To compare like with like, the most useful comparison between particular pumps or fans is the cost of energy (COE), measured in kWhr per cubic metre. This can be quite substantial, with energy consumption of a typical pump forming some 93 percent of its total life cycle costs (LCC) over 20 years.
Using VSDs in the system will increase initial purchase cost but will improve efficiency and give a better COE.
Achieving a good COE needs a good plan at the start of the pump or fan life cycle. For instance, everyone involved in the project must work together to ensure their decisions complement each other rather than conflicting and leading to a worse COE. Although a poor choice of pump or fan with a low efficiency cannot be 'cured' by variable-speed control, a good choice can be enhanced by variable-speed control.
The image shows how the influence of COE changes throughout the life-cycle of a pump or fan application.
A COE curve is determined by 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.
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. The fan and pumps curves shown are for a nominal design e.g. 50 Hz speed. These are starting points for assessing COE - the use of variable-speed allows operation outside of these curves.
Systems and their suitability for variable-speed drive operation
We can look at three examples of pump or fan systems to demonstrate how they may be suitable for variable-speed operation:
1. A retrofit fan design that can never benefit fully from variable-speed operation
The first example is a retro fit 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.
The best situation is achieved using two identical fans running at a lower speed, which will require using variable-speed together with and altered fan design. The fan operating point is on the 91.35 percent efficiency curve, giving a cut in flow rate of some 11 percent.
2. Multiple pump versus single pump operation
A pump application will operate on the same efficiency curve for increasing nominal flows. If we need to allow multiples of the nominal equivalent flow, two or three pumps will need to be run in parallel, operating at the same speed.
3. Higher operating speeds than nominal mains frequency speed and the consequence of Speed/Power limitation
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. If we want the system to operate beyond these limits we will need to use two or even three pumps working in parallel operation. The maximum value of speed/power applied to each pump motor will produce a curve with its own operational limits.
We also should consider the net positive suction head (NPSH), another important limit is that imposed by the pump. This is the net positive pressure of suction force into a pump intake after friction loss has been taken into account. These operational limits will affect the design decisions based from the COE curve(s) used.
From these considerations and examples, we can conclude that the concept of best efficiency flow (BEF) is not the most useful criteria when designing a pumping system and that a more realistic and practical measure is the concept of minimum COE. Although the choice of pump to achieve this may not work at the BEF, it will give us the lowest COE for that particular operating condition.