SlideShare a Scribd company logo
Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon-423603
(An Autonomous Institute Affiliated to Savitribai Phule Pune University)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 Certified
Unit-3: WASTE HEAT RECOVERY
SYSTEMS
Subject :- Waste Heat Recovery and Sustainable
Energy (ME 305A)
T.Y. B. Tech.(Mechanical)
By
Yogesh H. Ahire (Asst. Professor)
SRES’s Sanjivani COE, Kopargaon-423603, Maharashtra, India
Email: ahireyogeshmech@sanjivani.org.in
ahireyogesh@gmail.com
Mobile:- 9881290264
Department of Mechanical Engineering
Contents of the Course-WHSE
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (2)
COs Course Outcomes
Blooms Taxonomy
Level Descriptor
CO3
Analyze the waste heat recovery technologies
developed for various thermal systems. .
2 understand
Unit CONTENTS No. of
Hours
COs
3 WASTE HEAT RECOVERY SYSTEMS
Types of heat recovery systems (recuperators-regenerators-
economizers-plate heat exchangers-thermic fluid heaters, waste heat
boilers), Selection criteria for waste heat recovery technologies,
location, service conditions, design considerations.
7 Hrs. CO3
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (3)
• Evaluating the feasibility of waste heat recovery requires characterizing the
waste heat source and the stream to which the heat will be transferred.
• Important waste stream parameters that must be determined include:
– heat quantity,
– heat temperature/quality,
– composition,
– minimum allowed temperature, and
– operating schedules, availability, and other logistics
• These parameters allow for analysis of the quality and quantity of the stream
and also provide insight into possible materials/design limitations.
• For example, corrosion of heat transfer media is of considerable concern in
waste heat recovery, even when the quality and quantity of the stream is
acceptable.
• The following provide an overview of important concepts that determine waste
heat recovery feasibility.
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (4)
1. Heat Quantity :
• The quantity, or heat content, is a measure of how much energy is contained in
a waste heat stream, while quality is a measure of the usefulness of the waste
heat.
• The quantity of waste heat contained in a waste stream is a function of both the
temperature and the mass flow rate of the stream:
• Where,
– E is the waste heat loss (kJ/hr);
– m is the waste stream mass flow rate (kg/hr);
– and h(t) is the waste stream specific enthalpy (kJ/kg) as a function of temperature.
• Enthalpy is not an absolute term, but must be measured against a reference
state (for example, the enthalpy of a substance at room temperature and
atmospheric pressure).
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (5)
• Although the quantity of waste heat available is an important parameter, it is
not alone an effective measure of waste heat recovery opportunity. It is also
important to specify the waste heat quality, as determined by its temperature.
2. Waste Heat Temperature/Quality:
• The waste heat temperature is a key factor determining waste heat recovery
feasibility.
• Waste heat temperatures can vary significantly, with cooling water returns
having low temperatures around [40 - 90°C] and glass melting furnaces having
flue temperatures above [1,320°C].
• In order to enable heat transfer and recovery, it is necessary that the waste heat
source temperature is higher than the heat sink temperature.
• Moreover, the magnitude of the temperature difference between the heat
source and sink is an important determinant of waste heat’s utility or
• The source and sink temperature difference influences a) the rate at which heat
is transferred per unit surface area of heat exchanger, and
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (6)
b) the maximum theoretical efficiency of converting thermal from the heat source
to another form of energy (i.e., mechanical or electrical).
• Finally, the temperature range has important ramifications for the selection of
materials in heat exchanger designs
• Waste heat recovery opportunities are categorized by dividing temperature
ranges into low, medium, and high quality of waste heat sources as follows:
– High: [649ºC] and higher
– Medium: [232ºC] to [650ºC]
– Low: [232ºC] and lower
• Typical sources of low, medium, and high temperature waste heat are listed in
Table 4, along with related recovery advantages, barriers, and applicable
technologies.
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (7)
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (8)
2.1 Heat Exchanger Area Requirements :
• The temperature of waste heat influences the rate of heat transfer between a
heat source and heat sink, which significantly influences recovery feasibility.
• The expression for heat transfer can be generalized by the following equation:
• Where Q is the heat transfer rate; U is the heat transfer coefficient; A is the
surface area for heat exchange; and ΔT is the temperature difference between
two streams.
• Since heat transfer is a function of U, area, and ΔT, a small ΔT will require a larger
heat transfer.
• Figure 1 demonstrates the relative heat exchanger area required to transfer heat
from a hot gas at varying temperatures to liquid water. As shown, there is an
inflection point at lower temperatures where the required area for heat transfer
increases dramatically. The shape of the curve and the area required will vary
depending on the heat transfer fluids, heat transfer coefficient, and desired heat
transfer rate.
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (9)
2.2 Maximum Efficiency for Power Generation: Carnot Efficiency :
• Heat sources at different temperatures have varying theoretical efficiency limits
for power generation.
• Maximum efficiency at a given temperature is based on the Carnot efficiency,
which is defined as:
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (10)
Where TH is the waste heat temperature; and TL is the temperature of the heat sink.
• The Carnot efficiency represents the maximum possible efficiency of an engine at a given
temperature. The Carnot efficiency increases for higher temperatures and drops
dramatically for lower temperatures (Figure 2).
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (11)
• Since the temperature of waste heat has a dramatic impact on the feasibility of
heat recovery, it is important that an assessment of waste heat opportunities
considers both waste heat quantity and quality.
• In addition to analyze the quantity of waste heat lost from different processes,
the analysis of the work potential is also needed in order to account for variations
in waste heat temperatures.
• The work potential represents the maximum possible work that could be
extracted from a heat engine operating between the waste heat temperature and
ambient temperatures.
• This is calculated by multiplying the waste heat by the Carnot efficiency where
WP is the work potential of the heat source; E is the waste heat lost to the
environment; η is the Carnot efficiency; TH is the temperature of the waste heat
source; and TO is the ambient temperature, 77°F [25°C].
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (12)
2.3 Temperature and Material Selection :
• The temperature of the waste heat source also has important ramifications for
material selection in heat exchangers and recovery systems. Corrosion and
oxidation reactions, like all chemical reactions, are accelerated dramatically by
temperature increases.
• If the waste heat source contains corrosive substances, the heat recovery surfaces
can quickly become damaged. In addition, carbon steel at temperatures above
800°F [425ºC] and stainless steel above 1,200°F [650ºC] begins to oxidize.
• Therefore, advanced alloys or composite materials must be used at higher
temperatures.
• Metallic materials are usually not used at temperatures above 1,600°F [871ºC].
Alternatives include either bleeding dilution air into the exhaust gases to lower
the exhaust temperature, or using ceramic materials that can better withstand
the high temperature.
• In the case of air bleeding, the quantity of heat contained in the exhaust stream
remains constant, but the quality is reduced due to the temperature drop.
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (13)
3. Waste Stream Composition:
• Although chemical compositions do not directly influence the quality or quantity of the
available heat (unless it has some fuel value), the composition of the stream affects the
recovery process and material selection.
• The composition and phase of waste heat streams will determine factors such as
thermal conductivity and heat capacity, which will impact heat exchanger
effectiveness. Meanwhile, the process specific chemical makeup of off-gases will have
an important impact on heat exchanger designs, material constraints, and costs.
• Heat transfer rates in heat exchangers are dependent on the composition and phase of
waste heat streams, as well as influenced by the deposition of any fouling substances
on the heat exchanger. Denser fluids have higher heat transfer coefficients, which
enables higher heat transfer rates per unit area for a given temperature difference
(Table 5).
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (14)
• Another key consideration is the interaction between
chemicals in the exhaust stream and heat exchanger
materials.
• Fouling is a common problem in heat exchange, and
can substantially reduce heat exchanger effectiveness
or cause system failure.
• Figure-3 displays an abandoned recuperator
previously used in an aluminum melting furnace.
Deposition of substances on the heat exchanger
surface can reduce heat transfer rates as well as
inhibit fluid flow in the exchanger.
• In other cases, it will degrade the heat exchanger
such that it can no longer be used.
• Methods for addressing fouling are numerous and
include filtering contaminated streams, constructing
the exchanger with advanced materials, increasing
heat exchanger surface areas, and designing the heat
exchanger for easy access and cleaning.
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (15)
4. Minimum Allowable Temperature
• The minimum allowable temperature for waste streams is often closely connected
with material corrosion problems.
• Depending on the fuel used, combustion related flue gases contain varying
concentrations of carbon dioxide, water vapor, NOx, SOx, unoxidized organics, and
minerals.
• If exhaust gases are cooled below the dew point temperature, the water vapor in
the gas will condense and deposit corrosive substances on the heat exchanger
surface.
• Heat exchangers designed from low cost materials will quickly fail due to chemical
attack. Therefore, heat exchangers are generally designed to maintain exhaust
temperatures above the condensation point.
• The minimum temperature for preventing corrosion depends on the composition
of the fuel.
• For example, exhaust gases from natural gas might be cooled as low as [~120°C],
while exhaust gases from coal or fuel oils with higher sulfur contents may be
limited to [~150ºC] to [~175°C].
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (16)
• The most common method for preventing chemical corrosion is designing heat
exchangers with exhaust temperatures well above the dew point temperature.
• However, there are some cases where heat exchangers use advanced alloys and
composite materials to further recover low temperature heat.
• These systems have not seen much commercial application due to challenges such
as high material costs, large surface areas required for heat exchange, and lack of
an available end use for low temperature waste heat.
5. Economies of Scale, Accessibility, and Other Factors:
• Several additional factors can determine whether heat recovery is feasible in a
given application. For example, small scale operations are less likely to install heat
recovery, since sufficient capital may not be available, and because payback
periods may be longer.
• Operating schedules can also be a concern. If a waste heat source is only available
for a limited time every day, the heat exchanger may be exposed to both high and
low temperatures. In this case, one must ensure that the heat exchange material
does not fatigue due to thermal cycling.
Factors Affecting Waste Heat Recovery Feasibility
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (17)
• Additionally, it is important that the schedule for the heat source match the
schedule for the heat load. If not, additional systems may be required to provide
heat when the waste heat source is not available.
• Another concern is the ease of access to the waste heat source. In some cases, the
physical constraints created by equipment arrangements prevent easy access to
the heat source, or prevent the installation of any additional equipment for
recovering the heat.
• Additionally, constraints are presented by the transportability of heat streams. Hot
liquid streams in process industries are frequently recovered, since they are easily
transportable.
• Piping systems are easy to tap into and the energy can be easily transported via
piping to the recovery equipment.
• In contrast, hot solid streams (e.g., ingots, castings, cement clinkers) can contain
significant amounts of energy but their energy is not easily accessible or
transportable to recovery equipment. As a result, waste energy recovery is not
widely practiced with hot solid materials.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (18)
• Methods for waste heat recovery include transferring heat between gases and/or
liquids (e.g., combustion air preheating and boiler feedwater preheating),
transferring heat to the load entering furnaces (e.g., batch/cullet preheating in
glass furnaces), generating mechanical and/or electrical power, or using waste
heat with a heat pump for heating or cooling facilities.
1) Heat Exchangers :
• Heat exchangers are most commonly used to transfer heat from combustion
exhaust gases to combustion air entering the furnace.
• Since preheated combustion air enters the furnace at a higher temperature, less
energy must be supplied by the fuel.
• Typical technologies used for air preheating include recuperators, furnace
regenerators, burner regenerators, rotary regenerators, and passive air
preheaters.
1.1) Recuperator :
• Recuperators recover exhaust gas waste heat in medium to high temperature
applications such as soaking or annealing ovens, melting furnaces, afterburners,
gas incinerators, radiant tube burners, and reheat furnaces.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (19)
• Recuperators can be based on radiation, convection, or combinations:
• A simple radiation recuperator consists of two concentric lengths of ductwork, as
shown in Figure 4a. Hot waste gases pass through the inner duct and heat transfer
is primarily radiated to the wall and to the cold incoming air in the outer shell. The
preheated shell air then travels to the furnace burners.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (20)
• The convective or tube type recuperator, Figure 5a (heat exchanger) passes the
hot gases through relatively small diameter tubes contained in a larger shell. The
incoming combustion air enters the shell and is baffled around the tubes, picking
up heat from the waste gas.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (21)
• Another alternative is the combined
radiation/convection recuperator, shown in
Figure 4b and 5b.
• The system includes a radiation section followed
by a convection section in order to maximize
heat transfer effectiveness.
• Recuperators are constructed out of either
metallic or ceramic materials.
• Metallic recuperators are used in applications
with temperatures below [1,093ºC], while heat
recovery at higher temperatures is better suited
to ceramic tube recuperators.
• These can operate with hotside temperatures as
high as [1,538ºC] and coldside temperatures of
about [982ºC].
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (22)
1.2 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.
1) Furnace Regenerator:
• Regenerative furnaces consist of two brick
“checkerwork” chambers through which
hot and cold airflow alternately passes
(Figure 6).
• As combustion exhausts pass through one
chamber, the bricks absorb heat from the
combustion gas and increase in
temperature.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (23)
• The flow of air is then adjusted so that the
incoming combustion air passes through
the hot checkerwork, which transfers heat
to the combustion air entering the
furnace.
• Two chambers are used so that while one
is absorbing heat from the exhaust gases,
the other is transferring heat to the
combustion air.
• The direction of airflow is altered about
every 20 minutes.
• Regenerators are most frequently used
with glass furnaces and coke ovens, and
were historically used with steel
openhearth furnaces, before these
furnaces were replaced by more efficient
designs.
Figure 6 (a) Regenerative Furnace Diagram
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (24)
• They are also used to preheat the hot blast
provided to blast stoves used in
ironmaking; however, regenerators in blast
stoves are not a heat recovery application,
but simply the means by which heat
released from gas combustion is
transferred to the hot blast air.
• Regenerator systems are specially suited
for high temperature applications with
dirty exhausts.
• One major disadvantage is the large size
and capital costs, which are significantly
greater than costs of recuperators. (b) Checkerwork in Glass Regenerative
Furnace (Source: GS Energy & Environment,
2007)
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (25)
2) Rotary Regenerator/Heat Wheel :
• Rotary regenerators operate similar to
fixed regenerators in that heat transfer is
facilitated by storing heat in a porous
media, and by alternating the flow of hot
and cold gases through the regenerator.
• Rotary regenerators, sometimes referred
to as air preheaters and heat wheels, use
a rotating porous disc placed across two
parallel ducts, one containing the hot
waste gas, the other containing cold gas
(Figure 7).
• The disc, composed of a high heat
capacity material, rotates between the
two ducts and transfers heat from the
hot gas duct to the cold gas duct.
Figure 7 (a) Rotary Regenerator (Source:
PG&E, 1997),
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (26)
• Heat wheels are generally restricted to low
and medium temperature applications due
to the thermal stress created by high
temperatures.
• Large temperature differences between the
two ducts can lead to differential expansion
and large deformations, compromising the
integrity of ductwheel air seals.
• In some cases, ceramic wheels can be used
for higher temperature applications.
• Another challenge with heat wheels is
preventing cross contamination between
the two gas streams, as contaminants can
be transported in the wheel’s porous
material.
(b) Rotary Regenerator on a Melting
Furnace (Source: Jasper GmbH, 2007)
• One advantage of the heat wheel is that it can be designed to recover moisture as
well as heat from clean gas streams. When designed with hygroscopic materials,
moisture can be transferred from one duct to the other.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (27)
• This makes heat wheels particularly useful in air conditioning applications, where
incoming hot humid air transfers heat and moisture to cold outgoing air.
• Besides its main application in space heating and air conditioning systems, heat
wheels are also used to a limited extent in medium temperature applications.
• They have also been developed for high temperature furnace applications such as
aluminum furnaces, though they are not widely implemented in the United States
due to cost.
• They are also occasionally used for recovery from boiler exhausts, but more
economical recuperators and economizers are usually preferred.
 Case Study 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.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (28)
 Case Study Example
• A ceramic firm installed a heat wheel on the preheating zone of a tunnel kiln
where 7500 m3/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.
3) Passive Air Preheaters :
• Passive air preheaters are gas-to-gas heat
recovery devices for low-to-medium
temperature applications where cross
contamination between gas streams must be
prevented.
• Applications include ovens, steam boilers, gas
turbine exhaust, secondary recovery from
furnaces, and recovery from conditioned air.
• Passive preheaters can be of two types – the
platetype and heat pipe.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (29)
• 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:
• – Pasteurisation section in milk packaging plant.
• – Evaporation plants in food industry.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (30)
• The heat pipe heat exchanger consists of several pipes with sealed ends. Each pipe
contains a capillary wick structure that facilitates movement of the working fluid
between the hot and cold ends of the pipe.
• As shown in Figure 9 below, hot gases pass over one end of the heat pipe, causing
the working fluid inside the pipe to evaporate. Pressure gradients along the pipe
cause the hot vapor to move to the other end of the pipe, where the vapor
condenses and transfers heat to the cold gas.
• The condensate then cycles back to the hot side of the pipe via capillary action.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (31)
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (32)
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (33)
• 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.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (34)
 Finned Tube Heat Exchangers/Economizers :
• Finned tube heat exchangers are used to
recover heat from low to medium
temperature exhaust gases for heating
liquids.
• Applications include boiler feedwater
preheating, hot process liquids, hot water
for space heating, or domestic hot water.
• The finned tube consists of a round tube
with attached fins that maximize surface
area and heat transfer rates.
• Liquid flows through the tubes and receive
heat from hot gases flowing across the
tubes. Figure 10 illustrates a finned tube
exchanger where boiler exhaust gases are
used for feedwater preheating, a setup
commonly referred to as a boiler
“economizer”.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (35)
• In both the cases of economiser & air pre-
heater, there is a corresponding reduction
in the fuel requirements of the boiler.
• 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.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (36)
 Waste Heat Boilers:
• Waste heat boilers, such as the two-pass boiler shown
in Figure 11, are water tube boilers that use medium
to high temperature exhaust gases to generate steam.
• Waste heat boilers are available in a variety of
capacities, allowing for gas intakes from 1000 to 1
million ft3/min.
• In cases where the waste heat is not sufficient for
producing desired levels of steam, auxiliary burners or
an afterburner can be added to attain higher steam
output.
• The steam can be used for process heating or for
power generation.
• Generation of superheated steam will require addition
of an external superheater to the system.
• Waste heat boilers are built in capacities from 25 m3
almost 30,000 m3 / min. of exhaust gas.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (37)
• Typical applications of waste heat boilers are to recover energy from the exhausts
of gas turbines, reciprocating engines, incinerators, and furnaces.
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (38)
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (39)
Waste Heat Recovery Options and Technologies
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (40)
References
Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (41)
• “Waste Heat Recovery in Process Industries”, Hussam Jouhara, WILEY-VCH
• “WASTE HEAT RECOVERY: Principles and Industrial Applications”, Chirla Chandra
Sekhara Reddy, Gade Pandu, Rangaiah, World Scientific Publishing Co. Pte. Ltd.
• “Renewable and Waste-Heat Utilisation Technologies”, Nareshkumar B. Handagama,
Martin T. White, Paul Sapin, & Christos N. Markides, Cambridge University Press.
• “Energy Efficiency”, F. Kreith and R. E. West, CRC handbook, CRC Press,1999
• BEE India Books

More Related Content

PDF
FFR13.pdf
Riyashrivastava30
 
PDF
Waste Heat Recovery and Sustainable Energy
YOGESH AHIRE
 
PDF
Waste Heat Recovery and Sustainable Energy
YOGESH AHIRE
 
PDF
Waste Heat Recovery and Sustainable Energy
YOGESH AHIRE
 
PDF
IRJET- Comparsion of Heat Transfer Analysis of Double Pipe Heat Exchanger wit...
IRJET Journal
 
PDF
54b7f9370cf2c27adc47d1ea
ngothientu
 
PPTX
Thermal utilization (treatment) of plastic waste.
Om Prakash Rajak
 
PDF
IRJET- Experimental Evaluation of Shell & Tube Heat Exchanger with P – Toluid...
IRJET Journal
 
Waste Heat Recovery and Sustainable Energy
YOGESH AHIRE
 
Waste Heat Recovery and Sustainable Energy
YOGESH AHIRE
 
Waste Heat Recovery and Sustainable Energy
YOGESH AHIRE
 
IRJET- Comparsion of Heat Transfer Analysis of Double Pipe Heat Exchanger wit...
IRJET Journal
 
54b7f9370cf2c27adc47d1ea
ngothientu
 
Thermal utilization (treatment) of plastic waste.
Om Prakash Rajak
 
IRJET- Experimental Evaluation of Shell & Tube Heat Exchanger with P – Toluid...
IRJET Journal
 

Similar to Waste Heat Recovery and Sustainable Energy (20)

PDF
IRJET- Experimental Evaluation of Shell & Tube Heat Exchanger with P – Toluid...
IRJET Journal
 
PDF
Experimental Studies on Pool Boiling Heat Transfer Using Alumina and Graphene...
IRJET Journal
 
PDF
Performance Analysis of a Shell Tube Condenser for a Model Organic Rankine Cy...
IJERA Editor
 
PDF
Analysis of Double Pipe Heat Exchanger With Helical Fins
IRJET Journal
 
PDF
Study & Review of Heat Recovery Systems for SO2 Gas Generation Process in Sug...
IRJET Journal
 
PDF
REPORT-MSD
Helmy Rahman
 
PPTX
Heatreflex
Amjad Anvari-Moghaddam
 
PDF
Ip3415871592
IJERA Editor
 
PDF
Green fuel generation using waste heat exhausted from milk powder spray dryers
Otago Energy Research Centre (OERC)
 
PPTX
Introduction and 1st Laws of Thermodynamic - UNIT 3.pptx
MKMOHLALA
 
PDF
C1303071722
IOSR Journals
 
PPTX
Boiler performance (Part 2) - Boiler efficiency, Boiler trial and Heat balance
AVDHESH TYAGI
 
PDF
Flue gas low temperature heat recovery system for air conditioning
eSAT Journals
 
PDF
Research collaboration proposal
Vitalii Pertsevyi
 
PPTX
CH 515
hamzaabbas74
 
PDF
Experimentation of Heat Pipe Used In Nano-Fluids
paperpublications3
 
PDF
Energy and Exergy Analysis of a Cogeneration Cycle, Driven by Ocean Thermal E...
theijes
 
PPTX
Thermal analysis - TGA & DTA
Naresh Babu
 
PDF
Theoretical Analysis for Energy Consumption of a Circulation-Type Superheate...
IJMER
 
PDF
IRJET- Uncertainty Analysis of Flat Plate Oscillating Heat Pipe with Differen...
IRJET Journal
 
IRJET- Experimental Evaluation of Shell & Tube Heat Exchanger with P – Toluid...
IRJET Journal
 
Experimental Studies on Pool Boiling Heat Transfer Using Alumina and Graphene...
IRJET Journal
 
Performance Analysis of a Shell Tube Condenser for a Model Organic Rankine Cy...
IJERA Editor
 
Analysis of Double Pipe Heat Exchanger With Helical Fins
IRJET Journal
 
Study & Review of Heat Recovery Systems for SO2 Gas Generation Process in Sug...
IRJET Journal
 
REPORT-MSD
Helmy Rahman
 
Ip3415871592
IJERA Editor
 
Green fuel generation using waste heat exhausted from milk powder spray dryers
Otago Energy Research Centre (OERC)
 
Introduction and 1st Laws of Thermodynamic - UNIT 3.pptx
MKMOHLALA
 
C1303071722
IOSR Journals
 
Boiler performance (Part 2) - Boiler efficiency, Boiler trial and Heat balance
AVDHESH TYAGI
 
Flue gas low temperature heat recovery system for air conditioning
eSAT Journals
 
Research collaboration proposal
Vitalii Pertsevyi
 
CH 515
hamzaabbas74
 
Experimentation of Heat Pipe Used In Nano-Fluids
paperpublications3
 
Energy and Exergy Analysis of a Cogeneration Cycle, Driven by Ocean Thermal E...
theijes
 
Thermal analysis - TGA & DTA
Naresh Babu
 
Theoretical Analysis for Energy Consumption of a Circulation-Type Superheate...
IJMER
 
IRJET- Uncertainty Analysis of Flat Plate Oscillating Heat Pipe with Differen...
IRJET Journal
 
Ad

More from YOGESH AHIRE (11)

PDF
Refrigeration-and-air-conditioning_HVAC_ppt.pdf
YOGESH AHIRE
 
PPT
Unit-5_HVAC: Thermal Design of Refrigeration System Components
YOGESH AHIRE
 
PDF
UNIT-1_HVAC: Advanced Vapour Compression Cycles
YOGESH AHIRE
 
PDF
UNIT-1_NOTES-PKNAG-SPATI.pdf
YOGESH AHIRE
 
PDF
UNIT-2_Part1_NOTES-PKNAG-SPATI.pdf
YOGESH AHIRE
 
PDF
UNIT-2_Part3_RANKINE CYCLE.pdf
YOGESH AHIRE
 
PDF
Fluid Mechanics (Unit6) - External Flows by Prof. Y. H. Ahire
YOGESH AHIRE
 
PDF
Fluid Mechanics (Unit5) - Flow Through Pipes, by Prof. Y. H. Ahire
YOGESH AHIRE
 
PDF
Fluid Mechanics (Unit4) - Internal Flows by Prof. Y. H. Ahire
YOGESH AHIRE
 
PPT
FLUID MECHANICS - COMPUTATIONAL FLUID DYNAMICS (CFD)
YOGESH AHIRE
 
PPT
Pumps and pumping systems
YOGESH AHIRE
 
Refrigeration-and-air-conditioning_HVAC_ppt.pdf
YOGESH AHIRE
 
Unit-5_HVAC: Thermal Design of Refrigeration System Components
YOGESH AHIRE
 
UNIT-1_HVAC: Advanced Vapour Compression Cycles
YOGESH AHIRE
 
UNIT-1_NOTES-PKNAG-SPATI.pdf
YOGESH AHIRE
 
UNIT-2_Part1_NOTES-PKNAG-SPATI.pdf
YOGESH AHIRE
 
UNIT-2_Part3_RANKINE CYCLE.pdf
YOGESH AHIRE
 
Fluid Mechanics (Unit6) - External Flows by Prof. Y. H. Ahire
YOGESH AHIRE
 
Fluid Mechanics (Unit5) - Flow Through Pipes, by Prof. Y. H. Ahire
YOGESH AHIRE
 
Fluid Mechanics (Unit4) - Internal Flows by Prof. Y. H. Ahire
YOGESH AHIRE
 
FLUID MECHANICS - COMPUTATIONAL FLUID DYNAMICS (CFD)
YOGESH AHIRE
 
Pumps and pumping systems
YOGESH AHIRE
 
Ad

Recently uploaded (20)

PPTX
Civil Engineering Practices_BY Sh.JP Mishra 23.09.pptx
bineetmishra1990
 
PPT
1. SYSTEMS, ROLES, AND DEVELOPMENT METHODOLOGIES.ppt
zilow058
 
PDF
settlement FOR FOUNDATION ENGINEERS.pdf
Endalkazene
 
PDF
top-5-use-cases-for-splunk-security-analytics.pdf
yaghutialireza
 
PDF
The Effect of Artifact Removal from EEG Signals on the Detection of Epileptic...
Partho Prosad
 
PPTX
MT Chapter 1.pptx- Magnetic particle testing
ABCAnyBodyCanRelax
 
PDF
Advanced LangChain & RAG: Building a Financial AI Assistant with Real-Time Data
Soufiane Sejjari
 
PDF
FLEX-LNG-Company-Presentation-Nov-2017.pdf
jbloggzs
 
PDF
LEAP-1B presedntation xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
hatem173148
 
PPTX
22PCOAM21 Session 1 Data Management.pptx
Guru Nanak Technical Institutions
 
PPTX
IoT_Smart_Agriculture_Presentations.pptx
poojakumari696707
 
PPTX
MSME 4.0 Template idea hackathon pdf to understand
alaudeenaarish
 
PDF
EVS+PRESENTATIONS EVS+PRESENTATIONS like
saiyedaqib429
 
PPTX
Color Model in Textile ( RGB, CMYK).pptx
auladhossain191
 
PDF
July 2025: Top 10 Read Articles Advanced Information Technology
ijait
 
PDF
2010_Book_EnvironmentalBioengineering (1).pdf
EmilianoRodriguezTll
 
PDF
Unit I Part II.pdf : Security Fundamentals
Dr. Madhuri Jawale
 
PDF
Top 10 read articles In Managing Information Technology.pdf
IJMIT JOURNAL
 
PPTX
22PCOAM21 Session 2 Understanding Data Source.pptx
Guru Nanak Technical Institutions
 
PDF
Introduction to Data Science: data science process
ShivarkarSandip
 
Civil Engineering Practices_BY Sh.JP Mishra 23.09.pptx
bineetmishra1990
 
1. SYSTEMS, ROLES, AND DEVELOPMENT METHODOLOGIES.ppt
zilow058
 
settlement FOR FOUNDATION ENGINEERS.pdf
Endalkazene
 
top-5-use-cases-for-splunk-security-analytics.pdf
yaghutialireza
 
The Effect of Artifact Removal from EEG Signals on the Detection of Epileptic...
Partho Prosad
 
MT Chapter 1.pptx- Magnetic particle testing
ABCAnyBodyCanRelax
 
Advanced LangChain & RAG: Building a Financial AI Assistant with Real-Time Data
Soufiane Sejjari
 
FLEX-LNG-Company-Presentation-Nov-2017.pdf
jbloggzs
 
LEAP-1B presedntation xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
hatem173148
 
22PCOAM21 Session 1 Data Management.pptx
Guru Nanak Technical Institutions
 
IoT_Smart_Agriculture_Presentations.pptx
poojakumari696707
 
MSME 4.0 Template idea hackathon pdf to understand
alaudeenaarish
 
EVS+PRESENTATIONS EVS+PRESENTATIONS like
saiyedaqib429
 
Color Model in Textile ( RGB, CMYK).pptx
auladhossain191
 
July 2025: Top 10 Read Articles Advanced Information Technology
ijait
 
2010_Book_EnvironmentalBioengineering (1).pdf
EmilianoRodriguezTll
 
Unit I Part II.pdf : Security Fundamentals
Dr. Madhuri Jawale
 
Top 10 read articles In Managing Information Technology.pdf
IJMIT JOURNAL
 
22PCOAM21 Session 2 Understanding Data Source.pptx
Guru Nanak Technical Institutions
 
Introduction to Data Science: data science process
ShivarkarSandip
 

Waste Heat Recovery and Sustainable Energy

  • 1. Sanjivani Rural Education Society’s Sanjivani College of Engineering, Kopargaon-423603 (An Autonomous Institute Affiliated to Savitribai Phule Pune University) NAAC ‘A’ Grade Accredited, ISO 9001:2015 Certified Unit-3: WASTE HEAT RECOVERY SYSTEMS Subject :- Waste Heat Recovery and Sustainable Energy (ME 305A) T.Y. B. Tech.(Mechanical) By Yogesh H. Ahire (Asst. Professor) SRES’s Sanjivani COE, Kopargaon-423603, Maharashtra, India Email: [email protected] [email protected] Mobile:- 9881290264 Department of Mechanical Engineering
  • 2. Contents of the Course-WHSE Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (2) COs Course Outcomes Blooms Taxonomy Level Descriptor CO3 Analyze the waste heat recovery technologies developed for various thermal systems. . 2 understand Unit CONTENTS No. of Hours COs 3 WASTE HEAT RECOVERY SYSTEMS Types of heat recovery systems (recuperators-regenerators- economizers-plate heat exchangers-thermic fluid heaters, waste heat boilers), Selection criteria for waste heat recovery technologies, location, service conditions, design considerations. 7 Hrs. CO3
  • 3. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (3) • Evaluating the feasibility of waste heat recovery requires characterizing the waste heat source and the stream to which the heat will be transferred. • Important waste stream parameters that must be determined include: – heat quantity, – heat temperature/quality, – composition, – minimum allowed temperature, and – operating schedules, availability, and other logistics • These parameters allow for analysis of the quality and quantity of the stream and also provide insight into possible materials/design limitations. • For example, corrosion of heat transfer media is of considerable concern in waste heat recovery, even when the quality and quantity of the stream is acceptable. • The following provide an overview of important concepts that determine waste heat recovery feasibility.
  • 4. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (4) 1. Heat Quantity : • The quantity, or heat content, is a measure of how much energy is contained in a waste heat stream, while quality is a measure of the usefulness of the waste heat. • The quantity of waste heat contained in a waste stream is a function of both the temperature and the mass flow rate of the stream: • Where, – E is the waste heat loss (kJ/hr); – m is the waste stream mass flow rate (kg/hr); – and h(t) is the waste stream specific enthalpy (kJ/kg) as a function of temperature. • Enthalpy is not an absolute term, but must be measured against a reference state (for example, the enthalpy of a substance at room temperature and atmospheric pressure).
  • 5. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (5) • Although the quantity of waste heat available is an important parameter, it is not alone an effective measure of waste heat recovery opportunity. It is also important to specify the waste heat quality, as determined by its temperature. 2. Waste Heat Temperature/Quality: • The waste heat temperature is a key factor determining waste heat recovery feasibility. • Waste heat temperatures can vary significantly, with cooling water returns having low temperatures around [40 - 90°C] and glass melting furnaces having flue temperatures above [1,320°C]. • In order to enable heat transfer and recovery, it is necessary that the waste heat source temperature is higher than the heat sink temperature. • Moreover, the magnitude of the temperature difference between the heat source and sink is an important determinant of waste heat’s utility or • The source and sink temperature difference influences a) the rate at which heat is transferred per unit surface area of heat exchanger, and
  • 6. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (6) b) the maximum theoretical efficiency of converting thermal from the heat source to another form of energy (i.e., mechanical or electrical). • Finally, the temperature range has important ramifications for the selection of materials in heat exchanger designs • Waste heat recovery opportunities are categorized by dividing temperature ranges into low, medium, and high quality of waste heat sources as follows: – High: [649ºC] and higher – Medium: [232ºC] to [650ºC] – Low: [232ºC] and lower • Typical sources of low, medium, and high temperature waste heat are listed in Table 4, along with related recovery advantages, barriers, and applicable technologies.
  • 7. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (7)
  • 8. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (8) 2.1 Heat Exchanger Area Requirements : • The temperature of waste heat influences the rate of heat transfer between a heat source and heat sink, which significantly influences recovery feasibility. • The expression for heat transfer can be generalized by the following equation: • Where Q is the heat transfer rate; U is the heat transfer coefficient; A is the surface area for heat exchange; and ΔT is the temperature difference between two streams. • Since heat transfer is a function of U, area, and ΔT, a small ΔT will require a larger heat transfer. • Figure 1 demonstrates the relative heat exchanger area required to transfer heat from a hot gas at varying temperatures to liquid water. As shown, there is an inflection point at lower temperatures where the required area for heat transfer increases dramatically. The shape of the curve and the area required will vary depending on the heat transfer fluids, heat transfer coefficient, and desired heat transfer rate.
  • 9. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (9) 2.2 Maximum Efficiency for Power Generation: Carnot Efficiency : • Heat sources at different temperatures have varying theoretical efficiency limits for power generation. • Maximum efficiency at a given temperature is based on the Carnot efficiency, which is defined as:
  • 10. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (10) Where TH is the waste heat temperature; and TL is the temperature of the heat sink. • The Carnot efficiency represents the maximum possible efficiency of an engine at a given temperature. The Carnot efficiency increases for higher temperatures and drops dramatically for lower temperatures (Figure 2).
  • 11. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (11) • Since the temperature of waste heat has a dramatic impact on the feasibility of heat recovery, it is important that an assessment of waste heat opportunities considers both waste heat quantity and quality. • In addition to analyze the quantity of waste heat lost from different processes, the analysis of the work potential is also needed in order to account for variations in waste heat temperatures. • The work potential represents the maximum possible work that could be extracted from a heat engine operating between the waste heat temperature and ambient temperatures. • This is calculated by multiplying the waste heat by the Carnot efficiency where WP is the work potential of the heat source; E is the waste heat lost to the environment; η is the Carnot efficiency; TH is the temperature of the waste heat source; and TO is the ambient temperature, 77°F [25°C].
  • 12. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (12) 2.3 Temperature and Material Selection : • The temperature of the waste heat source also has important ramifications for material selection in heat exchangers and recovery systems. Corrosion and oxidation reactions, like all chemical reactions, are accelerated dramatically by temperature increases. • If the waste heat source contains corrosive substances, the heat recovery surfaces can quickly become damaged. In addition, carbon steel at temperatures above 800°F [425ºC] and stainless steel above 1,200°F [650ºC] begins to oxidize. • Therefore, advanced alloys or composite materials must be used at higher temperatures. • Metallic materials are usually not used at temperatures above 1,600°F [871ºC]. Alternatives include either bleeding dilution air into the exhaust gases to lower the exhaust temperature, or using ceramic materials that can better withstand the high temperature. • In the case of air bleeding, the quantity of heat contained in the exhaust stream remains constant, but the quality is reduced due to the temperature drop.
  • 13. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (13) 3. Waste Stream Composition: • Although chemical compositions do not directly influence the quality or quantity of the available heat (unless it has some fuel value), the composition of the stream affects the recovery process and material selection. • The composition and phase of waste heat streams will determine factors such as thermal conductivity and heat capacity, which will impact heat exchanger effectiveness. Meanwhile, the process specific chemical makeup of off-gases will have an important impact on heat exchanger designs, material constraints, and costs. • Heat transfer rates in heat exchangers are dependent on the composition and phase of waste heat streams, as well as influenced by the deposition of any fouling substances on the heat exchanger. Denser fluids have higher heat transfer coefficients, which enables higher heat transfer rates per unit area for a given temperature difference (Table 5).
  • 14. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (14) • Another key consideration is the interaction between chemicals in the exhaust stream and heat exchanger materials. • Fouling is a common problem in heat exchange, and can substantially reduce heat exchanger effectiveness or cause system failure. • Figure-3 displays an abandoned recuperator previously used in an aluminum melting furnace. Deposition of substances on the heat exchanger surface can reduce heat transfer rates as well as inhibit fluid flow in the exchanger. • In other cases, it will degrade the heat exchanger such that it can no longer be used. • Methods for addressing fouling are numerous and include filtering contaminated streams, constructing the exchanger with advanced materials, increasing heat exchanger surface areas, and designing the heat exchanger for easy access and cleaning.
  • 15. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (15) 4. Minimum Allowable Temperature • The minimum allowable temperature for waste streams is often closely connected with material corrosion problems. • Depending on the fuel used, combustion related flue gases contain varying concentrations of carbon dioxide, water vapor, NOx, SOx, unoxidized organics, and minerals. • If exhaust gases are cooled below the dew point temperature, the water vapor in the gas will condense and deposit corrosive substances on the heat exchanger surface. • Heat exchangers designed from low cost materials will quickly fail due to chemical attack. Therefore, heat exchangers are generally designed to maintain exhaust temperatures above the condensation point. • The minimum temperature for preventing corrosion depends on the composition of the fuel. • For example, exhaust gases from natural gas might be cooled as low as [~120°C], while exhaust gases from coal or fuel oils with higher sulfur contents may be limited to [~150ºC] to [~175°C].
  • 16. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (16) • The most common method for preventing chemical corrosion is designing heat exchangers with exhaust temperatures well above the dew point temperature. • However, there are some cases where heat exchangers use advanced alloys and composite materials to further recover low temperature heat. • These systems have not seen much commercial application due to challenges such as high material costs, large surface areas required for heat exchange, and lack of an available end use for low temperature waste heat. 5. Economies of Scale, Accessibility, and Other Factors: • Several additional factors can determine whether heat recovery is feasible in a given application. For example, small scale operations are less likely to install heat recovery, since sufficient capital may not be available, and because payback periods may be longer. • Operating schedules can also be a concern. If a waste heat source is only available for a limited time every day, the heat exchanger may be exposed to both high and low temperatures. In this case, one must ensure that the heat exchange material does not fatigue due to thermal cycling.
  • 17. Factors Affecting Waste Heat Recovery Feasibility Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (17) • Additionally, it is important that the schedule for the heat source match the schedule for the heat load. If not, additional systems may be required to provide heat when the waste heat source is not available. • Another concern is the ease of access to the waste heat source. In some cases, the physical constraints created by equipment arrangements prevent easy access to the heat source, or prevent the installation of any additional equipment for recovering the heat. • Additionally, constraints are presented by the transportability of heat streams. Hot liquid streams in process industries are frequently recovered, since they are easily transportable. • Piping systems are easy to tap into and the energy can be easily transported via piping to the recovery equipment. • In contrast, hot solid streams (e.g., ingots, castings, cement clinkers) can contain significant amounts of energy but their energy is not easily accessible or transportable to recovery equipment. As a result, waste energy recovery is not widely practiced with hot solid materials.
  • 18. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (18) • Methods for waste heat recovery include transferring heat between gases and/or liquids (e.g., combustion air preheating and boiler feedwater preheating), transferring heat to the load entering furnaces (e.g., batch/cullet preheating in glass furnaces), generating mechanical and/or electrical power, or using waste heat with a heat pump for heating or cooling facilities. 1) Heat Exchangers : • Heat exchangers are most commonly used to transfer heat from combustion exhaust gases to combustion air entering the furnace. • Since preheated combustion air enters the furnace at a higher temperature, less energy must be supplied by the fuel. • Typical technologies used for air preheating include recuperators, furnace regenerators, burner regenerators, rotary regenerators, and passive air preheaters. 1.1) Recuperator : • Recuperators recover exhaust gas waste heat in medium to high temperature applications such as soaking or annealing ovens, melting furnaces, afterburners, gas incinerators, radiant tube burners, and reheat furnaces.
  • 19. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (19) • Recuperators can be based on radiation, convection, or combinations: • A simple radiation recuperator consists of two concentric lengths of ductwork, as shown in Figure 4a. Hot waste gases pass through the inner duct and heat transfer is primarily radiated to the wall and to the cold incoming air in the outer shell. The preheated shell air then travels to the furnace burners.
  • 20. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (20) • The convective or tube type recuperator, Figure 5a (heat exchanger) passes the hot gases through relatively small diameter tubes contained in a larger shell. The incoming combustion air enters the shell and is baffled around the tubes, picking up heat from the waste gas.
  • 21. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (21) • Another alternative is the combined radiation/convection recuperator, shown in Figure 4b and 5b. • The system includes a radiation section followed by a convection section in order to maximize heat transfer effectiveness. • Recuperators are constructed out of either metallic or ceramic materials. • Metallic recuperators are used in applications with temperatures below [1,093ºC], while heat recovery at higher temperatures is better suited to ceramic tube recuperators. • These can operate with hotside temperatures as high as [1,538ºC] and coldside temperatures of about [982ºC].
  • 22. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (22) 1.2 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. 1) Furnace Regenerator: • Regenerative furnaces consist of two brick “checkerwork” chambers through which hot and cold airflow alternately passes (Figure 6). • As combustion exhausts pass through one chamber, the bricks absorb heat from the combustion gas and increase in temperature.
  • 23. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (23) • The flow of air is then adjusted so that the incoming combustion air passes through the hot checkerwork, which transfers heat to the combustion air entering the furnace. • Two chambers are used so that while one is absorbing heat from the exhaust gases, the other is transferring heat to the combustion air. • The direction of airflow is altered about every 20 minutes. • Regenerators are most frequently used with glass furnaces and coke ovens, and were historically used with steel openhearth furnaces, before these furnaces were replaced by more efficient designs. Figure 6 (a) Regenerative Furnace Diagram
  • 24. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (24) • They are also used to preheat the hot blast provided to blast stoves used in ironmaking; however, regenerators in blast stoves are not a heat recovery application, but simply the means by which heat released from gas combustion is transferred to the hot blast air. • Regenerator systems are specially suited for high temperature applications with dirty exhausts. • One major disadvantage is the large size and capital costs, which are significantly greater than costs of recuperators. (b) Checkerwork in Glass Regenerative Furnace (Source: GS Energy & Environment, 2007)
  • 25. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (25) 2) Rotary Regenerator/Heat Wheel : • Rotary regenerators operate similar to fixed regenerators in that heat transfer is facilitated by storing heat in a porous media, and by alternating the flow of hot and cold gases through the regenerator. • Rotary regenerators, sometimes referred to as air preheaters and heat wheels, use a rotating porous disc placed across two parallel ducts, one containing the hot waste gas, the other containing cold gas (Figure 7). • The disc, composed of a high heat capacity material, rotates between the two ducts and transfers heat from the hot gas duct to the cold gas duct. Figure 7 (a) Rotary Regenerator (Source: PG&E, 1997),
  • 26. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (26) • Heat wheels are generally restricted to low and medium temperature applications due to the thermal stress created by high temperatures. • Large temperature differences between the two ducts can lead to differential expansion and large deformations, compromising the integrity of ductwheel air seals. • In some cases, ceramic wheels can be used for higher temperature applications. • Another challenge with heat wheels is preventing cross contamination between the two gas streams, as contaminants can be transported in the wheel’s porous material. (b) Rotary Regenerator on a Melting Furnace (Source: Jasper GmbH, 2007) • One advantage of the heat wheel is that it can be designed to recover moisture as well as heat from clean gas streams. When designed with hygroscopic materials, moisture can be transferred from one duct to the other.
  • 27. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (27) • This makes heat wheels particularly useful in air conditioning applications, where incoming hot humid air transfers heat and moisture to cold outgoing air. • Besides its main application in space heating and air conditioning systems, heat wheels are also used to a limited extent in medium temperature applications. • They have also been developed for high temperature furnace applications such as aluminum furnaces, though they are not widely implemented in the United States due to cost. • They are also occasionally used for recovery from boiler exhausts, but more economical recuperators and economizers are usually preferred.  Case Study 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.
  • 28. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (28)  Case Study Example • A ceramic firm installed a heat wheel on the preheating zone of a tunnel kiln where 7500 m3/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. 3) Passive Air Preheaters : • Passive air preheaters are gas-to-gas heat recovery devices for low-to-medium temperature applications where cross contamination between gas streams must be prevented. • Applications include ovens, steam boilers, gas turbine exhaust, secondary recovery from furnaces, and recovery from conditioned air. • Passive preheaters can be of two types – the platetype and heat pipe.
  • 29. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (29) • 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: • – Pasteurisation section in milk packaging plant. • – Evaporation plants in food industry.
  • 30. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (30) • The heat pipe heat exchanger consists of several pipes with sealed ends. Each pipe contains a capillary wick structure that facilitates movement of the working fluid between the hot and cold ends of the pipe. • As shown in Figure 9 below, hot gases pass over one end of the heat pipe, causing the working fluid inside the pipe to evaporate. Pressure gradients along the pipe cause the hot vapor to move to the other end of the pipe, where the vapor condenses and transfers heat to the cold gas. • The condensate then cycles back to the hot side of the pipe via capillary action.
  • 31. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (31)
  • 32. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (32)
  • 33. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (33) • 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.
  • 34. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (34)  Finned Tube Heat Exchangers/Economizers : • Finned tube heat exchangers are used to recover heat from low to medium temperature exhaust gases for heating liquids. • Applications include boiler feedwater preheating, hot process liquids, hot water for space heating, or domestic hot water. • The finned tube consists of a round tube with attached fins that maximize surface area and heat transfer rates. • Liquid flows through the tubes and receive heat from hot gases flowing across the tubes. Figure 10 illustrates a finned tube exchanger where boiler exhaust gases are used for feedwater preheating, a setup commonly referred to as a boiler “economizer”.
  • 35. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (35) • In both the cases of economiser & air pre- heater, there is a corresponding reduction in the fuel requirements of the boiler. • 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.
  • 36. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (36)  Waste Heat Boilers: • Waste heat boilers, such as the two-pass boiler shown in Figure 11, are water tube boilers that use medium to high temperature exhaust gases to generate steam. • Waste heat boilers are available in a variety of capacities, allowing for gas intakes from 1000 to 1 million ft3/min. • In cases where the waste heat is not sufficient for producing desired levels of steam, auxiliary burners or an afterburner can be added to attain higher steam output. • The steam can be used for process heating or for power generation. • Generation of superheated steam will require addition of an external superheater to the system. • Waste heat boilers are built in capacities from 25 m3 almost 30,000 m3 / min. of exhaust gas.
  • 37. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (37) • Typical applications of waste heat boilers are to recover energy from the exhausts of gas turbines, reciprocating engines, incinerators, and furnaces.
  • 38. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (38)
  • 39. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (39)
  • 40. Waste Heat Recovery Options and Technologies Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (40)
  • 41. References Prof. Y. H. Ahire Department of Mechanical Engineering Sanjivani College of Engineering, Kopargaon (41) • “Waste Heat Recovery in Process Industries”, Hussam Jouhara, WILEY-VCH • “WASTE HEAT RECOVERY: Principles and Industrial Applications”, Chirla Chandra Sekhara Reddy, Gade Pandu, Rangaiah, World Scientific Publishing Co. Pte. Ltd. • “Renewable and Waste-Heat Utilisation Technologies”, Nareshkumar B. Handagama, Martin T. White, Paul Sapin, & Christos N. Markides, Cambridge University Press. • “Energy Efficiency”, F. Kreith and R. E. West, CRC handbook, CRC Press,1999 • BEE India Books