One of the most important problems of the engineer is the efficient and controlled transfer of fluids from one point to another. This transfer may be opposed by gravitational force, by some other external force, or by friction. Under certain conditions the gravitational force and other forces may act to aid the transfer, but friction always exists as a force opposing motion. The engineer attempts to reduce the effect of friction and at the same time takes advantage of useful forces to produce a motion of the fluids under conditions that can be controlled.
As previously defined, a fluid is a substance in a liquid; gaseous, or vapor state which offers little resistance to deformation. Common examples of the three states of a fluid are water as a liquid, air as a gas, and steam as a vapor. All these types of fluids have a tendency to move because of natural forces acting on them. A city may be supplied with water flowing by gravity from high ground. Air may circulate in an auditorium because of its own temperature difference. Steam rises through the water in a boiler owing to the difference in density or specific weight of steam and water. In many cases, however, the circulation is inadequate, and mechanical equipment must be built to supplement the natural circulation. Often mechanical circulation is the only means of obtaining the desired fluid flow. The equipment for producing this fluid flow is divided into two major classes: pumps for handling liquids, and fans, blowers, and compressors for handling gases or vapors.
The conditions under which liquids are to be transported vary widely and require a careful analysis before the proper selection of a pump can be made. Generally, the engineer purchasing a pump consults with pump manufacturers to obtain the best type for a particular job. However, a fundamental knowledge of the basic types of pumps that are available and a realization that there is a wide variety of the basic types are of great value to the prospective purchaser.
The conditions that will influence the selection of the type of pump are: 1) the type of liquid to be handled: that is, it's viscosity, cleanliness, temperature, and so on; 2) the amount of liquid to be handled; 3) the total pressure against which the liquid is to be moved; 4) the type of power to be used to drive the pump.
Pumps may be divided into four major classifications:
1. Piston pumps or reciprocating pumps driven by engines or electric motors.
2. Centrifugal pumps driven by steam turbines or electric motors.
3. Rotary pumps driven by steam turbines or electric motors.
4. Fluid-impellent pumps which are not mechanically operated but are fluid-pressure-operated.
The centrifugal pump consists of an impeller or rotating section to produce the flow and a casing to enclose the liquid and to direct it properly as it leaves the impeller. The liquid enters the impeller at its center or "eye" and parallel to the shaft. By centrifugal force the liquid passes to the impeller rim through the space between the backward curved blades. The velocity of the liquid with respect»to the impeller is in a direction opposite to the impeller motion. The impeller blades are curved backward to permit the liquid to flow to the rim of the impeller with a minimum of friction. As the liquid leaves the impeller, it is thrown in a spiral motion forward with a certain velocity.
The water is guided away from the impeller by two basic types of casing: the volute, and the turbine or diffuser. Liquid enters the impeller at the "eye", is thrown to the outside, and leaves the pump through the expanding spiral or volute casing. The casing has the volute shape to permit flow with a minimum of friction and to convert a part of the velocity head into static head. The static head is the head that overcomes resistance to flow.
The turbine or diffuser pump has the same type of impeller as the volute pump. The casing has a circular shape, and within the casing is a diffuser ring on which are placed vanes. The vanes direct the flow of liquid and a decrease in the velocity of the liquid occurs because of an increase in the area through which the liquid flows. Thus, part of the velocity head is converted into static head as in the volute pump. For a multistage pump, the diffuser pump has a more compact casing than the volute pump. The diffuser-pump design is adaptable to differences in flow conditions since the same casing can be used with various arrangements of diffuser vanes. In the volute pump a variation in the requirements of the volute casing demands alternations in the casing itself. Generally, the volute pump will be used for low-head high-capacity flow requirements and the diffuser pump for high-head requirements.
Both volute and diffuser pumps are classified by the type of impeller, the number of stages, and the type of suction or intake used. A pump having two "eyes" on the impeller is called a double-suction pump. The double suction, one "eye" located on each side of the impeller, permits forces acting on the impeller to be balanced, thus reducing the axial thrust on the shaft. Also, the double-suction pump is used for handling hot water where there is danger of water flashing into steam at points of low pressure. The double suction offers little resistance to flow; thus low-pressure areas are less apt to occur. The double-suction pump is used also for large capacities.
When two or more impellers are mounted on the same shaft and act in series, the pump is called a multistage pump, the number of stages corresponding to the number of impellers. A boiler-feed pump is capable of delivering 415,0001b of water per hr against a pressure of 1500 psi. Multistaging produces better performance, higher pump efficiency, and smaller impeller diameters for high-pressure heads. Usually each stage produces the same head, and the total head developed is the number of stages times the head produced per stage.
The types of impellers installed in centrifugal pumps are as numerous as the uses to which the pumps are put. Classification, however, can be made by designating the direction of flow of the fluid leaving the impeller. All pumps have the intake parallel to the impeller shaft. The discharge however, may be radial, partially radial and axial, or axial. In the radial-type impeller the suction and discharge are at right angles. The radial impeller may be of the closed or the open type. The term closed or open refers to the fluid passage within the impeller. The open impeller has one side of the flow path open to the pump casing or housing. The closed impeller has both sides of the flow path enclosed by the sides of the impeller. The partially radial impeller discharges at an angle greater than 90 degrees with intake and is of the open-impeller design.
The axial-flow impeller discharges at an angle of approximately 180 degrees with the intake and is generally of the propeller type.
Each of the impeller types has a specific purpose. The axial-flow type is used to pump large quantities of fluid against a relatively small static head.
It is not a true centrifugal pump but is designed on the principles of airfoitshapes. The radial pump is used for handling smaller quantities of fluid against a high head, because the centrifugal force is high but the flow path is small and restrictive. The open impeller is designed to handle dirty liquids such as sewage, where the flow path must be less restrictive. The partially radial impeller covers intermediate pumping conditions.
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