Dancing With the Dragon
Pump control using general-purpose drives is mainly implemented through the control of flow rate. As in many other variable-speed drive applications, this enables notable energy saving
Fig. 2: Power applied to the shaft is reduced significantly when drives rather than valves are used to control speed
c and d= characteristics of pipe for difference resistance.
Reducing flow by reducing the speed is much more energy efficient than reducing it by increasing the pipe resistance using valve.
1. With the pump running at the rotation speed n1, and the pipe resistance set to its lowest value 2c , the working point of the pump is at A (intersection of 2a and 2c ). The output flow rate Q1 is maximal and the power P1 applied to the shaft is proportional to the area Q1H1.
2. If the flow rate must be reduced from Q1 to Q2, the traditional method is to change a valve setting, increasing the resistance of the pipe from 2c to 2d .The working point hence changes to B. This increases the pump lift from H1 to H2. The power P2 now applied to the shaft is proportional to the area Q2H2.
3.Variable Speed Drive Way
The pipe resistance remains at 2c but pump rotation speed is decreased from n1 to n2. The new head-flow curve is thus 2b and the working point moves to C. The pump head is decreased substantially to H3.The power P3 applied to the shaft is directly proportional to the area Q2H3. This represents a reduction (compared to P2 ) proportional to the area Q2(H2-H3 ). The energy savings achieved will be similary impressive
To be continue...