Rotating gear pumps

F-G F Thermoforming, injection, blow, rotational and extrusion molds Business machine and camera housings, blowers, bearings, gears, pump impellers... 

Since discrete quantities are trapped and transferred, the delivery pressure and flow varies, as shown in Figure 32.21, which also illustrates how increasing the number of cylinders in a reciprocating pump reduces fluctuations. In the case of lobe and gear pumps the fluctuations are minimized by speed of rotation and increasing tooth number, but where, for control or process reasons, the ripple in pressure is still excessive, means of damping pulsations has to be fitted. Often a damper to cope with this and pressure pulses due to valve closure is fitted, and two types are shown in Figure 32.22. 

A rotating cam is mounted eccentrically in a cylindrical casing and a very small clearance is maintained between the outer edge of the cam and the casing. As the cam rotates it expels liquid from the space ahead of it and sucks in liquid behind it. The delivery and suction sides of the pump are separated by a sliding valve which rides on the cam. The characteristics again are similar to those of the gear pump. 

Rotary pumps forcibly transfer liquid through the action of rotating gears, lobes, vanes, screws etc, which operate inside a rigid container. Normally, pumping rates are varied by changing the rotational speed of the rotor. Rotary pumps do not require valves in order to operate. 

The use of spur gears in gear pumps will produce in the discharge pulsations having a frequency equiv ent to the number of teeth on both gears multiplied by the speed of rotation. The amplitude of these disturbances is a function of tooth design. The pulsations can be reduced markedly by the use of rotors with helical teeth. This in turn introduces end thrust, which can be eliminated by the use of doublehelical or herringbone teeth. 

As the gear teeth rotate and travel past the pump inlet, a partial vacuum forms. Oil is carried by the small chambers formed between the gear teeth. As the teeth mesh near the pump outlet, oil is delivered at a higher pressure. Also, flow from a gear pump is continuous and not intermittent. 

Fig. E6.1b Alternative design solution of a roll pump from building block 1. On the top we see two two-roll pumps one co-rotating and the other counterrotating. The latter is a toothless gear pump where the pumping mechanism is viscous drag rather than positive displacement. In the middle we have three three-roll configurations, and at the bottom a four-roll pump.

Next we examine two important continuous positive-displacement pumps the gear pump and the co-rotating fully intermeshing twin-screw pumps. First we show in the following Example how to convert building block 5 into a continuous intermeshing twin-screw pump. 

Fig. E6.16 Output versus gear frequency of rotation for a 5.6-in-diameter 4.5 in wide, double (30°) helical, 14-teeth 0.67-in-deep gear pump, and with LDPE (circles) and PS (triangles). Smooth curves are the theoretical ones at the respective densities. [Reprinted with permission from C. Y. Cheng, Farrel Corp., Ansonia, CT, private communication, 1972.]...

To increase the extremely poor efficiency in co-rotating extruders, special screw elements have been developed. These include the gear element , which allows the pressure generating benefits of gear pumps to be used in co-rotating extruders. A gear element for co-rotating extruders will be available from CWP in future. 

There are three basic types of devolatilization equipment that have been used for the commercial manufacture of polystyrene wiped film evaporators, devolatilizing extruders and flash evaporators. In wiped film evaporators, the polymer solution is fed into a vessel under vacuum. The solution is moved into thin films along the vessel walls by a set of rotating blades. These blades continue to move the polymer through the vessel while continually renewing the surface area. The tank walls are heated to supply the required energy for devolatilization. These units are typically mounted vertically with the polymer solution fed at the top. At the bottom is a melt pool where a gear pump transfers the melt to the next unit operation, typically pelletization. 

---