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COMPRESSOR TYPES, CLASSIFICATIONS, AND APPLICATIONS

by David H. Robison

Market Specialist

Sundstrand Fluid Handling

Arvada, Colorado and Peter J. Beaty

Senior Consultant

Du Pont

Newark, Delaware

concepts to all interested parties. This group will also David Robison is a Market Specialist, certainly need a working knowledge of aerodynamics, blade de­ specializing in new compressor products at sign (and repair), magnetic bearing theory, and advanced ther­ Sundstrand Fluid Handling in Arvada, modynamic concepts. The compressor types introductory program Colorado. does not address these more advanced topics. He is responsible for new compressor Compressor Types starts at the beginning of the user/manufac­ introduction and development and has been turer relationship with applications. Moreover, utilizing Balje's involved in positive displacementcompres­ [1] work on specific speed as a focal point, the following concepts sor along with positive displacement will be covered at entry level:

and centrifugalcompressor packaging, dur­ • , blower, compressor differentiation ing his 14 year career. • Curve shape and where to operate Mr. Robison received a Bachelor's de­ gree in Mechanical Engineeringfrom Tulsa University, is a member • Head, flow, horsepower; calculations of the Natural Gas Processor Supplier Association, and ASME. • The specific speed of positive displacement machinery

• The specific speed of single stage centrifugals

Peter J. Beaty is a Senior Consultant • Applications specializing in turbomachineryat DuPont's Vents Engineering Center in Wilmington, Dela­ ware. He is responsible for specification Flares and selection of process and Oxidizers drivers, and has been involved with numer­ ous compressor startup and troubleshoot­ Overhead recompression ing assignments during his 26 years with Process recompression DuPont. Transmission Mr. Beaty received a Bachelor's Degree in Mechanical Engineering fromVillanova The Compressor Types Program is an ongoing program offered University,is a registered Professional and a member of Pi by the Laboratory at Texas A&M University to Tau Sigma, Tau Beta Pi, and ASME. He is on the ASME B19 assist industry professionals in gaining exposure to elementary Committee for Compressor Safety,and represents DuPont onAPI' s compressor concepts to help them assume their broadening com­ Committee forRefinery Equipment. He is an active member of the pressor responsibilities in the 1990s. API Subcommittee on Mechanical Equipment. DEFINITION AND OPERATION Fans, blowers and compressors are designed to deliv­ ABSTRACT er gas at a pressure higher than that originally existing. Pressure rise, working pressure, specific speed and mechanical design form The compressor industry has emerged from the decade of the the basis of differentiation and classification. Initially, these ma­ 1980s right sized, streamlined, and computerized. Management chines can be divided into positive displacement and dynamic trends include a broadening of responsibility for all departments. categories (Figure 1). In order to satisfy these new responsibilities, maintenance, oper­ The methods employed to achieve compression are: ations, and engineering personnel need continuous review of compressor types, classifications,and applications. • Trap consecutive quantities of gas in some type of enclosure, Companies are discovering the void in talent that right sizing has reduce the volume, thus increasing the pressure, then push the created and most organizations retain a core group of experienced compressed gas out of the enclosure.

professionals who are utilized as a reference resource. The turbo­ • Trap consecutive quantities of gas in some type of enclosure, machinery grass roots introduction seeks to present elementary carry it without volume change to the discharge opening, com-

183 184 PROCEEDINGS OF THE TWENTY-FIRST TURBOMACHINERY SYMPOSIUM

COMPRESSORS Sliding Vane

Sliding vane compressors are rotary positive displacement com­ I pressors (Figure 3). A slotted cylinder is fitted with nonmetallic POS�IVE CONTINUOUS vanes and placed eccentric inside a tube. As the slotted cylinder is DISPLACEMENT FLOW turned, the vanes slide along the inner wall of the tube forming regions of changing volume.

rSCAnNG r=oR ROTARY DYNAMIC

LIQUID HELICAL AXIAL LOBE FLOW

SLIDING STRAIGHT CENTRIFUGAL. MIXED VANE LOBE FLOW

Figure 1. Compressor Types. pressing the gas by overcoming back flow from the discharge system and pushing the compressed gas out of the enclosure.

• Compress the gas by the mechanical action of rotating impel­ Figure 3. Sliding Vane Compressor. lers or bladed rotors that impart velocity and pressure to the flowing gas. Additional velocity energy in the gas is converted to pressure in an adjacent stationary diffuser or blade. Liquid Ring • Entrain the gas in a high velocity jet of another compatible gas and convert the high velocity of the mixture into pressure via a Liquid ring compressors utilizea squirrel cage fan type impeller diffuser. which is placed eccentric inside a tube (Figure 4). A compatible liquid is introduced into the chamber along with the gas to be COMPRESSOR TYPES DESCRIPTION compressed. Because of the centrifugalforce and the shape of the Positive displacements machines work by mechanically chang­ internal cavity, the liquid forms an eccentric shape producing ing the volume of the working fluid.Dynamic machines work by regions of changing volume. The liquid must be separated from the mechanically changing the velocity of the working fluid. compressed gas after the compression process and recirculated.

Reciprocating Piston

Reciprocating compressors are positive displacement machines in which the compressing and displacing element is a piston having a reciprocating motion within a cylinder. The overall cycle is shown in Figure 2 with the four typical phases of , compres­ sion, discharge, and expansion. Inlet valves are open from 4 to 1, and discharge valves are open from 2 to 3.

t a: :::::1 (/) (/) w a: c.

1 INTAKE

VOLUME� Figure 2. Reciprocating Compressors. Figure 4. Liquid Ring Compressor. TUTORIAL ON COMPRESSOR TYPES, CLASSIFICATIONS, AND APPLICATIONS 185

Rotary Lobe

Two straight mating lobed impellers trap the gas and carry it from intake to discharge (Figure 5). There is no internal compression.

DISCHARGE

Figure 7. Centrifugal Compressors.

DISCHARGE li

INLET

Figure 5. Straight Lobe, TwoRotor Compressor.

Helical Screw Figure 8. Axial Compressors. Two intermeshing rotors compress and displace the gas (Figure 6). The gas is trapped in the rotor pockets at one end; it is compressed between the intermeshing rotors and discharged at the is generally expressed through graphs of pressure produced, power opposite end. Some helical screw compressors operate with fluid required and efficiency vs flow. Organizing the family tree of and these are called floodedscrew compressors. The fluidprovides compressor types by pressure and flow in Figure 9 shows the a liquid seal around the rotors, absorbs the of compression, relative performance position of the various compressor types and allowing the to produce a greater pressure rise. The fluid makes visible the dilemma of application engineering; which must be removed from the gas after the compression process. compressor type is suitable for actual process problems. For flowrates greater than 10,000 cfm, the choice of a multistage axial is relatively simple. For pressure ratios greater than 20, the choice of a multistage reciprocating compressor is simple as well. How­ ever, most compressor applications have a pressure ratio less than 20 and a flowrateless than 10,000 cfm, which requires a compres­ sion method evaluation for a given process. All of the positive displacement and dynamic compressor types make contributions in this performance zone. For a given flowand pressure rise, each type of compressor can be satisfactory yet accomplish the task in a much different way. The graph of compressor type via curve shape in Figure 10 highlights the differences in curve shape between positive displacement and dynamic compressor . Figure 6. Helical or Spiral Lobe Compressors. Positive displacement designs tend to produce variable head at a

Centrifugal Compressors

Centrifugal compressors are dynamic machines in which the rapidly rotating impeller accelerates the gas (Figure 7). The pro­ cess flow propagates from axial to radial (perpendicular to shaft centerline)into a stationary diffuser converting velocity to pressure. Axial Compressors

Axial compressors are dynamic machines in which the gas flow is accelerated in an axial and peripheral direction by the rotation of specially shaped blades (Figure 8). The process flow is parallel to IIULTI8TAGE AXIAL shaft centerline. Stator blades allow the recovery of velocity to pressure. 2 Compressor Performance

A variety of factors such as type, discharge pressure, capacity, FLOW RATE (CFM) speed, power range, compression ratio, and specific speed can be used to differentiate fans, blowers and compressors. Performance Figure 9. Compressor Types. 186 PROCEEDINGS OF THE TWENTY-FIRST TURBOMACHINERY SYMPOSIUM

140 \ K/K(K-1) (K-1}/K � Required Head= foot pound per pound mass at design 120 w w a::cn ) 1545 K ( p2 )(K-1)/K �a: ----- H-zx x T x--x( - -1 (2) 13 � 100 ------.� -- I MW K-1 P a::W I a.::t: ==- �- .. t-a:: - - 80 - • • Paeihe I 3 ffi 0 - �ComprwMcn Actual flow rate at compressor inlet= ACFM act ft /minute 0 a: ----.flow � \ w D. 60 T 1 p ACFM=SCFM x- x � (3) T P std I 50 60 70 80 90 100 110 120 PERCENT FLOW

Figure 10. General Characteristic. Mass flowrate= Q* = SCFM/379xMW pounds mass per minute

relatively constant flow while dynamic designs tend to produce lmol MW variable flowat a relatively constant head. For the purposes of the Q* -SCFM X-- X-- (4) 379scf lmol turbomachinery program, the interest here is in dynamic designs and when to choose a dynamic fan, blower, or compressor. Q* x H Fan, Blower, Compressor Differentiation Hp- -�--- (5) 33,000 x EFF While differentiation of dynamic compressor types is possible utilizing dimensional analysis it is more common to loosely classify these machines by the pressure rise they produce. ( P (K-1)/K ( 2) • Fans produce a pressure rise of 0.25 psi to 3.0 psi P (6) Disch. temp = T2= T1 _.:... I ___ • Blowers produce a pressure rise of 1.0 psi to 8.0 psi EFF

• Compr�ssors produce a pressure rise greater than 5.0 psi AssumeEFF = 0.6 or, estimateEFF fromspecific speed graph. Curve Shape and Where to Operate Calculations are based upon mol range of 16 to 50.

Equipment selected to operate at, or near, its absolute design DIMENSIONAL ANALYSIS AND SPECIFIC SPEED limits generally requires a more aggressive maintenance program. It would seem prudent, then, to operate at the extremities of the Mathematicians have confirmed the notions of machine design­ centrifugal compressor curve only when absolutely necessary or ers that for each type of machine there is an optimum geometry when adequate system controls are included in the design. While which allows the machine to operate in its best efficiency range. the positive displacement compressor is traditionally pressure For compressors, dimensional analysis has been used to identify controlled centrifugal compressors are flow controlled. At the the speed in rpma given machine geometry must operate in order extreme right side of the centrifugal performance curve the pres­ to raise one cubic foot of gas one foot in head. Moreover, by sure rise tends to drop. This cutoffregion is called stone wall. For generalizing geometries into specific diameters a plot of specific a given impeller, diffuser and housing design, energy losses speed vs specific diameter for the compressor family tree in Figure increase dramatically as the gas flowthrough the machine increas­ 11 reveals that for each machine design there are ideal operating es. The ability to predict the pressure rise of a given design is ranges where compressor efficiencies are optimum. difficult and therefor unattractive in this unstable flow region. As Required Data system resistance increases flowthrough the centrifugal machine decreases and at low flow, which is particular to the given hard­ N rpm,operating speed ware design, the centrifugal will surge. Surge occurs when the Vl actual cubic feed per second, inlet flow process flowreverses around the impeller and is aerodynamically and mechanically undesirable.

HEAD, FLOW, HORSEPOWER, 1-.• "'"""' ... PITOT SPECIFIC SPEED: CALCULATIONS ,!==� 1::::, .1 �'�.. ' � . •.·� -� Required Data � ' ···� .... "' - �I P-1 psia, suction pressure kg/cm2 v, •Ad.t.'JMc. ' H111•fl.lbi/lb, P-2 psia, discharge pressure kg/cm2 ·' ... ::::--._....._� T-1 °R, Suction °K ·' I :�-� I ·rM.r�11�&lM[cJt_, MW, mol weight ·' .3 .• 1 3 • 10 2030 10100 3110 81101000 3000.:1111110000 k, adiabatic ratio of specific heat N. SPECFIC SPUD SCFM, Rate of Flow Std ft3/min Nm3/min Figure 11. Generalized N5 D5 Diagram for Single Stage Pumps Z, compressibility; if unknown, use 1 for estimating only and Compressors at Low PressureRatios. From Turbomachines, Calculate: O.E. Balje.Reprinted by permission of John Wiley & Sons, Inc., Pressure ratio (ratio of compression)= p2 I pl (1) Copyright ©1981 [1]. TUTORIAL ON COMPRESSOR TYPES, CLASSIFICATIONS, AND APPLICATIONS 187

H foot pound force per pound mass, adiabatic head the oil wellhead, gas , and refinery, have been D feet, diameter vented to atmosphere. Environmental and regulatory issues con­ firm the public notion that: SPECIFIC SPEED AND DIAMETER EQUATIONS If you can't breathe it, you shouldn't vent it. V1·5 Ns=NX (7) If you can't drink it, you shouldn't spill it. H.1s To meet these types of demands, destruction techniques are being utilized to dispose of noncommercial (nonbreathable) va­ pors. These low pressure, compliance projects are of interest from an application perspective. D X H-25 Ds= (8) Vt-5 VENTS Vent applications include: coal mine methane, bio-pond vapor SPECIFIC SPEED OF POSITIVE feed, landfillgas, barge vapors, tank, tanker, rail car vapors, pump DISPLACEMENT MACHINERY tandem seal vent drum vapors, and aeration. From Figure 8, the reader is reminded that there are numerous Piston type compressors generally operate in the 300 to 1,800 mechanical choices for these applications. The following proce­ rpm range utilizing 4.5 in to 26.0 in diameter and stroke dure may prove helpful in making a compressor selection: lengths of 3.0 in to 13.0 in. Piston compressor designers tend to • Obtain the process conditions and gas quality. think in t�flllS of linear stroking velocities of 500 ft/min to 1,000 ft/min. The optimum specific speed and diameter ranges for these • Calculate the design point head flow, horsepower and dis­ machines is: charge temperature.

• Utilize specific speed and diameter criteria to narrow the Ns= 0.003 to 3.0 Ds= 3.0 to 30.0 choice of compressors.

Rotary lobe compressors operate in the 600 to 3,600 rpmrange • Compare available seals, , control requirements utilizing 4.0 in to 14.0 in diameter lobes. The optimum specific installation costs, operating costs, price, delivery, and vendor speed and diameter ranges for these machines is: service reputation.

• Select the compressor. Ns= 2.5 to 150 Ds= 0.3 to 5.0 EXAMPLES SPECIFIC SPEED OF SINGLE STAGE CENTRIFUGALS P1 = 3 in Hg Vacuum, P2= 15 psig, MW= 30, k = 1.39, Z= 1 T1= 100°F,Q=504MSCFD@ 14.7 &60 F,Elevation; SeaLevel. Single stage centrifugals operate in the 3,600 to 50,000 rpm range utilizing 4.0 in to 30.0 in diameter impellers. Centrifugal Atmospheric pressure - 14.65 psia designers tend to think in terms of impeller rotational tip speeds of P1 = 14.65 psia - (3 in Hg x 0.491 psi/in Hg) = 13.18 psia 500 to 1,000 ft/sec. The optimum specific speed and diameter p2= 14.65+ 15 = 29.65 psia ranges for these machines are: MW = 30 k Ns = 7.0 to 150 Ds= 1.0 to 10 = 1.39 T1 =460+ 100 Mixed flowand axial flow machines operateat similar speeds to From Equation ( 1) centrifugals, utilizing diameter and stators. The optimum specific speed and diameter ranges for these machines are: Pressure Ratio= P/P1 = 29.65/13.18 = 2.25 (k-1) /k = .281 Ns= 150 to 6000 Ds= .4 to 3.0 k/(k-1) = 3.56

APPLICATIONS From Equation (2) Vapor containment process problems produce the need, to 1545 1 H - 1 X X 560X 3.56 X (2.25·28 -1) - 26,276 ft lbf/lbm analyze and apply various types of compressors. Vapor compres­ 30 sion can be divided by technique into two groups: From Equation (3) Destruction Recovery 504,000 SCF 1 day 560°R ACFM= x x x Vents Overhead recompression 1 day 1440 min 520°R Flares Process compression Oxidizers Transmission 14.65 psia = 419ACFM 13.18 psia Recovery techniques have substantial visibility because of the required capital investment along with payback potential and From Equation (4) usually command the attention of a more senior engineering 504•000 SCF 1 day management team. Vapors which have a high sales value, or are Mass Flow Rate Q* = x x present in sizable quantities, are recovered, compressed, and sold day 1440 min in a liquid or gaseous state. Historically, low pressure, dilute vapors, and aromatics, such as 1 mol x 30 Ibm = 27.7 lbm/min those present at your convenience store gasoline pump, as well as 379 SCFM 1 mol 188 PROCEEDINGS OF THE TWENTY-FIRST TURBOMACHINERY SYMPOSIUM

From Equation (5) Piston!Vane Lobe Centrifugal HPat 27 7x 26276 Design 27.57 49.01 Horsepower= · = 36.76 est. HP at design 33.93 33,000 X 0.6

From Equation (6) From Equation (6) Recalculate 2 25·281 -1 Discharge T 268.6 418 338 Disch. temp= 560 + 1)= 798°R - 460= 338°F ( · 0.6 Prepare installation, operation, life cycle cost matrix and com­ From Equation (7) plete evaluation. This example was created for comparison only. The choices are Piston!Vane Lobe Centrifugal more obvious if, for example, the flowrate is increased to 2.0 mmscfd, and the pressure is decreased to 5.0 psig. Typical rpm 1170 2,880 35,000 Note: For Balanced (2) Pistons Lobe Centrifugal Opposed Piston @1,200RPM @3,600RMP @15,000RPM Use 1/2ACFM Direct Connect Belt Drive GearDrive

0 For 2 MMSCFD; 3.75 126 527 1 < 2 9 ·5 )"5 2 o c-±!2..)" 5 ooo < .i!2..) " 5 11 0 88 35 NS and 5 psig Ns= __...:: 6=0- 60 , 60 26,276·75 26,276·75 26,276·75 discharge Q = 1663 Hd= 12,194

Ns= 1.059 3.69 44.82 A specific speed of 3. 75 is outside of the nominal piston design range and a better choice would be centrifugals or rotary lobes. Conversely, if the flowrateis decreased to 504 mmscfd and the From Figure 11 discharge pressure is increased to 30 psig: Optimum Ds 4.7 2.0 3.0 (2) Pistons Lobe Centrifugal @1,200 @2,000 @35,000

From Equation (8) Ns .728 1.76 31 Piston/Vane Lobe Centrifugal Q = 419 Hd= 43,294 0:0. 5 <2 5 )"5 * In this case, the rotary lobe is clearly not the best selection, and 4.7 3.0( ) 12 X- D = x _g_ while the centrifugal appears satisfactory, the specific diameter ft 26,276·25 26,276-25 ft calculation coupled with the 1000 to 2000 fps nominal tip speed limit probably the centrifugal into a series (two stage) ) D = (2 - 8.28" 4.98" 7.47" configuration making the piston or sliding vane design a more attractive selection.

From Figure 11 REFERENCE Eff. Estimate 0.8 0.45 0.65 1. Balje, 0. E., Turbomachines, New York: John Wiley & Sons, Inc. (1981). From Equation (5)

Recalculate 27.7 X 26,276 27.7 X 26,276 27.7 X 26,276 Horsepower 33,000 X 0.8 33,000 X 0.45 33,000 X 0.65