positive-displacement compressors compressors lesson one reciprocating compressors 46201 preview ......

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Table of Contents

Lesson One Reciprocating Compressors..............................................................3

Lesson Two Sliding-Vane Rotary Booster Compressors....................................25

Lesson Three Oil-Flooded Screw Compressors...................................................39

Lesson Four Screw Compressor Units................................................................57

Lesson Five Ammonia System Lubrication/Oil Management...........................73

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Positive-DisplacementCompressors

POSITIVE-DISPLACEMENT COMPRESSORS

Lesson One

ReciprocatingCompressors

46201

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1

4

Lesson

Reciprocating Compressors

History of Reciprocating Compressors Multicylinder Industrial Refrigeration

CompressorsDesign Features of Industrial Ammonia

Reciprocating CompressorsCapacity Control

Reciprocating Compressor LubricationTypical Compressor EfficiencyCompressor Application DataCompressor UnitsCompound Reciprocating Compressors

TOPICS

After studying this Lesson, you should be able to…

• Briefly describe the evolution of ammonia recipro-cating compressors.

• Describe typical design features of today’s recip-rocating compressors.

• Explain how capacity control and proper lubrica-tion are achieved in ammonia reciprocating com-pressors.

• Explain how to use volumetric and adiabatic effi-ciency data and the performance factor in sizingor selecting compressors for an application.

• Describe the function and basic design require-ments of internally compounded reciprocatingcompressors.

OBJECTIVES

Capacity control 1.40 the action of loading andunloading cylinders, usually in response to suctionpressure on industrial reciprocating compressors

Volumetric efficiency 1.51 the ratio of the actualvolume that a compressor can pump at a givenset of conditions to the theoretical displacement ofthe compressor, both in cfm

Compression ratio 1.52 the ratio of the dischargepressure to the suction pressure, both in psia

Adiabatic efficiency 1.54 the ratio of the powerrequired for isentropic compression to the actualshaft power delivered to the compressor, both inbhp

Performance factor 1.55 a measure of the rela-tionship between the actual power required at aspecific condition to the capacity rating of thecompressor

KEY TECHNICAL TERMS

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History of Reciprocating Compressors

1.01 The refrigeration systems being developed inthe mid-1800s were of three different forms:

• air machines, which cooled by the expansionof compressed air

• absorption machines, which used ammonia asthe refrigerant and water as the absorbent

• vapor-compression machines, which initial-ly used ether and later ammonia as therefrigerant.

The vapor-compression refrigeration system usingammonia reciprocating compressors soon became thedominant method for product cooling and making ice,far exceeding the earlier air-compression cycle andabsorption systems in efficiency and equipmentcapacity. These early reciprocating compressors wereof either vertical or horizontal design, driven by steamengines directly or belt connected. Operating speedswere about 150 to 300 rpm.

1.02 The earliest reciprocating refrigeration com-pressors typically consisted of two single-acting pis-tons arranged vertically. Figure 1-1 shows a two-cylin-der vertical compressor arranged with a direct-drivesteam engine. The vertical frame supports the cylindersand pistons and houses the vertical connecting rod and

its guide bearing. The catwalk provides access to thecylinder valves and also to the open-topped woodenwater jackets through which cooling water flowsaround the cylinders to provide cooling for the heatgenerated in the compression process. Note the con-necting rod guide bearing located just below the cat-walk deck and the piston rod packing gland at the baseof the cylinder. Also noticeable is the long stroke of thepiston and the valving at the top of the cylinder.

5

In the early- to mid-1800s, there was an interest in the artificial creation of coldtemperatures for the production of ice and refrigeration purposes. At that time,natural ice was already being distributed worldwide, cut from the northern lakesof Europe and America. In the 1870s both David Boyle, an American mechanic,and Carl von Linde, a German scientist, independently patented and built practi-cal ammonia compressors. Linde’s compressors soon outsold Boyle’s, and in1881 there were 750 Linde refrigeration systems in 445 breweries.

About this time, in the 1880s, two machinery companies from southern Pennsyl-vania, Frick and York, entered the ammonia compressor refrigeration field. Theyhad been making stationary steam engines and farm equipment and had the nec-essary facilities, knowledge, and skills for the manufacture of reciprocatingmachinery so that they could readily enter this new market.

In this Lesson, you will read about the commercial developments leading totoday’s industrial ammonia reciprocating compressors. Contemporary designfeatures are discussed in detail, including a commonly used capacity-controlsystem. This Lesson also discusses the lubrication system for reciprocatingcompressors and alternative compound reciprocating machines.

Fig. 1-1. Early Frick VSA two-cylinder compressor


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1.03 Single-acting refers to the action of the pistonand the compression cycle. The suction stroke occursas the piston descends, filling the cylinder with low-pressure ammonia vapor at the evaporator pressure.This is followed by a compression and dischargestroke as the piston ascends to top dead center. Duringevery revolution, there is one high-pressure dischargeper cylinder. The two cylinders are displaced by 180°so that, for the entire compressor, there are two dis-charge strokes per revolution.

1.04 Concurrently with the development of thevertical single-acting machines, horizontal double-acting compressors were also introduced. These couldalso be either belt driven or directly connected with asteam engine. These compressors used both sides ofthe piston by compressing gas on one side whiledrawing low-pressure vapor into the expanding cylin-der on the other side at the same time. Thus, each rev-olution produced two discharge strokes per piston.Characteristic of these machines is the dual suctionand discharge piping to both ends of the cylinder. Fig-ure 1-2 shows this arrangement in a cutaway view.

1.05 These early compressors were really nothingmore than rigidly supported, large compression cylin-ders with most of the components—connecting rods,crankshafts, and moving parts—located outside the

ammonia atmosphere. However, these machinesdeveloped into what became known as the enclosedcompressor, typically arranged vertically, with two orfour cylinders in line. These compressors werereferred to throughout the industry as HDI (heavy-duty industrial) or VSA (vertical single-acting) com-pressors.

1.06 The enclosed HDI compressor required con-siderably less maintenance than the early compres-sors, which consisted mainly of a structure supportingthe ammonia cylinders. The crankshaft and all run-ning gear of the enclosed compressor was containedwithin the housing, which also included an oil sumpfrom which the crankshaft, main bearings, connectingrods, and running gear were lubricated. This enclo-sure, which contained the ammonia atmosphere, elim-inated the need to hand-oil the compressor wear com-ponents, except for the pedestal bearing outboard ofthe flywheel. Figure 1-3 shows a cutaway of anenclosed HDI compressor.

1.07 The water-cooling jacket was also an inte-gral part of the cast compressor housing and sur-rounded the upper compression portion of the cylin-ders. The cooling jacket was no longer open, butnow was closed with the top cover. Cooling waterwas piped to and from the jacket. The cylinder suc-

6 Lesson One

Fig. 1-2. Early Frick horizontal double-acting compressor


1009

1008

1015

1037

1013

1014

1024

1000

1024

10101020

1019

10161017

1018

1021

1022

1023

1024

1025

1035

1026

10111006

1032

1034 1038

1034

1004

1032

1031

10111025

1001

1028

1039

1030

1005 1012

1033

1027 1028

1029

1003

1002

1014

1013

1000 COLUMN1001 BASE1002 BELT-WHEEL1003 OUT-BOARD BEARING CAP1004 CRANK SHAFT1005 BEARING-STUFFING BOX END1006 BEARING-OUTER1007 HAND-HOLE COVER (NOT SHOWN)1008 JACKET COVER 2-CYL.1009 AMMO. CYLINDER HEAD1010 PISTON1011 CONN. ROD1012 STUFFING BOX GLAND1013 DISCHARGE VALVE CAGE1014 DISCHARGE VALVE1015 SAFETY HEAD SPRING1016 DISCH. VALVE SPRING PLUG1017 " " "1018 SUCTION VALVE1019 SUCTION VALVE CAGE SCREWS1020 SUCTION VALVE SPRING1021 " " CAGE1022 CUSHION NUT1023 CROSSHEAD PIN LOCK PIN1024 PISTON RING1025 CASTLE NUTS1026 CONN. ROD BOX (CROSS HEAD END)1027 HOLLOW CUP POINT SET SCREWS1028 1/2" PIPE PLUG1029 OIL CHAIN1030 STUFFING BOX WASHER1031 CONN. ROD BOLT LINERS1032 DIE CAST BUSHING1033 KEY (BELT WHEEL)1034 END OIL SHIELD1035 CROSS HEAD PIN1036 CONN. ROD BOLT (NOT SHOWN)1037 WATER COVER GASKET1038 CONN. ROD BOX (CRANK END)1039 PEDESTAL1040 CONNECTING ROD LINER

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1.08 So far, the compressor prime movers weresteam engines, either direct or belt driven. At theturn of the century, however, with the productionand distribution of electric power and the commer-cial availability of electric motors, the electricmotor soon became the compressor driver ofchoice. The motors, which were dependent on thepower available, ranged from dc motors to large-diameter synchronous ac motors and eventually tothe squirrel-cage ac induction motors commonlyused today.

1.09 Most notable of the improvements duringthe first half of the 20th century was the use ofmechanical shaft seals to replace the packing glands

used in the original designs, thus reducing ammonialeakage and providing for longer run times withoutthe need for periodic packing-gland adjustment andreplacement. A second major improvement was theincorporation of an oil pump, driven at the crank-shaft, which provided force-fed lubrication to therunning gear and wear elements. A third improve-ment was the addition of methods of refrigerationcapacity control.

Reciprocating Compressors 7

Fig. 1-3. Sectional view of Frick enclosed two-cylinder compressor

Don’t think that all the old machines anddrivers are a thing of the past. In 1998 theauthor witnessed a 1920s-vintage four-cylinder Frick vertical HDI compressorstill in regular daily operation, beingdirectly driven by a slow-speed synchro-nous motor. The motor was a fairly opendesign, about 12 in. wide and 6 ft in diam-eter. The machine ran smoothly and quiet-ly, with only a characteristic click, click,click valve noise noticeable.

Application 1-1


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Multicylinder Refrigeration Compressors

1.10 Probably the influence of automotive enginedesign led to the opportunity for increasing the capac-ity of the compressors while reducing the physicalsize and system cost. Higher production rates of com-mon parts led to direct replacement and use of partsinterchangeably in all like-model compressors, in turnimproving reliability and serviceability.

1.11 The requirement for greater refrigerationcapacity and smaller equipment size dictated smaller-diameter pistons and higher speeds. The early large-bore compressors were limited in capacity by themaximum speeds at which they could run. This limi-tation was due to several causes:

• Flow areas at the suction and discharge cylin-der valves limited the volume of vapor flowinto and out of the cylinder, and for the largemachines, this volume was reached at the lowrunning speeds already attained.

• The weights of the pistons and connectingrods and their physical size made operation athigher speeds unacceptable because of forcescaused by unbalance vibration.

• The physical size of the two- and four-cylindermachines proved it was impractical to continueadding additional cylinders to obtain morecapacity. This approach would have driven thecompressor costs to a prohibitive level.

1.12 The direction in which to move became clear.If larger-size bores are a limiting factor, make the pis-tons smaller, decrease the stroke, run at a higher speed,and put more cylinders within a compressor housing—

and all this was done. It was accomplished at first withcylinder bores of about 6 to 8 in. and strokes just equalto or less than the bore. These compressors weredeveloped in the mid-1900s. Running speeds wereconsidered breakneck at 500 to 800 rpm, and compres-sors were manufactured in V and W arrangementswith up to eight or nine cylinders per compressor.

1.13 Machines in this new generation of industrialcompressors were much smaller than the large HDImachines and, because of the speed and multiple cylin-ders, had a much greater refrigeration capacity. Theyused the latest technology regarding shaft-sealing tech-niques and had automatic cylinder-unloading capabilityfor capacity control. In addition, the crankshafts con-tained counterweights and were balanced to run vibra-tion-free. This made installation much easier, becausethe compressor package could be mounted on an exist-ing sturdy concrete floor. The earlier HDI machinesrequired that a huge mass of concrete be installed underthe compressor to hold it in place during operation.

1.14 Following the mid-century mark, compressorbores again became smaller, speeds increased further,and some manufacturers increased the number of cylin-ders per compressor up to 16. The capacity of the com-pressors was also greater than before. Industrial com-pressors now had cylinder bores of 2.75 to 4.50 in. withmaximum speeds ranging from 1200 for most and upto 1800 rpm for certain R-12 and R-22 applications.

1.15 This is where the industry is now. It should benoted that this discussion has dealt with industrialammonia compressor development only. It has leftuntouched the concurrent development of the smallercommercial, residential, and appliance compressors,which are not suitable for industrial ammonia refrig-eration service.

Design Features of Reciprocating Compressors

1.16 Figure 1-4 shows a contemporary VilterModel 450XL series compressor. The 450XL seriesconsists of six compressor sizes with 2 to 16 cylin-ders, all with the same bore and stroke and usingcommon internal components. Most manufacturerstypically standardize on certain bore and strokedimensions for a line of compressors and then devel-op a range of compressors with two or four cylindersup to 12 or 16 cylinders to provide good coverageover a wide range of capacities.

8 Lesson One

Table 1-1. Refrigerating capacity of enclosed compressors

Compressorbore x stroke

No. ofcylinders RPM

Capacity (tonsat 0°F/96°F)

4 in. x 4 in.4 in. x 4 in.5 in. x 5 in.6 in. x 6 in.7 in. x 7 in.8 in. x 8 in.9 in. x 9 in.

1222222

290250250230220200200

1.62.85.68.9

13.118.225.8


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1.17 The external view of the compressor, shownin Fig. 1-4A, is typical of most industrial refrigera-tion compressors. Each top head contains two cylin-ders. This six-cylinder compressor has three topheads—the third is located symmetrically on thereverse side of the machine. The large angle valveon the left end is the suction valve through whichammonia vapor enters the compressor from theevaporator. The flanges of the smaller dischargevalve are just visible on the farther end of the com-pressor.

1.18 At that farther end below the valve is the belt-drive flywheel. The tank-type appendage below thesuction valve is the oil filter, and the small tubulardevice attached horizontally alongside the compressoris the oil cooler. Note the lubrication oil lines con-necting the oil filter housing cap to the oil cooler. Thetwo capped connections at the near end of the oilcooler are for the coolant (typically water), which isconnected at the jobsite.

1.19 The cylinder top head is made of two sec-tions. The lower thick section is the cylinder dis-charge head. The thinner domed section on top, con-taining the open elbow along its side, is the dischargecooling head, typically supplied with water or liquidammonia as the coolant to assist in removing some ofthe high discharge temperature associated withammonia compressors.

1.20 Figure 1-4B shows the internal section of thesame 450XL compressor. Note that these large indus-trial refrigeration compressors from different manufac-turers have quite similar construction and features,which you will also notice in sectional views later inthis Lesson. All compressors include an oil-supplysump at the bottom of the housing. The oil is directedthrough a fine-mesh strainer located in the sump up toan oil pump, which is driven directly by the crankshaft.This provides for a pressurized or force-fed lubricationsystem whenever the compressor is in operation.

1.21 Because the compressor must be in operationto establish and maintain the oil pressure, there is nolubrication oil flow for the first several revolutions ateach startup. All industrial compressors have oil-pres-sure cutout switches, sometimes with an alarm fea-ture, which stop the compressor should the oil pres-sure differential fall below a preset minimum. For rec-iprocating compressors, the ammonia pressure in thecrankshaft area, over the oil sump, is equalized to thesuction pressure and typically requires a minimumdifferential oil pressure of about 30 to 40 psi abovethe suction pressure.

1.22 Because there is no oil pressure differentialimmediately at startup, the oil-pressure cutout switchhas a timed bypass delay of at least 15 seconds. Thisdelay permits the compressor to establish oil pressureand avoid nuisance cutouts.


Spring loadedsafety heads

Suction anddischarge porting

Piston andconnecting rod

Double taperedroller bearings

Oil filter

Compression and oil rings

Crankshaft

Connectingrod bearings

Double bellowsshaft seal

A B

Fig. 1-4. Vilter 450XL series compressor–external and internal views


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1.23 The oil pump is generally an internal geartype with a crescent offsetting a center spur gear fromthe internal ring gear. Oil flow rates are about 3 to 5gpm, depending on compressor size. As alternatives,some manufacturers use external spur gear or vanedrotary pumps. Oil in the Vilter compressor is directedthrough an oil filter, then to the main tapered rollerbearings, and then through the crankshaft to lubricatethe connecting rods and the shaft seal area. Oil pres-sure is also used to actuate the compressor cylinderload/unload (capacity-control) mechanism, which isdiscussed later in this Lesson.

1.24 The main bearings on the Vilter compressorare double tapered roller bearings located at eitherend of the shaft. These bearings support the crank-shaft in the housing and carry the radial loadsimposed by the compressing action of the piston/con-necting rod set in each cylinder. The tapered rollerbearings also lock the crankshaft in position axially tocenter the connecting rods at each cylinder. Mostother manufacturers do not use tapered rollers, butinstead use journal bearings for the radial loads andthrust washers or collars for axial positioning.

1.25 The crankshafts are typically of forged steelor of ductile (nodular) cast iron. The ductile cast ironhas strength properties similar to those of steel forg-ings and permits economical production because itdoes not need forging dies, which are extremelyexpensive. Each crankpin carries from two to fourconnecting rods—three in the case of this Vilter450XL. At either end of the shaft, just inside the mainbearings, the shaft has counterweights that balanceout the primary imbalance associated with the recip-rocating action of the pistons and the upper portion ofthe connecting rods. The crankshaft bearing surfacesmay be flame- or case-hardened or given a specialsurface treatment to provide a microthin but hard-wear surface.

1.26 To be able to attach the shaft to a driver—via belt or direct connection to a motor—the shaftmust extend to the outside of the compressor. Theextension to the atmosphere must be sealed from theammonia environment within the compressor. Thisis accomplished with either a single or a doublemechanical shaft seal. One portion of the sealassembly is fastened to the rotating shaft and makesa static seal at the shaft. The other portion of the sealis fixed to the nonrotating housing and also makes astatic seal at the housing. The two highly polished,extremely flat, lapped mating surfaces are main-tained in flexible (spring-loaded) contact with oneanother to provide the running seal. The seal assem-bly runs in an oil bath that provides the necessarylubrication and cooling.

1.27 The connecting rods may be either forgedsteel or aluminum, typically with steel-backed babbittbearing shells at the crankshaft end and bronze orsteel-backed babbitt bushings at the wristpin end. Alubrication hole is generally drilled through the shankof the rod to lubricate the wrist pin area at both therod and the piston bosses.

1.28 The piston fits within the removable cylindersleeve and typically carries three to four piston rings,which are a combination of compression rings at thetop and one or two oil-scraper rings at the bottom.The manufacturer provides specific instructionsregarding the location and orientation of piston rings.They are important in providing the necessary cylin-der sealing for optimum capacity and minimum oilloss (carryover) from the compressor.

1.29 The shape of the head of the piston variesfrom manufacturer to manufacturer, depending on thedesign of the suction and discharge valve plates. Pis-ton heads may be flat, as on the Vilter compressor inFig. 1-4, or extended and dished, as on the Mycomcompressor shown later in this Lesson. In either case,the piston at top dead center will come as close aspractical to the cylinder valve service mounted on thetop of the cylinder liner. This clearance runs in therange of 0.030 to 0.045 in. and accounts for normalmanufacturing tolerances and some clearance for anoil film that might be carried on the top of the piston.It is important to come as close as possible to thevalving, but not to touch it with either the piston itselfor with any liquid film on the piston, because the filmis essentially incompressible.

10 Lesson One

CAUTION

Compressors that have been shut down for aprolonged period or for servicing must havesome oil manually pumped (via a hand ormechanical pump) through the lubricationsystem to ensure a supply of oil to all criticalbearing surfaces before startup. Failure tofollow this procedure may result in excessivewear and early compressor failure.


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1.30 In the top head are two large safety headsprings that hold the cylinder valve service assembliesin place. Their purpose is to provide a flexible clamp-ing of the valve plate to the cylinder sleeve, permit-ting movement of the valve plate from the sleeve ifexcess liquid oil or refrigerant is in the cylinder dur-ing the discharge stroke. This is to prevent componentdamage during slugging. Note that not all manufac-turers use this approach. Also, it is possible that thisfeature worked well on slow-speed (150 to 300 rpm)compressors, but is not effective at the higher speedsnow common. Proper operation and design shouldminimize the need for this feature, which should notbe depended on.

1.31 Figure 1-5 shows the internal section of aMycom 6WB compressor, including some additionaldetail that is not included on the Vilter 450XL section,but is typical of all industrial compressors. One fea-ture is the suction strainer screen basket on the com-pressor inlet. The other is the oil-level sight glassshown at the oil sump. Overall, the compressors lookquite similar, except that the main crankshaft bearingsare the journal type and the piston tops are sculpturedand scalloped to mate with the valving.

1.32 The journal bearings, which are commonlyused by most manufacturers, are easily seen on thesection of the Mycom 6WB. Also note that theinner ends of the journals become the thrust bear-

ings, which locate and maintain the axial positionof the crankshaft. This compressor also has a com-mercial filter with a spin-on cartridge (not shown),which is located externally along with the shell-and-tube oil cooler. Note the relative sizes of thesuction and discharge shutoff valves. The suctionvalve is always the larger because the inlet gas is ata lower pressure, is less dense, and takes up morevolume than the refrigerant at the discharge aftercompression.

1.33 The housings for virtually all industrial com-pressors—as well as the various top heads, coverplates, and bearing housings—are made of fine-grain,nonporous, leak-tight, high-strength gray iron or mee-hanite semisteel castings. The compressors are testedto be leak-tight and are given a strength test at 150%of the design working pressure of the compressor.

1.34 The compressor suction area surrounds thecylinders, and ammonia vapor enters the cylinder onthe piston downstroke. At bottom dead center the suc-tion valve closes, and on the upstroke the cylinderdiminishes, compressing the gas. As the pistonapproaches top dead center, the pressure increasesabove the discharge pressure, opening the dischargevalve. The compressed gas enters the top headthrough the cylinder discharge valve, is manifolded tothe discharge service valve, and continues on to thecondenser.


Head cover cooling

Gaskets

Unloadermechanism

Crankshaft

Crankcase

Oil-levelsight glass

Oil cooling

Oil filtration

Pistons andcylinder sleeves

Suctionstrainer

Plate valvemechanism

Spring-loadedsafety heads

Fig. 1-5. Mycom 6WB compressor


109

8

76

5

43

2 1

11

1213

14

15

16

17

1819

2021

1. Spring, safety head2. No. 2 nut, discharge valve cage3. No. 1 nut, discharge valve cage4. Discharge valve cage5. Bolt, discharge valve cage6. Spring, discharge valve7. Discharge valve8. Guide, discharge valve cage9. Seat, discharge valve10. Valve plate11. Spring, suction valve12. Suction valve13. Piston ring14. Cylinder sleeve15. Piston16. Assembly, connecting rod17. Piston pin18. Spring, lift pin19. Lift pin20. Cam ring21. Retaining ring

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1.35 The cylinder suction valve assembly con-sists of a flat suction valve disk or ring, which restson inner and outer valve seats that are formed intothe top and are part of the cylinder liner. The suc-tion valve ring is held in place with a number ofsmall helical springs that are recessed into the suc-tion valve plate. The suction valve plate contains arecessed pocket into which the suction valve ringrises during the suction stroke to permit suction gasto enter the cylinder. The depth of the recess is crit-ical, because it controls the flow rate of refrigerantgas and also the impact stresses and motion of thering.

1.36 The discharge valve assembly is mountedabove the suction valve plate. The discharge assemblyconsists of the discharge valve cage, a mushroom dis-charge seat, a flat discharge valve disk or ring (small-er than the suction ring), several small helical springs,and a bolt to clamp the mushroom, ring, and springsto the cage. The inner discharge seat is located on themushroom, but the outer discharge seat is machinedinto the top of the suction valve plate. The dischargecage also has a machined recess that permits the dis-charge valve ring to rise the desired lift amount, con-

trolling the rate of discharge gas flow and the motionof the valve ring.

1.37 Figure 1-6 shows how the top of the piston iscontoured to mate with the shape of the cylinder valveservice. This is the clearance space, which must be keptas small as practical to minimize the trapped high-pres-sure gas that cannot be discharged out of the cylinder.The space must be sufficiently large, however, to avoidphysical contact between the piston and valve plateseven with some liquid oil or refrigerant on the piston.

1.38 Regardless of the manufacturer, certain oper-ating characteristics are signs of valve failure. Dis-charge valve failure is indicated by high dischargetemperatures. Suction valve failure is indicated byreduced power consumption.

The Programmed Exercises on the next page willtell you how well you understand the material youhave just read. Before starting the exercises,remove the Reveal Key from the back of yourbook. Read the instructions printed on the RevealKey. Follow these instructions as you workthrough the Programmed Exercises.

12 Lesson One

Fig. 1-6. Mycom GWB cylinder sleeve and valving


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1-1. Important improvements to early com-pressors included mechanical

seals, a(n) pump,and a -control system.

1-2. Increased refrigerating capacity fromsmaller machines resulted from using

pistons that were smaller andrunning at speeds.

1-3. Today’s ammonia reciprocating com-pressor has up to cylindersand runs at a maximum speed rangingup to about rpm.

1-4. The oil-pressure cutout switch musthave a timed bypass to per-mit compressor startup without nui-sance .

1-5. Most manufacturers use journal bear-ings for the loads and thrustwashers or collars for posi-tioning.

1-6. When used, springs hold thecylinder valve service assemblies inplace.

1-7. Ammonia vapor enters the cylinder onthe piston .

1-8. The discharge valve assembly ismounted above the .

1-1. SHAFT; OIL; CAPACITY

Ref: 1.09

1-2. MORE; HIGHER

Ref: 1.12

1-3. 16; 1200

Ref: 1.14

1-4. DELAY; CUTOUTS

Ref: 1.22

1-5. RADIAL; AXIAL

Ref: 1.24

1-6. SAFETY HEAD

Ref: 1.30

1-7. DOWNSTROKE

Ref: 1.34

1-8. SUCTION VALVE PLATE

Ref: 1.36

Programmed Exercises 13


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14 Lesson One

Capacity Control

1.39 Notice the long slender pins (item 19 in Fig.1-6 on page 12) that rest on a rotatable band locatedon the outside of the cylinder sleeve. The pins fitthrough holes in the cylinder sleeve and have a cap-tured spring that forces the pins down against theband. The upper end of the pins extend into the suc-tion-gas entry area just under the suction valve ring.The purpose of these lift pins is to permit loading andunloading of individual cylinders to control the capac-ity of the compressor and thus match the system loadrequirements.

1.40 The action of loading and unloading cylin-ders is called capacity control. Capacity control onindustrial reciprocating compressors is generally inresponse to the refrigeration system suction pressure.The compressor capacity-control system can be set torespond to a predetermined pressure, and deviationcaused by a varying system load will result in thecompressor automatically loading and unloading tomaintain the required pressure and therefore theevaporating temperature.

1.41 To load a cylinder or a bank of cylinders, oilpressure is directed to a small capacity-control piston,which moves a rod attached to the rotatable band.Rotating the band causes the pins to follow down theangular ramp on the band, withdrawing the lift-pinupper surface below the suction valve ring. In this posi-tion the suction valve is free to actuate freely inresponse to the piston stroke position, drawing suctiongas on the downstroke and closing and permitting com-pression and discharge on the upstroke. The term for acylinder in this position is loaded. This condition is dueto oil pressure acting on the capacity-control piston.

1.42 To unload a cylinder, the oil pressure isremoved from the capacity-control piston and a suit-ably sized spring returns the rod, rotating the band,causing the lift pins to rise up the ramp on the band.The lift pins lift the suction valve ring off its seat andforce it against the backstop. In this position, suctiongas enters the cylinder on the piston downstroke, butthe valve cannot close at bottom dead center, remain-ing open during the upstroke. No compression or dis-charge takes place, because the gas is merely pushedout of the cylinder as the piston rises back through theopen suction valve. The term for a cylinder in thisposition is unloaded. This condition is due to the

removal of oil pressure from the capacity-control pis-ton and the action of the return spring.

1.43 It should be noted that the compressors do nothave unloaders on all cylinders, except in some specialcases, to prevent operation with no flow through thecompressor. Two- and four-cylinder compressors havea minimum of one cylinder loaded. Six- and eight-cylinder compressors have a minimum of two cylin-ders loaded. Twelve- and sixteen-cylinder compressorshave at least four cylinders loaded at all times.

1.44 Compressor starting torque, and thereforemotor starting requirements, are also minimized by thisunloading feature. There is no oil pressure immediatelyat start. The pump does not function until the compres-sor turns, and a few seconds are required to establishsufficient oil pressure. In other words, whether or notthe system demands that all cylinders be loaded, thecompressor always starts on the minimum number ofcylinders—those that do not have unloader mecha-nisms—reducing starting torque for the critical secondsof transition from 0 rpm to operating speed.

Reciprocating Compressor Lubrication

1.45 Compressor lubrication is essential to the suc-cessful operation of all industrial compressors. Thelubrication is provided under pressure, as force-fed orpressure-fed lubrication, which results from a mechani-cal oil pump that is located at the crankshaft and direct-ly driven by the crankshaft. Oil is drawn from the oilsump in the compressor crankcase, typically through awire-mesh strainer to prevent foreign material fromentering the pump. Many systems also include a coa-lescing oil separator, as is discussed in Lesson Five.

1.46 The remainder of the lubrication system con-sists of the internal and external oil passages from thepump to the various points of lubrication—thecrankpins, main journals, connecting rods, and shaftseal. The lubrication system on most compressors alsoprovides the hydraulic pressure to activate the capacitycontrol. In addition, most compressors have an oil filtra-tion system, which often filters all the oil pumped, or abypass system, which filters only a bypass stream of thetotal oil flow. Some manufacturers provide oil filtrationonly as an option. If your compressor does not have afilter on the lubrication system, take measures to addone, even if it is only on a bypass. This can be accom-plished with a suitable spin-on 15- to 20-micron pleated


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paper filter handling from 0.5 to 1 gpm of oil. Within afew minutes of operation, the entire oil sump quantitywill have passed through the filter and the compressorwill be safe from the grit in the unfiltered oil.

1.47 The oil also sprays from the connecting rodsat the crankpins and both lubricates and removes heatfrom the cylinder sleeves, inside the pistons, and theinner deck area of the compressor. The heat pickup bythe oil increases its temperature enough that mostammonia compressors (except boosters) have someprovision for oil cooling. This oil cooling is generallyprovided by a water-cooled or refrigerant-cooledshell-and-tube heat exchanger.

1.48 The lubricants used for ammonia reciprocat-ing compressors have a viscosity of 300 to 350 SUS(Saybolt universal seconds), about the same viscosityas an SAE 20 motor oil. This viscosity is also indicat-ed as ISO VG 60, the International Standards Organi-zation designation of 60 centistokes viscosity grade.The lubricants may be of naphthenic or paraffiniccrude oil origin and should be specifically designatedfor refrigeration compressor service. The more accept-able lubricant is a hydro-treated paraffinic oil, whichhas stable characteristics, a high coking temperature, alow pour point, and a low oil carryover rate throughthe oil separator. More specific information aboutlubricants will be covered in Lesson Five of this Unit.

1.49 Reciprocating compressors typically containa crankcase oil heater inserted below the oil level inthe oil sump. The purpose of the heater, which isenergized only when the compressor is not running, isto keep the oil in the crankcase slightly warmer thanthe machine room or surrounding area so that duringprolonged off cycles, liquid ammonia does not con-dense in the oil sump. These heaters are typicallyrated at about 300 watts and often contain inherentthermostats to turn them off when the oil is sufficient-ly warm, at about 120 to 140°F. Heaters directlyimmersed in the oil must have sufficient surface toprevent the oil from coking on the heater. (Coking is aparticular form of oil degradation.) This is accom-plished by specifying heaters with a maximum wattdensity of about 15 to 17 watt/in2.

Typical Compressor Efficiency

1.50 Two efficiency terms used in conjunctionwith positive-displacement compressors are impor-

tant to understand—volumetric efficiency, VE, andadiabatic (ideal) efficiency, Na. However, a thirdrelationship, called the performance factor, or PF, isthe most useful and practical for comparing similar-sized industrial compressors from several manufac-turers.

1.51 You may recall from the previous Unit thatvolumetric efficiency, VE, is the ratio of the actualvolume (Acfm) that a compressor can pump at agiven set of conditions to the theoretical displacementof the compressor, Dcfm.

1.52 The volumetric efficiency data is generallypresented in graphical form plotted against compres-sion ratio, where the compression ratio, CR, is equalto the ratio of the discharge pressure (psia) to the suc-tion pressure (psia), as follows:

Figure 1-7 on the following page is a generalized plotof volumetric efficiency for industrial reciprocatingcompressors.

1.53 Note how the VE decreases as the CRincreases and approaches 10.0. At that point the CR is down to 50% of the theoretical displacement of thecompressor. This coincides with the limitation ofoperation of most ammonia reciprocating machines,because at these conditions the discharge temperatureis in excess of 300°F, and the oil will char through thevalving, leaving a black sooty deposit that results infrequent servicing and loss of performance.

1.54 Adiabatic efficiency, Na, is the ratio of thepower required for isentropic (constant entropy, ideal)compression to the actual shaft power delivered to thecompressor. For industrial open compressors, theactual power is given in motor brake horsepower, notthe motor power input.

The adiabatic efficiency is determined by manufactur-ers by test and is used to establish power requirements

Nisentropic powershaft input powera =

CRdischarge pressuresuction pressure

=

VEAcfmDcfm

=



16 Lesson One

Volu

met

ric

effi

cien

cy (

VE

, %)

100

90

80

70

60

50

40

30

20

10

01 2 3 4 5 6 7 8 9 10 11 12

Compression ratio CR =Discharge psia

Suction psia( )

Fig. 1-7. Typical industrial ammonia reciprocatingcompressors volumetric efficiency

A. York model RW-4A compressors

B. Mycom N2-12W compressors at 95°F condensing

Unit model

Saturateddischargetemp. °F

Saturatedsuctiontemp. °F

RW44A RW64A RW84A RW124A RW164A

Tons Bhp MBhTons Bhp MBh Tons Bhp MBh Tons Bhp MBh Tons Bhp MBh

80

90

-15-10

-505

101520

253035404550

17.620.824.328.132.336.941.947.4

51.057.364.171.579.588.1

26.431.136.442.148.455.362.871.0

76.586.096.2

107.2119.232.1

35.241.548.556.164.573.783.794.7

102.0114.7128.3143.0158.9176.1

48.852.255.458.361.063.465.667.4

76.678.779.880.982.083.0

32.935.237.339.241.042.644.145.3

51.452.853.654.355.055.7

295338385436491550614683

742822906996

10931198

441506577653735825920

1024

111312321357149316391796

64.869.373.5 77.481.084.287.189.6

101.8104.6106.1107.6108.9110.3

587674769870980

109912261364

148316421809198921842394

52.862.372.884.296.8

110.6125.6142.1

153.0172.0192.4214.5238.4264.2

97.2104.0110.3116.1121.4126.3130.7134.4

152.7156.8159.2161.4163.4165.5

8811012115413051470164918402047

222424632714298432773592

70.483.097.0

112.2129.0147.5167.5189.5

204.0229.4256.6286.0317.9352.3

129.1138.2146.5154.3161.4167.9173.8178.7

203.1208.6211.8214.7217.4220.1

11731348153717391959219724522728

296532833618397843684788

Ref

rig

eran

t

Model

Displace-ment at

1,200 rpm

Refrigeration capacity at 95°F condensing temperature Brake horsepower at 95°F condensing temperature

Suction temperature °F

–20 –10 0 10 20 30 –20 –10 0 10 20 30

R-7

17

N2WAN4WAN6WAN8WAN4WBN6WBN8WBN12WB

Cfm U.S. tons Bhp

41.891.2

136.8182.4224.4337.2450.0674.4

3.98.4

12.616.920.831.241.662.3

5.612.218.324.430.145.160.190.2

7.716.825.233.741.562.282.9

124.4

10.322.533.744.955.383.0

110.7166.0

13.429.243.858.472.0

108.0143.8215.9

17.137.255.874.491.7

137.6183.4275.1

9.821.432.142.952.879.2

105.6158.4

11.324.737.049.460.891.3

121.7182.5

12.727.741.655.568.4

102.6136.7205.1

13.930.445.660.874.9

112.4149.9224.8

14.932.548.765.080.1

120.1160.1240.2

15.533.750.667.583.1

124.7166.3249.4


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Fig. 1-8. Capacity/bhp ratings for typical ammonia compressors

Ton

s (K

cal/h

r x

103 )

Bh

p (

kW)

130 (393)

120 (363)

110 (333)

100 (302)

90 (272)

80 (242)

70 (212)

60 (181)

50 (151)

40 (121)

30 (91)

20 (60)

10 (30)

75 (56)

70 (52)

65 (48)

60 (45)

55 (41)

50 (37)

45 (34)

40 (30)

35 (26)

30 (22)

25 (19)

20 (15)

15 (11)°F°C

Saturated suction temperature

-40-40

-30-34

-20-29

-10-23

0-18

+10-12

+20-7

+30-1

+40+4

+50+10

+60+16

Tons (Kcal/hr x 103)

Bhp (kW)

R-502

R-22

R-717

R-717

R-22R-290

R-12

R-502

R-290

R-12

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for their equipment throughout the range of operation.It is normally not published.

1.55 The performance factor, PF, is a measure ofthe relationship between the actual power required ata specific condition to the capacity rating of the com-pressor.

The performance factor—bhp/ton—provides apractical way to analyze and assist in choosingbetween several alternative compressor selectionsfor a given operating condition. The calculation iseasily accomplished directly from the manufactur-er’s rating information. Simply determine the brakehorsepower (bhp) required for the application andthen divide the bhp by the refrigeration rating in

tons. Note that you must be sure to correct the man-ufacturer’s data for your specific condition—including actual speed, superheat, liquid subcool-ing, and so on. The lower the performance factor,PF in bhp/ton, the more energy-efficient is theselection.

Compressor Application Data

1.56 Manufacturers typically supply compressorapplication data, limits of operation, and rating tablesor graphs of capacity in their engineering-guide litera-ture. Some manufacturers also provide computerdisks with complete rating information so the cus-tomer can obtain ratings at his or her specific condi-tions, without interpolation of tabular data. Samplesections of typical rating and application data fromvarious manufacturers are presented in Figs. 1-8, 1-9,and 1-10 on the following page.

PF = actual bhp

rated capacity in tons


Fig. 1-9. Capacity/bhp rating graph for Vilter 454XL compressor at 1200 rpm and 95° F condensing


ITEM 452XL 454XL 456XL 458XL 4512XL 4516XL

Number of cylindersMaximum rpmBore & Stroke – in.

(mm)Cfm @ maximum rpm (m3/hr)

Suction connection – in. (mm)Discharge connection – in. (mm)Unit weight less motor – lb (kg)Oil charge – gallons (liters)Standard steps of unloading (%)Option 1 steps of unloading (%)Option 2 steps of unloading (%)Maximum discharge temp. – °F (°C)Crankcase oil temp. range – °F

(°C)

21200

41/2 x 41/2(114 x 114)99.4(169)

24(73)27(82)29(88)9(27)

16(48)21/2(64)

2(51)1900(862)

5(19)0

50100

300(149)110-130(43-54)

41200

41/2 x 41/2(114 x 114)199(338)49(148)55(166)59(178)19(57)31(94)3(76)

21/2(64)2700(1225)

7(27)50

25/50/7550/100

300(149)110-130(43-54)

61200

41/2 x 41/2(114 x 114)298(507)73(221)82(248)88(266)28(85)

47(142)4(102)3(76)

3100(1406)7(27)33/66

–33/66/100300(149)110-130(43-54)

81200

41/2 x 41/2(114 x 114)398(676)97(293)

109(330)117(354)37(112)62(187)4(102)3(76)**

3400(1542)7(27)25/50

25/50/7525/50/75/100

300(149)110-130(43-54)

121200

41/2 x 41/2(114 x 114)597(1014)146(442)164(496)176(532)56(169)94(284)5(127)

(2)3(76)5300(2404)

14(53)33/66

–33/66/100300(149)110-130(43-54)

161200

41/2 x 41/2(114 x 114)796(1352)195(590)219(662)235(711)75(227)

125(378)5(127)*(2)3(76)

5800(2630)14(53)25/50

25/50/7525/50/75/100

300(149)110-130(43-54)

R-717 (10°F)R-12 (40°F)R-22 (20°F)R-502 (–20°F)R-290 (0°F)

Tons (Kcal/hr x 103)Refrigeration @95°F condensing

*Add 1 in. (25.4 mm) for halocarbon units. **Add 1 in. (25.4 mm) for ammonia units.

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1.57 Most of this Lesson has discussed reciprocat-ing compressors, but except for service replacementsand those sold for further packaging into refrigerationsystems, most compressors reach the customer as partof a compressor unit. The compressor unit is a muchmore convenient way to obtain the compressor, readyfor tie-in to your system.

1.58 The compressor unit typically consists of thefollowing:

• compressor

• base assembly—belt or direct drive

• controls—electromechanical or microprocessorwith operating and safety devices

• motor and drive package installed

• optional equipment required.

Figure 1-11 shows a factory-packaged compressorunit.

Compound Reciprocating Compressors

1.59 There is a special class of reciprocating com-pressors known as compound or internally compoundedcompressors. These compressors are unique in that theycontain within a single housing both the low (booster)and high stages of a two-stage compressor system. Thisis accomplished by the following:

• internal divisions within the compressor bodyto separate the two pressure stages

• provisions for both the booster low-pressureand the intermediate high-stage suction valveconnections

• provisions for compressor discharge valve con-nection to the condenser.

1.60 Companies that manufacture compound com-pressors typically use a modification of their existingcompressor housings and running gear (crankshaft,liners, rods, pistons, cylinder valving, and so on) toprovide the internally compounded machine economi-cally. The advantage of these compressors is for thesmaller industrial plant where one or two compound

18 Lesson One

Fig. 1-10. Vilter general specifications and limitations


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compressors are all that are required to maintain thelow-temperature load. The system is simplified as arethe controls. It takes only a single control panel andone motor to operate each compressor.

1.61 The two criteria that mark two-stage systemsare prepackaged with compound compressors. Theycontain an interstage desuperheater and a liquid sub-cooler, both within the same heat exchanger, and usesome condenser liquid via a thermal expansion valveto accomplish both desuperheating and subcooling.The intercooler is mounted between the low-stagecylinder discharge and the high-stage suction. Thebooster discharge is typically cooled to within about10°F of the interstage saturation temperature. Thehigh-pressure liquid is also subcooled to within about10 to 15°F of the interstage saturation. This is similarto the operation of a standard two-stage refrigerationsystem with two compressors.

1.62 The basic difference, except for size, is thatthe interstage pressure is determined by (1) the ratioof low-stage to high-stage cylinders per compressorand (2) the absolute low-side suction pressure. It isnot normally possible to introduce additional high-

side loads into the intermediate pressure (high-stagesuction). Six- and twelve-cylinder compressors typi-cally have a basic 2:1 ratio, with cylinder ratios of 4:2and 8:4 respectively. Eight- and sixteen-cylinder com-pressors are typically divided into a 3:1 ratio with 6:2and 12:4 low:high cylinders.

1.63 The crankcase of the compressor is normallyequalized to the suction (low-stage) pressure. Thispermits the low-stage cylinders to operate much thesame as a single-stage compressor. For the high-stagecylinders, however, the load on the wrist pin andcrankpin bearings is considerably higher, becauseeven the lower intermediate pressure at the intake ofthe high-stage cylinders is above the crankcase (suc-tion) pressure. The force on the piston and connectingrod is continually down toward the shaft at all times.It becomes necessary to use pressure-lubricated nee-dle or roller bearings at the small end of the connect-ing rod to obtain adequate bearing life.

1.64 Figure 1-12 on the following page shows acompound compressor. The illustration also shows thefour possible intermediate cooling systems for thiscompressor.


Fig. 1-11. York model R compressor unit


20 Lesson One

Intermediatecoolingsystem

LP HP

Evaporator

Expansion valve

Condenser

Compressor Compressor

A. Injection interstage gas cooling

B. Injection interstage gas and liquid cooling

System

C. Open flash interstage cooling

D. Closed flash interstage cooling

Fig. 1-12. Sabroe compound compressor


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22 Programmed Exercises

1-9. Compressor capacity control isachieved by means of loading andunloading cylinders, usually in responseto pressure.

1-10. No compression takes place on theupstroke in a(n) cylinder.

1-11. All ammonia reciprocating compressorsshould have a(n) installed toensure that oil fed to the bearings is

.

1-12. Crankcase oil heaters, which work onlywhen the compressor is ,usually warm the oil to about

°F.

1-13. The most useful efficiency term is theperformance factor, the relationshipbetween actual power in tothe compressor capacity rating in

.

1-14. Limits of compressor operation are pro-vided in graphs and tables from manu-facturers’ or .

1-15. A compound compressor requirescontrol panel(s) andmotor(s).

1-16. The booster discharge of a compoundcompressor is typically cooled to withinabout °F of the interstage sat-uration temperature.

1-9. SUCTION

Ref: 1.40

1-10. UNLOADED

Ref: 1.42

1-11. OIL FILTER; CLEAN

Ref: 1.46

1-12. NOT RUNNING; 120 TO 140

Ref: 1.49

1-13. BHP; TONS

Ref: 1.50, 1.55

1-14. LITERATURE; COMPUTER DISKS

Ref: 1.56

1-15. ONE; ONE

Ref: 1.60

1-16. 10

Ref: 1.61


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1-1. Force-fed lubrication to the compressor wearelements became possible with the additionof

� a. lift pins� b. packing glands� c. the oil pump� d. the shaft seal

1-2. All industrial reciprocating refrigeration com-pressors are equipped with a(n)

� a. oil cooler� b. oil filter� c. oil-supply sump� d. suction strainer

1-3. For reciprocating compressors, the minimumdifferential oil pressure is usually about

� a. 10 to 15 psi above suction pressure� b. 10 to 15 psi below discharge

pressure� c. 30 to 40 psi above suction pressure� d. 30 to 40 psi below discharge

pressure

1-4. The devices that provide cylinder sealing toprevent oil carryover are the

� a. piston rings� b. safety head springs� c. suction valve plates� d. unloader elements

1-5. The suction valve is always larger than thedischarge valve because the inlet gas is

� a. at a higher flow rate� b. at a higher pressure� c. less dense and takes up more

volume� d. more dense and takes up more

volume

1-6. The discharge valve opens when the piston

� a. approaches bottom dead center� b. approaches top dead center� c. begins the downstroke� d. begins the upstroke

1-7. When lift pins lift the suction valve ring off itsseat, the cylinder is and the valveis during the upstroke.

� a. loaded; closed� b. loaded; open� c. unloaded; closed� d. unloaded; open

1-8. Suitable viscosity for lubricants used withammonia reciprocating compressors is

SUS or ISO VG .

� a. 20; 60� b. 60 to 65; 325� c. 225; 100� d. 300 to 350; 60

1-9. Which of the following is the equation for themost practical way to compare the suitabilityof various compressors for an application?

� a. CR = discharge pressure/suction pressure

� b. Na = isentropic power/shaft input power

� c. PF = actual bhp/rated capacity in tons

� d. VE = Acfm/Dcfm

1-10. Which of the following components arerequired on compound reciprocating compres-sor connecting rods because of additionalforces resulting from higher pressures?

� a. Journal bearings� b. Needle or roller bearings� c. Safety head springs� d. Thrust collars

Self-Check Quiz 23

Answer the following questions by marking an “X”in the box next to the best answer.


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1-1. c. The oil pump. Ref: 1.09

1-2. c. Oil-supply sump. Ref: 1.20

1-3. c. 30 to 40 psi above suction pressure. Ref: 1.21

1-4. a. Piston rings. Ref: 1.28

1-5. c. Less dense and takes up morevolume. Ref: 1.32

1-6. b. Approaches top dead center.Ref: 1.34

1-7. d. Unloaded; open. Ref: 1.42

1-8. d. 300 to 350; 60. Ref: 1.48

1-9. c. PF = actual bhp/rated capacity in tons. Ref: 1.50, 1.55

1-10. b. Needle or roller bearings. Ref: 1.63

Contributions from the following sources are appreciated:

Figure 1-1. Frick CompanyFigure 1-2. Frick CompanyFigure 1-3. Frick CompanyFigure 1-4. Vilter Manufacturing Corp.Figure 1-5. Mycom America Corp.Figure 1-6. Mycom America Corp.

Figure 1-8. York International; Mycom America CorpFigure 1-9. Vilter Manufacturing Corp.Figure 1-10. Vilter Manufacturing Corp.Figure 1-11. York InternationalFigure 1-12. Sabroe Refrigeration A/S

Early ammonia reciprocating compressors wereslow-speed vertical or horizontal single-actingmachines with either belt or direct drive, followedby enclosed HDI or VSA compressors. Laterimprovements included shaft seals, oil pumps,and capacity-control systems, followed byreduced piston size and stroke and increased run-ning speeds and number of cylinders. Today’sammonia reciprocating compressors typicallyhave 2 to 16 cylinders, an oil-supply sump, an oilpump, bearings, shaft seals, pistons with varyingnumbers of compression and oil-scraper rings,sometimes safety head springs, usually a suctionstrainer and oil filter, and often an oil cooler. Thesuction valve is always larger than the dischargevalve.

A typical capacity-control method works by load-ing and unloading cylinders as oil pressure isapplied and removed. Almost all compressorshave a minimum number of cylinders loaded at alltimes, which prevents operation with no flowthrough the compressor and also reduces start-ing torque. Reciprocating compressor lubrication

is achieved by means of a crankshaft-driven oilpump. The oil filtration system removes grit. Mostnon-booster ammonia compressors provide foroil cooling. Lubricants have a viscosity of 300 to350 SUS (ISO VG 60). A crankcase heater warmsthe oil when the compressor is not running.

Three efficiency terms are used with positive-dis-placement compressors—volumetric efficiency(VE), adiabatic efficiency (Na), and performancefactor (PF). PF, the actual bhp divided by the ratedcapacity in tons, is most useful for comparing theoperation of various compressors. The lower thePF, the more energy-efficient is the selection.Also, manufacturers provide compressor applica-tion data.

Most compressors are provided as part of a com-pressor unit, which includes the base, drive, con-trols, and other components. Compound com-pressors contain both booster and high stages inone housing, along with a desuperheater/subcool-er, thermal expansion valve system, and inter-cooler.

24 Lesson One

SUMMARY


positive-displacement compressors compressors lesson one reciprocating compressors 46201 preview ......

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