Energy SavingThe Norgren guide to saving energy incompressed air systems.

Compressed air is

often wrongly

assumed to be a

cheap or even free

source of power.

It is not.

A typical 1 000 cfm (500 litres/sec)installation will consume 20 000of electricity in a year. During itslifetime energy represents 75% of thetotal cost of buying and running acompressor.

Numerous independent studiesconfirm that industry wastes around30% of the compressed air itgenerates, equivalent to 6 000 inour typical 1 000 cfm installation.

The aim of this guide is to help theend user minimise wastage, byimproving existing installed systems. It will highlight key areas forsavings, and offer practical adviceon an action plan.

For further information or advicecontact your local Norgren TechnicalSales Centre. Telephone: 0345 662266.

2

LEAKAGE 4

Leakage is the major source of energy loss in compressed air systems. A typical plant may lose 20% of its compressed air through poorlyconnected pipe joints, fittings, couplings etc. Fixing the leaks andintroducing planned maintenance can produce substantial savings.

MISUSE 6

The second major wastage of compressed air is to use it as a powersource just because it is available. There may be better alternatives formoving, drying or cleaning products.

Where compressed air is used selecting correct equipment such asnozzles and use of control circuits can minimise wastage.

OVER PRESSURISATION 7

A considerable saving both in energy and equipment life can be madeby using devices at the minimum pressure required for the applicationrather than full line pressure. Simple use of pressure regulators offersvery fast payback.

PRESSURE DROP 8

Loss in pressure due to blocked filter elements and undersizedpipework can mean pressure starvation at the end of compressed airlines. The guide shows examples of how to choose and maintainequipment to minimise pressure drop in systems.

ENERGY AND SAFETY 10

Components fitted for safety reasons such as preset regulators and shutoff valves can also help energy saving. This section reviews relevant partsof BS EN 983 and other standards linking them to energy issues.

GENERATION 12

The correct selection of control equipment to multiple compressor setups, attention to inlet cooling and after treatment of the compressed aircan realise good energy savings. Regular and correct maintenance ofcompressors, filters and driers is also vital.

ACTION PLAN AND FURTHER INFORMATION 14

A simple checklist for action and sources of further information.

REFERENCE TABLES 15

Energy Saving in Compressed Air Systems

3

The process of air preparation hasbeen the core of Norgrens business for over70 years.

The guide reviews each of the majoropportunities for energy saving so that youcan take practical measures in your ownplant.

Each section covers: What it is ? Where to look for savings What to note or measure How much does it cost ? What are the solutions ? How do we maintain good practice ?

Throughout the guide you will finddetailed examples of how to calculate thesavings potential indicated by.

These are based around a modelfactory Example Engineering, which hasmany of the problems commonly found incompressed air systems.

The factory has installed compressorcapacity of 1 500 cfm (750 litres/sec), andaverage demand of 1 000 cfm (500 l/s). Itoperates 24 hours per day, 7 days per week,for 50 weeks a year. Electricity costs4,5p/kWh. At 75% compressor utilisation,total cost is 78 400 per year.

The basis for most of the calcula-tions is the wastage formula. This costs flow at: 0,4 x hours x flow l/s x energy cost/kWh.At Example Engineering, typical leakage is20% and equals 100 l/s, which costs: 0,4 x 8 400 x 100 x 0,045 = 15 120.

The calculation examples in thisguide are based on one section of thefactory, the workshop area.

The workshop operates for 2 500hours per year, but the ring main is pres-surised all the time the factory is open. Thetotal savings identified equal 30% of the aircurrently used by the workshop area.

4 machines each with4 x sequence valves

Drilling machines10 drills rated 4 litres/sec. @ 4 bar10 blow guns

Filter1"

2 x lathes + 2 blow guns2 x millers + 2 blow gunsGrinder rated 15 litres/sec. @ 4 barLaser cutter rated 10 litres/sec. @ 4 bar

4 x 2 mm nozzles cleaning material

TEST RIG

Gauging and instrumentation10 litres/sec. constant bleed1 mm @ 4 bar

Oil removal filter 1"

Filter/reg1/4" set at 4 bar

Filter 2"

7 bar ring main

Filter1"

EXAMPLE ENGINEERING

WORKSHOP AREA

HOW TO USE THIS GUIDE

Figure 1.

Leaks can be a significant source ofwasted energy in an industrial compressedair system.

If compressed air were hydraulicfluid, leaks would be so visible that we wouldensure their reduction. As it isl we accept alow level hiss in our work places as part ofthe job

At a price which is roughly compa-rable to that of domestic gas, this attitudecosts industry dearly.

It is estimated that leaks cost UKindustry 20 million per annum.

In addition to being a source ofwasted energy, leaks can also contribute toother operating losses. Leaks causepressure loss in systems, which can meanpressure is too low to the application leadingto more reject product. Frequently thegeneration capacity is increased to compen-sate, rather than simply fixing the leaks.

WHERE TO FIND LEAKS

Leaks occur everywhere ! PIPEWORK

Ageing pipework is a prime source ofleaks. Replace any corroded pipeworksections - for safety as well as energysaving.

FITTINGS, FLANGES AND MANIFOLDSLarge leaks are often found at connectionpoints, both in the main distributionsystem and in off takes. Sometimeswhen several snap connectors are usedtogether to form manifolds they can be asource of leakage due to worn connec-tors and poorly jointed pipe work.

FLEXIBLE HOSES AND COUPLINGSLeaks can be caused by damage to hosedue to abrasion by surrounding objects,deterioration of the hose material andstrain on the joint because the hose istoo long or too short.

OLD COMPONENTS NOT MAINTAINED -SEALS START TO LEAKCheck all pneumatic components eg oldcylinders and regulators, for worninternal air seals which can cause largeleaks.

4

CONDENSATE DRAINAGE VALVESLarge amounts of air can be lost whendrain valves are stuck open or even leftopen intentionally. These can often befound in remote parts of the systemwhere condensate collects.

SYSTEMS LEFT PRESSURISED WHENNOT IN USEWhere subsystems have a large amountof leakage which cannot be avoided egpresses and drop hammers, isolate themfrom the air supply when not in use.Simple shut-off valves (figure 2), orelectrically operated soft start dumpvalves (figure 3) offer cost effective waysto isolate leaky systems, or areas of aplant when not in use.

Figure 2.Manual shut-off valve.

Figure 3.Soft start dump valve used to isolateequipment, preventing leaks.

MEASURING THE LEAKAGE

You can measure the base leakageeasily using one of several methods : Install a flowmeter and pressure trans-

ducer in the compressed air feeding main(after any receivers). Connect the outputof the flowmeter and the pressuretransducer to a chart recorder and takereadings over a representative period oftime. Measure the flow from thecompressor when the system is notworking eg at a weekend.

Use a compressor of known capacity topump the system up to normal operatingpressure during non production hours.The compressor will unload at theoperating pressure. As the systempressure drops due to leakage thecompressor will load at its minimumrunning pressure. You can then estimatethe leakage rate from the average loadedand unloaded times over a representativeperiod.

Pump the system up to pressure andmeasure the time taken for the pressureto decay to the lower limit. If you knowthe total volume of the piping networkand the receivers, you can calculate theleakage rate.

Use a small flowmeter in branchlines to identify real problem areas.

But is it worth it ? How much doleaks really cost ?

LEAKAGE

WHAT DOES IT COST ?

Figure 4.Leakage rate for different hole diameters.

A single leak from a hole diameter2 mm can cost 600 per year, in ourworkshop example. Use orifice flow chart(figure 19) to calculate leakage at differentbore sizes and pressures.

REDUCING LEAKAGE

Set targets for leakage reduction.Publicise how much money the leakage iscosting the organisation and how much youintend to save. Implement an ongoingmaintenance programme - have leak tagsavailable and encourage their use.

Carry out a survey of thecompressed air system. Inspect during quiethours. Listen for pipework or tool leaks andexamine hoses and couplings. Sprayspotleak on pipe joints and watch forbubbles.

The average leak will take aroundhalf a man hour to fix, and offer very quickpayback.

Fixing the leaks will clearly savesignificant amounts of money, but how do wemake sure they stay fixed ?

Implementing a site-wide awarenessprogramme leads to long term savings on abig scale.

Dividing the site into areas, fittingair consumption meters and charging eacharea for its air usage will soon focus theattention of energy users. Targets can theneasily be set to reduce energy loss due toleaks.

litres/sec. @ 6,3 bar

0,5 1 2 3 4 mmBore Size

468

121416

21

10

Figure 5.Use a plug-in flow meter to measureleakage in machinery.

5

LEAKAGE After surveying the workshop area a number of leaks were found.

1 x 2 mm leak @ 4 barand 11 x 1 mm leak @ 7 barUsing the orifice flow table, that equates to 4,8 l/s and 11 x 1,2 l/s

Total leaks = 18 l/s

Potential for saving

0,4 x 8 400* x 18 x 0,045 = 2 722* assumes system stays pressurised for 24 hours per day, 50 weeks per year.

Cost of solution:

Estimate 1/2 a man hour to fix each leak @ 15.00 per hour = 90.00

Savings

2 722.00

Expense

90.00

Nett saving

2 632.00

The second major wastage ofcompressed air is to use it as a power sourcejust because it is available.

Some examples of this are ineffi-ciently creating vacuum, ejecting faultyproducts and removing water/dirt/powderfrom products. There may be better alterna-tives for these applications. If compressedair is chosen the correct equipment andcontrol must be employed to keep usage to aminimum.

WHERE TO FIND MISUSE

In an existing plant new misuses canoften be seen by an increase in air demandand/or compressor running hours. Toidentify existing misuses all areas of theplant need to be surveyed, asking thequestion - is this an effective use of air ?

COSTING THE MISUSE

Where a process has air passing toatmosphere, such as rejection of underweight or faulty product in a canningprocess, a flowmeter can be installed in theline to measure the air usage. Then by usingthe wastage formula the cost of this processcan be found.

RECOMMENDED PRACTICE

Another way to calculate costs is touse the exit orifice or nozzle diameter and theapplied pressure to calculate the flow (seetable, figure 19 orifice flow).

Where nozzles must be used, forexample blowing loose flour off loaves ofbread, then ensure that the distance betweenthe exit nozzle and the product is as short aspossible as this will allow the supplypressure to be reduced. The nozzle shouldbe directed only at the area needed giving acone (circular area) or fan spray (long narrowband) etc. Where a very long narrow areaneeds to be covered use nozzles in parallel toproduce a curtain reducing the distance tothe furthest point. Ensure the mains feed lineto a number of nozzles is of sufficientdiameter so as not to restrict the outlet flow.

6

Air saver nozzles can entrain, oraccelerate air within their mechanism toproduce the desired outputs with reducedsupply pressures, giving savings of up to atwenty fold reduction in compressed airusage.

Finally where such solutions mustbe used ensure control valves and sensorsare fitted to the system, so that flow onlyoccurs when the product is at the applicationpoint, with no flow during the gaps betweenproduct on the conveyor, or at work breaksetc.

In some cases the solution is not touse compressed air. Dedicated air blowersor vacuum pumps may well prove more costeffective.

All such solutions can be costed andcompared to the air usage/wastage and inalmost every case savings can be made

Once misuse has been determinedwithin a plant, ensure that when newprocesses are installed due consideration tothe power source and its control are made.

Figure 6.Blow gun with air saver nozzle

Figure 7.Example of poor practice. Nozzle should becloser to the bread and of an air saverdesign. A control circuit is needed to stopair when no bread is under the nozzles.Consider local filtration and membranedryer for high quality dry air.

MISUSE

MISUSEMaterial is cleaned prior to being cut by the laser using 4 x 2 mm at linepressure.4 x 2 mm nozzle presets 4 x 4,8 l/s at 7 bar

4 x 1,81 l/s at 2 barUsing the orifice flow table, figure 20.So a reduction in pressure to 2 bar will give a flow saving of 11,96 l/s.

Potential for saving

0,4 x 8 400* x 11,96 x 0,045 = 1 808.00* assumes no isolation valves and system is continually pressurised.

Cost of solution:

Fit one pressure regulator = 18.00Estimate 1/2 a man hour @ 15.00 per hour = 7.50Total 25.50

Savings

1 808.00

Expense

25.50

Nett saving

1 782.50

Many systems run at full linepressure with the only control being thepressure switch on the compressor.

Every item of pneumatic equipmenthas an optimum operating pressure and flow.Usage outside of these conditions willshorten the equipment life due to increasedloading and wear, and will increase therunning costs. A device running at 7 bar willconsume twice as much air as it would at3 bar.

IDENTIFYING OVERPRESSURISATION

The absence of pressure regulatorsin a system indicates that equipment is beingused at excessive pressures. Savings can berealised in many areas, including air tools,control valves, clamping cylinders and on thereturn stroke of large double acting cylinders.

Where pressure regulators arepresent but outlet pressure is the same as theinlet, this often indicates poor lubricationwith extra pressure being applied toovercome the friction slowing down theprocess. This is costly in extra wear andenergy.

CALCULATING THE COSTS

All air tools are rated for their flowand optimum pressure. The air wastage canbe calculated by using the pressure ratio(absolute), and then multiplying by the ratedair flow i.e. consumption at 3 bar is 8 l/s at7 bar his will be

(7 + 1) x 8 = 16 l/s(3 + 1)This can then be substituted into the

annual wastage formula to calculate savings.

Double acting cylinders usuallyonly do work on the out stroke (work stroke).When no work is being done or longer resettimes are possible, the return stroke can beat a lower pressure. Where large bore, longstroke or multiple cylinder systems exist,considerable air savings can be made.Using a regulator to reduce return stroke

pressure can be a fast payback solution.Consumption with reduced pressure

return stroke for double acting cylinder canbe measured using the formulaAir saving = 0,7854 x d2 x L (P1 - P2) x 10-6

T x 60d = cylinder diameter (mm)L = stroke length (mm)T = time for 1 stroke (sec)P1 = applied pressure (bar) - outstrokeP2 = applied pressure (bar) - return stroke

Valves have a rated conductance Cin litres/sec per bar absolute. Any flowsaving is calculated by simply reducing theapplied pressure. It is important to note thevalve operating duration (i.e. time that flowoccurs) to ensure that the correct flow savingis arrived at. Usually this figure is small butfor multiple valve installations and/or rapidlycycling valves with long pipe runs the totalsaving can be significant.

Once over pressure examples havebeen identified within a factory, ensure thatall new plant, processes and equipment areexamined for optimum operating conditionsand pressure control equipment prior toinstallation. This should be reflected inincreased tool life as well as reduced energycosts.

Figure 8.Use of a regulator to reduce pressure on thereturn stroke of large bore cylinders savesenergy.

7

OVER PRESSURISATION

OVER PRESSURISATION

10 air tools rated @ 4 barThese drills are all supplied with 7 bar line pressure, and each is used onaverage for around 1 000 hours per year. The air consumption of each drill at 4 bar is 15 l/s.

\ at 7 bar each tool will be consuming: 8 x 15 = 24 l/s5

So by using a lower pressure there is a potential saving of 9 l/s per tool.Over the total 2 500 hours of annual usage, the average flow saving =

1 000 x 9 = 3,6 l/s2 500

Potential for saving

0,4 x 2 500 x 3,6 x 0,045 = 1 620

Cost of solution:

Fit one pressure regulator per tool = 18.00 x 10 = 180.001/2 man hour for fitting

@ 15.00 per hour = 10 x 7.50 = 75.00Total 225.00

* calculations need to be done with absolute pressures i.e. 1 bar higher than reading

Savings

1 620.00

Expense

225.00

Nett saving

1 395.00

Pressure drop can be defined as theloss in a system of power available to dowork. In practice it is evidenced by lowpressure in parts of the system. It is oftencompensated for by increasing generationpressure or turning up regulators.

The potential energy generated bycompressing the air is dissipated throughfriction and heat losses as it is pushedthrough all the components of the system.We need therefore to design and maintainsystems to minimise the amount of pressuredrop. Every 1 bar of unnecessary pressuredrop leads to an increase of 7% in gener-ating costs. This means around 3 500 perannum to our typical factory.

The two main areas where pressuredrop occurs are pipework and filtration.

PIPEWORK

Pressure drop occurs in pipeworkmainly as a result of friction of the airmolecules with the surface of the pipe. If thepipe is too small for the volume of flow thevelocity of the air will be very high and therewill be a big loss in power.

Energy is also lost when there is achange in flow direction i.e. elbows,junctions and shut off valves. Simple pipesystems will minimise pressure drop.

8

How to calculate pressure drop inpipework:

Method 1

Measure supply pressure. Measure the pressure at furthest point

from supply. The difference is the system pressure

drop.

Method 2

Estimate the flow usage - eg. calculatethe swept volume of working cylinders.

Note the supply pressure and thediameter of the pipe.

Use published normagraphs to arrive atthe pressure drop.

Method 3

Use a small flowmeter to measure theflow.

Note the supply pressure and thediameter of the pipe.

Use the tables on pages 14/15 to seewhether flow is within recommendedrange.

RECOMMENDATIONS

Dont over flow the pipework. Keepvelocity below 6 m/s in mains.

Simplify the pipework. Avoidelbows as a 90 degree elbow is equivalent to1,6 m of straight pipe.

Fit low resistance valves a full flowball valve equates to 0,4 m of pipe, less thanhalf the resistance of a gate valve.

FILTRATION

Filtration is an essential part of theconditioning in a compressed air system. Ifnot protected from water, particles anddegraded compressor oils, machines willquickly breakdown.

To keep pressure drop as small aspossible:

Look for the right size filter unit

As with pipework if the filter unit istoo small for the flow required then it willgive a higher pressure drop. When new ageneral purpose filter should give no morethan 0,1 bar pressure drop. Fitting a smallerfilter is a false economy, as it will give higherinitial pressure drop and also block morequickly because the surface area of theelement is smaller.

Figure 10.Selecting the right size filter is important.

ECONOMIC FILTER SIZE

200

150

100

50

Cost

1/4

3 6 9 12Months

1/2

PRESSURE DROP

Figure 9. Example of poor pipework on aproduction test rig.

An extra 0,35 bar of pressuredrop in a line can cost as much as 400per year.

Fitting pressure drop indicators -simple pneumatic or electrical - can indicateimmediately when pressure drop isincreasing. Changing the elements at thispoint means significant energy saving.

Figure 12.Cost saving through regular elementchanges.

It is good practice to change thefilter elements at regular intervals. This willensure that energy wastage is kept tominimum and that correct air quality isdelivered.

Any new plant should be installedwith the right level of air quality in mind -instrument quality only where the applicationdemands it.

Delivering very dry high quality airto all areas of the site is costly and should beavoided.

ANNUALENERGYCOSTS

500

400

300

200

100

SAVINGS

Increasing pressure drop

No replacement

6 12 18 24Months

9

Look for the right level of filtration

A very fine filter will have a greaterresistance to flow than a coarse filter. Mostair tools for example will only requirefiltration to around 40 micron. It makessense therefore not to use a 5 micron or evena 0,01 micron filter in this application. Seefigure 21 for guidelines on quality of airneeded for common applications.

Where applications needing highergrade filtration exist, place the higher gradefilters as close to the application as possible.This ensures that the size of filter determinedby the flow is as small as possible. Do notfilter the whole of the air line or branch lineto this standard, since this will increase theflow requirement, increasing the size of thefilter, its purchase price, replacement elementprice and incur extra pressure loss for thewhole of the system downstream of it.

Figure 11.The effect of filter grade on pressure drop.

Look for dirty filter elements- check pressure drop indicators

After some time in service particleswill be trapped within the filter mediacausing the element to become blocked.This means pressure is lost at the applica-tion. What usually happens at this stage isthat the pressure is increased to compensateby turning up a regulator. Increasing thepressure increases the costs.

Initialpressure drop (bar)

Line Pressure = 6,3 barFlow = 30 litres/sec.

0,3

0,2

0,1

40 m 5 m CoalescingFilter Grade

Figure 13.Filters fitted with electrical andmechanical service indicators.

PRESSURE DROP

A 2" filter flowing 400 l/s @ 7 barwhen new, pressure drop = 0,15 barin 2 years this could increase to 0,4 barThis extra 0,25 bar creates an extra power demand of 1,8 Kwh

Potential for saving

For 2 500 hours total extra power = 1,8 x 2 500 Kwh @ 0,045 per Kwh extra cost = 202.50

Cost of solution:

Replace filter element = 57.001/2 a man hour labour @ 15.00 per hour = 7.50

Savings

202.50

Expense

64.50

Nett saving

138.00

How can Safety be an Energy Issue ?In compressed air systems compo-

nents fitted for valid safety reasons have acost, however there are some that offer apayback resulting from the benefits gained inenergy savings.

There are several documents thatdeal with safety of compressed air systemsand pneumatic components. Some areinternational standards whilst others thoughnot having legal status, offer best practiceguidance from safety organisations andleading fluid power trade organisations.

BS EN 983 - LEAKS

Leaks (internal or external) shall notcause a hazard.

In systems where air pressure isused to maintain a load, such as in a press,braking or clamping application, a leak couldpotentially constitute a hazard.

BS EN 983 - FILTRATION

Filter condition monitoring. Ifdeterioration of filter performance could leadto a hazardous situation, clear indicationshould be given.

A blocked filter, leading to reduceddownstream pressure could have a similareffect to a leak in systems where the pressureis used to maintain loads.

Pressure drop indicators will showwhen the filter is blocking and needschanging. This also minimises energy costsby keeping pressure drop to an acceptablelevel.

BS EN 983 - TAMPERRESISTANT DEVICES

Pressure and flow control devicesor their enclosures shall be fitted with tamperresistant devices where an unauthorisedalteration to pressure or flow can cause ahazard.

Frequently pressure is increased tomachines or systems in the hope that theincrease will speed up the process. Usuallythere are other factors within the system

which will limit this and increasing thepressure will only increase the air consump-tion.

In some cases increasing thepressure can be unsafe such as when usingpneumatic clamps. The force generated iscalculated to clamp the component, anyincrease in that force may result in crushingof the component which may shatter orexplode.

Simple tamper evident covers whichcan be padlocked can be fitted to regulatorsto ensure systems remain safe. Lockableshut off valves prevent someone accidentallyturning off the air to a system, or turning onthe air while a machine is being maintainedcreating a potential hazard.

Figure 14.Use of a lockable tamper evident cover ona filter/regulator.

BS EN 983 - SOFT STARTDUMP VALVES

Machines should be designed sothat at start up any moving componentsreach their working position in a safemanner. There must also be a safe way ofreleasing the system air very quickly whensignalled.

Combined soft start dump valvesachieve both these functions in one unit.They also have the added benefit that thesignal can be linked to one power downoperation which will isolate the machinewhen not in use. This means any leaks orconstant bleed devices will not drain themain system.

HSG 39 - CORRECT USE OFBLOW GUNS

Blow guns consisting simply of areduced orifice in direct line with the supplyhose can be extremely dangerous, unlesspreceded by a pre-set tamperproof pressureregulator set at a reduced pressure from thenormal 80 psi air line supply.

Blow guns are commonplacethroughout industry and whilst most peopleare familiar with their use , the very realhazards they present are often not appreci-ated. As an example a pressure of 0,4 barcan penetrate human skin with possible fatalresults if air gets into the bloodstream.

Many blow guns are operated at fullline pressure and can even be home madei.e. short pieces of copper tube with diame-ters up to 6 mm. This situation is clearlydangerous. A secondary concern is thesheer volume of air that is wasted.

Good practice would be a blowgunwith built in side vents to prevent pressurebuild up if the nozzle becomes blocked,preceded by a small preset non adjustableregulator (see figure 15).

If reduced pressure presentsproblems with an operation such as cleaningswarf from a component, then blow gunswith efficient nozzles can be used to entrainsome atmospheric air. This equipment willprovide a safe working situation with theadded benefit that it will pay for itself veryquickly in reducing air usage.

Figure 15.Recommended arrangement for blow gun.

10

ENERGY & SAFETY

BS 6005 - 1997 SAFETY OFPOLYCARBONATE BOWLS

Polycarbonate is commonly used forbowls on filters, filter-regulators and lubrica-tors, offering clear visibility of bowl contents.However in an industrial environment itneeds to be treated with some care. Thestandards says:

A.4.1.2 Bowls which on visualinspection show signs of mechanicaldamage, cracking, or hazing should bereplaced.

A.4.1.3 Bowls which have beencontaminated with paint should also bereplaced; they should not be cleaned.

A.4.1.4 All bowls which have beenin service for 10 years should be replaced,even though they may appear acceptable bythe visual inspection mentioned in A.4.1.2

Whilst changing bowls which haveany of the above problems will not directlysave energy, it should be included in amaintenance plan which also checks thecondition of filter elements and drains toreduce pressure drop and leaks.

Filters are notorious for being badlymaintained and it is important to raiseawareness of the safety implications ofneglect of these units.

PUWER - ISOLATION FROMAIR SUPPLY

Regulation 19 Every employer shallensure that where appropriate work equip-ment is provided with suitable means toisolate it from its sources of energy.

A variety of valves are available tohelp meet this requirement: ball valves shut off valves included in FRL units

Figure 17.Lockable shut-off valve

electrical operated control valves pneumatically operated control valves

Use of these has the added benefitthat any leakage in the system downstreamwill not be continually draining the mains airsupply.

AIR FUSE

The use of air fuses can also havean affect on energy saving. The device isdesigned to prevent pneumatic hoseswhipping around, exhausting high pressureair in the event of a hose fracture. The fusereduces the flow to atmosphere, so that onlya very small amount of air escapes,compared to full line failure flow. Danger ofinjury from the hose is eliminated and energywastage is minimised.

In situations where isolating valvesand air fuses do not exist it would benecessary to bleed down the system, wastingall the compressed air before the hose failurecould be repaired.

11

Figure 16.Filter removed from a breathable air set inan automotive paint shop.

ENERGY & SAFETY

18 blow guns with 4 mm hole, supplied with 7 bar line pressure.Blow guns should be regulated to a lower pressure using the orifice flow table:

Flow through 4 mm @ 7 bar = 19 l/sFlow through 4 mm @ 2 bar = 7 l/sPotential flow saving per gun = 12 l/s

Gun is used for 300 hours per year (around 10 minutes in every hour)Average saving per year = 300 x 12 = 1,4 l/s

2 500Total for 18 blow guns = 25 l/s

Potential for saving

0,4 x 2 500 x 25 x 0.045 = 1 125.00

Cost of solution:

18 preset regulators = 306.001/2 man hour to fit each = 135.00Total 441.00

Savings

1 125.00

Expense

441.00

Nett saving

648.00

At best only 5% of the input energyto an air compressor remains in the air afterit is compressed. This is due to the heatrejected by the compressor in its coolingsystems.

Most compressor locations willcontain the compressor, the treatment systemand the control system. Each element of thecompressor station, the installation and itsmaintenance has an effect on energy effi-ciency.

COMPRESSOR SIZE AND CON-FIGURATION

The size and configuration ofcompressor is important in terms of energyefficiency.

Depending on the demand pattern itis normal to have the largest and mostefficient machine on line to handle the baseload and other machines coming on and offline to meet changes in demand.

Most modern installations use rotarycompressors of the oil injected vane andscrew types. When higher quality and largervolumes of air are needed oil free screw orcentrifugal machines can be used and theseusually have better efficiencies.(See figure 22)

Although not so popular for newapplications, unless they are for specialgases or high pressure, there are manypiston machines still in operation. Thesemachines particularly in the larger sizes haveexcellent efficiency and part load control.

Variable speed drives are becomingquite common as are two stage oil injectedmachines.

INSTALLATION

Cooling is most important with allcompressors. The inlet air should be as coolas possible, ideally taken from a shadedoutside location. In general a 4C reductionin inlet temperature will give an improvementof 1% in efficiency.

A simple check on a compressorshealth is to measure the differences intemperature between the cooling medium

and the discharge air from the aftercooler.For air-cooled compressors this

should not exceed 15C.For water-cooled compressors this

should not exceed 10C.If greater temperature differences are

found the machines efficiency will be lowerthan design. The cooling systems should beimproved.

Make sure all the feeding mains arecorrectly designed with flow velocities notgreater than 6 metres per second. Use swepttees and long radius elbows at all pipejunctions.

Use electronic level sensing traps onall condensate collection points and ensurecondensate recovery conforms to the regula-tions.

HEAT RECOVERY

Use the waste heat of compressionfor space heating, domestic water heating orprocess water. Large savings can beachieved by doing this.

MAINTENANCE

The way compressors are lookedafter in the field has a major impact ongeneration efficiency. Machines shouldalways be maintained strictly in accordancewith the manufacturers instruction book.

It is a false economy to run rotaryvane and screw units past the manufacturersrecommended compression element lifecycle. Typically this is 24 000 hours withoil-injected machines and 40 000 hours foroil free machines.

Regularly inspect the intercoolerpressure on two stage piston and screwcompressor. This should be around 2 to2,5 bar when the final discharge pressure is7 bar. Any deviation shows stage imbalancegiving poor efficiency.

Similarly check the pressure dropacross the oil separator system.

If the maintenance of yourcompressor is conducted by a third partyfirm make sure you use a manufacturersaccredited agent. Only use genuine spare

parts, items which are not of the originaldesign or poorly refurbished will have aserious effect on energy efficiency. A smallapparent saving in these areas can give afalse economy in the long term.

CONTROL

Where a number of compressors,possibly of different types and sizes are usedto meet varying air demands then a controlsystem should be employed. This willoptimise the number and mix of compressorsto meet the demand, giving close pressurecontrol with the most energy efficient mix ofmachines.

TREATMENT

Only treat the air to the minimumstandard required. Refrigerated air dryersgiving +3C dewpoint and filters add 3% tothe energy cost. Desiccant air dryers andfilters giving -40C dewpoint add between 8and 15% to the running costs.

Use desiccant or membrane dryersat the point of use to save energy. Usedewpoint sensing controls with desiccantdryers.

Keep treatment system pressurelosses to 0,5 bar. Size filters for themaximum flow, do not allow reduced flangesizes. Maintain filters regularly.

OPERATING PRESSURE

Establish the minimum acceptablepressure at the point of use and ensure thepiping network is designed such that thepressure drop with the system on full loaddoes not exceed 0,5 bar.

If possible reduce the generationpressure. A reduction of 1 bar can save 7%of the generation cost. Reduced pressurealso reduces the unregulated air demand ofthe plant. A reduction from 8 bar to 7 barwill reduce the unregulated demand byaround 12%.

Generation section kindly supplied byEric Harding, Air Technology Ltd.

12

GENERATION

Measure System Flow Demand:methods

Survey Factory In 3 Areas:compressor housering mainbays/point of use

Focus In Each Area On:leaksmisuseover pressurepressure dropsafety issues

locally measure usage if possible Having Identified Areas for Savings:

cost out corrective action andpaybackimplementcheck/flow pressure drop to validatemeasure

Use Revised Flow Figures to ModifyControl System for Compressors wherenecessary

Implement Ongoing PreventiveMaintenance and Periodic Re-audit(Leaks Come Back)

Clean Compressed AirNorgrens Guide to Effective Air Preparation

ETSU PUBLICATIONS

ETSU the energy efficiency branch ofthe Department of Environment Transport andRegions, offer a range of free publications onall aspects of energy saving. Forcompressed air information refer to:

Good Practice Guides

126 Compressing Air Costs216 Energy Saving in the

Filtration and Drying ofCompressed Air

238 Heat Recovery from AirCompressors

241 Energy Savings in theSelection, Control andMaintenance of AirCompressors

Good Practice case studies

Various. Contact ETSU for details.ETSU contact number 01235 436747.

Energy consumption guides

ECG 40 Compressing Air Costs:-Generation

ECG 41 Compressing Air Costs:-Leakage

ECG 42 Compressing Air Costs:-Treatment

13

OTHER PUBLICATIONS

HSG 39 Compressed Air SafetyBS 6005 1997 Specifications for

Moulded TransparentPolycarbonate Bowls usedin Compressed Air Filtersand Lubricators

PUWER Provision and Use of WorkEquipment Regulations1998

BS EN983 - 1996 Safety of machinery -safety requirements for fluid powersystems and their components -pneumatics

A European Norm standardthat supports the essential healthand safety requirements of theEuropean Machinery Directive.

It identifies hazards whichaffect the safety of systems and theircomponents when put to theirintended use.

It is not a manufacturingstandard so does not give guidancein the manufacturing of pneumaticcomponents.

ISO 8573 Filtration

ACTION PLAN FURTHER SOURCES OF INFORMATION

Figure 21.

RECOMMENDED FILTRATIONLEVELS.

Application Typical Quality ClassesOil Dirt

Air agitation 1 3Air bearings 2 2Air gauging 2 2Air motors 4 4Brick and glass machines 5 4Cleaning of machine parts 3 4Conveying, granular products 2 4Conveying, powder products 1 3Foundry machines 4 5Food and beverages 1 1Hand operated air tools 5 5Machine tools 5 4Mining 5 5Micro-electronics manufacture 1 1Packaging and textile machines 5 3Photographic film processing 1 2Pneumatic cylinders 3 5Pneumatic tools 5 4Pneumatic tools (high speed) 4 3Process control instruments 2 3Paint spraying 1 1Sand Blasting 4 5Welding machines 5 5General Workshop air 5 4

14

Figure 19.

ORIFICE FLOW CHARTOrifice Size (hole) litres/sec - ANR (dm3/s)(mm) 2 bar 4 bar 6 bar 7 bar 8 bar0,2 0,02 0,03 0,04 0,05 0,060,3 0,04 0,05 0,10 0,11 0,120,5 0,11 0,19 0,26 0,30 0,391,0 0,45 0,73 1,05 1,20 1,351,5 1,02 1,70 2,37 2,69 3,052,0 1,81 3,05 4,20 4,80 5,403,0 4,00 6,77 9,46 10,81 12,164,0 7,27 12,04 16,82 19,16 21,675,0 11,35 18,83 26,32 30,00 33,826,0 16,34 27,16 37,82 43,32 48,658,0 29,16 48,15 67,30 76,90 86,50

10,0 43,32 75,30 105,10 120,10 135,1015,0 102,10 169,90 236,60 269,90 304,00

Figure 20.

AIR QUALITY CLASSIFICATIONS ISO 8573Quality Class Dirt Water Pressure Dew-point Oil

Particle Size in Microns C (ppm vol.) at 7 bar g (including vapour) mg/m3

1 0,1 -70 (0,3) 0,012 1 40 (16) 0,13 5 20 (128) 14 15 +3 (940) 55 40 +7 (1 240) 256 +10 (1 500)

Figure 22.

COMPRESSOR EFFICIENCIESConfiguration Capacity Specific Power Part Load

litres/sec Kwe/50 l/s Efficiency225 24 Good

Lubricated piston 25250 20 Good2501 000 17 Excellent

225 26 GoodOil-free piston 25250 22 Good

2501 000 19 Excellent225 24 Poor

Oil injected 25250 22 Fair

rotary vane and screw2501 000 19 Fair to good

2525 20,5 GoodOil free toothed

250100 18 Goodrotor and screw

1 0002 000 18 Good2501 000 21 Good

Oil-free centrafugal 1 0002 000 18 ExcellentAbove 2 000 17 Excellent

15

Figure 23.

FRICTION LOSS IN PIPE FITTINGS IN TERMS OF EQUIVALENTMETRES OF STRAIGHT PIPE.

8 mm 10 mm 15 mm 20 mm 25 mm 32 mm 40 mm 50 mmTee (straight through) 0,15 0,15 0,21 0,34 0,46 0,55 0,67 0,92Tee (side outlet) 0,76 0,76 1,01 1,28 1,62 2,14 2,47 3,1890 elbow 0,43 0,43 0,52 0,64 0,79 1,07 1,25 1,5945 Elbow 0,15 0,15 0,24 0,30 0,38 0,49 0,58 0,73Ball valve* 0,01 0,03 0,09 0,12 0,15 0,22

* Self exhausting full open

Figure 24.

MAXIMUM RECOMMENDED FLOW * THROUGH ISO 65 MEDIUMSERIES STEEL PIPE.

Applied Nominal Standard Pipe Size (Nominal Bore) mmGauge 6 8 10 15 20 25 32 40 50 65 80Pressure Approximate Pipe Connection inchbar 1/8 1/4 3/8 1/2 3/4 1 1 1/4 1 1/2 2 2 1/2 30,4 0,3 0,6 1,4 2,6 4 7 15 25 45 69 1201,0 0,5 1,2 2,8 4,9 7 14 28 45 80 130 2301,6 0,8 1,7 3,8 7,1 11 20 40 60 120 185 3302,5 1,1 2,5 5,5 10,2 15 28 57 85 170 265 4704,0 1,7 3,7 8,3 15,4 23 44 89 135 260 410 7256,3 2,5 5,7 12,6 23,4 35 65 133 200 390 620 140858,0 3,1 7,1 15,8 29,3 44 83 168 255 490 780 14375

10,0 3,9 8,8 19,5 36,2 54 102 208 315 605 965 14695

*Air flow rates in dm3/s free air at standard atmospheric pressure of 1 013 mbar.General notes

The flow values are based on a pressure drop (DP) as follows:10% of applied pressure per 30 metres of pipe 6 15 mm nominal bore inclusive5% of applied pressure per 30 metres of pipe 20 80 mm nominal bore inclusive

Figure 25.

ADDITIONAL COSTS FOR TREATING COMPRESSED AIRPressure Dew-point Dryer Type Filtration Added Cost Over Initial

C Generation CostTypical 10 Membrane Pre 10 - 15% Low

3 Refrigerated General purpose 3% Medium40 Heatless desicant Pre and after 8 - 15% High40 Heated desicant Pre and after 10 - 15% High70 Heatless desicant Pre and after 15 - 21% High