All about Blowers Series: Positive Displacement Blowers

5/29/2018

Although blowers are commonly used in manufacturing, it can be difficult to find good sources of information on the different types of blowers and how to choose the appropriate one. The purpose of this article series is to give a good, basic understanding of the different types of blowers and provide you with the technical information required to make a good decision for your application.

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What are the different types of blowers?

At its most basic level a blower is a tool that draws in air at an inlet and pushes air out as a steady stream at the outlet. Blowers can largely be classified into two categories: impeller based and positive displacement. Impeller based blowers have fins that radiate outwards from a rotating central axis. Positive displacement blowers use a mechanism of filling and emptying chambers at the inlet and outlet, respectively, to create flow. The fundamental difference between the two is that impeller based blowers have output flow that varies with pressure whereas positive displacement blowers have a more constant output flow regardless of change in pressure.

2. Positive Displacement Blowers

Positive displacement blowers create flow by filling and emptying chambers of air; these blowers produce flow which is relatively independent of operating pressure. There are several variations available which use the same premise but slightly different design. Here we discuss two popular types: rotary vane and rotary lobe.

Rotary Vane

The simplest rotary vane blowers consist of a circular rotor rotating inside of a larger circular cavity. The centers of these two circles are offset, causing eccentricity. Vanes are located in the centre of the rotor and slide into and out of the rotor. In the example here, there are two vanes which seal on all edges, creating three-walled chambers between the wall of the cavity, the rotor, and the vane. In practice there can be greater than two vanes creating more chambers. On the intake side of the pump, the volume of the vane chamber increases, sucking in air until the trailing vane passes the inlet. The size of the chamber changes as rotation continues. On the discharge side of the pump, the volume of the vane chamber decreases, increasing the pressure until it reaches that required to overcome the external pressure. The volume of gas trapped on the inlet side is constant which creates a constant operating flow. Compression in the cavities allow for high pressures to be obtained.

Rotary vane pump. Key: 1. Body 2. Rotor 3. Piston valve 4. Spring Source: "Rotary vane pump" by Rainer Bielefeld is licensed under CC-BY-SA-3.0.

Rotary Lobe (Roots Blower)

Rotary Lobe blowers work on a similar principal as rotary vane blower. Two impellers (shaped as figure eights in the photo on the right, although tri-lobe designs are also common) rotate in opposite directions which cause the blower to trap gas on the inlet side of the blower and then, as the lobes rotate, exhaust it from the outlet. The gas travels in the cavities around the periphery of the lobe chamber from the inlet to the outlet and the volume of the air does not change between the inlet and the outlet of the blower. The volumetric flow is dependent on the size of the chambers and the speed of rotation. Because of their design, a two lobe rotary lobe blower outputs air in pulses. This can be mitigated by increasing the number of lobes.

Rotary Lobe blowers have no lubrication or oil in contact with the air and therefore are useful for applications where the delivered air must be free from contaminants.

The next article in our series compares the two categories of blower and discusses when they are best suited for use.

A rotary lobe blower with two-lobed rotors. Modern blowers can have 3- or 4-lobed rotors. Key: 1. Rotary vanes 2. Pump body a. Intake b. Pumping c. Output 3. Rotary vanes Source: "Roots blower - 2 lobes" by Inductiveload is licensed under CC-BY-SA- 3.0.