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Energy and Reliability Considerations For Adjustable Speed Driven Pumps
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Centrifugal devices such as pumps, fans, and compressors follow a general set of speed affinity laws: Q2= Q1*(N2/N1) [Equation 1]; H2=H1*(N2/N1)2 [Equation2]; P2=P1*(N2/N1)3 [Equation 3] where Q= flow rate, N = speed, H =head, and P = shaft power. The subscripts 1 and 2 represent two different speed conditions. In fluid systems where there is no static head, the system head, like the pump head, is proportional to the velocity, or flow rate, squared. As a result, the pump affinity laws also indicate how the pump and system will work together to define the operating point in such a system; in other words, the system flow rate is linear with pump speed. Stable control of system flow rate is easily achieved. In these all-frictional systems, the energy efficiency merits of using an adjustable speed drive (ASD) is apparent if one simply considers the effect of a 20% reduction in speed: the flow rate drops by 20%, but the pump input power drops almost in half. But not all fluid systems are all-frictional. In fact, some fluid systems are dominated by static, rather than frictional head. When there is static head in a fluid system, the system no longer follows the same rule that head is proportional to flow rate squared. Instead, there is a constant term (static head), to which is added a variable term (dynamic, or frictional) head. The operational response to a speed change in a system with static head is both system and device dependent, which means that both the system and the centrifugal device characteristic curves must be known to predict the result of a speed change. This interdependence between the pump and system is often overlooked in pump selection. Much work has been done historically in defining control characteristics of valves (the alternative, less energy efficient means of flow control). There is a need to develop a better awareness of the sometimes nonlinear behavior of pumps under ASD flow control. This paper explores the behavior of ASD-controlled pumps in systems with varying levels of static head. Some potential parametric and single value figures of merit for evaluating the suitability of pump, drive, and system combinations are discussed, and an important pump selection technique in adjustable speed applications is described.
Casada, D. (1999). Energy and Reliability Considerations For Adjustable Speed Driven Pumps. Energy Systems Laboratory (http://esl.tamu.edu). Available electronically from