1. Is the waste heat dissipation from the plant
too large ?
In thermal processes there are always a heat loss - processes
without waste heat dissipations are purely theoretical, and do not exist the real world.
Energy arises from thermal processes and it always proceed while it seems to vanish in
other thermal or thermodynamic processes - whether it might be in conventional heat
transfer, engines, tubines or refrigeration plants (just to mensions some examples). But
although the heat loss always exists, it is important to evaluate the size of this loss,
in relations to what is considered as being reasonable from an economical and/or
enviromental point of view - thus to consider investing in heat recovery.
The task in good heat recovery is to create maximum cut in the
operation expenses with a minimum of investment.
Modern boilers, heaters and heating systems are developed with
only modest heat loss. They have what you call a high efficiency. The efficiency tells how
much of the total amount of potential energy is being utilized for the main purpose. A
modern e.g. boilers can for instance have an efficiency of 0,8 - 0,9 (or 80 - 90%),
meaning that the boilers has got a loss - mainly due to the chimney loss - of 10 - 20%. An
older boiler might however have an efficiency of only 0,6 - 0,7 (or 60 - 70%) equal to a
heat loss of 30 - 40%.
Still heat losses can be very large even in modern boilers, both
considering the amount in it self, as well as relatively to the potential total energy
(the efficiency). There are many reasons for this, but common for all existing industrial
heating systems are, that they originally were determined and adapted to the demands at
the time when they were erected. For instance an enlargement of the production plant might
result in a demand for other larger amount and/or other types of process heating. New
lines requiring high temperature processes, unsufficient heat supply, or a surplus of low
temperature process heat leading to a low utilization of capacity of the old old boiler,
price increase on electricity, possiblity of using new fuel (e.g. natural gas). All these
circumstances might result in a situation which is not optimal for the old heating system,
thus the overall efficiency might drop significantly.
2. The Advantages of Heat Recovery for an
Industrial Production Plant
The heat loss from a fired boiler/heater is primary dissipated
through the chimney i.e. it comes from the hot flue gas. The amount of flue gas and the
temperature of this, is proportional to the heat loss. Often a considerable economical
advantage can be achieved by cooling the flue gas, and thereby utilizing the energy
amount, which under normal circumstances would have been let out as heat loss.
In industrial production plants, large amount of energy for
process heating are often used, and it is characterized by quite high temperatures. The
heating media (the carrier of the process heating) can be steam, thermal fluid (hot
circulating oil), pressurized/unpressurized hot water or air/gas - as well as more
sophisticated heating media for special tasks, and used maybe locally in the production
plants.
Generally a high temperature level on the heating media often
means a high chimney temperature (flue gas temperature). Consequently large oil or gas
fired industrial boiler/heaters very often makes efficient and thus economical attractive
solutions possible. By inserting a heat exchanger in the flue gas flow - just before the
chimney - process heat can be generated at almost no costs.
Heat produced in a flue gas recuperator (exhaust gas boiler) will
in many new installations be of a lower temperature level than the existing boiler/heater
can provide - simply because the boiler/heater already has utilized most of the the energy
developed, by combustion of fuel. The smaller heat loss through the chimney can be seen by
the lower flue gas temperature, which thereby gives some restriction i.e. limitation for
the temperature level of the produced/recovered heat.
In most production plants and heat consuming factories it is
often advantageous to establish a secondary heating system - for instance for production
of low pressure steam for low temperature applications. Eighter indirectly such as
preheating of air / liquid / oils - or directly such as injection of steam in tanks and
reactors, or for cleaning/sterilization in general.
Nevertheless, still in many installations, the recovered heat can
be used in the primary heating system. This means a better utilization of the fuel, which
gives a higher overall efficiency - eighter by a higher amount of heat produced on the
same amount of fuel - or by the same amount of heat produced by a smaller amount of fuel
(see illustrations next page).
3. Investment & Benefit
Like any other kind of investments, the purpose of investing in
heat recovery, is to get a net profit which is as large as possible compared to the total
expenses related to the investment. Technically likewice - the task in heat recovery is to
utilize as much of the energy bound in the fuel as possible by the most simple solution
i.e. optimizing the size of the heat recovery system. For instance to take out as much
heat from the flue gas as possible.
In other investment models, especially the purely finacial
models, the profit is often difficult to determine exactly. They often depends and rely on
weak elements such as expectation on up or down going figures.
Investment in heat recovery is easier to survey by the following
facts:
Heat cost money
Any amount of heat represents an amount of money
Not utilized heat is loss of money
Heat recovery investments is actually less risky. Only real
uncertainty in determine the size of profit in heat recovery investments, is if the fuel
prices drops significant. But is that likely to happen?
The art of heat recovery is to evaluate the system individually,
and based on these investigations - to get the maximum amount of heat recovered by the
smallest alteration of the system.
This also emphasizes the fact that there are an optimum for each
system, and that requires to be determine in order to maximize profit compared to the
investment ("price/earning"). This is best illustrated by the extreme cases,
where the size of the investment in both cases are wrong ( too large and too small
respectively). The basic costs for heat recovery equipment make a too small investment
insufficient, because the capacity of the heat exchanging part simply becomes
insignificant and the costs for accessoriesbecomes too high. In contradistiction to this
is the too ambitious investment, which does not correspond at all what is actually
possible to save in fuel.
It takes a infinite large heat
exchanger part
- to recover all of the heat loss
It takes an almost infinite large
heat exchanger part
- to recover almost all of the heat loss
It means that the heat recovery amount practically represents a
limited amount of heat, which is sometimes far less than the overall heat loss.
Therefore the optimum heat recovery is very often found by
evaluating the annual gained (from saved fuel cost) per invested $ - combined with what is
totally gained during an estimated life time of the heat recovery plant.
When a non-optimum is choosen despite a serious and throughout
evaluation, it is often due to a decision of limiting the investment anyhow - for
different reasons. These could be e.g. uncertainty about the futuring use of the oil/gas
fired heater, or to observe the feasability in a small scale investment.
5. The Concept from AB+Co. Thermal Transfer Ltd.
AXA Exhaust Gas Recuperators, produced by AB+Co. Thermal Transfer
Ltd. In Denmark, are based on a new and price attractive concept, where heat loss from
older industrial heating systems are recovered simply and effectively, for the purpose of
producing process heat - as steam, hot water, high temperature pressurized water, hot oil
(thermal fluid), air/gas or as other heat carrying fluids.
Basicly the consept is that a heat exchanger are build-on the
flue gas outlet of the gas/oil fired heater, engine or turbine. In the heat exchanger of
the recuperator, a smaller or larger part of the heat loss is converted into accessable
heat - due to the heating of the heat carrying fluid (like above mensioned) or production
of low pressure steam.
The news is firstly the heat exchanger part (module 1), which is
adapted individually to the actual circumstances - not only in size and performance, but
mainly in the design of the heat transferring surface. It means that the design of the
basic heat exchanger part, are chosen from a broad program of heat exchanger principles
(fin tubes vs. plain tubes, straight tubes vs. winded coils, sections w. rectangular face
and cross flow vs. circular face and counter flow etc.).
A correct choice of heat exchanger principle for actual task and
circumstances is essential in order to obtain the best technical solution for the smallest
investment.
The choice of heat exchanger principle is considered in relation
to a.o. the nature of the flue gas, the heat carrying fluid, temperatures, pressures and
the the capacity. For instance, a resuperator for heating of low temperature water, and
cooling non-agressive flue gas, might be designed with a fin tube heat exchanger - while a
heat exchanger for production of steam (exhaust gas boiler) most likely will be designed
as coils.
6. A Simple and Economical Solution
The other new topic is the modular consept, which makes the
installation simple and significant more economical than common known recuperator
solutions. Furthermore the modular design means that the installation can be build-out
later within very reasonable economical means.
Due to the flexibility in the modular consept, a wide range of
operating temperatures are optional within the limits of (i.e. below) the flue gas inlet
temperature ofcourse.
As heat recovery creates additional heat, it is also optional to
eighter produce extra heat on the existing oil /gas fired heater - or to run reduced load
on the oil /gas fired heater with the produced heat than before. Running on reduced load
gives a better efficiency of the system, but also on the oil/gas fired heater itself,
because the combustion chamber (the convection and radiant heat part) becomes larger
relatively to the combusted amount of fuel. The recuperator forms an extention of the
oil/gas fired heater - which utilize the heat in the flue gas to make up the reduced heat
output from the combustion chamber of the oil/gas fired heater. Totally the oil/gas fired
heater together with the recuperator give the same heat output - but with less fuel.
7. Heat Recovery delivered as a unit - or as a
complete system / turn-key
AXA Exhaust Gas Recuperators™ & Heat Recovery
Systems™ can be delivered optional eighter as units, as system solutions (packages)
or just one or few heat exchanger "components". Generally heat recovery requires
tailormade solutions, thus not two recuperators are identical. However the modular consept
from AB+Co. Thermal Transfer Ltd. utilizes the similarity in the different kind of heat
exchanger design and instrumentation as mensioned on page 8 and 9.
The unit design offers a complete skidmounted heat recovery
system with piping, tank, armatures, instrumentation and controls - ready to be connected
to the existing heating system. This is a good alternative for uncomplicated small and
medium size heat recovery.
The system solution are complete heat recovery system as well.
But due to the size or because it might involve heat recovery in general of the whole
heating systems, it is delivered as a package of separate components/item and complete
instruction for building up the ssystem on site. This alternative is recommended e.g. for
erection of complete secondary heating systems and for large scale heat recovery.
The third alternative is just to use one or a few items for
"self-made" heat recovery systems. Often this requires assistance from AB+Co.
Thermal Transfer Ltd. for lay-out, design, engineering and supervision. Sometimes only the
AXA heat exchanger part - the heart of the recuperator - is requires, whereas the rest of
the system including the other items and the engineering are made without assistance from
AB+Co. Thermal Transfer Ltd.
8. Is this worth an investment ?
You oftengain a lot when investing in heat recovery. Still it
should be evaluated carefully to avoid unpleasant surprises. Special circumstances might
disturb a nice picture of heat recovery.
To make a first step evaluation, the table on page 7 indicating
the positive and negative indicators might give an idea. In the below simplified
feasibility studies, the calculations might give a better impression.
Generally it is a good idea to make some considerations on how
potential the present heat loss is related to the potential heat recovery. E.g. how much
flue gas is avaiable and how much higher the temperature of the flue gas is compared to
the temperature of the existing or new heating system.
- What heat carrying fluid is required in the recuperator, and what
is preferred (steam, thermal fluid (oil), water or gas/air) ?
- What temperature level of above mensioned heat carrying fluid is
possible and necessary related to the flue gas temperature ?
- Should the heat recovery aim towards a reduction of the fuel
consumption with same heat output - or towards a larger heat output and maintaining the
fuel consumption ?
- Is a new secondary system acceptable if it is economical (better
utilization of recuperator) - or should only heat recovery within the limit of the
existing heating system be considered (requires higher flue gas temperature) ?
9. Public economical support
Heat recovery is ofcourse healthy for the economy - sometimes
even for the national economy (despite less fuel tax income). In enviromentally terms, the
heat recovery contribute to less emmision, and that has made the public interested in heat
recovery too.
In many countries or regions it is possible to get subsidies
(economical support) for stimulating heat recovery initives. It does varies a lot - in
some countries it is forced by law to keep the temperatures and the amount of flue gas
(emmision) below a certain level. But the tendency is that public interest developes
rapidly, thus also the possibility of public subsidies.
E.g. in Denmark, a large so called "CO2 Fund" has
through taxes on fuel, collected a large amount of money, which is now being allocated
back to the industry as subsidies for public recognized (approved) heat recovery systems.
Similar arrangement exist in other countries and region, and might very well make the
feasability calculations turn out even better.
10. Doing the first feasability calculations
AB+Co. Thermal Transfer Ltd. offers a prelimineary and general
evaluation free of charge. Based on a developed questionairy the actual circumstances are
enlightened. When it is filled out, it will give a hint on whether heat recovery it worth
an investment. The result can be calculated directly by using formulars on the sheet, or
it can be faxed, mailed or E-mailed directly to AB+Co. Thermal Transfer - Project Dept.
and the answer will be returned within two working days.
The result of this calculation might lead to inquiry for a heat
recovery system, and/or a more detailed investigation on consultant basis.
11. AB+Co. Thermal Transfer Ltd.
AB+Co. Thermal Transfer Ltd. is a one of Europe's leading
manufacturers of units for industrial heating, cooling and heat recovery. A wide range of
heat exchangers and complete units for process heating and heat recovery can match almost
any task. The solutions that is offered are customized to the actual circumstances, but
still always economical as an investment, and considering the operation costs as well.
Products delivered from AB+Co. Thermal Transfer A/S and in
operation all over the world:
- Heat Exchangers (designed and customized for the task)
- Process Air Heaters (steam, thermal fluid, electrical)
- Process Air Coolers & Dehumidifiers
- Gas-to Gas Heat Exchangers (single and multi pass recuperators)
- Exhaust Gas Coolers (for e.g. steam production/heating of fluid
and gas)
- Heater Units/Systems (complete skidmounted) for special tasks
within industrial heating.
