The first chapter about the gear pump
It is very likely that you have heard or read about some of the introductions and articles concerning the use of melt pressurization metering gear pumps on extrusion lines. However, there are indeed many reports about the advantages of installing such pumps. However, what is the value of a gear pump for a specific process? What are the actual benefits and costs? How can I know? These problems are of concern to people.
For the application of gear pumps in extruders, this interest has been increasing over the past decade, especially in similar extrusion applications such as films and sheets. Of course, the real reason for this interest is that gear pumps can improve product quality, increase production, and reduce consumables, especially when the melt density or product changes.
But how should these benefits be obtained? How large is this benefit for any kind of application? How can you see these benefits on the production line? What kind of return can you get? All of these are issues that we are concerned about when we discuss this application.
In this article, we will explore these and other related issues and try to explain how this gear pump affects an extrusion line. We will mainly discuss the transformation of the production line, that is, the effect of adding a metering pump to the original extrusion line. Of course, if such a gear pump has been designed into a new production line, its benefits will naturally be available, but this topic will have to be reversed.
In this article, we will use the PEP-II gear pump from the Zenith plant in the United States as an example to illustrate the application principles and effects of the gear pump.
The Zenith PEP-II gear pump is designed specifically for the production of polymers, especially extrusion production processes, and its excellent performance can be used for a variety of process conditions. The PEP-II pump has a comparison with any pump type on the market today. The highest pump efficiency. Its manufacturing materials are the newly developed tool steels, and the whole-body hardening process is used to obtain longer life and better pressure bearing capacity.
The process at Zenith's plant to handle high hardness materials is more precise in terms of dimensional errors than other pump manufacturers.
Chapter 2 How Gear Pumps Work
The concept of gear pump is very simple, that is, its basic form is that two gears with the same size rotate in mesh with each other in a close-fitting housing. The inside of the housing resembles an “8†shape. Two gears are mounted on the gear. Inside, the outer diameter of the gear and both sides fit tightly with the housing. The material coming from the extruder enters the middle of the two gears at the suction inlet, fills this space, moves along the teeth as the teeth rotate, and finally exits when the two teeth mesh.
In terms of terminology, a gear pump is also called a positive displacement device, that is, like a piston in a cylinder, when a tooth enters the fluid space of another tooth, the liquid is mechanically squeezed out. Because the liquid is incompressible, the liquid and the teeth cannot occupy the same space at the same time, so that the liquid is eliminated. Due to the constant meshing of the teeth, this phenomenon occurs continuously and thus also provides a continuous removal at the outlet of the pump. For each revolution of the pump, the amount of discharge is the same. With the uninterrupted rotation of the drive shaft, the pump also continuously discharges fluid. The flow of the pump is directly related to the speed of the pump.
In fact, there is a small amount of fluid loss in the pump, which does not allow the pump to operate with efficiency of 100%. Because these fluids are used to lubricate the bearings and the sides of the gears, the pump body must never be fit without clearance, so that 100% of the fluid cannot be discharged from the outlet, so a small amount of fluid loss is inevitable. However, the pump can still perform well. For most extrudates, it can still achieve 93 to 98% efficiency.
For fluids with varying viscosities or densities in the process, this type of pump will not be affected much. If there is a damper, such as placing a strainer or a restrictor on the discharge side, the pump pushes fluid through them. If this damper changes during operation, ie if the filter becomes dirty, clogged, or the back pressure of the limiter rises, the pump will still maintain a constant flow until the mechanical limit of the weakest part of the device is reached (usually equipped with a torque limiter).
There is actually a limit to the rotational speed of a pump, which depends mainly on the process fluid. If the oil is conveyed, the pump can be turned at a very high speed, but when the fluid is a highly viscous polymer melt In physical terms, this restriction will be greatly reduced.
It is very important to push the highly viscous fluid into the two-tooth space on the side of the suction inlet. If this space is not full, the pump will not be able to discharge accurate flow, so the PV value (pressure * flow rate) is another limiting factor, and Is a process variable. Due to these limitations, gear pump manufacturers will offer a range of products, namely different specifications and displacement (discharged per revolution). These pumps will be combined with specific application processes to optimize system capabilities and prices.
The gear and shaft of the PEP-II pump are all in one unit, and the hardened whole process is used to obtain a longer working life. The “D†type bearing incorporates a forced lubrication mechanism that allows the polymer to pass over the bearing surface and return to the inlet side of the pump to ensure effective lubrication of the rotating shaft. This feature reduces the possibility of polymer retention and degradation. The precision-machined pump body can precisely match the “D†type bearing with the gear shaft to ensure that the gear shaft does not decenter to prevent gear wear. The Parkool seal structure and the PTFE lip seal together form a water-cooled seal. This seal does not actually touch the surface of the shaft. Its sealing principle is to cool the polymer into a semi-molten state and form a self-sealing seal. A Rheo seal can also be used. It has a reverse spiral groove on the inner surface of the shaft seal that allows the polymer to be back pressed back into the inlet. For ease of installation, the manufacturer has designed an annular bolt mounting surface to mate with the flange mounting of other equipment, which makes the manufacturing of the cylindrical flange easier.
The PEP-II gear pump is equipped with a heating element that matches the specifications of the pump and is available to the user. This guarantees rapid heating and heat control. Unlike heating in the pump body, damage to these components is limited to one plate, independent of the entire pump.
The gear pump is driven by an independent motor, which can effectively block upstream pressure fluctuations and flow fluctuations. The pressure pulsation at the outlet of the gear pump can be controlled within 1%. The use of a gear pump on the extrusion line can increase the flow output speed, reduce the shear and residence time of the material in the extruder, reduce the extrusion temperature and pressure pulsation to improve productivity and product quality.
The third chapter of the utility test of gear pump
The following analysis will mainly demonstrate the unique role of the gear pump in the extrusion production process. These analyses were performed on a single screw extruder.
Pressure uniformity characteristics
Figure 1 shows an example of how an extruder can eliminate inlet pressure pulsations. The curve was plotted by running a 50 cc/rev Zenith gear pump at a constant speed and changing the feed rate of a 2-1/2 caliber single screw extruder. PETG. The left curve shows the inlet pressure pulsation value (250 psi per cell), and the right curve shows the corresponding outlet pressure pulsation value (50 psi per cell). This graph shows that for inlet pressure pulsation, the output of the gear pump is not affected. In this test, we also manually applied a 750 PSI pulse at the inlet of the pump, and the corresponding outlet response was only 10 PSI. This confirms that a working gear pump can attenuate the pressure pulsation at the die head. Therefore, it can be concluded that the gear pump can effectively isolate the irregular pulsation caused by the extruder screw jitter, speed instability, poor feeding, etc., so as to minimize the impact on the final molding device.
Further research also proves this. Using a CDPE single-screw extruder without a gear pump, the pressure pulsation was ±4.3% of 1800 psi; with a gear pump, it was reduced to ±1.2%.
Figure 2 shows an “active metering†concept, where the gear pump guarantees a steady stream of pressure and speed to the die. In many production processes, high inlet pressure and low outlet pressure are common. The purpose of establishing a high back pressure for the extruder is to enhance mixing and melting, while the flow through the die is regulated by the pump to achieve the final product size.
This plot was plotted in the same experiment as in Figure 1.
Zenith pumps are generally not very sensitive to inlet pressure over a wide range (500-1500 psi), depending on the polymer itself and the pressure drop across the gear pump. Based on the data from these tests, gear pumps can reduce the momentary pressure pulsations that occur during melting. Due to these characteristics, the dimensional stability at the top of the mold can be improved, and the quality of the product is improved.
2. Increase production capacity
In extrusion lines with gear pump assist, the inlet pressure of the pump is kept low (typically 500 psi), so during the pressure transfer from the screw of the extruder to the gear pump, the return flow of the material can be reduce. From a hydraulic point of view, a gear pump is a device that can build pressure more efficiently than an extruder.
Thus, the output of a screw extruder at a given die pressure can be greatly improved. The Zenith PEP-II pump can easily set up the pressure required for the die, which reduces the back pressure on the extruder. As a general rule, for each 1000 PSI decrease in the back pressure of the extruder, the output at a given speed can be increased by approximately 10%.
In a LDPE extrusion experiment, using a gear pump, the extruder output increased by 29% over the same screw speed and die pressure (3400 PSI, see Figure 3). This is easy to understand because the extruder does not output material at 3400 PSI pressure at this time, and the gear pump only requires the extruder to provide a small inlet pressure (500 PSI in this experiment).
Obviously, with a gear pump, the extruder's productivity can be greatly increased at the same speed.
3. Temperature control
It needs to be emphasized again that, as a result of the use of Zenith PEP-II gears, the reduction of extruder back pressure will reduce the residence time of the polymer in the machine and make the plasticization process more stable. When the extruder speed and pressure are constant, the temperature at the pump inlet of the gear pump extrusion system is much lower than the temperature of the conventional extrusion system at the outlet of the extruder. Even if there is a temperature increase in the pump, the total temperature is still lower than in conventional extruder systems.
Figure 4 shows the benefits of running a large-displacement pump at low speeds compared to running a small-displacement pump at high speeds in a production process. From this figure, we can see that at a flow rate of 80 lbs/hr, three pumps with different displacements have the following temperature results:
Displacement (cc/rev) Speed ​​(rpm) Temperature Rise (°C)
10 to 80 30
20 to 40 12
30 to 27 8
Similar results were obtained for other polymers based on specific heat and thermal conductivity. The temperature rise of the pump can be calculated as follows:
T (°C)=2Π×TV×N/16800×Q×SH
Where TV = viscous torque (lb-in) Q = flow rate (1b/min)
N = pump speed (rpm) SH = specific heat
Figure 5 shows an example of temperature rise. The pump used was Zenith PEP-II with a displacement of 100 cc/rev. This figure shows the changes in temperature rise at different speeds and pressures.
4. Reduce shear energy
The gears in the Zenith PEP-II pump have a crowned root design that reduces shear energy and power consumption compared to conventional standard tooth forms. In addition, the use of a low aspect ratio tooth profile also reduces the distance the polymer has to pass through the middle of the gear. These features greatly reduce the energy delivered to the material, protect its mechanical properties, and maintain a lower temperature throughout the process. At the same time the life of the pump is extended and the drive is reduced for most extrusions
The system provides a highly efficient and energy-saving stabilizer. The crown-shaped tooth profile can effectively increase the output by 5 to 10%.
Figure 6 shows the results of comparing a gear with a crowned root to a standard gear on a PS production line. This curve shows that its growth is about 30%.
(To be continued)