Why Waste Heat
Recovery Is So Important
ncreased energy efficiency
is a constant challenge
for industry, energy prices
are rising globally so there
is a short term necessity
to reduce fuel bills in order to remain
competitive. There is also a prescient
environmental issue which cannot be
ignored. These are two strong arguments
why power generation operators and
energy consumers operating hot water
or steam boilers 'without' waste heat
recovery systems in place need to act now. boiler economisers, "Using an economiser
provides a guaranteed method of reducing
fuel usage while providing the same
boiler output, typically achieving 4% to
6% fuel saving with a return in investment
between 6 to 18 months. Even if you are
only concerned about the immediate
economics, it still makes a lot of sense.
We also frequently see monitoring figures
before and after installation, so we know
it works."
Opinions vary on the environmental
impact of carbon dioxide emissions,
however the general consensus on global
warming is that doing nothing is no longer
an option - and if action is not taken
now the situation may be irreversible as
soon as 2020. A 2016 report produced
by respected scientific journal Climate
Dynamics* analysing current CO2 global
emissions suggests that devel oped nations
need to reverse carbon emissions at the
latest by 2030 and other nations by 2050. In an industrial boiler system heat is
usually considered 'waste' when it is
below 100°C as it can't be used in main
processes for boiling, steam creation or
sterilisation. This spent heat could be
used for ambient heating of the factory or
building, but its use is restricted to colder
climates or during colder times of the year
in temperate climates, plus, installations to
provide this type of heating are often large
and costly.
I
The process of reducing carbon emissions
also frequently involves reducing other
forms of pollution at the same time.
Simply because the release of unburnt
fuels into the environment and exhaust
emissions from low efficiency burning
cycles involving fossil fuel derivatives into
the atmosphere can be very damaging to
natural life-cycles, including our own.
Why heat recovery?
One of the key areas for potential energy
saving in existing systems is waste heat
recovery. It is important because by
reducing the amount of fuel used to
produce heat in a boiler installation,
heating efficiency increases, resulting
in lower fuel use. This serves both the
requirement to increase efficiency and
reduce carbon emissions.
Best practice for business dictates that
ways to improve operational efficiency
should be constantly sought out. While
the humble boiler became popular over
100 years ago, it remains a major energy
consumer in many factories and other
types of business, generating waste heat
from the spent gases used to heat the
water.
Luke Hall, Project Manager at established
heat recovery specialist ThermTech,
supports the universal application of
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PECM Issue 27
Applying boiler economisers
A more useful alternative technology is to
apply a boiler economiser. Spent heated
gases from the outlet or flue of the boiler
burner are returned to the inlet of the
boiler to pre-warm the in-feed water, thus
providing a higher inlet temperature to the
boiler. This results in less fuel demand to
reach a usable water heat. This technique
is often used in modern domestic boilers
but is not common for industry. Many
industrial boiler systems are often older
installations and awareness of this energy
efficient technique as a retrofit option
is not always known, therefore energy
savings are looked for elsewhere.
How do economisers work?
Installed between the boiler outlet and the
flue or chimney, economisers operate on
the principle of heat exchange, salvaging
boiler gases to pre-heat the boiler in-feed
water. This reduces the energy required
to heat the boiler feed water to operating
temperature. Usually constructed from
carbon steel, the economiser design
incorporates finned tubes containing the
boiler feed water running perpendicular
to the flow of hot outlet gases. By heat
transfer the flue gas temperature reduces
as it increases the feed water temperature.
The extent to which the economiser
pre-heats the feed water depends on the
design of the economiser and existing
boiler parameters. The key to efficient
heat transfer rate however is the design of
the finned tube surface, which needs to
maximise total thermal contact with the
hot gas.
Off-the-shelf or custom built economisers
Since every application is different, each
economiser should be purpose built to
achieve the best transfer efficiency for the
boiler design and the way it operates. This
may include minor changes to an existing
design, which is common for some more
traditional designs based on boilers that
operate continuously at maximum flow
rates. More modern designs with variable
flow-rates however, will usually require
something more specifically suited to the
application.
ThermTech has been providing industry
advice on heat recovery for over 25 years
and is accredited by CHAS, ISO 9001,
Achilles and ASME Boiler and Pressure
Vessel Code Certification. The company's
engineers' objective is to help assess the
quantity and quality of available heat and
matching it to the most effective re-use
cycle, using the most cost effective and
practically suitable forms of heat recovery
equipment.
*Climate Dynamics is a peer-reviewed
scientific journal published by Springer
Science & Business Media
Photo Caption By reducing the amount
of fuel used to produce heat in a boiler
installation, the heating efficiency
increases, resulting in lower fuel use. This
serves both the requirement to increase
efficiency and reduce carbon emissions.
www.thermtech.co.uk