Screw Compressor by Vilter: The Inside Story

Vilters single screw compressor- the inside story. In the mid-1980s Vilter decided to build its own screw compressor and became a licensee to fabricate both twin and single screw compressors and evaluating both technologies many inherent benefits of the single screw were recognized. Thus, Vilter decided to produce single screw compressors.

To this date with thousands of compressors operating in the field, Vilter team members are confident that they made the right choice in screw compressor technologies. A single screw compressor unit is comprised of typical components including an oil separator, lubrication pump and piping and oil cooler, oil filter, micro controller, suction stop check valve strainer assembly, a motor whose shaft is connected by a coupling to a compressor.

The main rotor of the single screw compressor is a simple cylinder with a shaft through it. There are six threads or grooves or flutes cut in cylinder. Each flute is opened at one end of the cylinder diminishing to zero volume out the side of the cylinder before reaching the opposite end.  The open end of the grooves allows low pressure suction gas to fill the flutes in operation– each flute becomes a compression chamber

There are four holes drilled through the main rotor, these vent holes allow the cavity on the opposite end of the rotor at low suction pressure. Depending on the size of the compressor there are either two, four or six vent holes drilled through the main rotor.  The main rotor rotates on the shaft typically have 3600 revolutions per minute- yet with some models at half speed or 1800 rpm

Two Stargate Rotors intermesh with the main rotor. The Stargate Rotors are identical in shape and are positioned opposite of each other across the diameter of the main rotor. While the main rotor rotates, the star gate rotors go along for the ride. As a tooth of the gate rotor intermeshes with a flute of the main rotor, the tooth serves to close the gate trapping the gas in the flute.

As the tooth travels through the flute, the trapped gas is compressed to a smaller volume and higher pressure. With six total flutes, three flutes are engaged in compression on the top half of the compressor and the opposing three flutes are simultaneously engaged in compression on the bottom half of the compressor.

The gate rotor on the opposite side is oriented identically to the gate rotor on the near side just flipped over. The gas that enters the flutes on the top half of the compressor is compressed by the gate rotor on the near side and the gas that enters the flutes on the bottom half of the compressor is compressed by the opposing gate rotor on the far side.

A casing wrapped around the main rotor encloses and traps the gas in the flutes.  High-pressure compressed gas is discharged from the casing’s triangular ports out the side of the compressor. The blue cloud at the right end of the main rotor represents the low pressure of suction gas. The vent holes drilled to the main rotor allow the same low-pressure condition to exist in the cavity at the left end of the main rotor.

The pressure of the suction gas exerts a force in an axial direction on the main rotor. But since suction pressure resides at both ends of the main rotor, the axial forces cancel resulting in a zero-net axial force on the main rotor of the compressor.

Rotating the compressor to an axial view, we see that the compressed gas is discharged simultaneously from two opposing discharged ports out opposite sides of the compressor. The radial forces exerted on the top of the compressor are identical and opposed to the radial forces exerted on the bottom of the compressor. Thus, the radial forces cancel both axial and radial forces are canceled. The only net force on the main rotor of the single screw compressor is gravity.

Low bearing loads result in extremely long compressor life and very high reliability. the Vilter single screw compressor is the only industrial refrigeration screw compressor offered with a standard 15-year bearing warranty.

Now let’s talk about the capacity and volume control. A fixed port casing wrapped around the main rotor would be similar to the casing of an air compressor where a fixed volume of gas is trapped, compressed and discharged out of the fixed port.

A Chicago pneumatics fixed port air compressor with a split casing has a triangular shaped discharge port in the casing through which the compressed gas is discharged. The Vilter single screw compressor does not have a fixed port casing rather a portion of the casing is removed from both sides of the single screw’s frame.

The portions of the frame that were removed are replaced with two bars of steel or slide valves on each side of the compressor. The patented parallax slide valves retain the curved shape of the casing they replaced. The casing of a Vilter single screw compressor has the voids in the frame that accommodate the parallel slide valves. The slide valves are supported by carriage assemblies which are mounted in the compressors frame.

Each slide valve is able to move or slide in the direction of the axis of the main rotor. The position of the right air to the capacity slide valves determines the amount of gas that is trapped and compressed. The infinitely variable capacity slide valves are capable of modulating from full load when positioned to the right to 10% of full load when positioned to the left.

The position of the left edge of the volume ratio slide valves determines the pressure or volume at which the gas is released from the compressor. The volume slide valve enables infinitely variable volume ratio control from a low one point to VI when positioned to the right to as high as 7.0 VI when positioned to the left- the broadest volume ratio range in the industry.

Parallel slide valves are interlinked so that the position of the capacity slide valve on one side of the compressor is identical to the position of the capacity slide valve on the opposite side of the compressor. Likewise, the position of the volume ratio slide valves is identical on both sides of the compressor thus the parallax slide valve design maintains the balanced forces around the main rotor of the compressor

The capacity and volume ratio slide valves are positioned in parallel to each other; not in series. but the single screws slide valves there are no slide stops or mechanical limitations to restrict their functionality. Both slide valves are able to modulate within their full range of travel simultaneously. Thus, the Vilter single screw is the only industrial screw compressor capable of operating at optimized efficiency through its full capacity range

So far, we’ve presented the two primary benefits of Vilters single screw compressor: balanced forces for extremely long bearing life and high reliability and the parallax slide valves for optimized efficiency through its full capacity range.

Besides these primary benefits, there are also many additional benefits of the single screw compressor. All the single screw compressors’ bearings are at suction pressure. with the single screw, there is no risk of bearing degradation or failure due to liquid ammonia washing oil off the bearings in the discharge gas stream since there are no bearings in the discharge gas stream. This feature of the single screw allows the success of Vilter’s cool compression technology.

Further lubrication of the bearings is easily achieved due to the high differential pressure between the oil that’s fed to the bearings and the low pressure at which the bearings operate.

The single screw’s length to diameter ratio or the L \ D (Length \ Diameter) of the rotors is proportional. The single screws rotors are stocky and rugged and their L \ D (Length \ Diameter) proportions remain consistent throughout the entire product line.

The robust construction of single screw rotors is not subject to the challenges presented by mid rotor deflection of other technologies. With the single screw cascading is minimal. cascading is when gas slips across the land from a high-pressure flute to the adjacent lower pressure flute.

The land of the single screw is not a sealing tip but rather a flat wide surface. When the rotor rotates and pressure increases, the land increases in width and cascading is minimized. Minimal cascading results in high volumetric efficiency.

The single screws Gate Rotors are constructed of a non-wearing high-performance engineered composite material having a chemical composition polyphenylene sulfide or PPS and is commonly known as Ryton by its producer Philips Chemical Company. this material is incredibly strong.

Some characteristics of Ryton include high temperature stability, dimensional reliability and flame retardance. Ryton holds up to any gas or liquid.in fact it has such high chemical resistance that there are no known solvents of Ryton below 400 Degrees Fahrenheit.

The Gate Rotors are tolerant of extreme conditions. They can take the hit of eleven hundred and sixty foot-pounds of energy from an ak-47 rifle at 50 yards. In an experiment, the bullet hit the inflicted damage did not extend beyond a three-quarter inch diameter hole.

The Gate Rotors are tooled to conformance by the main rotor. There is no need for match marking or mating pairs of rotors with a single screw compressor.  Gate Rotors are easy to access and replace if necessary right through the side access covers of the compressor without removing the compressor from the unit.

Replacement of the Gate Rotors restores the compressor to original capacity. The single screw compressor is quiet and vibration free. It’s quiet and vibration free primarily due to its balanced design and the flow of its discharged gas pulsations.

Consider the gas pulsations of three different compressors operating at 3,600 revolutions per minute. A four lobes twin screw compressor produces four discharged gas pulsations per revolution multiplied by 3,600 revolutions per minute or 14400 gas pulsations per minute discharged from a single port.

A five lobes twin screw compressor produces 18,000 gas pulsations per minute discharged from a single port. A single screw with its six flutes in two dis charged ports produces 12 discharged gas pulsations per revolution or 43,200 gas pulsations per minute discharged simultaneously from dual opposing discharge ports. As a small volume of gases discharged from one port, a simultaneous small volume of gas is discharged from the opposing discharge port.

Any induced vibration to the frame of the compressor due to a discharged pulse on one side is cancelled by the identical volume discharge pulse on the opposite side. It’s very unusual for vibration monitoring and analysis to be applied to a single screw compressor because the single screw is inherently a vibration free compressor.

In summary, the single screw compressors ballast forces provide an extremely long life and very low maintenance. It’s parallel slide valves result in peak energy efficiency. all of the compressors bearings are at suction pressure for reliability. The L \ D (Length \ Diameter) is proportional throughout the product line.

The land is wide for minimal cascading and high volumetric efficiency. gate rotors are durable yet forgiving. It’s a quiet and vibration free compressor and the single screw is the only screw compressor unit with a five-year compressor and 15-year barring warranty.

The single screw product line consists of two series of units: the VSM with nine models ranging from 150 CFM up to 700 CFM and the larger VSS units with 11 models ranging from 750 CFM up to 3,000 CFM. these units are built around six different rotors from the smallest 205 mm rotor about 8 inches in diameter up to the large 395 mm rotor which is nearly 16 inches in diameter.

The 205mm rotor provides us with six models from the VSM 152 up to the VSM 401. The model number indicates the compressors capacity in CFM. The three smallest models run at 1,800 rpm and all of the larger models operate at 3,600 rpm. From the 240 mm rotor, three models are available: the VSM 501, VSS 601 and VSS 701.

In summary, When it comes to durable and budget air compressor, Vilter’s air compressor will be in the list.

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