Screw Pump | Working , Design , Advantages and Disadvantages
A screw pump is a positive-displacement (PD) pump that use one or several screws to move fluids or solids along the screw(s) axis. In its simplest form (the Archimedes’ screw pump), a single screw rotates in a cylindrical cavity, thereby moving the material along the screw’s spindle. This ancient construction is still used in many low-tech applications, such as irrigation systems and in agricultural machinery for transporting grain and other solids.
These pumps have two or more gear-driven helical meshing screws in a close fitting case to develop the desired pressure. These screws mesh to form a fluid-type seal between the screws and casing.
A schematic diagram of a screw pump is shown in Fig . A two-screw pump consists of two parallel rotors with inter-meshing threads rotating in a closely machined casing. The driving screw and driven screw are connected by means of timing gears. When the screws turn, the space between the threads is divided into compartments. As the screws rotate, the inlet side of the pump is flooded with hydraulic fluid because of partial vacuum.When the screws turn in normal rotation, the fluid contained in these compartments is pushed uniformly along the axis toward the center of the pump, where the compartments discharge the fluid. Here the fluid does not rotate but moves linearly as a nut on threads. Thus, there are no pulsations at a higher speed; it is a very quiet operating pump.
In a screw pump, a chamber is formed between thread and housing as shown in Fig. The following expression gives the volumetric displacement
Advantages and disadvantages of screw pump
The advantages are as follows:
1.They are self-priming and more reliable.
2. They are quiet due to rolling action of screw spindles.
3.They can handle liquids containing gases and vapor.
4. They have long service life.
The disadvantages are as follows:
1.They are bulky and heavy.
2.They are sensitive to viscosity changes of the fluid.
3. They have low volumetric and mechanical efficiencies.
4. Manufacturing cost of precision screw is high.
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