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To have a chance at being efficient, a boiler must
first be adequately loaded. This is a system issue.
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Services (sometimes called “utilities”)
in manufacturing plants are typically a large contributor to site energy
consumption. With these services, an energy carrier such as a fossil fuel or
electricity is used to transfer energy into a fluid, which is then distributed
to point of use.
Some examples of common services are compressed air, steam,
chilled water, thermic oil and refrigerants. Of course, the supply of
electrical power may itself be considered a service.
In general, manufacturers tend to
focus more on core manufacturing processes than on services, and since often
the services section of a manufacturing site is managed by someone other than
the managers responsible for production processes, there is poor integration between
services and manufacturing when it comes to energy efficiency. The result can
be a piecemeal, component-level approach to energy efficiency, when what is
required is a systems approach. I started this post by saying that services
tend to be large users, but we need to take a step back to appreciate that
while the energy input is to the service, the drivers of that energy use lie in
various plant areas, and even include environmental factors. When resource
efficiency practitioners talk about “systems” with respect to services,
generally we mean the generation, distribution and user side of the service
infrastructure, considered as an integrated whole. In the case of some services,
there is an additional system element, that of recovery (e.g. returning
condensate, thermic oil or refrigerant vapour).
Let’s consider for a moment why a
systems approach is important. The first point to note is that by considering
an individual element of a system in isolation, one can make small improvements
but miss the far larger opportunities available. Hence there could be focus on
boiler efficiency which could reduce steam system costs by 5%, when all the
while an opportunity exists to reduce the quantity of steam consumed,
potentially reducing system costs by 25%.
A second problem with taking a
component-level approach would be that in attempting to save energy, there
could be unintended consequences which lead to a net decline in performance,
either in energy use or in some other operational area. A simple example would
be to reduce the fan speed on a refrigerant condenser in order to reduce the
energy consumed by the fan motors, but to inadvertently increase the energy
consumption of the vapour compressors, thereby increasing system-level
refrigeration energy consumption.
A third and vital reason to take
a systems approach to services optimisation is that the individual elements of
the system interact with each other, and when implementing a basket of
solutions comprising projects in each individual area, the implementation of
each solution will influence the impact and viability of the others. A heat
recovery project for a compressed air system will reduce in attractiveness
should the amount of air required be significantly reduced, for example. The
point here is that the benefit of implementing a portfolio of solutions for a
given system will generally always be less than the sum of the individual
solutions. This implies that it is useful to have a view of the full portfolio
of planned solutions before proceeding with implementation, and while this is
not universally true (e.g. one should not hesitate to fix air or steam leaks),
it is important when considering capital-intensive options.
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Reducing the pulse frequency for this bag filter
would reduce compressed air consumption. This
would reduce the loading of the air compressors.
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Almost without exception, the approach
to energy efficiency I see in most manufacturing plants as regards services is
as follows:
i. Tinkering with the distribution network e.g.
insulating distribution pipelines, fixing leaks and the like;
ii.
Investing heavily in generation e.g. an
“efficient” new air compressor, replacement of slide valve control with VSD
control for refrigeration compressors etc.
There is no harm in dealing with
obvious inefficiencies in distribution. The costs for implementation in this
area tend to be low. At very large facilities, the savings can be significant.
In general, however, projects involving the distribution network tend to yield
only modest savings, and if this is all you are going to do, don’t expect breakthroughs
on the bottom line. There are some distribution problems that do however
require system-level considerations – an example would be a project to increase
the size of air distribution pipelines to reduce pressure drop, which may
become unnecessary should air use be significantly reduced.
The focus on generation I referred to above is in
part due to the fact that there is a relentless focus on technological
development in this area by OEM’s. Shiny new machines are attractive, and tend
to come with assurances from suppliers with respect to increased levels of
efficiency. Exercise extreme caution however. Many technologies only operate
optimally when used in the correct context. Establishing this context requires
a consideration of the system. As a simple example, a VSD air compressor as a
replacement for a fully-loaded fixed-speed machine will not yield savings. A
second reason that organisations tend to focus on the generation side of the
system is the common misconception that efficiency needs to be “bought”, and
that older installations cannot operate efficiently without investment. Since
the generation side of the system is where capital tends to be concentrated,
the tendency is to spend in this area. The reality is however that there are
any number of no-cost and low-cost options that may be employed to drive energy
efficiency.
The recovery aspect of some
systems offers significant opportunities. In steam systems, condensate and flash
steam recovery options can offer large savings at relatively low capital cost. Their
viability is however impacted on by system-level considerations. A change in the
steam pressure supplied to a user could significantly reduce the amount of
flash steam produced, for example.
Given the above, you have most
likely figured out by now that the place to start when optimising services is
on the user end of the system. This means finding ways to reduce the amount of
service fluid required. This comes with a lot of good news. In general,
user-side opportunities abound. And in many cases, a common-sense approach can
yield large benefits. Close those freezer doors. Insulate those process
vessels. Recover energy from the leaving process stream. Automate the operation
of that system so that switching off equipment is not dependent on someone remembering
to do so. It may also be necessary to challenge paradigms in your business to
reap these benefits. Does that process really need to operate at that
temperature? Do we really need to wash this pipeline with hot water after each
and every batch? Does the air pressure to that nozzle really have to be 8 barg.
or can we get away with 5 barg?
When you take the time out to do
this, you could be surprised by the quantum of the savings possible. That is
one big reason to start here. The second is that there will be knock-on effects
for the rest of the system. You may now only require two vapour compressors in
your refrigeration plant room instead of three. The back-pressure turbine you
thought was viable is now no longer so, since the amount of steam used has been
significantly reduced. Your fully-utilised chiller plant is now a candidate for
the implementation of a VSD compressor, since you now operate at part-load
conditions for much of the time. You thought that you needed a VSD air compressor,
but can now switch one of your compressors off. Had you made investments in
generation, distribution and recovery before optimising your users, you could
have ended up with a sub-optimal system.
Copyright © 2016, Craig van Wyk, all rights reserved
