Energy efficiency in Thermal utilities
(Chapter 8: Waste heat recovery)
Introduction
Waste heat is heat, which is generated in a process by way of fuel combustion or chemical reaction,
and then “dumped” into the environment even though it could still be reused for some useful and
economic purpose. The essential quality of heat is not the amount but rather its “value”. The strategy
of how to recover this heat depends in part on the temperature of the waste heat gases and the economics
involved.
Large quantity of hot flue gases is generated from Boilers, Kilns, Ovens and Furnaces. If some of this
waste heat could be recovered, a considerable amount of primary fuel could be saved. The energy lost
in waste gases cannot be fully recovered. However, much of the heat could be recovered and loss
minimized by adopting following measures as outlined in this chapter.
Heat Losses —Quality
Depending upon the type of process, waste heat can be rejected at virtually any temperature from that
of chilled cooling water to high temperature waste gases from an industrial furnace or kiln. Usually
higher the temperature, higher the quality and more cost effective is the heat recovery. In any study
of waste heat recovery, it is absolutely necessary that there should be some use for the recovered heat.
Typical examples of use would be preheating of combustion air, space heating, or pre-heating boiler
feed water or process water. With high temperature heat recovery, a cascade system of waste heat
recovery may be practiced to ensure that the maximum amount of heat is recovered at the highest
potential. An example of this technique of waste heat recovery would be where the high temperature
stage was used for air pre-heating and the low temperature stage used for process feed water heating
or steam raising.
Heat Losses — Quantity
In any heat recovery situation it is essential to know the amount of heat recoverable and also how it
can be used. An example of the availability of waste heat is given below:
1.Heat recovery from heat treatment furnace
In a heat treatment furnace, the exhaust gases are leaving the furnace at 900°C at the rate of 2100
m*/hour. The total heat recoverable at 180°C final exhaust can be calculated as
By installing a recuperator, this heat can be recovered to pre-heat the combustion air. The fuel savings
would be 33% (@ 1% fuel reduction for every 22°C reduction in temperature of flue gas).
Classification and Application
In considering the potential for heat recovery, it is useful to note all the possibilities, and grade the
waste heat in terms of potential value as shown in the following Table 8.1
High Temperature Heat Recovery
The following Table 8.2 gives temperatures of waste gases from industrial process equipment in the
high temperature range. All of these results from direct fuel fired processes.
Medium Temperature Heat Recovery
The following Table 8.3 gives the temperatures of waste gases from process equipment in the medium
temperature range. Most of the waste heat in this temperature range comes from the exhaust of directly
fired process units.
Low Temperature Heat Recovery
The following Table 8.4 lists some heat sources in the low temperature range. In this range it is usually
not practical to extract work from the source, though steam production may not be completely excluded
if there is a need for low-pressure steam. Low temperature waste heat may be useful in a supplementary
way for preheating purposes.
Benefits of Waste Heat Recovery
Benefits of “waste heat recovery’ can be broadly classified in two categories:
Direct Benefits:
Recovery of waste heat has a direct effect on the efficiency of the process. This is reflected by reduction
in the utility consumption & costs, and process cost.
Indirect Benefits:
a) Reduction in pollution: A number of toxic combustible wastes such as carbon monoxide
gas, sour gas, carbon black off gases, oil sludge, Acrylonitrile and other plastic chemicals etc, releasing to atmosphere if/when burnt in the incinerators serves dual purpose i.e. recovers heat
and reduces the environmental pollution levels.
b) Reduction in equipment sizes: Waste heat recovery reduces the fuel consumption, which
leads to reduction in the flue gas produced. This results in reduction in equipment sizes of all
flue gas handling equipments such as fans, stacks, ducts, burners, etc.
c) Reduction in auxiliary energy consumption: Reduction in equipment sizes gives additional benefits in the form of reduction in auxiliary energy consumption like electricity for fans, pumps etc.
Development of a Waste Heat Recovery System
Understanding the process
Understanding the process
Understanding the process is essential for development of Waste Heat Recovery system. This can be
accomplished by reviewing the process flow sheets, layout diagrams, piping isometrics, electrical and
instrumentation cable ducting etc. Detail review of these documents will help in identifying:
a) Sources and uses of waste heat
a) Sources and uses of waste heat
b) Upset conditions occurring in the plant due to heat recovery
c) Availability of space
d) Any other constraint, such as dew point occurring in an equipments etc.
After identifying source of waste heat and the possible use of it, the next step is to select suitable heat recovery system and equipments to recover and utilise the same.
After identifying source of waste heat and the possible use of it, the next step is to select suitable heat recovery system and equipments to recover and utilise the same.
Economic Evaluation of Waste Heat Recovery System
It is necessary to evaluate the selected waste heat recovery system on the basis of financial analysis
such as investment, depreciation, payback period, rate of return etc. In addition the advice of experienced
consultants and suppliers must be obtained for rational decision.
Next section gives a brief description of common heat recovery devices available commercially and
its typical industrial applications.
Commercial Waste Heat Recovery Devices
Recuperators
In a recuperator, heat exchange takes place between the flue gases and the air through metallic or ceramic walls.
Duct or tubes carry the air for combustion to be pre-heated, the other
side contains the waste heat stream. A recuperator for recovering waste heat
from flue gases is shown in Figure 8.1.
The simplest configuration for a recuperator is the metallic radiation recuperator, which consists of two concentric lengths of metal tubing as shown in Figure 8.2.
The inner tube carries the hot exhaust gases while
the external annulus carries the combustion air from
the atmosphere to the air inlets of the furnace
burners. The hot gases are cooled by the incoming
combustion air which now carries additional energy
into the combustion chamber. This is energy which
does not have to be supplied by the fuel; consequently,
less fuel is burned for a given furnace loading. The
saving in fuel also means a decrease in combustion
air and therefore stack losses are decreased not only
by lowering the stack gas temperatures but also by
discharging smaller quantities of exhaust gas. The
radiation recuperator gets its name from the fact
that a substantial portion of the heat transfer from
the hot gases to the surface of the inner tube takes
place by radiative heat transfer. The cold air in the
annuals, however, is almost transparent to infrared
radiation so that only convection heat transfer takes
place to the incoming air. As shown in the diagram,
the two gas flows are usually parallel, although the
configuration would be simpler and the heat transfer more efficient if the flows were opposed in
direction (or counterflow). The reason for the use of parallel flow is that recuperators frequently
serve the additional function of cooling the duct carrying away the exhaust gases and consequently
extending its service life.
A second common configuration for recuperators is called
the tube type or convective recuperator. As seen in the
Figure 8.3, the hot gases are carried through a number of
parallel small diameter tubes, while the incoming air to
be heated enters a shell surrounding the tubes and passes
over the hot tubes one or more times in a direction normal
to their axes.
If the tubes are baffled to allow the gas to pass over them
twice, the heat exchanger is termed a two-pass recuperator;
if two baffles are used, a three-pass recuperator, etc.
Although baffling increases both the cost of the exchanger
and the pressure drop in the combustion air path, it
increases the effectiveness of heat exchange. Shell and tube type recuperators are generally more compact and have a higher effectiveness than radiation
recuperators, because of the larger heat transfer area made possible through the use of multiple tubes
and multiple passes of the gases.
Radiation/Convective Hybrid Recuperator:
For maximum effectiveness of heat transfer, combinations of radiation and convective designs are
used, with the high-temperature radiation recuperator being first followed by convection type.
These are more expensive than simple metallic radiation recuperators, but are less bulky. A convective/
radiative hybrid recuperator is shown in Figure 8.4
Ceramic Recuperator
The principal limitation on the heat recovery of metal recuperators is the reduced life of the liner at
inlet temperatures exceeding 1100 °C. In order to overcome the temperature limitations of metal
recuperators, ceramic tube recuperators have been developed whose materials allow operation on
the gas side to 1550 °C and on the preheated air side to 815 °C on a more or less practical basis.
Early ceramic recuperators were built of tile and joined with furnace cement, and thermal cycling
caused cracking of joints and rapid deterioration of the tubes. Later developments introduced
various kinds of short silicon carbide tubes which can be joined by flexible seals located in the air
headers.
Earlier designs had experienced leakage rates from 8 to 60 percent. The new designs are reported
to last two years with air preheat temperatures as high as 700 °C, with much lower leakage rates.
Regenerator
The Regeneration which is preferable for large
capacities has been very widely used in glass and
steel melting furnaces. Important relations exist
between the size of the regenerator, time between
reversals, thickness of brick, conductivity of
brick and heat storage ratio of the brick.
In a regenerator, the time between the reversals
is an important aspect. Long periods would mean
higher thermal storage and hence higher cost.
Also long periods of reversal result in lower
average temperature of preheat and consequently
reduce fuel economy. (Refer Figure 8.5).
Accumulation of dust and slagging on the
surfaces reduce efficiency of the heat transfer as
the furnace becomes old. Heat losses from the walls of the regenerator and air in leaks during the gas
period and out-leaks during air period also reduces the heat transfer.
Heat Wheels
A heat wheel is finding increasing applications in low to medium temperature waste heat recovery
systems. Figure 8.6 is a sketch illustrating the application of a heat wheel.
It is a sizable porous disk, fabricated with material having a fairly high heat capacity, which rotates
between two side-by-side ducts: one a cold gas duct, the other a hot gas duct. The axis of the disk is
located parallel to, and on the partition between, the two ducts. As the disk slowly rotates, sensible
heat (moisture that contains latent heat) is transferred to the disk by the hot air and, as the disk rotates,
from the disk to the cold air. The overall efficiency of sensible heat transfer for this kind of regenerator can be as high as 85 percent. Heat wheels have been built as large as 21 metres in diameter with air
capacities up to 1130 m3/ min.
A variation of the Heat Wheel is the rotary regenerator where the matrix is in a cylinder rotating across
the waste gas and air streams. The heat or energy recovery wheel is a rotary gas heat regenerator,
which can transfer heat from exhaust to incoming gases.
Its main area of application is where heat exchange between large masses of air having small temperature
differences is required. Heating and ventilation systems and recovery of heat from dryer exhaust air
are typical applications.
Case Example
A rotary heat regenerator was installed on a two colour printing press to recover some of the heat,
which had been previously dissipated to the atmosphere, and used for drying stage of the process. The
outlet exhaust temperature before heat recovery was often in excess of 100°C. After heat recovery the
temperature was 35°C. Percentage heat recovery was 55% and payback on the investment was estimated
to be about 18 months. Cross contamination of the fresh air from the solvent in the exhaust gases was
at a very acceptable level.
Case Example
A ceramic firm installed a heat wheel on the preheating zone of a tunnel kiln where 7500 m*/hour of
hot gas at 300°C was being rejected to the atmosphere. The result was that the flue gas temperature
was reduced to 150°C and the fresh air drawn from the top of the kiln was preheated to 155°C. The
burner previously used for providing the preheated air was no longer required. The capital cost of the
equipment was recovered in less than 12 months.
Heat Pipe
A heat pipe can transfer up to 100 times more thermal energy than copper, the best known conductor.
In other words, heat pipe is a thermal energy absorbing and transferring system and have no moving parts and hence require minimum maintenance.
The Heat Pipe comprises of three elements — a sealed container, a capillary wick structure and a working
fluid. The capillary wick structure is integrally fabricated into the interior surface of the container
tube and sealed under vacuum. Thermal energy applied to the external surface of the heat pipe is in
equilibrium with its own vapour as the container tube is sealed under vacuum. Thermal energy applied
to the external surface of the heat pipe causes the working fluid near the surface to evaporate
instantaneously. Vapour thus formed absorbs the latent heat of vapourisation and this part of the heat
pipe becomes an evaporator region. The vapour then travels to the other end the pipe where the thermal
energy is removed causing the vapour to condense into liquid again, thereby giving up the latent heat
of the condensation. This part of the heat pipe works as the condenser region. The condensed liquid
then flows back to the evaporated region. A figure of Heat pipe is shown in Figure 8.7
Performance and Advantage
The heat pipe exchanger (HPHE) is a lightweight compact heat recovery system. It virtually does not
need mechanical maintenance, as there are no moving parts to wear out. It does not need input power
for its operation and is free from cooling water and lubrication systems. It also lowers the fan horsepower
requirement and increases the overall thermal efficiency of the system. The heat pipe heat recovery
systems are capable of operating at 315°C with 60% to 80% heat recovery capability.
Typical Application
The heat pipes are used in following industrial applications:
a.) Process to Space Heating: The heat pipe heat exchanger transfers the thermal energy from
process exhaust for building heating. The preheated air can be blended if required. The
requirement of additional heating equipment to deliver heated make up air is drastically
reduced or eliminated. b.) Process to Process: The heat pipe heat exchangers recover waste thermal energy from the
process exhaust and transfer this energy to the incoming process air. The incoming air thus
become warm and can be used for the same process/other processes and reduces process
energy consumption.
c.) HVAC Applications:
Cooling: Heat pipe heat exchangers precools the building make up air in summer and thus
reduces the total tons of refrigeration, apart from the operational saving of the cooling system.
Thermal energy is supply recovered from the cool exhaust and transferred to the hot supply
make up air.
Heating: The above process is reversed during winter to preheat the make up air.
Heating: The above process is reversed during winter to preheat the make up air.
The other applications in industries are:
1. Preheating of boiler combustion air
2. Recovery of Waste heat from furnaces
3.Reheating of fresh air for hot air driers
4.Waste Heat Recovery
5.Recovery of waste heat from catalytic deodorizing equipment
6.Reuse of Furnace waste heat as heat source for other oven
7.Cooling of closed rooms with outside air
8.Preheating of boiler feed water with waste heat recovery from flue gases in the heat pipe
economizers.
9.Drying, curing and baking ovens
9.Drying, curing and baking ovens
10.Waste steam reclamation
e Brick kilns (secondary recovery)
11.Reverberatory furnaces (secondary recovery)
12Heating, ventilating and air-conditioning systems
Savings in Hospital Cooling Systems
Volume =140 m3/min Exhaust
Recovered heat= 28225 kcal/hr
Plant capacity reduction = 9.33 Tons of Refrigeration
Electricity cost (operation) = Rs. 268/Million kcal (based on 0.8 kW/TR)
Plant capacity reduction cost (Capital) =Rs.12,000/TR
Capital cost savings = Rs. 1,12,000/-
Payback period = 16570 hours
Economiser
In case of boiler system, economizer can
be provided to utilize the flue gas heat for
pre-heating the boiler feed water. On the
other hand, in an air pre-heater, the waste
heat is used to heat combustion air. In both
the cases, there is a corresponding reduction
in the fuel requirements of the boiler. An
economizer is shown in Figure 8.8.
For every 22°C reduction in flue gas
temperature by passing through an
economiser or a pre-heater, there is 1%
saving of fuel in the boiler. In other words, for every 6°C rise in feed water temperature through an economiser, or 20°C rise in combustion air
temperature through an air pre-heater, there is 1% saving of fuel in the boiler.
Shell and Tube Heat Exchanger:
When the medium containing waste heat is a liquid or a vapor which heats another liquid, then the
shell and tube heat exchanger must be used since both paths must be sealed to contain the pressures
of their respective fluids. The shell contains the tube bundle, and usually internal baffles, to direct
the fluid in the shell over the tubes in multiple passes. The shell is inherently weaker than the tubes
so that the higher-pressure fluid is circulated in the tubes while the lower pressure fluid flows through
the shell. When a vapor contains the waste heat, it usually condenses, giving up its latent heat to
the liquid being heated. In this application, the vapor is almost invariably contained within the shell.
If the reverse is attempted, the condensation of vapors within small diameter parallel tubes causes
flow instabilities. Tube and shell heat exchangers are available in a wide range of standard sizes
with many combinations of materials for the tubes and shells. A shell and tube heat exchanger is
illustrated in Figure 8.9.
Typical applications of shell and tube heat exchangers include heating liquids with the heat contained
by condensates from refrigeration and air-conditioning systems; condensate from process steam;
coolants from furnace doors, grates, and pipe supports; coolants from engines, air compressors,
bearings, and lubricants; and the condensates from distillation processes.
Plate heat exchanger
The cost of heat exchange surfaces is a major cost factor when the temperature differences are not
large. One way of meeting this problem is the plate type heat exchanger, which consists of a series
of separate parallel plates forming thin flow pass. Each plate is separated from the next by gaskets
and the hot stream passes in parallel through alternative plates whilst the liquid to be heated passes
in parallel between the hot plates. To improve heat transfer the plates are corrugated.
Hot liquid passing through a bottom port in the head is permitted to pass upwards between every second plate while cold liquid at the top of the head is permitted to pass downwards between the odd plates. When the directions of hot & cold fluids are opposite, the arrangement is described as counter current.
A plate heat exchanger is shown in Figure 8.10.
Typical industrial applications are:
o Pasteurisation section in milk packaging plant.
o Evaporation plants in food industry.
Run Around Coil Exchanger
It is quite similar in principle to the heat pipe exchanger. The heat from hot fluid is transferred to the
colder fluid via an intermediate fluid known as the Heat Transfer Fluid. One coil of this closed loop
is installed in the hot stream while the other is in the cold stream. Circulation of this fluid is maintained
by means of la circulating pump.
It is more useful when the hot land cold fluids are located far away from each other and are not easily
accessible.
Typical industrial applications are heat recovery from ventilation, air conditioning and low temperature
heat recovery.
Waste Heat Boilers
Waste heat boilers are ordinarily water tube boilers in which the hot exhaust gases from gas turbines,
incinerators, etc., pass over a number of parallel tubes containing water. The water is vaporized in the
tubes and collected in a steam drum from which it is drawn off for use as heating or processing steam.
Because the exhaust gases are usually in the medium temperature range and in order to conserve space,
a more compact boiler can be produced if the water tubes are finned in order to increase the effective
heat transfer area on the gas side. The Figure 8.11 shows a mud drum, a set of tubes over which the
hot gases make a double pass, and a steam drum which collects the steam generated above the water
surface. The pressure and rate at which the steam is generated depends on the temperature of waste
heat. The pressure of a pure vapor in the presence of its liquid is a function of the temperature of the
liquid from which it is evaporated. The steam tables tabulate this relationship between saturation
pressure and temperature. If the waste heat in the exhaust gases is insufficient for generating the
required amount of process steam, auxiliary burners which burn fuel in the waste heat boiler or an after-burner in the exhaust gases flue are added. Waste heat boilers are built in capacities from 25 m*
almost 30,000 m?/ min. of exhaust gas.
Typical applications of waste heat boilers are to recover energy from the exhausts of gas turbines,
reciprocating engines, incinerators, and furnaces.
Case Example
Gases leaving a carbon black plant rich in carbon monoxide which are vented to the atmosphere.
In the various commercial options previously discussed, we find waste heat being transferred from a
hot fluid to a fluid at a lower temperature. Heat must flow spontaneously “downhill”, that is from a
system at high temperature to one at a lower temperature. When energy is repeatedly transferred or transformed, it becomes less and less available for use. Eventually that energy has such low intensity
(resides in a medium at such low temperature) that it is no longer available at all to perform a useful
function. It has been taken as a general rule of thumb in industrial operations that fluids with
temperatures less than 120°C (or, better, 150°C to provide a safe margin), as limit for waste heat recovery
because of the risk of condensation of corrosive liquids. However, as fuel costs continue to rise, even
such waste heat can be used economically for space heating and other low temperature applications.
It is possible to reverse the direction of spontaneous energy flow by the use of a thermodynamic system
known as a heat pump.
The majority of heat pumps work on the principle of the vapour compression cycle. In this cycle, the
circulating substance is physically separated from the source (waste heat, with a temperature of T, )
and user (heat to be used in the process, T. ,) streams, and is re-used in a cyclical fashion, therefore
called ‘closed cycle’. In the heat pump, the following processes take place:
1. In the evaporator the heat is extracted from the heat source to boil the circulating substance;
2. Thecirculating substance is compressed by the compressor, raising its pressure and temperature;
the low temperature vapor is compressed by a compressor, which requires external work.
The work done on the vapor raises its pressure and temperature to a level where its energy
becomes available for use
3. The heat is delivered to the condenser;
4. The pressure of the circulating substance (working fluid) is reduced back to the evaporator
condition in the throttling valve, where the cycle repeats.
The heat pump was developed as a space heating system where low temperature energy from the
ambient air, water, or earth is raised to heating system temperatures by doing compression work with
an electric motor-driven compressor. The arrangement of a heat pump is shown in Figure 8.12.
The heat pumps have the ability to upgrade heat to a value more than twice that of the energy consumed
by the device. The potential for application of heat pump is growing and number of industries have
been benefited by recovering low grade waste heat by upgrading it and using it in the main process
stream.
Direct Contact Heat Exchanger
Low pressure steam may also be used to preheat the feed water or some other fluid where miscibility
is acceptable. This principle is used in Direct Contact Heat Exchanger and finds wide use in a steam
generating station. They essentially consist of a number of trays mounted one over the other or
packed beds. Steam is supplied below the packing while the cold water is sprayed at the top. The
steam is completely condensed in the incoming water thereby heating it. A figure of direct contact
heat exchanger is shown in Figure 8.13. Typical application is in the deaerator of a steam generation
station.
Solved Example:
In a crude distillation unit of a refinery, furnace is operated to heat 500 m*/hr of crude oil from 255°C
to 360°C by firing 3.4 tons/hr of fuel oil having GCV of 9850 kcal/kg. As an energy conservation
measure, the management has installed an air preheater (APH) to reduce the flue gas heat loss. The
APH is designed to pre-heat 57 tonnes/hr of combustion air to 195°C.
Calculate the efficiency of the furnace before & after the installation of APH. Consider the following
data:Specific heat of crude oil =0.6 kcal/kg°C
Specific heat of air = 0.24 kcal/kg°C
Specific gravity of Crude oil = 0.85
Ambient temperature = 28°C.
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Chapter 9
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