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Co-ordinating motor drive systems for best efficiency

14 September 2012

When it comes to pumps and fans, cost of energy is a more useful measurement than more traditional efficiency measures such as best efficiency flow. Frank Griffith explains the reasoning behind this assumption.

Figure 1

When considering how to achieve the best possible efficiency from a fan or pump installation, the only viable option is variable-speed drives (VSDs). Only VSDs allow the system to be optimised for the best duty operating speed, rather than being restricted to the compromises inherent in a fixed speed design.

An ability to vary the speed of a pump or fan, allows for the complex interplay between its operation and the associated system, helping achieve the optimum operating conditions that provide the best solution for the end user. The most useful, and directly practical, metric for a particular pump or fan is cost of energy (CoE), measured in kWh per cubic metre. 

The significance of this metric is illustrated by figures presented at a recent conference, where a pump vendor gave the following period life cycle costs (LCC) for a typical pump:

Utilisation                          66 percent
Time period                         20 years
Energy consumption                 93 percent
Capital cost                          2 percent
Service                                 4 percent
Maintenance                         1 percent

Clearly, CoE is the major factor as represented here by the energy consumption making up 93 percent of the total LCC over 20 years.

Obviously, adding more efficiency and thus higher CoE by using VSDs will add to the initial cost of the system, but this is more than paid for by the reduction in life cycle power consumption.

Plan for good CoE
As an example, a 20 percent increase in initial cost (1 percent overall) is equivalent to a 20 percent saving in LCC. This gives a reduction of 24 percent in power consumption, giving a net saving of 19 percent of LCC. This saving is best achieved at the start of the life cycle.

If good decisions are made at the beginning of a project, then a good CoE can be achieved for the lifetime of the plant. Bad choices will give a poorer CoE which the end-user must live with, and pay for, the rest of the plant life.

It must be stressed that a good CoE cannot be achieved by one participant in the project. The system designer, pump/fan designer, variable-speed control designer/supplier must work together for a good solution. A decision made unilaterally, for example by the architect designing the pump room can have a significant impact on the choices made by others, such as the size of the equipment, and could well result in a less than satisfactory CoE.

Similarly, a poor choice of pump/fan with a low efficiency cannot be 'cured' by variable-speed control. But a good choice is enhanced by variable-speed control, providing flexibility of design for the system.

Two factors determine a CoE curve. The first is the system curve type, either friction only, static only or a combination of the two. The second factor is the pump curve, defined by the head against flow and forming starting points for iso-efficiency curves, joining up points of the same efficiency.

Traditionally, a fan or pump is specified for a given duty point, based on defined values of head and flow. Usually, the pump design is selected for best efficiency at the duty point, suitable for a fixed speed pump (see Figure 2).

It should be noted that all examples of fan and pumps curves shown are for a nominal design; for example, 50Hz speed. These are starting points for assessing CoE - the use of variable speed allows operation outside of these curves.

However, variable speed operation allows a pump/fan to work at all points along the system curve, not all of which will have the same efficiency as the duty point, and the change in efficiency will be reflected in changes of the associated power.

Let’s take a look at three examples of pump or fan systems and their suitability for variable speed operation: (1) a retrofit fan design that can never benefit fully from variable speed operation; (2) multiple pump versus single pump operation, and (3) higher operating speeds than nominal mains frequency speed and the consequence of speed/power limitation

The first example is a retrofit fan design, in which the use of variable speed operation gives limited benefit to the application. Fan design is selected such that the throttled duty sits on the best efficiency point (BEP). Using variable speed merely puts the duty on a lower iso-efficiency curve. 

By using variable speed and altering the fan design, we can achieve the optimum situation using two identical fans running at a lower speed. The fan operating point sits on the 91.35 percent efficiency curve and gives an 11 percent reduction in flow rate. 

In the second example (multiple pump versus single pump), a pump application will operate on the same efficiency curve for increasing nominal flows. Multiples of the nominal equivalent flow will require two or three pumps in parallel operating at the same speed.

In the third example, each point on a CoE curve has a corresponding operating speed and power. The maximum values of motor speed and/or power will then define the working limits of operation along a CoE. Operation beyond these limits automatically implies parallel operation, with two or even three pumps. This in turn will have its own operational limits due to the maximum value of speed/power applied to each pump motor.

Another important limit is that imposed by the pump net positive suction head (NPSH), the net positive pressure of suction force into a pump intake after friction loss has occurred. Being aware of these operational limits affects whatever design decisions result from the CoE curve(s) used.

Overall, we can conclude that the concept of best efficiency flow (BEF) as always being the best choice criteria should be replaced by the concept of minimum CoE. Although the choice of pump to achieve this may not work at the BEF, it will nevertheless deliver the least CoE for that particular operating condition.

Frank Griffith is a drives consultant engineer with ABB 

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