The concept of a gear pump is straightforward: it consists of two identical gears rotating in mesh within a closely fitted housing, which resembles the shape of an "8" with the gears inside. The outer diameter of the gears and the inner walls of the housing are designed to maintain a tight seal. As the fluid from the extruder enters the space between the two gears at the suction port, it gets trapped and carried along the housing as the gears rotate. When the teeth of the gears mesh, the fluid is forced out through the discharge port.
In technical terms, a gear pump is classified as a positive displacement device, similar to how a piston moves within a cylinder. As one gear tooth enters the fluid space of the other, the fluid is pushed out because it cannot be compressed. This action repeats continuously due to the constant rotation of the gears, resulting in a steady and consistent flow of fluid with each revolution of the pump. Since the drive shaft rotates without interruption, the pump delivers a continuous flow of material, directly proportional to its speed.
However, no pump operates at 100% efficiency. A small amount of fluid is inevitably lost due to leakage between the gears and the housing, as well as for lubrication purposes. These losses are necessary for the proper operation of the bearings and gears. Despite this, most gear pumps achieve efficiencies between 93% and 98%, depending on the material being processed.
Gear pumps are also capable of handling fluids with varying viscosities or densities. Even if there's a damper, such as a screen or a restrictor, on the outlet side, the pump can still push the fluid through. If the resistance changes during operation—like a clogged screen or increased backpressure—the pump will continue to deliver a constant flow until it reaches its mechanical limit, usually protected by a torque limiter.
There is a limit to how fast a gear pump can operate, primarily determined by the properties of the fluid. High-speed rotation is feasible with low-viscosity fluids like oil, but when dealing with high-viscosity materials such as polymer melts, the maximum speed must be reduced to prevent cavitation and inefficiency.
It's crucial that the fluid fills the space between the two gears at the suction side. If this area isn't fully filled, the pump may not deliver the exact flow rate required. Additionally, the PV value (pressure multiplied by flow rate) becomes a critical factor, influencing the pump’s performance. Due to these constraints, manufacturers offer a range of gear pumps with different sizes and displacements to suit various applications, optimizing both system capacity and cost.
The PEP-II gear pump features a monolithic design where the gear and shaft are integrated and hardened, ensuring a longer service life. It uses a special "D" bearing with a forced lubrication system, allowing polymer to pass through the bearing surface and return to the inlet, ensuring effective lubrication. This design minimizes polymer buildup and degradation. Precision machining of the pump body ensures accurate alignment of the gears and shafts, preventing eccentricity and reducing wear.
The PEP-II also includes a Parkool seal system combined with a PTFE seal, creating a water-cooled sealing mechanism. Instead of direct contact with the shaft, the seal cools the polymer to a semi-molten state, forming a self-sealing effect. Alternatively, Rheoseal technology can be used, featuring reverse spiral grooves in the seal to redirect the polymer back into the inlet.
For easier installation, the pump is equipped with a ring bolt mount that matches standard flange connections, simplifying integration with other equipment. The PEP-II also comes with a heating element tailored to its size, enabling quick and precise thermal control. Unlike traditional heating methods, this system limits potential damage to just one component.
Driven by a separate motor, the gear pump isolates it from pressure fluctuations upstream, maintaining stable output. The pressure pulsation at the outlet can be kept below 1%, improving overall system performance. In an extrusion line, the use of a gear pump increases output while reducing shear stress and residence time in the extruder, lowering extrusion temperature and minimizing pressure fluctuations, thus enhancing productivity and product quality.
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