Rotary Positive Displacement Pumps
Rotary positive displacement pumps draw fluid into some type of chamber, which is then moved around by the rotary action of the pump from the inlet to the outlet of the pump, where the fluid is discharged. The clearances between the moving parts are typically close, so the pump speed must be kept relatively slow. If these pumps operate at excessive speeds, erosion will be caused by the fluids, like polishing of stones by ocean waves. Excessive clearances are created due to such erosion that permits slip of liquids, decreasing the pump efficiency. Several types of rotary positive displacement pumps are described next.
Gear pumps consist of gears that are arranged with the teeth meshed, as shown in the diagram at the left. The gears rotate in opposite directions, so that they pull fluid into the spaces between the gear teeth and the pump casing. The fluid is finally released through the pump discharge due to the movement of the teeth. A fairly constant fluid flow is maintained by smaller teeth, while bigger teeth will produce a more pulsating fluid flow pattern.
Image Credit: RPI Chem Engr Dept
The principle of the lobe pump is very similar to that of the gear pump, except that a smaller number of larger "lobes" mesh together as shown in the diagram of a disassembled lobe pump at the right. In this case the fluid is moved through the pump in spaces between the lobes and the pump casing.
Image Credit: MSU Engineering
Screw pumps are fitted with two screws that have opposite threads, one turning clockwise while the other turns counterclockwise. The screws are fixed on parallel shafts that have meshed gears. The shafts are rotated due to the gear movements. The fluid is drawn from the pump due to the screws turning. The clearance between the moving components and the pump casing is necessarily quite small.
Image Credit: Wikimedia Commons
Peristaltic pumps, also sometimes called tubing pumps, don't quite fit the general description of rotary positive displacement pumps given above, but this is still the closest classification for them. For this type of pump, a rotor with lobes squeezes tubing against the inside of a cylinder as it rotates and thus draws fluid through the tube. The principle is illustrated in the diagram at the right.
Image Credit: H. Bengtson