In many applications, the pump does more than simply transport ambient temperature and pressure water media. Centrifugal pumps become ineffective as viscosity increases, and users need to consider using positive displacement pumps (PD pumps, or positive displacement pumps). When pressure needs to rise, some positive displacement pumps are unsustainable. When the temperature rises. Other pumps will also fail. So when you need more than 35 kg of pressure, or 300 degrees Celsius, or viscosity up to hundreds of thousands or millions of centipoises how to do? Maybe some pumps have been specifically designed or modified to meet one or both of these requirements, but what if the condition requires the pump to meet all these harsh conditions? This requires a high performance external gear pump designed for these harsh conditions. This pump can handle any or all of these conditions through specially engineered materials, clearances and designs. External gear pump has two same size gear shaft. The drive shaft connects the motor or reducer (via flexible coupling) and drives the other shaft. In heavy duty industrial gear pumps, the gears are usually one piece with the shaft and the journals have small tolerances. Gear shaft as a whole is to withstand high pressure, high viscosity under high torque load. The four journal bearings are dynamically supported and lubricate the bearings with pumped media. There are three common forms of gear: straight teeth, helical teeth and herringbone teeth. The three forms have their own advantages and disadvantages, have different applications. Straight teeth are the simplest form and are optimally used under high pressure conditions because there is no axial thrust and the delivery efficiency is high. Helical teeth have the least amount of pulsation during delivery and are quieter at higher speeds because the meshing of the teeth is gradual. However, due to the axial force, the selection of bearing material may result in limited pressure difference between inlet and outlet and lower viscosity. Because the axial force will push the gear to the bearing section and friction, so only choose the higher hardness of the bearing material or in the cross-section of the special design to cope with this axial thrust. Herringbone teeth are back to back helical forms that provide slightly lower pulsation than straight teeth, and axial forces can be balanced. However, the manufacturing cost is high, and assembly / disassembly is difficult because it must be installed in pairs. In high viscosity applications, liquids are easy to cure, or in very large pumps, which is a real drawback. External gear pump operation principle is very simple, the liquid into the suction side of the pump, the meshing interdental cavity inhalation, and then driven within the cavity between the teeth, along the outer edge of the gear shaft to reach the exit side. The re-engaged tooth pushes the liquid out of the hole into the back pressure. In theory, the nominal displacement of a positive displacement pump is independent of the pressure. However, volumetric failure or internal leakage is inherent in the form of positive displacement pumps. In order to achieve a high pressure differential and the required nominal flow rate, the gear pump must overcome this internal leakage. There are four kinds of internal leakage: 1: Between the gear journal and the bearing 2: Between the gear end face and the bearing face, 3: Between the tooth top and the pump housing, 4: Between the meshing teeth. In order to maximize the pressure bearing capacity of the pump, the clearance between these mating components must be as small as possible to limit internal leakage. However, just narrowing the gap is not as simple as it sounds, and other factors such as temperature, viscosity, and material selection must also be considered. Leakage from inside is not all bad. In gear pumps, some internal leakage is required to lubricate the internal passages and form a fluid film in the sliding bearings to dynamically support the gear shaft. The correct design should be, the internal leakage is 1-3% of the flow. Material selection is high temperature industrial pump selection is very important. Gear pumps are often used to deliver fluids that are highly corrosive, wear resistant or variable. Pump housing, shaft and bearing material must first match with the pumped liquid. Pump design becomes more complicated when extra heat is taken into account, even considering the thermal expansions of various materials. As mentioned earlier, the smaller the internal clearance, the better to achieve the highest pressure capability. In high temperature conditions, the pump needs to "expand" within the existing clearance due to the thermal expansion of the components. This is beyond the usual considerations of most general purpose gear pump manufacturers. Over-estimating the material's thermal expansion can cause the pump's clearance to be too loose to produce the required pressure; underestimating the thermal expansion can cause the pump to lock when it reaches the process temperature. For this reason, pumps designed for high or low temperatures often do not function well at non-design temperatures. For example, if the pump body is 316 stainless steel, the gear shaft is 440B stainless steel, and the bearing is graphite. The 316 stainless steel has an expansion rate of 17x10-6mm / mm / deg C, 440B 11x10-6mm / mm / deg C, and carbon 3.6x10-6mm / mm / deg C. Pump manufacturers must have the ability to calculate pump clearances at high temperatures. Preheating of the pump is necessary to prevent damage to the components by high temperature shocks. Preheat is recommended when the pumped fluid temperature is above 150 ° C. When using a mechanical seal, the pump must be preheated to within 30 ° C of the operating temperature to prevent damage to the sealing surface. Jacket pump can be steam, heat medium and electric heating to preheat. Viscosity is the resistance to fluid flow. The first problem with high viscosity conditions is how to pump fluid. The pump must be turned very slowly to allow the fluid to enter the non-intermeshing cavities, which create a suction that draws the fluid into the pump. The tighter the clearance, the better the seal of the pump and the stronger the suction. Once the fluid enters the pump, the internal gap needs to be properly defined in terms of viscosity. The gap is too small will limit the flow of fluid in the passage, so that the lack of lubrication and bearing overheating; gap is too large, the strength of the liquid film can not support and lubricate the gear shaft, causing journal and bearing direct contact, leading to failure. Another important factor in handling high-viscosity fluids is the high torque of the drive gear, which must be strong enough to transmit the high torque of the drive. Toothed design is very important, too large, the delivery efficiency is not enough, too small can not afford high torque. The torque applied to the gear shaft and the shearing force on the teeth increase with increasing viscosity and differential pressure. When these factors are combined with high temperatures, the design of the gear shaft and toothing becomes extremely important because Metal parts with the temperature coefficient of elasticity decreased. Involving high pressure, high viscosity, high temperature conditions so that users no longer choose to use centrifugal pumps and positive displacement pump (PD pump). And when these conditions become particularly harsh, many other forms of PD pump to use the limit, the only option is the external gear pump. There are many manufacturers of external gear pumps, but few can cope with these conditions. The sensible user choice should be that there is a high demand for application testing and that there are suppliers that successfully process traceable records of these conditions.
Compared with single-channel pipettes, multi-channel pipettes simplify all the tasks associated with microtiter plates that often occur in immunology, biochemistry, clinical diagnosis and food analysis. Multi-channel pipettes generally have 8 and 12 heads. The gun body can be rotated at 360°C, and each part can be disassembled and repaired separately. The lower half can be sterilized at 121°C. The dimpled housing ensures a firmer grip for the operator. Can be quickly calibrated.
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