3.4.4 Lobe rotor pump
Lobe rotor pump
A Lobe rotor pump has two rotors, which unlike gear pumps, operate without metallic contact with each other. Both rotors are driven by synchronized gears which are completely separated from the pump chamber, see figure 3.44a. The shaft bearings are also situated in the gear housing. The pumped fluid does not therefore come into contact with the bearings.
Figure 3.44a Lobe rotor pump (rotating piston pump)
Synchronized drive gears cause the rotors to rotate in opposite directions. The inlet fluid flow is divided into two halves, trapped in the space formed between the rotor and the pump casing and transferred, without change in volume, towards the outlet where the rotors meet, thereby reducing the cavity and forcing the fluid out, see figure 3.44b.
Figure 3.44b Working principle of Lobe rotor pump
The absence of metallic contact between the surfaces of the rotors themselves or between the pump casing and the rotors means that wear of the rotating parts is insignificant. The only wear which occurs is due to friction with the pumped fluid.
Since the shaft bearings are normally mounted outside the pump casing, the shaft ends are relatively long and unsupported which imposes limits in terms of pumping pressure. For pressures in excess of 1,2-1,5 MPa some manufacturers fit plain support bearings inside the pump casing. Others mount an extra support bearing housing in front of the pump casing. The latter case giving rise to four shaft seals.
The shape of the rotors varies from manufacturer to manufacturer. The most usual is shown in figure 3.44c. The shape does not alter the principle of operation. It can be said however that rotors with one or two lobes give rise to greater pulsation than rotors having three lobes. For gentle handling of fluids, rotors having one or two lobes should be chosen.
Figure 3.44c Various rotor shapes
Lobe rotor pumps are suitable for both fluent and viscous products. Special feeding arrangements being necessary for extremely viscous fluids which cannot otherwise flow. Fluid temperatures of up to 200°C can be handled if the
clearances between the rotors and the front cover are increased. By increasing the clearances behind the rotor it is possible to handle fluid at temperatures of down to minus 40°C. To maintain constant temperature pumps may be fitted with heating or cooling jackets.
Lobe rotor pumps handle the pumped fluid very gently. Examples of this phenomenon can be found within the foodstuffs industry where lobe rotor pumps are used for pumping cooked pea soup, preserves containing whole berries and other similar applications. In these cases the lobe rotor pumps are specially constructed to fulfill the hygiene requirements; being easily dismantled and suitable for washing by hand or in accordance with the CIP (Clean In Place) method. Lobe rotor pumps can also be equipped for completely aseptic pumping for use within the pharmaceutical industry. They are also extensively employed in the chemical industry for handling both aggressive and neutral products.
Special dense-pulp pumps have been developed for the cellulose industry, these being suitable for pulp concentrations ranging from 6 to 30%, see figure 3.44d.
Figure 3.44d Dense pulp pump with feed screw.
Lobe rotor pumps usually operate at relatively low speeds and they are often used for high viscosity fluids. The pump is quiet running and delivers a largely pulsation-free flow. For most lobe rotor pumps the components in contact with the fluid are manufactured in acid-resistant steel. Simpler types have a cast iron pump casing with rotors and shafts made of steel. Sometimes the rotor material can be varied in order to increase the pumps suction capacity, by the use of nitrile rubber rotors, for example.
Shaft seals consist of various forms of mechanical seals and stuffing boxes. Since the pump has two shafts which pass through the pump casing, two sets of seals are needed for each pump. Rotating piston pumps are available in a large variety of sizes up to 300 m³/h. Pressure increases are normally 0,3-1,5 MPa. Suction capacities vary between 1 and 5 m, depending upon internal clearances, pump size and speed.