Energy efficiency - beyond the motor

Energy efficiency is more than just high efficiency motors: gearboxes, pumps and compressors must also be considered, says Mike Smith of the Deritend Group.

The recent rapid rise in industrial energy costs is refocusing works managers - and accountants - onto the energy saving role that can be played by high efficiency electric motors.

This focus is entirely justified as research shows that energy costs can represent up to 99% of the lifecycle costs of electrical motors.

However, it does not take into account the instances where high efficiency motors are routinely fitted as part of a larger power transmission chain with gearboxes, or where motors are used to drive pumps or compressors.

In these types of applications the choice of gearbox or pump - or rather the wrong choice - can easily wipe out the extra efficiencies provided by the energy efficient motor.

A quick example illustrates this.

Deritend recently conducted an energy audit for a customer in the food and beverage industry.

One of the items monitored was a conveyor driven by a standard motor and 1.5kW helical worm gearbox.

The overall efficiency of this drive system was 59%.

Deritend replaced this system with a Flender helical bevel gearbox and an Eff1 motor.

The overall efficiency of the new system is 81%, producing a saving for the customer, on a single drive system, of GBP 375 pounds sterling (assuming energy costs of 4.43p/kWh).

What this example illustrates is that it is vital to choose a gearbox with the highest possible efficiency, usually helical or bevel helical units rather than worm drives.

In addition, the system design should ensure that the drive system train be as direct as possible to the machine it serves.

This means avoiding the efficiency losses that result from belt and chain power transmission systems, and choosing shafts instead.

Where the equipment is pumps rather than gearboxes, reductions in energy requirements of between 1 and 5% can be achieved by replacing standard pump motors with high-efficiency types.

However, much higher efficiency gains, of 25 to 30%, are possible by actually analysing the pumping application itself.

This involves determining if the installed pump is actually the right unit for the job.

If not, the pump should be replaced by one whose performance curve matches the demands of the application.

Replacement can be expensive of course, but with the potential savings of 25 to 30% over the life of the pump, it does make economic sense.

The process of selecting a centrifugal pump for maximum energy efficiency involves examining the pump performance curves that are provided by pump manufacturers.

Pump curves show the pump's best efficiency point (BEP): ie the point where the pump operates most cost effectively.

Centrifugal pumps can generally be ordered with a variety of impeller sizes, with each having its own performance curve.

In order to minimise energy consumption within the pumping system, a pump should be selected that has a system curve which intersects the pump curve within 20% of its BEP; then select a midrange impeller that can be easily replaced to meet higher or lower flow rate requirements.

Even with high efficiency motors and pumps, initial cost savings can be lost if the pump output is varied by throttling, an inefficient mechanism at best.

An alternative to throttling flow that improves both performance and energy efficiency is the use of variable-speed drive (VSD).

VSD are particularly effective for pump control because of the "square law".

This states that the torque required to turn a load decreases as a function of the square of the speed; in other words, at 50% speed; the torque requirement will be 0.52 or 25%.

Since the power required from the motor is a function of torque and speed, it follows that the load in terms of kilowatts - which is what the user pays for - increases or decreases as the cube of the load speed.

In the terms of the above example, driving the load at 50% speed only requires an eighth of the power needed to run at maximum speed, even though the flow rate will still be 50%.

The benefit of the square law is evident from an example where a 22kW centrifugal pump runs continuously at 50% flow.

The comparison of operating costs between pump throttling and inverter control, using typical losses at 50% flow, is as follows.

The power required for throttle system is 55% of 22kW, or 12.1kW.

The power required for inverter system is 25% of 22kW, or 5.5kW.

As the example shows, using inverter control produces savings of 6.6kW - or more than 50% in energy costs.

If the pump operates on a duty cycle of 10 hours per day, 5 days per week, for 50 weeks per year, the annual saving is a considerable 16.5MW.

Assuming an energy cost for this example of GBP 0.045 per kilowatt-hour, the annual savings are GBP 742.50.

Another major equipment sector that benefits greatly from improvements in energy efficiency as a result employing VSD is compressors.

It is a fact that compressors are left running most of the time even when they are not required.

This is wasteful of energy and also expensive.

However, employing a VSD and a suitable controller can eliminate this off-load running, which usually represents up to 50% of full load power.

A compressor installation equipped with a VSD operates by varying air compressor flow in response to changes in detected air system pressure, in order to maintain an exact and constant pressure level.

As air system demand falls, and more air is delivered into the air system than is being used, the system pressure will begin to rise and the VSD compressor will reduce speed, and hence output, to maintain the target pressure level.

In this way best efficiency operation is achieved, and energy costs are reduced considerably.

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