The Roots type blower is a positive displacement lobe pump which operates by pumping, or moving, a fluid, usually air, but in some cases a process gas, using a pair of inter-meshing lobed rotors, which have a profile that looks not unlike the number 8. When operating, the fluid is trapped in the outer pockets surrounding the lobes and is carried, or progressed, from one side of the machine to the other.
The lobe shaped rotors (usually two lobes, but it can be 3 or 4) work in tandem and move the fluid, usually air, from one side of the machine to the other. The lobes do not grab, or scoop the air out of the inlet pipe, but rather as they pass the inlet opening they cause a local low pressure area inside inlet port of the machine into which the fluid will then flow. As the rotors continue to turn the fluid that has entered the machine is trapped by the rotors and progressed from the inlet side to the outlet side. On reaching the outlet side the fluid has no choice but to be pushed out into the discharge pipe.
Each time one of the rotor tips passes either the intake or exhaust port, the passing of that tip causes a pressure pulse to be generated. Therefore in the case of a blower with rotors that have 2 lobes there will be 4 pulses for each complete revolution of the pair of rotors (2 pulses from each rotor for each revolution). This allows for a relatively easy estimation of the principal frequency of pulsation that the machine will generate.
Whilst the frequency of the noise from the passing of the rotor tips at the intake and exhaust ports is well understood, there is a second noise mechanism where the fluid tries to pass back through the machine from the high pressure side to the low pressure side by "slipping" around the rotors at their tips. This is an aerodynamic noise mechanism and is less easy to predict as the noise level changes with the speed of the blower and the gap between not only the rotors, but also between the gap between the rotors and the casing.
The noise generated by these two mechanisms are quite different.
The sound from the passing of the rotor past the open inlet port will have a certain frequency, or tone, which on large low speed machines can literally sound like a large helicopter, whilst on smaller machines which operate at higher speeds this tone may sound like a tuning fork.
The aerodynamic sound from the slippage of the fluid back past the rotors is not usually obviously tonal in nature, but rather has a more broadband nature to it, not unlike a valve or pipe exhausting to atmosphere at high velocity.
In this respect the design requirements for a blower intake silencer are quite different from that of a blower 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. It can also be said that generally, the noise from the passing of the rotor across the inlet port generates a lower pressure wave than for the same rotor passing across the outlet port.
Additionally the pressure drop design for an inlet silencer becomes more important as the size and power of the machine increases.
This does not mean that it is not important to minimise pressure drop for an outlet silencer, it is just that if the pressure drop for an inlet silencer is too high it has a greater effect on capacity and power than if the outlet silencer had a similarly high pressure drop.
When particularly low noise levels are required an in-line absorptive silencer can be included before the inlet to the inlet pulsation dampener. This arrangement is most often found when machines are drawing air directly from the atmosphere and the intake is nearby to where workers have access.
The most complicated arrangement of silencers and pulsation dampeners is found on VSA (Vapour Swing Absorption) plants where roots blowers are often found continuously supplying air to a process in which the oxygen and nitrogen are separated for industrial purposes. In these systems the machines are large, the power of the motors driving them is considerable, and the noise and pulsation levels damaging if not properly controlled.
