over when wornproceduresEfficient multi-stagecompression available

Screw Simple operation High energy useEfficiency: Lower temperatures Low air quality6.4 -7.8 Low maintenancekw/m3/min Quiet

CompacVibration freeCommercially availablevariable speed units with relatively good turndown.

Vane Simple operation Limited range of capacity.Lower temperatures Low air qualityQuietLow maintenance

Centrifugal Energy Efficient High initial costEfficiency: Large Capacity Inefficient at low capacity5.8 – 7 Quiet Specialised maintenancekW/m3/min High air quality Only water-cooled models

available

4.6.2 Using multiple compressors

Depending on your load profile, the addition of another(differently sized) compressor may deliver a moreefficient outcome than replacement. For example, ifyour load profile contains a certain level of constantdemand with a small amount of fluctuating demand andyou currently have one, fixed-speed screw compressor,you could add a variable screw or vane compressor toprovide for the fluctuating demand while the fixed speedscrew compressor provides for the base load.

Figure 7: Graph of a Load Profile suitable for a fixedspeed and variable compressor setup.

4.6.3 Compressor control systemsThere are different types of control systems available forcompressors. Your current system may not be using themost efficient method, or may have no control systemat all.

Start/StopThis method of control will start the compressor oncethe pressure drops below a certain level and stop itonce the desired pressure has been reached. Thismethod should not be used when there is a frequentincrease in demand, as continually starting andstopping the compressor can be detrimental to the lifeof the compressor (increasing maintenance costs).

While energy is saved during the periods that thecompressor is not running, the compressor mustcharge the receiver to a higher pressure than is requiredby the end uses so that a minimum pressure ismaintained until the compressor starts again.

Load/UnloadUsing this method, the compressor will charge thesystem to the desired pressure, then the compressormotor will continue to run at constant speed while thecompressor action is disengaged. While this means thatthe compressor is using less power for a large amountof the time, the motor can use between 15 and 35% offull load power, meaning this scheme can also beinefficient. Figure 8 below shows a typical powerconsumption profile for a compressor using aload/unload control system. You can see that thecompressor still uses roughly 30% of peak power whileunloaded. During the time immediately after thecompressor enters the unloaded stage, the oilseparator performs a blow down to clean outaccumulated oil. This leads to the compressor runningat part load for some time, meaning that quite asignificant amount of energy can still be used duringunloaded operation.

Figure 8: Typical power curve of a compressor using a load/unload control system2

Base load

Fluctuating load

Time (seconds)Oil/Air separator

pressure blowing down

30 60 90 120 150 180

UnloadedLoaded

ThrottlingThrottling control varies the degree to which the inletvalve is open, controlling the amount of air intake to thecompressor. This method is only effective with screw orvane compressors that run at 70% load or above.

Centrifugal compressor control Centrifugal compressor control systems are usuallymore complex and involve throttling and recirculating airload, as well as venting of air to the outsideenvironment.

Variable-speed screw compressorsVariable-speed screw compressors control the motorspeed whilst keeping the slide valve to the compressorfully open to meet fluctuating air demand. Whencombined with a base load compressor providingconstant air supply at optimum efficiency, a variable-speed and fixed-speed combination can be the mostefficient for real-world, fluctuating, air-demand patterns.

Figure 9: Typical performance of rotary oil lubricatedcompressors with different control systems.2

Compressor power consumption with flowThe power drawn by compressors changes significantlywith changes in airflow. Figure 9 shows the power-flowcurves for a number of compressor control systems.This graph shows how the different control systems canaffect the compressor’s efficiency at low flow levels. Thecurve shows the amount of power consumed (relativeto maximum power) as the flow capacity through thecompressor changes. The steeper the compressorcurve, the more efficiently it performs at part loads.

Performance comparisonrotary oil lubricated air compressors

Total kW Input -vs- Capacity

100Variable inlet controlledrotary oil injected air compressors with modulation blowdown at 40 and 60%

Variable outlet controlledrotary oil injected air compressors

Load/Unload controlledrotary oil injected air compressors

Variable speed drivenrotary oil injected aircompressors

Flow capacity (%)

4.6.4 Compressor maintenanceEnsuring regular maintenance is carried out on yourcompressor will also reduce energy costs. Ensure theinlet filter is not blocked and keep coolers clean andproperly lubricated.

4.6.5 Variable speed drivesVariable speed drives can be fitted to the motors ofscrew and vane compressors, and most suppliers arenow offering factory-fitted, variable-speed units. Theseallow the motor to be run at the rate required in order tofulfil demand at that time. This method can saveconsiderable amounts of energy. However, variable-speed drives should only be fitted in cases where airdemand varies. Using a variable-speed drive for acompressor that runs at full load constantly willconsume more energy. Some companies have electedto install one variable speed unit, with two or more baseload units to save on capital cost.

4.6.6 Multi-stage compressorsMultiple stages can be used to improve the efficiency ofscrew and centrifugal compressors. Additional coolingcan be placed between the stages to further increasethe efficiency of the second stage.

4.6.7 Compressor operating conditionsOnce you have selected the type of compressor systemthat you wish to install, you should ensure that the plantservices the new compressor will require are availableand that operating conditions are suitable.

4.6.8 LocationWhen installing a new air compressor, the location ofinstallation is important. There should be sufficient roomfor the equipment as well as access for maintenance.The compressor should also be as close as possible tothe plant. If the compressor is to be located outside orin a compressor house that is detached from the mainbuilding, it is best to place the compressor house in themost shaded area. This is usually on the south side ofthe main building.

4.6.9 Inlet air temperatureLower air inlet temperatures allow your compressor tooperate more efficiently. Section 5.5 (in Solution 2)illustrates how important this is and what sort of energysavings to expect. In essence, compressors producelarge amounts of heat. If this is not exhausted to theatmosphere via cooling towers (water-cooling) or

through exhaust stacks, you are wasting energy.When selecting a new compressor, it is important tolocate the air inlet to use the lowest temperature airpractically available and to discharge the waste heat tothe atmosphere if possible.

4.6.10 PowerThere should be suitable access to electrical power forthe compressor. The distribution board from which thecompressor is run should be well within its rated powerlimits and the cables supplying the compressor shouldalso be rated for the current required by thecompressor.

4.6.11 WaterAccess to suitable water should be available for coolingthe compressor if required. This water should be ascool as possible, so a supply from pipes that areexposed to the sun is not recommended.

4.6.12 BlowdownAir compressors often have blowdown mechanisms,which will blow any condensed water or filtered oil fromfilters or condensate traps. This waste should beconsidered as contaminated, so there should be accessat the compressor for disposal of this waste.

4.7 Step 7: Measure theimprovement

By following these steps, you have done almosteverything possible to improve the energy efficiency ofyour compressed air system. Repeat the measurementsyou made when determining the current old usage.By comparing the result, you can determine thereduction in energy used to run the system (kW/L/s)and hence the reduction in energy costs of the system.

Design a new system 18

5.1 Establish air demand(quantity and quality)

Using the techniques described in Section 4.1 (inSolution 1), air demand quantity and quality can beestimated for your new or extended facility. The keyprinciples that should be adhered to are:

• Design ‘out’ inappropriate uses of compressed air.Take the opportunity to consider substitution ofcompressed air for other technologies (for example,electric drives instead of compressed air cylinders,low-pressure blowers for water removal).

• Separate services using differing air quality.Establishing air-quality requirements is critical, as itcan dictate the types of compressor and filtrationused. Higher quality air is more expensive to produceand you should consider multiple compressors withseparate duties.

• Minimise compressed air demand spikes.By considering how equipment operates together,you can minimise pressure spikes, which in turnaffects how efficiently your compressors can operate.Bear in mind that you will also be using receivers,which can also help to minimise pressure spikes.

• Separately itemise the equipment that uses high pressure.If your high-pressure requirements are a relativelysmall proportion of your overall air demand, it may bepossible to supply them via a separate compressor.

• Minimise pressure requirements for your system.A compressor’s efficiency is linked to its pressuresetting. Less energy is required to compress air to alower pressure, so once you have itemised the air-quality requirements of your equipment, consider howyou can minimise that even further through equipmentselection and substitution.

5.2 Design your piping and fittings

Pipes and fittings also affect the pressure set point ofyour compressor. A compressed air system that isperforming well should have a pressure drop of lessthan 10% between the compressor outlet and all pointsof use. There are ways to ensure a minimal pressuredrop is obtained, including:

• Optimise pipe diameter, length and the number ofbends. Pressure drop is a function of a number ofthings – importantly: the velocity of the air down thepipe (which is dictated by your air demand combinedwith pipe diameter); the distance of the pipingbetween the compressor and the end uses; and thenumber and type of bends. By paying attention tothese things, total pressure drop can be minimised.

• Select valves and fittings with low pressure drop.Section 4.3.3 and Section 4.3.4 in Solution 1 havemore information on piping layout and pipe diameter.

5.3 Select and locate air receivers

Receivers have a key role in maximising the efficiency ofyour compressor. They achieve this by storingcompressed air to meet rapid increases in demand,which reduces the impact of air demand surges on thecompressor. This allows compressors to run at theiroptimum load position and minimises the number ofload/unload cycles (if using fixed-speed compressors).Often, a single main receiver is used, but secondaryreceivers should be considered if specific equipment orwork areas can be identified that induce intermittent,high, air demand.

5 Solution 2 – Design a new system

Design a new system 19

5.4 Select dryers, separators and filters

Refrigerant dryers add electrical consumption, pressuredrop dessicant dryers waste compressed air throughpurging and increasing pressure and separators andincreasing pressure and separators and filters alsointroduce pressure drop to your system. Carefulselection can minimise the negative impact on energyefficiency. Section 4.5 (in Solution 1) explores details ofwhy and how dryers, separators and filters are used,with the key points being:

• Treating your compressed air to a higher quality thanis required wastes energy and may also result inhigher maintenance costs.

• Although desiccant dryers do not use energy to run,they require compressed air for purging. These energycosts should be included in any comparison betweendessicant and refrigerative dryer systems.

• Regularly checking and replacing filters will minimisepressure drop across them (these can be monitoredto optimise the energy costs with filter replacementcosts).

• Electronic level sensing drains help to optimise thetime that drains are open (drain valves are fitted onequipment in which water condenses, such asaftercoolers, receivers, dryers and filters).

Solution 2 Design a new system 20

Table 5: Annual Energy and Cost Savings with Reduced Compressor Inlet Temperature8

Air Intaketemperaturereduction 3°C 6°C 10°C 20°CComp- kWh/yr $/yr kWh/yr $/yr kWh/yr $/yr kWh/yr $/yrarative savings savings savings savings savings savings savings savingsaverage load (kW)

4 80 8 160 16 264 26 528 53

7.5 150 15 300 30 495 50 990 99

11 220 22 440 44 725 73 1450 145

15 300 30 600 60 990 99 1980 198

22 440 44 880 88 1450 145 2900 290

30 600 60 1200 120 1980 198 3960 396

37 740 74 1480 148 2440 244 4880 488

55 1100 110 2200 220 3625 363 7251 725

75 1500 150 3000 300 4950 495 9900 990

110 2200 220 4400 440 7260 726 14520 1452

160 3200 320 6400 640 10550 1055 21100 2110

5.5 Locate inlet and discharge air outlet

The efficiency of the compressor can be greatlyimproved by providing cooler air at its intake. This canbe as simple as ducting air from outside thecompressor house or another location on-site. Anotherimportant consideration is where the waste heat fromyour compressor is discharged to. Is it exhausted intothe compressor house? Or into the atmosphere? Bestpractice would ensure that waste heat does not find itsway back into heating the inlet air to the compressor.Table 5 illustrates energy savings arising from reducingthe air inlet temperature to your compressor.

5.6 Select a compressor and control system

Selecting the control philosophy may be an iterativeprocess that includes:• choosing a compressor, or range of compressors

(different sizes, types and so on)• selecting how each compressor will operate in

relation to the others given its range of operationaloptions. Section 4.6 (in Solution 1) discussesfactors to be considered in selecting a compressor.Suffice to say that the aim is to meet air demand atthe lowest total cost, including the energy costs.

Selecting a service provider 21

Upgrading and improving your compressed airsystem can take considerable time depending onyour circumstances. While you may want to follow thesteps in this guide, you may not have the time orresources available to do so. Compressed air serviceproviders can supply the services required to assess,upgrade or install your compressed air system. Youmay wish to ask them to assist you with some or allof the process. In either case, there are somequestions you should ask before you begin.

6.1 Questions to ask serviceproviders

Will the provider take a systems approach?It is important that your service provider considershow to optimise your entire compressed air system,not only one or two of its components. Ensure thatthe provider will include the following in theirinvestigation:

• air treatment level specifications• leak management assessment• pressure levels throughout the system• flow throughout the system• control system optimisation• heat recovery potential.

Will the provider examine the demand side as well as the supply?While the supply side of equipment such as thecompressor, dryer, receiver and filters are importantconsiderations, the provider should also beinvestigating the demand side of your system,including the distribution network, pressureregulators, the end uses and the profile of thedemand.

What analysis services do they offer?In order to ensure your system runs as efficiently aspossible, the provider must first conduct a detailedanalysis of various aspects of your system. While leakdetection is important, your provider should also beable to measure and analyse the load profile of yoursystem, the pressure at various points around thesystem and the power consumption of thecompressor and dryer. Other questions to ask of yourprovider include:

• What training do the staff have?• Are they qualified to work on all compressor types?• Can they service and install equipment such as

filters, drains and piping?• Do they provide emergency service response?• Will they take care of parts shipping?• Will they contract out any of the work themselves?• Do they have the capability to remotely monitor

your system?• Can they provide emergency rental compressors?

6.2 Database of compressed air service providers

A database of professional compressed air serviceproviders can be found in the SustainableManufacturing Directory provided by theSustainability Victoria. This directory can be found at the Sustainability Victoria website: www.sustainability.vic.gov.au

6 Selecting a service provider

Appendix A Compressed air system overview 22

A compressed air system consists of a number ofmain stages: compression – the air is put underpressure; treatment – the air quality is improved;storage – reservoirs of air to meet rises in demand;and distribution – piping of air to end uses. Eachcomponent in a typical system helps to deliver clean,dry, compressed air that is free of pressurefluctuations at its point of use. If any component isworking inefficiently, the system’s performancesuffers and operating costs rise. A brief description ofeach component follows.

• Inlet filter: removes particles from the air enteringthe compressor. Usually part of the compressorpackage.• Compressor: compresses air to a small volume,

increasing the pressure.• Motor: drives the compressor – also known as

prime mover.• Compressor controller: directs the compressor’s

output. It may be microprocessor,electromechanical or pneumatically based.Advanced controllers include machine protectionand information management.

• Aftercooler: compression leaves the air hot and wet.The aftercooler lowers the temperature of the airleaving the compressor and removes water thatcondenses as the air cools.

• Separator: removes liquids from the compressedair.

• Receiver: stores a large reserve of compressed airto maintain a smooth flow to the plant, especiallywhen air demand rises.

• Air line filter: removes solids and liquids from thecompressed air stream. Can be placed throughoutthe system.

• Dryer: helps to eliminate any remaining moisture inthe compressed air by using either a refrigeratedcondenser or a desiccant. Refrigerated condenserscool the air to condense water vapours into a liquidthat is then drained from the system. Desiccantsare powders or gels that remove water byabsorbing it.

• Condensate trap: collects and discharges liquidthat condenses out of the air stream. It is an integralpart of aftercoolers, dryers and separators.

• Distribution piping: links the components. Itdistributes the air from a main header to branchlines and subheaders to drop points connected toindividual tools.

• Pressure regulator: controls air pressure and flow atindividual points of use.

Appendix A Compressed air system overview

Appendix B Methods for estimating compressor energy consumption 9 23

One of the two methods below can be used. Ensurethat all electrical meters are installed by qualifiedelectricians.

Method 1A power demand analyser or a suitable meter can beused to measure the average power (kW) of thecompressor system over a test period, or kWh usedover that period.

Method 2If the compressor is operating at a steady load,measurement of the compressor circuit three-phasecurrents can be used to determine compressor energyconsumption. A “clip on ammeter” can be used tomeasure the instantaneous currents of each phase andan average phase current can be calculated.Power for the three-phase circuit:

power (W) = 1.732 × E × Iav × power factor, whereE = supply voltage (usually 415V)Iav = average phase current (amps)

Power factor is usually about 0.85 at full load reducingto 0.7 at half load and to about 0.2 at no load.

monthly energy use (kWh) = power (kW) × number of operatinghours/month

An accurate assessment of compressor power usagemay require “no load” and “full load” tests to beconducted to determine power at full and no-loadconditions. This can then be multiplied by the hoursthe compressor runs in each load condition to obtainkWh/annum.

Alternatively, some modern compressors record totalkWh consumed in their control systems, which mayneed to be read on regular basis to obtain annualconsumption.Test type Description

“NO LOAD” test Shut the valve between thecompressor and the air receiver. Measure the powerconsumed by the processor.

“FULL LOAD” test Expel some air from the airreceiver. Measure power consumed by thecompressor as it builds up air pressure.

On the basis of the above electrical measurements, an approximate compressor performance graph can be drawn, as indicated below, and used with theoperating hours to obtain kWh/annum.

Figure 10: Typical Compressor Performance Graph9

Appendix B Methods for estimatingcompressor energy consumption 9

Free air delivery (L/s)

Measuredfull load

Measuredaverage load

Measuredno load

No loadcondition

Air leakagecondition

Average loadcondition

Max ratedair

delivery

Averageair

demand

Full loadcondition

Performance graph

20 40 60 80 100 120 140 160 180 200

Appendix C Measuring leaks 24

This section describes a method for measuring leaks inyour compressed air system.

Method 11.Let the compressor build up the system pressure until

it reaches the shutoff point and the compressorstops. If the compressor NEVER reaches the shutoffpoint, the air leakage rate must be VERY HIGH.

2.For a period of half an hour, record the time thecompressor runs and the time that it is off.

3.The quantity of air leaking from the system can beestimated as follows (F.A.D. = Free Air Delivery):

Method 21.Run the compressor until the pressure in the air

receiver builds up to its maximum operating pressure.2.Switch off the compressor and measure the time

taken for the receiver pressure to drop by, say, 100kPa (The pressure should drop enough to allow thecompressor to restart for the second half of the test).

3.Switch on the compressor and record the time takento build up the 100 kPA pressure loss.

4.The quantity of air leaking from the system can beestimated as follows:

5.Using Figure 10, a power use corresponding to themeasured air leakage level (kW) and a powerrequirements corresponding to the ‘no load’ condition(kW2) can be established. From these graph readings,the actual net power use attributable to air leakagecan then be calculated as follows:

Appendix C Measuring leaks

Appendix D Cost savings from the installation of a DDS system 25

Table 6 outlines the potential savings with DDSretrofitted to a typical heatless dessicant air dryerinstalled with a 55kW rotary screw air compressor.

Adsorption dryers – operating costs

Min. Inlet Pressure: 7.0 bar g

Ambient Temperature (std ref conditions): 21.0 °C

Inlet Temperature (Air compressor disch temp - +10 deg. C) 31.0 °C

Outlet Pressure Dewpoint: -40 °C

Inlet Moisture Content: 4.1385 g/m3

Max inlet water content: 3.3751 kg/hr

Operating costs

Energy required to produce purge air: 12.64 kWh

Energy cost per kWh: $ 0.100

Operating hours per annum: 3840

Annual running cost: $4,853.76

Operating Costs with Dewpoint Dependent Switching System

Average Inlet Temperature (at std ref conditions): 21.0 °C

Average Inlet Pressure: 7.0 bar g

Average Flow rate (70% of Max flow rate): 9.90 m3/min

Inlet Moisture Content: 2.2849 g/m3

Average inlet water content: 1.3572 kg/hr

Cost savings: 59.78%

Estimated annual savings: $2,901.58

Estimated annual running costs: $,1952.18

Table 6: Cost savings from the installation of a DDS system.2

Appendix D Cost savings from the installation of a DDS system

Appendix E Glossary 26

Air Compressor A machine that puts air under pressure by compressing a volume of air.

Artificial Demand Air demand that is not actually used by the end uses of the compressed air, such as loss causeby pressure drop.

Control System The mechanism that controls how the compressor meets demandthrough switching on/off and by varying its operation.

Demand Profile A graph of the air demand from the system over time.

Dessicant Dryer A dryer that uses two separate chambers to remove moisture from the air.

Inlet The air intake for the compressor.

Leak Management An action plan of how to check for and deal with leaks on a regular basisso that they do not cause significant loss.

Load The demand for air that the air compressor experiences.Piping Layout The arrangement of the compressed air piping network.

Refrigerant Dryer A dryer that removes moisture from the air bycooling the air so that water condenses. It usesa typical refrigeration principle and equipment.

Appendix E Glossary

Appendix F Further reading/references 27

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