Problem
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Solution
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| The National Gallery in Trafalgar Square, London,
houses a permanent collection of over 2,300 paintings by many of the
world's most famous artists. Chilled water was being continuously
re-circulated with the pumps running at full speed, resulting in
considerable waste of energy. |
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Eight variable speed drives were fitted to the
system, two at 3kW, four at 4kW and two at 15kW. These were
installed to control the flow of chilled water to the air handling
units, ensuring that the temperature stays within +- 1oC
of the set point. Stable temperature and humidity levels are
required for the conservation of the pictures. |
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Now output varies with demand, down to 45% of the maximum pump speed .
The new drive system gives a closer control of each individual area of
the Gallery. It is interface to a Trend Building Management
System, which controls the speed of the pumps by giving the variable
speed drive a 0-10 volt signal.
The drives are controlling the air temperature throughout the
Gallery's West Wing, protecting the work of artists such as
Michelangelo, El Greco, Titian and Vernoese.
"Variable speed drives are extremely accurate devices and so are
ideal for applications where close control is required." explains Mike
Carman, UK Sales Manager HVAC at ABB. "They are extremely
versatile when it comes to receiving input from feedback devices and
immediately adjusts the fan or pump speed to the required output.
In addition, it causes less wear and downtime on mechanical components
than the continuous stopping and starting of traditional on-off
control".
Variable speed drives are more efficient than traditional control
devices because they control output by regulating the motor spped,
rather than running the motor at full speed and using restrictions to
reduce flow. The reason that the difference in consumption is so
staggering is that to increase the speed of a centrifugeal load, such as
a pump or a fan, requires a power increase which is the cube of the
speed increase. This means that to double the flow, the power
input required is the cube of the desired flow increase, 23=8
times the power; to treble the speed takes 33=27
times the power. Conversely, this also means that a small
reduction in the flow results in a large cut in the energy consumption;
for instance, reducing the flow to 80% cuts the energy consumption by
half. |
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