Nikola Stosic is Royal Academy ofEngineering Professor of Positive

Displacement Compressor Technologyat City University, London, where he

has established the Centre forPositive Displacement CompressorTechnology. In parallel with this

appointment in 1995, he held thepost of Head of the Thermo

Engineering Department at theInstitute of Process, Power and

Environmental Engineering inSarajevo. In 1992, Professor Stosictook up a research appointment at

City University to investigate theperformance of two-phase expanders.

He has carried out research workon a range of applications of heat

and fluid flow, including boilers,furnaces and compressors. Arisingfrom this work, he is the author

and co-author of over 150 refereedpublications and more than 130

major industrial reports, a textbookon screw compressors, a textbook

on computers in engineering and amajor text on boilers and furnaces. Professor Stosic’s research interestsare in mathematical modelling andcomputer simulation of thermal and

fluid flow processes, including:turbulence models in heat enginesand flow machines; experimental

methods for investigating theperformance of thermal machineryand systems, especially of positive

displacement compressors andboilers; the application of analyticaland experimental techniques to the

design of positive displacementcompressors, especially screw

compressors, boilers, furnaces andpipe networks; and industrial

investigations of large-scale thermalpower plants. He graduated inmechanical engineering at theUniversity of Sarajevo. He then

obtained an MSc at the Universityof Zagreb, followed by a doctorate

at the University of Sarajevo.

a report by

N i k o l a S t o s i c

Royal Academy of Engineering Professor of Positive Displacement Compressor Technology,

Centre for Positive Displacement Compressor Technology, City University

Screw compressors of the type that is employed in theprocess and gas industries are large and expensive,while their continuing function is usually essential forcontinuation of the entire process in which they play apart. The reliability of their operation is thus at least asimportant as their efficiency. In the past few years,significant advances have been made in the design andmanufacture of the main components of machines ofthis type, such as the rotors and the bearings, as well aslesser components. These have resulted in previouslyunthinkable improvements to both performance andreliability that have been widely applied to both air andrefrigeration compressors. Despite this, process gascompressors have not yet widely benefited from theseadvances because, for these applications, the numbersproduced are far fewer and they have a far longerdevelopment cycle than other compressors. Thus,improvements in process gas compressors is nowoverdue. Moreover, experience already gained in otherapplications can be incorporated at the design stage,with the minimum of added development time andcost, by simultaneous consideration of all the relevantvariables that affect their operation.

R e l i a b i l i t y – A V i t a l D emand i n G a s a n d P r o c e s s S c r ew C omp r e s s o r s

Screw compressors have a growing role in the gasand process industry, where their power requirementis high and machine sizes are large. They are simplemachines in which the movement of the parts ispurely rotational.1 Therefore, they are typically up tofive times lighter than their reciprocatingcounterparts of the same capacity and have a nearlyten times longer operating life between overhauls.Moreover, provided that the running clearancesbetween the rotors and between the rotors and theirhousing are small, they can maintain high volumetric

and adiabatic efficiencies over a wide range ofoperating pressures and flows.2–4

Specialised machine tools now enable the mostcomplex screw rotor shapes to be manufactured withhigh tolerances at an affordable cost. Their use in screwcompressor manufacture, together with advancedrolling element bearings, has led to substantialimprovements in performance and reliability. Thedevelopment of rotor profiles has recently beenenhanced by advances in mathematical modelling andthe computer simulation of the thermodynamic andfluid flow processes within the compressor.5 Theseanalytical methods may be combined to form powerfultools for process analysis and optimisation and aresteadily gaining in credibility as a means of improvingdesign procedures. As a result, the design of screwcompressors has evolved substantially over the past 10years and is likely to lead to additional improvementsin machine performance in the near future.

Screw compressors in normal use today are based onrotors whose outer diameters range betweenapproximately 75mm and 620mm. These deliverbetween 0.6m3/min and 120m3/min of compressedgas when operating as oil-flooded machines andbetween 5m3/min and 600m3/min in the dry mode.The normal pressure ratios attained in a single stage are3.5:1 for dry compressors and up to 15:1 for oil-flooded machines. Usual stage pressure differences areup to 15 bars, but maximum values sometimes exceed40 bars. Typically, the volumetric efficiency of thesemachines is now over 90%, while specific power inputsthat are both size- and performance-dependent havebeen reduced to values that were regarded asunattainable only a few years ago. The followinginformation is mainly derived from material publishedby the author, Smith and Kovacevic.6

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1. Arbon I M, The Design and Application of Rotary Twin-shaft Compressors in the Oil and Gas Process Industry,IMechE, MEP London (1994).

2. Sakun I A, Vintovie kompresorii (Screw Compressors), Mashinostroenie, Leningrad (1960).3. Rinder L, Schraubenverdichter (Screw Compressors), Springer Verlag, New York (1979).4. O’Neill P A, Industrial Compressors, Theory and Equipment, Butterworth-Heinemann, Oxford (1993).5. Xing Z W, Screw Compressors, Machine Press, Beijing (2000).6. Stosic N, Smith I K, Kovacevic A, Screw Compressors: Mathematical Modeling and Performance Calculation,

Springer Verlag, Heidelberg (2005).

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Inc reas ing the Re l iab i l i t y o f Sc rew Compres sor s in the Gas and Proces s Indus t r i e s

R equ i r emen t s a n d Me a s u r e s f o r H i g h R e l i a b i l i t y

A key requirement for the successful design of alltypes of compressor is the ability to predict accuratelythe effects on performance of the change in anydesign parameter, such as clearance, rotor profileshape, oil or fluid injection position and rate, rotordiameter and proportions and speed. In the case ofscrew compressors, the main requirement is toimprove the rotor profiles so that the internal flowarea through the compressor is maximised while theleakage path is minimised. Also, internal friction dueto relative motion between the contacting rotorsurfaces should be made as small as possible in orderto permit higher rotor speeds without excessivemechanical losses. Recent improvements in bearingdesign make process fluid lubrication possible in someapplications. Moreover, seals are more efficient today.

All these developments can be utilised to produceeven more efficient, lighter and cheaper screwcompressors. Full compressor optimisation, whichhitherto has not been used extensively, is especiallyimportant and, when properly applied, leads to aunique compressor design for each application. Thisincludes simultaneous consideration of thefollowing factors.

T h e r m a l E x p a n s i o n o f t h e

R o t o r s a n d H o u s i n g

Modern rotor manufacturing methods, such asgrinding with simultaneous measurement, controland correction of the profile, enable the profiletolerance to be maintained within ±5µm. Thisenables the clearances between the rotors to be keptbelow 15µm. With such small clearances, rotorcontact is very likely and, hence, the profile and itsclearance distribution must be generated in such amanner that damage or seizure will be avoidedshould this occur.

In order to maximise screw compressor delivery ratesand efficiencies, interlobe clearances must be made assmall as possible without the likelihood of hard rotorcontact between the rotors. The traditional practiceto avoid rotor contact and seizure was to makeclearances larger than required by manufacturinglimitations. However, these clearances can beminimised by making their distribution non-uniformaround the profile so that if hard rotor contactoccurs, it will not be in rotor areas where slidingmotion between the rotors is dominant.

The following main effects must be allowed for inthe design in order to ensure this.

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Although the temperature range over which screwcompressors operate is not large, the effects ofthermal expansion are highly significant if the smallclearances required between the rotors and betweenthe rotors and the housing are to be maintainedunder working conditions. Thus, the rotor clearancesobtained under manufacturing conditions must beestimated while taking account of thermal distortionthat will occur when the compressor reaches itsoperating temperature and pressure.

C l e a r a n c e D i s t r i b u t i o n o n t h e

R o t o r L o b e F l a n k s

Oil-flooded compressors have direct contact betweentheir rotors. In well-designed rotors, the clearancedistribution will be set so that this is first made alongtheir so-called contact bands, which are positionedclose to the rotor pitch circles. As the relative motionbetween the contacting lobes in this region is almostpure rolling, the danger of their seizing, as a result ofsliding contact, is thereby minimised.

The traditional approach is to maintain a high so-called positive gate rotor torque, which ensuresround flank contact. What is not widely appreciated isthat there are significant advantages to be gained bymaintaining a negative gate rotor torque to ensure thatcontact, when it occurs, will be on the flat face. Thus,minimising the clearance on the flat flank will reducethe interlobe leakage more than minimising the roundflank clearance. Also, negative gate torque is achievedby making the gate rotor lobes thicker and the mainrotor lobes correspondingly thinner. The displacementis thereby increased. Thus, both these effects lead tohigher compressor flows and efficiencies.

D i s p l a c e m e n t o f t h e B e a r i n g C e n t r e s

Due to the fact that there must be some clearance inthe bearings, the pressure loads will tend to push therotors apart and displace their centres from their designposition with respect to the compressor housing. Thus,if the bearing centres are set to be the same as those ofthe rotors, the rotors will be eccentric and, as a result,the clearance between the rotors and housing will besmaller at the low-pressure side of the rotors and largerat the high-pressure side. As leakage is caused by thepressure difference, this displacement creates the leastfavourable rotor position for efficient compressoroperation. Also, the resulting rotor displacements fromtheir design positions may cause contact between therotor tips and the housing unless allowance is made forthem during the design. The situation can be remediedby making the bearing centre distance smaller than thatof the rotor housing and aligned to maintain a uniformclearance between the rotors and housing. Tominimise the rotor interlobe clearance, the bearingcentre distance must be even further reduced.

L o a d S u s t a i n a b i l i t y

A general feature of screw compressors is that thepressure difference through them causes high rotorloads, especially in low-temperature process compres-sors, where these are large. Therefore, to maintain theirrigidity and minimise deflection, the rotors areregularly profiled with a relatively small male rotoraddendum in order to increase the female rootdiameter. This sometimes leads to very shallow andclumsy rotors. An alternative possibility is to increasethe female rotor lobe thickness; this greatly increasesthe rotor moment of inertia and thereby reduces therotor deflection more effectively. In some compressordesigns, multiple cylinder roller bearings or multipleball bearings are located at the high-pressure end of therotors to withstand the large radial forces reliably overa long operating life. Frequently, two or more bearingsare also employed for axial loads. As only one axialbearing works, the role of the other is usually to clamprotor and prevent it bouncing in the axial direction.

Comp r e s s o r O p t im i s a t i o n

Even a simplified analysis of rotor characteristics canshow that a number of these lead to conflictingdesign requirements. This implies that, given thecompressor duty, simultaneous optimisation of all thevariables involved in the design process must beperformed to obtain the best possible performance.The full rotor and compressor geometry can becalculated from the rotor transverse plane co-ordinatesand the rotor length and lead. The built-in compressorvolume ratio must also be included as an optimisationvariable. The values of these parameters should beused as input parameters for the calculation of thescrew compressor thermodynamic process, normallyby using a mathematical model. Minimisation of theoutput from the process equations leads to theoptimum screw compressor geometry and operatingconditions. These can be defined as either the highestflow and compressor volumetric and adiabaticefficiencies or the lowest compressor specific power.

Con c l u s i o n

The performance of process gas screw compressors ishighly dependent on their rotor profiles and clearancedistribution. Moreover, detailed attention is required inthe design of other compressor components, such asthe housing ports, bearings, seals and the lubricationsystem in order to obtain the best results. If all thesefactors are considered simultaneously, the compressorreliability will also be increased. Using up-to-date rotorgeneration methods, specialised compressor designs arepossible that are superior in performance and reliabilityto most of those in contemporary use and well suitedto a growing range of applications and operatingconditions in the process and gas industries. ■38

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