Energy Saving With VSD Controlled Compressors

7/25/2019 Energy Saving With VSD Controlled Compressors

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Energy sav ing with VSD controlled compressors2011-06-01 - Using a variable-speed AC drive is o ne o f the most effective ways to save energy. Whileother energy saving methods m ay shave singular percentage points o ff the overall consumption, a

variable-speed drive frequently s aves 3 0 percent or more of the energy in many centrifugalcompressor applications

Electric motors u se two-thirds o f all electricity i n industry, so any c hance to reduce this l oad, even bysingle gures, is h ighly s ignicant. Offered the chance to cut energy c onsumption by up to half,industry ought to jump at the opportunity. Yet, variable-speed drives (VSDs) ar e only installed in aminority o f applications. The reasons o ften boil down to the fact that the people who design theseapplications a re not responsible for the energy b ill. At the same time, the people in charge of theenergy bill, in most cases, are not engineers a nd are not aware of the benets VSDs ca n offer.

How VSDs can help

The main a pplications for VSDs ha ve b een p ump a nd fan applications. But as V SD technology ha sprogressed other app lications, such as compressors, are beginning to reap the benets o f VSDcontrol.

A VSD controlled air compressor, for example, uses a n AC drive to control the speed of the unit, whichin turn sa ves e nergy com pared to a xed speed e quivalent.

VSDs r educe the energy o utput of a c ompressor, by controlling the speed of the motor, ensuring itruns n o faster than n ecessary. The t raditional way of controlling a compressor is b y running the m otor

at full speed and then stopping it when the air has b een compressed to the correct pressure. It is t henstored in a reservoir at a slightly h igher pressure than is n eeded, to allow a hysteresis i n the pressure.This on-off method is w asteful, because the m otor keeps r unning a t its n ominal speed regardless o fthe requirement. Some compressors a re designed with a bypass system which returns air from outputto input, which is again wasteful.

The p roblem of energy waste is m ade worse b y the fact that many motors ar e u ndersized. This isbecause they a re short term rated and only p ull high currents w hen they load up. For example, arecent a 75 kW motor on a compressor was m onitored a nd found to have an absorbed power of 83kW on load. This is very com mon in sc rew applications.

The 'oversizing' with compressors i s r elated to the actual output of the compressor not the motor.Compressor manufacturers o ften sell compressors f or worst case scenarios t hat are essentially t oolarge for t he applications.

These 'oversized co mpressors' can e asily be identied by the off load running hours w hen co mparedto on load hours. Compressors w ith high run hours a nd low load hours a re ideal for conve rsions.

VSDs w ork b y co ntrolling the waveform of the current and voltage supplying the motor. A VSDconverts t he incoming AC power to DC and then b ack t o a quasi-sinusoidal AC power using aninverter switching c ircuit. The m ovement of the motor shaft can b e adjusted w ith great accuracy,ensuring that the application gives t he performance needed. The benets o f this t echnology includedreducing p ower cost, reducing p ower surges ( from starting AC motors), and d elivering a more


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constant pressure.

Typically, a fth of a factory's e lectricity b ill is a ttributed to the production of compressed air. Themajority o f modern factories a re heavily involved in cutting costs, and energy a wareness i s a keyconcern. For example, 10-12 percent of all power generated in t he U K is d edicated to the p roductionof compressed air, and a portion of this p ower is w asted energy.

Large electrical cost savings c an be achieved by installing a VSD compressor in place of an existingrotary scr ew or piston m achine. Because of this, many g overnments a re p ushing the industries t omove towards this t echnology in hopes of reducing wasted energy.

So which type of compressors ca n benet the most from the application of VSDs?

There are t wo different basic co mpressor designs - positive d isplacement and d ynamic and, assuch, applying VSDs t o compressor applications a re b it more challenging than with pumps and fans.

Positive d isplacement compressorsThere are t wo main t ypes o f positive d isplacement compressor r eciprocating a nd rotary-screw.

Reciprocating compressors u se pistons d riven by a crankshaft. They ca n be either stationary o rportable, can be single or multi-staged, and can be driven by e lectric m otors o r internal combustionengines. Discharge p ressures ca n range from low pressure t o ve ry h igh pressure ( >35 MPa). Incertain a pplications, such as a ir compression, multi-stage d ouble-acting c ompressors a re said to bethe most efficient compressors a vailable, and are typically l arger, noisier, and more costly t hancomparable rotary u nits.

Rotary scr ew compressors u se two m eshed rotating p ositive-displacement helical screws t o force thegas into a smaller space. These are u sually used for continuous o peration in c ommercial andindustrial applications a nd may be either stationary o r portable. Their application can be from 2 kW toover 375 kW and from low pressure t o ve ry high pressure ( >8.3 MPa).

These kind of compressors, piston and screw type, for example, have the capability t o create pressureincrease, independent of driven speed.

This i mplies t hat this t ype of compressor does n ot need any m echanical volume c ontrol devices, butthat the required capacity c an be controlled directly by operational speed.

Energy co nsumption of positive displacement compressors i s d irectly p roportional to the speed andthis t ype of compressor, unlike a centrifugal compressor, does n ot have limitations t o its s peed range,especially w hen lubrication and cooling will not be hazardous.

Positive displacement compressors w ith VSD control save energy a t any load condition.Care must be taken w hen installing VSDs t o e nsure that the m achine is su itable for variable speedoperation, by examining minimum speed for the lubrication system and for the seals, but this d oes n otnormally create problems.

Dynamic compressorsThe m ost common dynamic co mpressor is t he centrifugal type which uses a rotating disk o r impeller


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in a shaped housing to force the gas t o the rim of the impeller, increasing the velocity o f the gas. Adiffuser section converts t he velocity e nergy t o pressure energy. These compressors a re primarilyused for continuous, stationary s ervice in industries s uch as o il reneries, chemical and petrochemicalplants a nd natural gas p rocessing plants. Their application can be from 75 kW to thousands of kW.With multiple s taging, they can a chieve extremely h igh o utput pressures g reater than 6 9 MPa.

The m ost common way t o c ontrol the c apacity of centrifugal compressor is t o m odulate inlet guidevanes. As t he l oad decreases, the mass o w of the a ir needs t o be reduced. Traditionally thecompressor is r un at the same speed, but the air ow is r educed by closing inlet vanes to match therequired capacity. Centrifugal compressors follow the same affinity l aws a s t hat associated withcentrifugal fans a nd p umps.

This i mplies t hat speed reduction also reduces t he compressors c apability o f generating pressureincrease, or lift. This means that a centrifugal compressors operational range through speed variationis l imited.

However, remarkable s avings ca n b e a chieved thanks t o the cu bic relationship b etween sp eed a ndpower w hich states that the need for power increases with the cube of the speed. This m eans that asmall increase i n s peed requires a l ot more p ower, but also that a m odest speed reduction c an g ivesignicant energy savings. A compressor r unning at half speed consumes only o ne-eighth of thepower compared to o ne running at full speed.

Applying VSDs t o centrifugal compressors, while still maintaining inlet vanes, brings e nergy s avingsthat can be estimated by comparing the required pressure to the pressure generating capability o fcompressor at a ce rtain sp eed.

Benets of VSDsSo the advantages o f using VSD control are that production of compressed air precisely follows t hevarying demand. As a result, direct energy savings o f between 5 to 35 percent can be achieved withlower energy co nsumption a t partial loads. Furthermore, precise control allows l ower dischargepressure. For example reducing system pressure from 7 to 6 bar reduces en ergy co nsumption by 7percent.

Indirect energy sa vings a dd u p to a nother 10 p ercent due t o lowered net pressure, thereby reducingsystem pressure and reducing leakages.

VSD control of compressors a lso brings i mproved process co ntrol by way of a stable pressureindependent of air consumption a long with less m echanical stress.

Other benets i nclude a stable s ystem pressure through a r apid reaction to pressure changes,typically w ithin 0.1 bar; smooth starting leading to no peak c urrent penalties, less stress onmechanical parts a nd unlimited start-ups; and a xed power factor.

Proven VSD-controlled c ompressorsDespite VSD-controlled compressors b eing relatively n ew applications, several installations h ave beensuccessfully t ried and tested.

In Belgium, a 2 75 kW xed spe ed compressor was con verted to a 315 kW variable spe ed compressor.


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During testing with three different load proles, savings b etween 18 and 25 percent resulted in savingsof over 15,200 per year, with a payback of just three years.

Meanwhile a European biochemical company h ad a d emand for oxygen with very con stant pressurebut where the oxygen volume is h ighly va riable. The system was e arlier controlled by switching on andoff two compressors o f different sizes. Because of problems w ith high power consumption, noise leveland m aintenance co st the larger compressor was eq uipped with a VSD. This resulted in e nergysaving o f about 1,700,000 kW h/year with a resulting reduction in C O2 e missions o f 850,000 kg /year.

Life cycle costsMany use rs favour traditional control methods b ecause these m ethods a re e asy to implement andstraightforward to understand. Many w ill also justify t his a pproach by a ssuming that the cost of thewasted energy is l ess t han that of buying a VSD. But this i s n ot true. Unlike many other energy sa vingmethods which may o nly reduce energy consumption by o ne or two percent, the very si gnicantsavings p roduced by a VSD means payback i s o ften achieved in a year or less. The application will

then continue to make a very s ignicant contribution to energy sa vings, year after year, for as l ong asit remains i n use.

Energy appraisalIn order for a c ompany to r educe energy co sts, it needs t o e valuate how it uses e nergy a nd a ir. Aprofessional air audit is t he best way to see if a VSD compressor is r ight for your application. Theseaudits a re available from various co mpressed air specialists, which can tell you how much yourcompany ca n save by installing a variable-speed drive compressor.

In addition, VSD manufacturers, such as A BB, offer an energy appraisal which is a systematicexamination of key a pplications t hat include the monitoring of energy co nsumed before and after thechange to VSDs.

An e nergy a ppraisal denes w here energy can be saved and quanties how much e nergy can besaved with the installation of VSDs. These gures a re then translated into a possible monthly sa ving,the amount of money that will be saved, in energy bills alone, if the equipment is installed.

It is n ot unusual for users t o d ismiss t he p romise of 50 p ercent energy sa ving o n a 20 p ercent speedreduction as the exaggerated claims o f a manufacturer. However the savings ca n be veried and thebest way to start is w ith an energy s urvey. This w ill enable you to see the potential savings i n blackand white, enabling you to make the decisions t hat bring your company improved protability.

Steve R uddellABB LimitedTel: 01925 741 111; Fax: 01925 741 212;Email: [email protected]; Web: www.abb.co.uk/energy

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Energy Saving With VSD Controlled Compressors