Because of the greater system head, the volumetric flow rate is actually less than twice the flow rate achieved by using a single pump.The maximum achievable flow rate of a HotEnd is dependent on a number of factors and as a result, cannot be accurately portrayed in one single value. As shown in Figure 12, a greater system head loss occurs with the increased fluid velocity resulting from the increased volumetric flow rate. When the system characteristic curve is considered with the curve for pumps in parallel, the operating point at the intersection of the two curves represents a higher volumetric flow rate than for a single pump and a greater system head loss. The total flow rate in the system, however, is the sum of the individual flow rates for each pump. Since the inlet and the outlet of each pump shown in Figure 11 are at identical points in the system, each pump must produce the same pump head. Figure 11 depicts two identical centrifugal pumps operating at the same speed in parallel.įigure 11: Pump Characteristic Curve for Two Identical Centrifugal Pumps Used in Parallel Centrifugal Pumps in Parallel In order to increase the volumetric flow rate in a system or to compensate for large flow resistances, centrifugal pumps are often used in parallel or in series. In addition, centrifugal pumps provide a high volumetric flow rate with a relatively low pressure. Centrifugal pumps are typically small in size and can usually be built for a relatively low cost. These prime movers include AC and DC electric motors, diesel engines, steam turbines, and air motors. Volumetric flow rate (\( \dot_1 \)) at a smaller pump head ( ΔP 1).Ī typical centrifugal pump has a relatively low number of moving parts and can be easily adapted to a variety of prime movers. Pump head, on the vertical axis, is the difference between system back pressure and the inlet pressure of the pump ( ΔP pump). When a centrifugal pump is operating at a constant speed, an increase in the system back pressure on the flowing stream causes a reduction in the magnitude of volumetric flow rate that the centrifugal pump can maintain.įigure 7: Typical Centrifugal Pump Characteristic Curve PSID can also be used to describe a pressure drop across a system component (strainers, filters, heat exchangers, valves, demineralizers, etc.). In this context, it is the pressure difference between the suction and discharge of a pump. The term PSID (Pounds Force Per Square Inch Differential) is equivalent to ΔP. This pressure increase can be anywhere from several dozen to several hundred psid across a centrifugal pump with a single stage impeller. Normally, a centrifugal pump produces a relatively low pressure increase in the fluid. Operating Characteristics of a Centrifugal Pump As a result of the interdependence of these parameters, the fluid flow in the volute, similar to subsonic flow in a diverging nozzle, experiences a velocity decrease and a pressure increase. The key parameters influencing the energy conversion are the expanding cross-sectional area of the volute, the higher system back pressure at the discharge of the volute, and the incompressible, subsonic flow of the fluid. The mathematical analysis of flow through the volute is based on the general energy equation, the continuity equation, and the equation relating the internal properties of a system. The mechanism of this energy conversion is the same as that for subsonic flow through the diverging section of a nozzle. The volute is a region of continuously increasing cross-sectional area designed to convert the kinetic energy of the fluid into fluid pressure. Fluid of high kinetic energy is forced out of the impeller area and enters the volute. As the fluid's velocity increases its kinetic energy increases. A transfer of momentum to the moving fluid increases the fluid's velocity. As the impeller and blading rotate, they transfer momentum to incoming fluid. In order to understand how a fluid system containing a centrifugal pump operates, it is necessary to understand the head and flow relationships for a centrifugal pump.įluid entering a centrifugal pump is immediately directed to the low pressure area at the center or eye of the impeller. Centrifugal pumps are one of the most common components found in fluid systems.
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