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Screw pumps are a unique type of rotary positive displacement pump in which the flow through the pumping elements is truly axial. The liquid is carried between the screw threads on one or more rotors. The liquid is then displaced axially as the screws rotate and mesh. In other types of rotary pumps, the liquid is forced to travel circumferentially, however the screw pump has an axial flow pattern and low internal velocities.
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It provides a number of advantages in many applications where liquid agitation or churning is objectionable. Screw pumps are classified as two different types: the single rotor and the multiple rotor. The multiple rotor is further divided into timed and untimed categories. Timed rotors rely on outside means for phasing the mesh of the threads and for supporting the forces acting on the rotors. Untimed rotors rely on precision and accuracy of the screw forms for proper mesh and transmission of rotation (Fraser, et. al., 1986.).
The screw pump is the oldest type of pump. The first applications, dating back to the third century B.C., included irrigation and land drainage. The screw pump is thought to have been first used in Egypt (Ewbank, 1972). After several other types of pumps were invented, the screw pump was not used as much because these other pumps could handle higher head capacities. However, later it was found that these pumps could not handle wastewater like the screw pump could. Because of this, the screw pump became widely used for such an application. The Dutch were the first to design a spiral lift screw in 1955. After this, double screw units were put into operation for flood control in the Netherlands and in municipal sewage installations in Europe. Based on excellent results from the pumps used in Europe, the trend extended to Canada and United States and are currently used today (Cheremisinoff, et. al., 1992).
Applications
There are several applications of the screw pump that include a wide range of markets: utilities fuel oil service, industrial oil burners, lubricating oil service, chemical processes, petroleum and crude oil industries, power hydraulics, and many others (Fraser, et. al., 1986). Listed below are some typical situations where a screw pump is used. The benefits of using a screw pump in each of these situations are discussed (Cheremisinoff, et. al., 1992).
Single Screw
The single screw pump is more commonly known as the Archimedean screw. It is quite large; typical dimensions include a diameter of 12 inches or greater, and a length up to about 50 feet. It is normally used as a water-raising pump with the screw arranged at an angle of 30 degrees. It can also be used for handling liquids containing solids in suspension with either vertical lift or horizontal transport. The design of single screw pumps allows very little fracturing of particles and little abrasion damage to the pump. One disadvantage is the considerable bulk necessary to achieve high capacities since rotational speeds are of the order of 30-60 rpm (Warring, 1984).
Intermeshing Screw Pump
The intermeshing screw pump is commonly called a rigid-screw pump. This type of pump is suitable for a wide range of sizes, and can be run at high speeds. The larger screw pumps are used for bulk handling of oils and similar fluids. The basic type is suitable for handling most clean fluids with low flow velocities and at low heads (Warring, 1984).
Eccentric screw pump
The eccentric screw pump is versatile. It is capable of handling a variety of liquids and products with high efficiency. It comprises of a rigid screw form rotor rolling in a resilient internal helical stator of hard or soft rubber with a moderately eccentric motion. It can handle viscous liquids, slurries, pastes, solids in suspension, and delicate products. This is because of the low flow velocities through the pump (Warring, 1984).
View Pictures of Other Screw Pumps!
The delivered capacity of any screw pump is the theoretical capacity minus the internal leakage. In order to find the capacity of a screw pump the speed of the pump must be known. The delivered capacity of any rotary screw pump can be increased several different ways. The capacity can be increased by simply increasing the speed, increasing the viscosity, or decreasing the differential pressure. The capacity of the pump depends on several factors (Cheremisinoff, et. al., 1992):
Flights: Single, double, and triple flights are often used. Flights are also known as helixes. With each increase in flights, there is a 20% increase in capacity. Therefore, a single flight pump has a capacity that is 80% of a double flight pump, which in turn has a capacity that is 80% of a triple flight capacity. The three-flight pump can handle the most capacity in the least amount of space.
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