A rotary screw compressor is a positive displacement type of gas compressor which employs two rotors that have an inter-meshing helical screws machined into the surface of the rotors to progress gas, usually air, from one side of the machine to the other, to achieve compression.
The rotors are normally close fitting with small gaps between them. They rotate continuously and progress, or sweep, the gas, usually air, from the inlet to the outlet.
Although the two rotors inter-mesh and rotate in tandem their shapes whilst looking similar, will in fact be different. One rotor, often called the male rotor, will have one less lobe than the second rotor, which is often called the female rotor.
As the rotors rotate they will progress the gas along the length of the rotor to the outlet. This is unlike the roots type blower where the gas is usually progressed across the machine.
Also unlike the roots type lobed blower which rotate at low speeds, usually less than 500 rpm, the rotors in a screw compressor will rotate at much higher speeds with rotational speeds of 3000 rpm not uncommon.
As a consequence of these differences screw compressors the levels of noise and pulsation generate by these machines will be different to that generated by a lobed roots blower.
If the noise and pulsation mechanisms for a roots blower are compared to a screw compressor of the same capacity, then we find very similar mechanisms, in that a slug of gas is induced into the inlet port to be progressed through the machine to the outlet port where it is pushed out into the discharge line, whilst at the same time the gas it trying to slip back past the rotors to the low pressure side of the machine.
Although these mechanisms are comparable, the noise and pulsation levels are quite different as we find that the pulsation amplitude is much less than that produced by a roots type blower whilst the frequency of that pulsation is much higher for a screw compressor because the rotational speed of the rotors is much higher.
In this respect the design requirements for a screw compressor intake silencer are quite different from that of a screw compressor discharge silencer. As whilst the frequency of the noise from the rotation of the rotors is the same on both sides of the machine, the aerodynamic slip noise does not propagate into discharge lines as easily as it does into the intake.
Additionally the pressure drop across and inlet silencer becomes more important as the size and power of the machine increases, if the pressure drop is too high then the machine will be "starved" of flow and overall performance will be reduced.
