It's cool to be an energy saver
Paul Pryor explains what to look for in a drive destined for an
application in the food industry, and describes how one company in this
sector saved a lot of energy by installing them to control fans in its
refrigeration systems
Typically, manufacturing plants in the food and drink industries involve
tens, if not hundreds, of electric motors, nearly all of which would
benefit from being controlled by a variable speed drive (VSD). For
example, using VSDs to give smooth speed control on mechanical handling
equipment minimises the risk of product damage, while also cutting plant
running costs by reducing wear and tear on the equipment itself. Even
bigger benefits are possible with motors used to drive compressors, pumps
and fans, where the installation of VSDs can dramatically reduce energy
consumption.
Under the provisions of the government's enhanced capital allowance (ECA)
scheme for energy-saving plant and machinery, businesses can claim 100%
first-year capital allowances for money spent on qualifying equipment.
This means that they can write off the whole of the capital cost of their
investment against taxable profits for the period in which they make the
investment, boosting cash flow as well as reducing the pay-back period
for the new equipment.
There is, in fact, a very effective method for food and drink
manufacturers to find out exactly how much they stand to gain by
installing VSDs: the energy audit. There are many organisations offering
these audits, but like all consultancy work, they can be costly. Some
companies, however (the author's company included), arrange audits
free-of-charge, provided that they result in the purchase of a VSD from
that company.
Although, in most instances, the decision to use VSDs is an easy one to
make, choosing the most suitable drives requires a little more thought.
True, it would be hard to find a modern drive that didn't provide
adequate speed control, at least in straightforward applications, but the
best products have much more to offer.
For example, the modern trend in plant design is to adopt a distributed
control system architecture, where the VSDs are mounted close to the
motors they control, rather than in a central control panel. In such
systems, VSDs supplied ready-mounted in enclosures have much to recommend
them, but unfortunately, such products often lack the flexibility needed
to match the end user's needs. New developments, like the customisable
enclosed drive, now supplied by the author's company, comes
ready-mounted, together with Class A EMC filters, in an IP55 enclosure
which has adequate room for additional components, such as switch
disconnectors, pushbuttons and pilot lights.
Of course, distributed control requires that the VSDs offer versatile and
easy-to-use communication options. Users want a system that is
straightforward to integrate with modern fieldbus systems, such as Modbus
and CANopen. They may also be looking for the possibility of connecting
to high-level supervisory systems, in which case, they should be looking
for Ethernet connectivity in their drives.
Of course, it's all very well to talk about the potential benefits of
installing VSDs, but what real quantifiable benefits are companies in the
food and drink sector achieving? The experiences of Cavaghan & Gray, a
major supplier of ready-meals, provide at least some of the answers. Like
all manufacturing companies, Cavaghan & Gray is constantly looking for
ways to increase the energy efficiency of its operations, and to this
end, the company turned its attention to the refrigeration systems
associated with its production processes.
The production of ready-meals requires large-scale carefully controlled
refrigeration. In the Cavaghan & Gray plant, the refrigerant gas used to
cool the products is itself cooled by passing it through a water-filled
heat exchanger. The water is, in turn, cooled by a fan-driven forced-air
system. Prior to the installation of the new VSDs, the fans used in this
system ran at a fixed speed, and control over the cooling was provided by
a simple mechanical system that throttled the air flow. This arrangement
was updated by installing 17 Telemecanique Altivar 58 VSDs, with power
ratings from 0.75kW to 5kW, to control the fans. The drives were selected
and supplied by Park Gate, a franchised Telemecanique distributor, who
also assisted with the initial installation and commissioning.
Engineers from Park Gate implemented a simple closed-loop control system,
based on feedback from a temperature probe in the water-cooling tower.
The signal from the probe is scaled to produce 0-10V input for the
drives, which use it in conjunction with their integral PI (proportional
integral) facilities, to control the speed of the fans. This arrangement
allows the fans to operate at whatever speed is necessary - from zero to
maximum - to meet the immediate cooling requirements of the plant. It was
quickly noted that, in the winter months, the fans were hardly ever
called upon to run. Clearly, useful energy savings were being achieved,
but both Park Gate and Cavaghan & Gray wanted to quantify these savings.
A kilowatt-hour meter was, therefore, temporarily installed to measure
energy usage over typical operating periods for the old and new control
systems. The results were impressive. The new system reduced energy costs
by 50%, and a quick calculation showed that it would pay for itself in
just four months.
As this example shows, installing VSDs can be a very positive move in
applications throughout the food and drink industry, but to maximise
their benefits, it is important to choose those VSDs carefully. Selecting
modern ECA-qualifying products from a supplier that is prepared to offer
a high level of expertise and support is a sure route to those twin goals
of enhanced productivity and profitability. Paul Pryor is with
Telemecanique, a division of Schneider Electric
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