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Working Principle of a Rotary Screw Air Compressor

What are Rotary Screw Air Compressors?

Rotary screw air compressors are designed to produce a consistent flow rate which is ideal for a variety of industrial applications. Popular applications include, small machine shops, food & beverage production, automotive manufacturing, pharmaceutical, etc.

Rotary screw air compressors are ideal for industrial and commercial businesses because of the durability, efficiency and energy savings. Whether you are in need of an air compressor with direct or belt driven, fixed or variable speed, low or high horsepower and CFM, Gardner Denver has a comprehensive selection of models to choose from.

Visit our Product Page to discover our full range of rotary screw air compressor products, and learn more about how our technologies best suit your application needs.

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How do Rotary Screw Air Compressors work?

The oil injected rotary screw air compressor is a versatile industrial machinery that efficiently converts power into compressed air through a continuous rotary motion. Commonly known as a twin-screw compressor (figure 1), this type of compressor consists of two rotors, each featuring a set of helical lobes attached to a shaft.

One rotor is called the male rotor and the other rotor is the female rotor. The number of lobes on the male rotor, and the number of flutes on the female, will vary from one compressor manufacturer to another.

However, the female rotor will always have numerically more valleys (flutes) than the male rotor lobes for better efficiency. Male lobe acts like a continuous piston rolling down the female flute which acts like a cylinder trapping air and reducing space continuously.

GDP-En-Application-Photo-screw-compressors — Figure 1: Twin screw compressor

With the rotation, the leading strip of the male lobe reaches the contour of the female groove and traps the air in the pocket previously formed. The air is moved down the female rotor groove and is compressed as the volume is reduced. When the male rotor lobe reaches the end of the groove, the trapped air is discharged from the airend. (figure 2)

GDP-En-Product-Photo-screw-compressors-V2 — Figure 2

This type of twin-screw compressors can be oil free or oil injected. In the case of the oil lubricated compressor oil is injected.

The oil performs four crucial functions:

Cooling
Lubrication
Sealing
Noise dissipation

Oil cooling and separation

The purpose of a compressor is to convert shaft work into a useful output, that is, air flow. As compressing air generates heat, all of the heat is retained within the compression chamber; this is adiabatic compression. If heat is added or taken away during the compression process this is called isothermal compression.

Oil injected screw compressors have a near isothermal compression process as the heat generated by the compression process is almost dissipated by the oil.

The temperature of the oil injected into the compression chamber is generally controlled between 60-700C. The discharge temperature must remain above the pressure dew point to avoid condensation of moisture that would mix with the oil.

A thermostatic valve controls the quantity of the oil being circulated to the oil cooler or to the bypass in order to maintain the desired temperature over a wide range of ambient temperatures.

A mixture of compressed air and oil leaves the air end and is passed to a separator where most of the oil is removed from the compressed air using directional and velocity changes. Then coalescent filters are used to remove the remaining oil resulting in very low oil carry over (usually 2-5 ppm).

GDP-En-Product-Photo-screw-compressors-V3 — Figure 3: Oil cooling and separation

Air after coolers

In addition to cooling oil, an air aftercooler is used to cool the discharged air and to remove excess moisture.

In most applications, radiator type coolers (figure 4) are used to provide heat recovery from the compression process and use it for facility heating. Water cooled heat exchangers are also available on most rotary screw compressors.

GDP-En-Product-Photo-screw-compressors-V4 — Figure 4: Air cooler

Compressor efficiency

All air compressors are less than 100% efficient. The best way to measure the efficiency of the compression is to look at its isentropic efficiency. The Isentropic efficiency of compression is the ratio of the ideal isentropic work to the actual work:

n_c = isentropic compressor work (ws)

Actual compressor work (wa)

When we raise the pressure of the air, the temperature increases and therefore so does the entropy of the fluid. The entropy is a measure of energy loss. A real compressor requires more work to raise the pressure of the air to the same outlet pressure than a theoretical compressor.

But the compression efficiency isn‘t the whole story. A compressor depends on the airend‘s efficiency and the compression chamber but its total energy consumption depends also on several losses and efficiencies of the components included in the package. (See Figure 5)

In reality there needs to be one measure to compare efficiency of one compressor to another.

GDP-En-Product-Photo-screw-compressors-V5 — Figure 5. Compressor total power consumption

Therefore the specific energy that equals the work required to compress a given amount of air (cubic meter) in a specific amount of time (minute) to a specified pressure (barg) is used. It is measured in kW.

Specific Energy = energy / unit volume

= kW per m³/min

The specific energy and the compressor package’s efficiency is dependable on all its component’s efficiency as well as the complete system’s pressure drop. The pressure drop can be measured on the intake valve and suction box, air filter, piping and oil separator.

What are the benefits of rotary screw air compressors?

Efficiency: They deliver a continuous and steady supply of compressed air, which is essential for applications that require a consistent flow of air. Their design minimizes fluctuations in pressure, leading to improved efficiency and reduced energy consumption.
Continuous Operation: Rotary screw compressors can operate continuously without the need for frequent starts and stops, which can extend the lifespan of the compressor and improve overall system reliability.
Adaptability: Rotary screw compressors can operate in both high and low conditions, even in areas where safety restricts other energy sources.
Easy to maintain: Their minimal moving and contacting parts make maintaining compressors easy, reducing wear, extending service intervals, and simplifying routine checks and repairs.
Low Noise Levels: These compressors are generally quieter than reciprocating compressors, making them suitable for environments where noise is a concern, such as indoor workplaces.

Gardner Denver Rotary Screw Air Compressors Types

Two-stage Compressors

Two-stage lubricated rotaries compress air in two steps. Step or stage one takes atmospheric air and compresses it part way to the discharge pressure target. Step or stage two ingests the air at the inter-stage pressure and compresses it to the discharge pressure target. Compression in two stages improves efficiency, but adds cost and complexity given the additional rotors, iron and other componentry involved. Two-stage is generally offered in the higher HP ranges (100 to 500 HP) because the improved efficiencies result in larger dollar savings when air use is large.

Single-stage Compressors

Single stage versus two-stage, it is a relatively straightforward calculation to determine what the payback will be from the more efficient but more costly two-stage unit.

Remember that the energy cost of operating a compressor is the largest expense over time, so evaluation of a two-stage machine is certainly worth a look.

Lubricated

The lubricated rotary screw compressor has been the most popular technology for the majority of industrial plant air applications from 20 to 500 HP and from 80-175 PSIG. These compressors offer unmatched versatility and adaptability to various operational demands. Their efficient design ensures a continuous and reliable supply of compressed air, crucial for sustaining seamless production processes.

Gardner Denver Rotary Screw Air Compressors, stand out as a superior choice in performance for a variety of reasons. Our compressors are rigorously tested to ensure accurate performance and quality, guaranteeing performance numbers are accurate, easy-to-understand. Reach out to our experts for assistance in selecting the ideal compressor series tailored to your specific needs! Contact Us.