Vilter Manufacturing Corporation

5555 South Packard Ave.Cudahy, Wisconsin 53110-8904

www.vilter.com

Telephone: (414) 744-0111Fax: (414) 744-3483

E-mail: custserv@vilter.com

An employee-owned company

District Sales Offices &Distributors Worldwide

Vilter reserves the right to make changes in design and specifications without notice. Copyright Vilter Manufacturing Corporation 2001• Bulletin No. 0140 DLC/3.5m/5-02 • Printed in U.S.A.

Tradition & TechnologyTogether. Working For You.

VSMMini ScrewCompressor

Featuringsingle screw design

and 5/15 warranty

The Vilter Single Screw compressor is arotary, positive displacement compres-sor which incorporates a main screwand two gaterotors. Compression of thegas is accomplished by the engage-ment of the two gaterotors with the hel-ical grooves in the main screw. Thedrive shaft imparts rotary motion to themain screw which in turn drives theintermeshed gaterotors.

The compressor is comprised of threefundamental components which rotateand complete the work of the compres-sion process. This typically includes acylindrical main screw with six helicalgrooves and two planar gaterotors,each with 11 teeth. The rotational axesof the gaterotors are parallel to eachother and mutually perpendicular to theaxis of the main screw.

Single Screw Compressors Design & Operation

The standard Vilter Single Screw Compressor (as shown in this endview drawing) consists of two rotating gaterotor assemblies and a mainscrew assembly. All bearings are pressure fed with oil.

This side view cross-sectional drawing of a Vilter oil flooded SingleScrew illustrates the suction and the discharge ports as well as the various seals and driveshaft.

Single Screw Compressors Design & Operation

GaterotorSupport

Gaterotor

Bearing

Bearings Parallex Slide System

Main Screw

Main Bearings Main Screw

Driveshaft

Shaft Seal

Labyrinth Seal

The Vilter Mini Screw (VSM) is the latest

in Vilter’s long, respected line of com-

pressors for industrial cooling applica-

tions. The VSM, featuring Vilter’s signa-

ture single screw design, has one main

rotor and either one or two gate rotors,

depending on the model. The VSM is

quieter than twin screws, especially the

gear-driven models.

Its compact

design has no

external tubing

on the compres-

sor, no gear box

and an optional

oil pump (based

on application),

making the VSM

a very cost-efficient model. Plus, it fea-

tures the Vilter Vantage micro-controller

and our exceptional 5/15 warranty.

Block andbleed valvesare standard

VantageTM micro-controller—The Vilter VantageTM micro-controller comes standard on the VSM. Developed in Windows CETM

utilizing Think and DoTM software for user-friendlyoperation, the Vantage features a smaller profilebox and passive screen. But just like Vilter’s othermicro-controllers, the Vantage features dual processing power, multiple languages in the controller (English, Spanish and French), as well as English or SI units of measure.

Need a more powerful controller? No problem. Youcan upgrade to the VissionTM micro-controller,2000 winner of start magazine's third annualTechnology & Business Award competition. It hasbeen developed in Windows CETM utilizing Think and DoTM software for user-friendly operation, easeof custom programming and maximum flexibility.

It features dual processing power providing unmatched reliability. Standard languages in the

controller are English, Spanish and French, as well as English or SI units of

measure.

Tradition & TechnologyTogether. Working For You.

ParallexTM slide system — It’s the keyto part load efficienciesfar superior to twinscrew compressors.Capacity and volumeslides (with an expandedvolume ratio of 1.2 to7.0) move independentlyof each other based onload, eliminating over orunder compression and saving motor horsepower.

Multi-position suction strainer cover and stopcheck valve — Can be mounted on either side infive rotated positions.

Oil Cooling Options— • Thermosyphon• Water Cooled• Liquid Injection

VSM Specifications

The VSS

The VSM, a morecompact model

Vilter Base Rating (a) Std. Conn Sizes Unit Dimensions (Approximate) Approx.Model Ammonia R-22 A (b) B (b) C (b) D (c) opt. E(c) Shipping (d)Number CFM Tons BHP Tons BHP Suction (b) Discharge(b) Length Width Height Dual Oil Filter Optional Oil Cooler Weight (lbs)

VSM-71 * 75 27 30 25 28 3 3 8'-8" 3'-8" 5'-2" 9.0" 8-3/4" 3400

VSM-71E 28 31 26 29

VSM-91 * 87 31 35 28 33 3 3 8'-8" 3'-8" 5'-2" 9.0" 8-3/4" 3400

VSM-91E 33 36 30 34

VSM-101 * 99 37 40 34 37 3 3 8'-8" 3'-8" 5'-2" 9.0" 8-3/4" 3400

VSM-101E 39 41 36 38

VSM-151 153 56 61 51 57 3 3 8'-8" 3'-8" 5'-2" 9.0" 8-3/4" 3400

VSM-151E 60 63 55 59

VSM-181 176 65 71 60 66 3 3 8'-8" 3'-8" 5'-2" 9.0" 8-3/4" 3400

VSM-181E 69 73 64 68

VSM-201 201 75 81 69 75 3 3 8'-8" 3'-8" 5'-2" 9.0" 8-3/4" 3400

VSM-201E 79 83 74 78

VSM-301 306 114 129 104 120 3 3 8'-8" 3'-8" 5'-2" 9.0" 8-3/4" 3600

VSM-301E 121 133 113 125

VSM-361 351 134 149 123 138 3 3 8'-8" 3'-8" 5'-2" 9.0" 8-3/4" 4150

VSM-361E 142 153 132 143

VSM-401 402 154 168 141 156 4 3 8'-8" 3'-8" 5'-2" 9.0" 8-3/4" 4150

VSM-401E 164 173 153 162

VSM-501 493 190 202 177 204 4 3 8'-11" 3'-8" 7'-6" 9.0" 10-3/4" 5500

VSM-501E 208 215 213 235

VSM-601 591 228 236 210 229 4 4 9'-8" 4'-0" 7'-10" 9.0" 10-3/4" 6000

VSM-601E 252 256 256 272

VSM-701 680 262 268 242 260 5 4 9'-8" 4'-0" 7'-10" 9.0" 10-3/4" 6000

VSM-701E 290 291 295 309

(a) Tons and BHP based on 20°F and 95°F; 10°F liquid subcooling, saturated suction. Ratings for other refrigerants are available - Consult home office.(b) Dimensions shown are approximate and should not be used for construction.(c) Addition to compressor package(d) With standard motor and liquid injection oil cooling. Units with external cooler will weigh approximately 500 pound more."E" models above are econ-o-Mizer® models* Models operate at 1775 RPM; all others operate at 3550 RPM

isolated from the main screw. Since the gaterotorassemblies are independent and do not interferewith the rest of the main screw body, bearings canbe sized for maximum reliability.

Sealing During CompressionSealing is accomplished by the combination

of precision running clearances and an injectedliquid (coolant/lubricant) which is allowed to leak through and thus seal the clearances duringthe compression process. In the Single Screwcompressor, this liquid must also have adequateviscosity to lubricate the bearings. The liquid isswept into the groove during the suction processand also injected into the compression groove during the compression process to maximize sealing of the running clearances.

Due to the rotation of the screw, centrifugalforce impels the injected liquid to the circumferen-tial clearance volume between the screw and thehousing. This minimizes the leakage described ascascading. Cascading is defined as the leakagefrom the high pressure groove past the land

separating the grooves into the trailing low pressure groove. Another inherent attribute of the Single Screw design is that there is more surface area on the lands near the discharge end of the groove than near the middle of the groove.This attribute also minimizes leakage from thehighest pressure region of the groove. Anotherarea where the leakage is minimized is between thehigh pressure end of the groove to the volumebehind the screw which is at suction pressure.This potential leak path is sealed by means of anon-contacting hydrodynamic seal known as a viscoseal, windback seal, or labyrinth seal.

Shaft Seal SystemA shaft seal system prevents any of the process gas

from leaking around the drive shaft of the mainscrew to the environment. The oil flooded SingleScrew compressor has two seal types; the standardsingle mechanical face seal or a triple mechanicalface seal with purge capabilities depending on theprocess requirements. The stationary carbon faceof the seal rides on a hydrodynamic film of oil on

the rotating mating ring which is fixed on the shaft.The optional triple seal allows various optionsincluding a purge and vent to be connected to thehousing thus adding a secondary safety buffer duringoperation. The incorporation of this seal is shownin the cross-section of the oil flooded gas end.

DesignEach rotating assembly within the gas end has

two sets of bearings. A typical oil flooded SingleScrew compressor consists of two rotating gaterotorassemblies and a main screw assembly, each havingone pair of angular contact bearings to maintainaxial position of the assembly and a cylindricalroller bearing to support the opposite end. All ofthe bearings are pressure fed with oil. The oil,upon draining from the bearings, is drawn into thesuction of the main screw and is discharged withthe process gas and injected oil. Since the mainscrew has no loads except for gravity, the bearingsare considered over designed since they are deter-mined by the required shaft diameter for theapplied horsepower. The Single Screw design does

not restrict the bearing sizes for the gaterotorsupports. As a result, the bearings are optimizedfor maximum reliability.

Slide DesignThe dual slide design on the Vilter Single

Screw compressor offer the highest level of flex-ibility and performance optimization for screwcompressors. This design actually has twoslides per compression side of the gas end. The two slides are commonly referred to as the capacity slide and the volume slide. Thecapacity slide moves from positions of 20% to100% of flow to allow the compressor to matchthe system flow requirements. Although lowerflow rates are possible, they are not recom-mended since this reduces the amount of oilflowing through the gas end and may result inoverheating. The volume slide allows the dis-charge port to be positioned in the optimumlocation depending on the capacity slide loca-tion, the properties of the gas and the injectant.

A unique feature of the dual slide design is

that it allows the compressor to start completelyunloaded. This is unlike any other screw com-pressor. When both slides are in the open posi-tion an unrestricted flow path through the com-pressor is created.. If for any reason the gas endis completely full of oil, the position of the slideson startup will allow the oil to be swept out ofthe gas end thus preventing the possibility ofhydraulic lock. The slides also allow the operation at extremely low ratios down to 1.2.However, the recommended operating points foroptimum design efficiency occurs at pressureratios of 2.0 and greater. Due to their design,Single Screw compressors are able to operatemore efficiently and reliably with higher suctionpressures and lower ratios than other types ofscrew compressors.

Since the capacity and volume slides operatein parallel (not in series like other types ofscrew compressors), an important feature of theSinge Screw compressor is the ability to operatewith optimum efficiency even at part load condi-tions. Other types of screw compressors have

dual slides which operate in series. This resultsin one of the slides blocking off some of theporting behind the other slide creating a restriction and performance penalty at part load conditions.

ConclusionThe Vilter single screw compressor with the

5/15-year warranty and ParallexTM slide systemmakes it the most efficient and reliable compres-sor in the world. It’s superior to any other singlescrew and most certainly every twin screw.

Thousands of single screws are in operationworldwide for gas compression, air conditioning,refrigeration, and petrochemical industries.

DISCHARGE PROCESS

The gas in the grooveis at discharge pressure. The mainscrew aligns with thedischarge port in thehousing and pushesthe gas through theport into the dis-charge chamber.

All of the gas thatwas trapped in thegroove is pushed out.The volume of thegroove is reduced to zero.

CAPACITY CONTROL

The maximum volumeof gas is trapped inthe groove.

Approximately 50% of the volume of thegroove is trapped and allowed to becompressed.

Approximately 20% of the volume of thegroove is trapped and allowed to becompressed.

VOLUME RATIO CONTROLS

The discharge portopens early in thecycle allowing only asmall reduction involume and a smallincrease in pressure.

The discharge portopens later in thecycle allowing a significant reductionin volume and a significant increasein pressure.

The slide and resulting dischargeport location are inthe maximum positionresulting in the maximum reductionin volume and a maximum increase indischarge pressure.

CAPACITY & VOLUME RATIO CONTROLS

The capacity slide isset to compress themaximum (100%) ofthe swept volumewhile the volumeslide is set to allow aminimal increase indischarge pressure.

The capacity slide isset to compress themaximum (100%) ofthe swept volumewhile the volumeslide is set to allow amaximum increase indischarge pressure.

Rotation

Rotation

Rotation

Capacity Slideat Maximum

Capacity

➡

Capacity Slideat Part Load

Capacity

➡

Capacity Slideat Minimum

Capacity

➡Maximum

Volume Ratio

IntermediateVolume Ratio

MaximumCapacity and

MaximumVolume Ratio

MaximumCapacity and

MinimumVolume Ratio

MinimumVolume Ratio

➡

➡ ➡

➡➡

Rotation

Rotation

Rotation Rotation

RotationRotation

Rotation

Discharge Port

Opening

Gas Discharge

➥

START OFDISCHARGE

END OFDISCHARGE

Discharge Port

Opening

TheoryThe compression cycle begins after suction gas

fills the top and bottom grooves of the main screwat the suction end of the casing. Since the screwcompressor has two gaterotors, the compressionprocess occurs simultaneously on opposite sidesof the screw; the top and bottom. As the mainscrew rotates, it in turn drives the gaterotors. The engagement of the gaterotor with a screwgroove traps the suction gas and begins the compression process. As the screw rotates, the engagement of the gate rotor continues, thus reducing the initial volume of the groove and increasing the pressure in the groove. Once again this occurs simultaneously on opposite sides of the screw.

Finally, as the main screw rotates toward thecompletion of the compression cycle, the groovealigns with a port in the housing at the dischargeend of the casing. The gas and any liquid in thegroove are radially discharged through the portinto the discharge plenum. Since there are sixgrooves in the main screw, the compression

process simultaneously occurs six times in twolocations per revolution of the screw. Operationat 3600 RPM results in 21,600 simultaneouscompression strokes at the top and bottomgrooves per minute and a relatively smooth flow of discharge gas.

Balanced LoadingOne advantage of the Single Screw compressor

is the fact that there are no net radial or axialforces exerted on the mains screw or drive shaftcomponents due to the work of compression.Since the compression process occurs symmetri-cally and simultaneously on opposite sides of thescrew, the forces due to compression are can-celed out. The only vertical loads exerted on themain screw bearings are due to gravity. Since thedischarge end of the screw is vented to suction,the suction gas pressure is exerted on both endsof the screw resulting in balanced axial loads.

The Single Screw has an inherent design advan-tage of reduced loading during the compressionprocess. This is due to the fact that the gaterotor

tooth area decreases as the gas pressure in thegroove approaches discharge pressure. When the gaterotor first engages with the mainscrew the compression process begins. As rotation continues, the gate rotor tooth areaexposed to the gas pressure increases. The result-ant force creates the axial loads on the gate rotorassembly. Approximately half way through thestroke, or when the radial axis of the gaterotor isperpendicular to the rotational axis of the mainscrew, the maximum area of the gate rotor isexposed to the gas pressure. As the compressioncycle continues, the pressure within the grooveincreases but the area of the gate rotor exposedto the discharge pressure continues to decrease.The lower loads transmitted to the componentsand bearings result in higher reliability. At theend of the stroke, the area of the gate rotor hasbeen reduced to zero as it disengages from themain screw.

Another design feature of the Single Screwcompressor that enhances reliability is the loadson the gate rotor assemblies are well defined and

SUCTION PROCESS

As the grooveincreases to maximum, the gas flows into theopen grooves.

When the groovereaches its maximumlength or volume, thegroove is closed bythe gaterotor andcontained within thehousing.

COMPRESSION PROCESS

The gas is trapped inthe groove at suctionpressure.

As the groove volumeis decreased, thepressure in thegroove increases.

The gas continues to be compresseduntil it reaches theminimum volume andmaximum pressurefor the application.

Rotation

GasIntake

Rotation

➥

Rotation

Rotation

Rotation

SUCTION

END OFSUCTIONPROCESS

START OFCOMPRESSION

COMPRESSION

END OF COMPRESSION