Energy
A big fan of energy saving
Want to save energy? Look to your pumps and fans. They are awash with
opportunities for savings, says Chris Cusick
Sixty-five percent of all electricity generated in this country goes to
driving pumps and fans. Many of these are run constantly at full speed,
creating excessive flow, which is mechanically damped or throttled in
order to match the demand. Often, and particularly in industrial
processes, a high flow is required for only a tiny part of the operating
cycle, or perhaps only during an emergency. So the energy wastage is
obviously going to be significant.
The physics behind centrifugal pumps and fans is centred on a 'cube law'
such that the power consumed increases with the cube of the speed of the
driving motor. That is to say: if you halve the speed you halve the power
consumption, then halve it again, then halve it a third time, i.e.
running at half speed requires only one-eighth of the energy. So, if you
have any pumps or fans that could be turned down, it would be very
'green' of you to do so. It is also very efficient to see if you can
simply turn them off rather than have them idling for extended periods.
And for those pumps and fans that you need to be able to 'crank up',
consider fitting a variable speed inverter to the drive motor. This will
allow you to vary the speed from zero to a defined maximum and thus
exactly match the demand of the moment. Modern inverters can be
programmed with a time sequence to run at specific speeds at different
points in the duty cycle. Alternatively they can be wired up to monitor
given variable parameters and adjust their speed in relation to the
readings of the moment.
For instance, a fan in the air handling plant of an airport's departure
lounge would typically be programmed to slow down during the night when
the ambient temperature is low and the building is typically sparsely
populated. In this case, a temperature and/or presence sensor could be
used to alert the drive to speed up if the night was particularly warm or
if cancelled flights lead to large numbers of people in the lounge.
In one particular car plant, the pressure in the paint spraying system
used to be held constantly high, as this ensured a quality finish in
minimum time. However, observations showed that spraying time exceeds
idling time. So now it is usual to fit a highly responsive drive to the
pump, so that energy is saved during non-operating time, yet pressure is
ramped up virtually instantaneously when it is required.
Mitsubishi recently installed inverters to control a 45kW boiler fan at a
Glasgow hospital. Energy usage dropped from 20,200 kWh the month
beforehand to 6,500 kWh, a 13,700 kWh or 68% saving. At 4.5p per
kilowatt, the financial saving was £617 a month - not bad for a £3,000
installation that will run for 60-120 months. Full details at
www.energycentre.org.
Chris Cusick is with the MELSMART Energy Centre