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.  

How Do Rotary Screw Air Compressors Work?

Oil-injected rotary screw air compressors are versatile industrial machines that efficiently convert power into compressed air through a continuous rotary motion. Commonly known as twin-screw compressors, these devices feature two rotors with helical lobes attached to shafts. One rotor is the male rotor, and the other is the female rotor. The number of lobes on the male rotor and the number of flutes on the female rotor vary by manufacturer, but the female rotor always has more valleys than the male rotor has lobes for better efficiency. The male lobe acts like a continuous piston, rolling down the female flute, which acts like a cylinder trapping air and continuously reducing space.

As the rotors rotate, the leading edge of the male lobe reaches the contour of the female groove, trapping air in the pocket formed between them. The air moves down the female rotor groove and is compressed as the volume decreases. When the male rotor lobe reaches the end of the groove, the compressed air is discharged from the airend.

Twin-screw compressors can be either oil-free or oil-injected. In oil-injected compressors, oil is introduced into the compression chamber.

The oil performs four crucial functions:

1. Cooling: The oil absorbs the heat generated during compression, helping to maintain optimal operating temperatures.

2. Lubrication: It lubricates the moving parts, reducing friction and wear.

3. Sealing: The oil helps to seal the gaps between the rotors and the housing, improving efficiency.

4. Noise Dissipation: It reduces noise by dampening vibrations within the compressor.

Oil Cooling and Separation

The primary function of a compressor is to convert shaft work into a useful output, namely airflow. Compressing air generates heat, and in an adiabatic compression process, all the heat is retained within the compression chamber. If heat is added or removed during the compression process, it is called isothermal compression.

Oil-injected screw compressors achieve near-isothermal compression, as the heat generated by compression is largely dissipated by the oil.

The temperature of the oil injected into the compression chamber is generally controlled between 60°C and 70°C. The discharge temperature must remain above the pressure dew point to avoid condensation, which would cause moisture to mix with the oil. A thermostatic valve controls the quantity of oil being circulated to the oil cooler or to the bypass, maintaining the desired temperature over a wide range of ambient conditions.

The mixture of compressed air and oil exits the air end and enters a separator, where most of the oil is removed from the compressed air through directional and velocity changes. Coalescent filters then remove the remaining oil, resulting in very low oil carryover (usually 2-5 ppm).

Air Aftercoolers

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

In most applications, radiator-type coolers are employed to recover heat from the compression process, which can then be used for facility heating. Water-cooled heat exchangers are also available on most rotary screw compressors, offering an alternative method for cooling and moisture removal.

Benefits of Rotary Screw Air Compressors

1. Efficiency: Rotary screw air compressors deliver a continuous and steady supply of compressed air, essential for applications requiring a consistent flow. Their design minimizes pressure fluctuations, improving efficiency and reducing energy consumption.

2. Continuous Operation: These compressors can operate continuously without frequent starts and stops, extending the lifespan of the compressor and enhancing overall system reliability.

3. Adaptability: Rotary screw compressors can function in both high and low conditions, even in areas where safety restrictions limit other energy sources.

4. Easy Maintenance: With minimal moving and contacting parts, rotary screw compressors are easy to maintain. This design reduces wear, extends service intervals, and simplifies routine checks and repairs.

5. Low Noise Levels: Generally quieter than reciprocating compressors, rotary screw compressors are suitable for environments where noise is a concern, such as indoor workplaces.