Waste Heat Recovery Devices by Varun Pratap Singh

Waste Heat Recovery Devices
by
Varun Pratap Singh

CONTENT
• Availability and Irreversibility
• Availability Function
• Second Law Efficiencies
• Work Potential Associated with Internal Energy
• Waste Heat Recovery
• Heat Losses – Quality vs. Quantity
• Principle of Heat Recovery Units
• Classification of WHRS on Temperature Range Bases
• Commercial Viable Waste Heat Recovery Devices
• Benefits of Waste Heat Recovery
• Development of a Waste Heat Recovery System
• Commercial Waste Heat Recovery Devices
• West Heat Recovery Boiler (WHRB)
• Recuperators- Regenerative, Ceramic, Regenerative Heat Exchanger
• Thermal wheel/ Heat Wheel
• Heat Pipe
• Economiser
• Feed Water
• Heat Pump
• Shell and Tube Heat Exchanger
• Plate Heat Exchanger
• Run-around coil
• Direct Contact Heat Exchanger
• Advantages and Limitations of WHRD’s

Availability and Irreversibility
Available and Unavailable Energy
The second law of thermodynamics tells us that it is not possible to convert all the heat absorbed by a
system into work.
Suppose a certain quantity of energy Q as heat can be received from a body at temperature T.
The maximum work can be obtained by operating a Carnot engine (reversible engine) using the body at T
as the source and the ambient atmosphere at T0 as the sink.
Where Ds is the entropy of the body supplying the energy as heat.
The Carnot cycle and the available energy is shown in figure.
The area 1-2-3-4 represents the available energy.

Suppose a finite body is used as a source. Let a
large number of differential Carnot engines be
used with the given body as the source.
If the initial and final temperatures of the
source are T1 and T2respectively, the total
work done or the available energy is given by
The shaded area 4-3-B-A represents the energy, which is discarded to the ambient
atmosphere, and this quantity of energy cannot be converted into work and is
called Unavailable energy.

Loss in Available Energy
Suppose a certain quantity of energy Q is transferred from a body at constant temperature T1 to
another body at constant temperature T2 (T2<T1).
Initial available energy, with the body at T1,
Final available energy, with the body at T2,
Loss in available energy
where Dsuni is the change in the entropy of the universe.

Availability Function
The availability of a given system is defined as the maximum useful work that can be obtained
in a process in which the system comes to equilibrium with the surroundings or attains the
dead state.
(a) Availability Function for Non-Flow process:-
Let P0 be the ambient pressure, V1 and V0 be the initial and final volumes of the system
respectively.
If in a process, the system comes into equilibrium with the surroundings, the work done in
pushing back the ambient atmosphere is P0(V0-V1).
Availability= Wuseful=Wmax-P0(V0-V1)
Consider a system which interacts with the ambient at T0. Then,
Wmax=(U1-U0)-T0(S1-S0)
Availability= Wuseful=Wmax-P0(V0-V1)
= ( U1-T0 S1)- ( U0-T0 S0)- P0(V0-V1)
= ( U1+ P0V1-T0 S1)- ( U0+P0V0-T0 S0)
= f1-f0
where f=U+P0V-T0S is called the availability function for the non-flow process. Thus, the
availability: f1-f0.
If a system undergoes a change of state from the initial state 1 (where the availability is (f1-f0)
to the final state 2 (where the availability is (f2-f0), the change in the availability or the change
in maximum useful work associated with the process, is f1-f2.

(b) Availability Function for Flow process:-
The maximum power that can be obtained in a steady flow process while the control volume
exchanges energy as heat with the ambient at T0, is given by:
Sometimes the availability for a flow process is written as:
which is called the Darrieus Function.
Availability Function

Second Law Efficiencies
The second law efficiency (h2) of a process, h2=Change in the available energy of the
system Change in the available energy of the source
(a) Compressors and Pumps:-
Change in the availability of the system is given by:
where T0 is the ambient temperature
The second law efficiency of a compressor or pump is
given by,

(b) Turbines and Expanders:-
The change in the available energy of the system=W
The change in the available energy of the source=Wrev=B1-B2
The second law efficiency of the turbine h2T/E is given by,
Second Law Efficiencies

Work Potential Associated with Internal Energy
The total useful work delivered as the system undergoes a reversible process from the given
state to the dead state (that is when a system is in thermodynamic equilibrium with the
environment), which is Work potential by definition.
Work Potential = Wuseful= Wmax- P0(V0-V1)
= ( U1-T0 S1)- ( U0-T0 S0)- P0(V0-V1)
= ( U1+ P0V1-T0 S1)- ( U0+P0V0-T0 S0)
= f1-f0
The work potential of internal energy (or a closed system) is either positive or zero. It is
never negative.

Work Potential Associated with Enthalpy, h
The work potential associated with enthalpy is simply the sum of the energies of its
components.
The useful work potential of Enthalpy can be expressed on a unit mass basis as:
here h0 and s0 are the enthalpy and entropy of the fluid at the dead state. The work potential of
enthalpy can be negative at sub atmospheric pressures.

Waste Heat Recovery
• Introduction
A waste heat recovery unit (WHRU) isan energy recovery heat exchanger that transfers heat from
process outputs at high temperature to another part of the process for some purpose, usually increased
efficiency. The WHRU is a tool involved in cogeneration. Waste heat may be extracted from sources
such as hot flue gases from a diesel generator, steam from cooling towers, or even waste water from
cooling processes such as in steel cooling.
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 vs. Quantity
• 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:
• Heat recovery from heat treatment furnace
In a heat treatment furnace, the exhaust gases are leaving the furnace at 900 °C at the rate ofv2100
m3/hour. The total heat recoverable at 180oC final exhaust can be calculated as
Q = V × ρ × Cp × ∆T ; where Q is the heat content in kCal and V is the flowrate of the substance in m3/hr
ρ is density of the flue gas in kg/m3, Cvp is the specific heat of the substance in kCal/kg °C
∆T is the temperature difference in °C and Cp (Specific heat of flue gas) = 0.24 kCal/kg/°C
Heat available (Q) = 2100 × 1.19 × 0.24 × ((900-180) = 4,31,827 kCal/hr
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

Principle of Heat Recovery Units
• Waste heat found in the exhaust gas of various processes or even from the exhaust stream of
a conditioning unit can be used to preheat the incoming gas. This is one of the basic methods for
recovery of waste heat. Many steel making plants use this process as an economic method to
increase the production of the plant with lower fuel demand.

Classification of WHRS on Temperature Range
Bases
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.

Classification of WHRS on Temperature Range
Bases
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.

• There are many different commercial recovery units for the transferring of energy
from hot medium space to lower one:
• Recuperators: This name is given to different types of heat exchanger that the exhaust
gases are passed through, consisting of metal tubes that carry the inlet gas and thus
preheating the gas before entering the process. The heat wheel is an example which
operates on the same principle as a solar air conditioning unit.
• Regenerators: This is an industrial unit that reuses the same stream after processing.
In this type of heat recovery, the heat is regenerated and reused in the process.
• Heat pipe exchanger: Heat pipes are one of the best thermal conductors. They have
the ability to transfer heat hundred times more than copper. Heat pipes are mainly
known in renewable energy technology as being used in evacuated tube collectors. The
heat pipe is mainly used in space, process or air heating, in waste heat from a process
is being transferred to the surrounding due to its transfer mechanism.
• Thermal Wheel or rotary heat exchanger: consists of a circular honeycomb matrix
of heat absorbing material, which is slowly rotated within the supply and exhaust air
streams of an air handling system.
• Economizer: In case of process boilers, waste heat in the exhaust gas is passed along a
recuperators that carries the inlet fluid for the boiler and thus decreases thermal energy
intake of the inlet fluid.
Classification of WHRS on basis of Type of
Equipment's

• Heat pumps: Using an organic fluid that boils at a low temperature means that energy
could be regenerated from waste fluids.
• Run around coil: comprises two or more multi-row finned tube coils connected to each
other by a pumped pipework circuit.
Classification of WHRS on basis of Type of
Equipment's

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 equipment's 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 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
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 equipment's etc.
After identifying source of waste heat and the possible use of it, the next step is to select
suitable heat recovery system and equipment's 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
1. Waste Heat Recovery Boilers
2. Recuperators
a) Radiation/Convective Hybrid Recuperator:
b) Ceramic Recuperator
3. Regenerators
4. Heat pipe exchanger
5. Thermal Wheel
6. Economizer
a) Shell and Tube Heat Exchanger
b) Plate heat exchanger
7. Heat pumps
8. Heat Wheels
9. Heat Pipe
10. Run Around Coil Exchanger
11. Thermo-compressor
12. Direct Contact Heat Exchanger
a) Hot-Hot mixing
b) Different Phase mixing
c) Hot-Cold Mixing

waste heat recovery boiler (WHRB)
West Heat Recovery Boiler (WHRB)

Fig: Schematic Diagram of WHRB system in Combine Cycle Power Plant

• 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 steam
generated above the water surface. The pressure at which the steam is generated and the rate of
steam production depends on the temperature of waste heat. The pressure of a pure vapour 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 m3 almost
30,000 m3 / min. of exhaust gas.

Recuperators
• Recuperators is a special purpose counter-flow energy recovery heat exchanger positioned within the supply and
exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover
the waste heat. Generally, they are used to extract heat from the exhaust and use it to preheat air entering the combustion
system. In this way they use waste energy to heat the air, offsetting some of the fuel, and thereby improves the energy
efficiency of the system as a whole.
• 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, 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 counter flow). 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.
Radiative Recuperators

• 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.
Radiative Recuperators

• 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
Radiation/Convective Hybrid
Recuperators

• 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.
•
Ceramic Recuperators

Recuperators can be used to increase the efficiency of gas turbines for power generation,
provided the exhaust gas is hotter than the compressor outlet temperature. The exhaust heat
from the turbine is used to pre-heat the air from the compressor before further heating in the
combustor, reducing the fuel input required. The larger the temperature difference between
turbine out and compressor out, the greater the benefit from the recuperator. Therefore, micro
turbines (<1MW), which typically have low pressure ratios, have the most to gain from the use
of a recuperator. In practice, a doubling of efficiency is possible through the use of a
recuperator. The major practical challenge for a recuperator in micro turbine applications is
coping with the exhaust gas temperature, which can exceed 750 °C (1,380 °F).

Regenerative Heat Exchanger
• A regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where
heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to
the cold fluid. To accomplish this the hot fluid is brought into contact with the heat storage medium,
then the fluid is displaced with the cold fluid, which absorbs the heat.
• In regenerative heat exchangers, the fluid on either side of the heat exchanger can be the same fluid.
The fluid may go through an external processing step, and then it is flowed back through the heat
exchanger in the opposite direction for further processing. Usually the application will use this
process cyclically or repetitively.
• Regenerative heating was one of the most important technologies developed during the Industrial
Revolution when it was used in the hot blast process on blast furnaces, It was later used in glass and
steel making, to increase the efficiency of open hearth furnaces, and in high pressure boilers and
chemical and other applications, where it continues to be important today.
• 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.

Thermal wheel/ Heat Wheel
• A thermal wheel, also known as a rotary heat exchanger, or rotary air-to-air enthalpy
wheel, or heat recovery wheel, is a type of energy recovery heat exchanger positioned
within the supply and exhaust air streams of an air-handling system or in the exhaust gases
of an industrial process, in order to recover the heat energy. Other variants
include enthalpy wheels and desiccant wheels. A cooling-specific thermal wheel is
sometimes referred to as a Kyoto wheel.

Case Examples for Heat Wheel
• Case Example:1
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:2
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.

Heat Pipe
• A heat pipe is a heat-transfer device that combines the principles of both thermal
conductivity and phase transition to effectively transfer heat between two solid interfaces.
• 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.
• At the hot interface of a heat pipe a liquid in contact with a thermally conductive solid
surface turns into a vapor by absorbing heat from that surface. The vapor then travels along
the heat pipe to the cold interface and condenses back into a liquid – releasing the latent
heat. The liquid then returns to the hot interface through either capillary action, centrifugal
force, or gravity, and the cycle repeats. Due to the very high heat transfer coefficients
for boiling and condensation, heat pipes are highly effective thermal conductors. The
effective thermal conductivity varies with heat pipe length, and can
approach 100 kW/(m⋅K) for long heat pipes, in comparison with
approximately 0.4 kW/(m⋅K) for copper.

Heat Pipe Mechanism
Figure: Heat Pipe

• 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 vaporisation 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.
Heat Pipe

Structure, Design and Construction
• A typical heat pipe consists of a sealed pipe or tube made of a material that is compatible with the working fluid
such as copper for water heat pipes, or aluminium for ammonia heat pipes. Typically, a vacuum pump is used to
remove the air from the empty heat pipe. The heat pipe is partially filled with a working fluid and then sealed. The
working fluid mass is chosen so that the heat pipe contains both vapour and liquid over the operating
temperature range.
• Below the operating temperature, the liquid is too cold and cannot vaporize into a gas. Above the operating
temperature, all the liquid has turned to gas, and the environmental temperature is too high for any of the gas to
condense. Whether too high or too low, thermal conduction is still possible through the walls of the heat pipe, but at
a greatly reduced rate of thermal transfer.
• Structure, Design and Construction
• A typical heat pipe consists of a sealed pipe or tube made of a material that is compatible with the working fluid
such as copper for water heat pipes, or aluminium for ammonia heat pipes. Typically, a vacuum pump is used to
remove the air from the empty heat pipe. The heat pipe is partially filled with a working fluid and then sealed. The
working fluid mass is chosen so that the heat pipe contains both vapour and liquid over the operating
temperature range.
• Below the operating temperature, the liquid is too cold and cannot vaporize into a gas. Above the operating
temperature, all the liquid has turned to gas, and the environmental temperature is too high for any of the gas to
condense. Whether too high or too low, thermal conduction is still possible through the walls of the heat pipe, but at
a greatly reduced rate of thermal transfer.

Heat Pipe Materials and Working Fluids
• Heat pipes have an envelope, a wick, and a working fluid. Heat pipes are designed for very long term
operation with no maintenance, so the heat pipe wall and wick must be compatible with the working fluid.
Some material/working fluids pairs that appear to be compatible are not. For example, water in an aluminum
envelope will develop large amounts of non-condensable gas over a few hours or days, preventing normal
operation of the heat pipe.
• Since heat pipes were rediscovered by George Grover in 1963, extensive life tests have been conducted to
determine compatible envelope/fluid pairs, some going on for decades. In a heat pipe life test, heat pipes are
operated for long periods of time, and monitored for problems such as non-condensable gas generation,
material transport, and corrosion.
• The most commonly used envelope (and wick)/fluid pairs include:[
• Copper envelope with water working fluid for electronics cooling. This is by far the most common type of
heat pipe.
• Copper or steel envelope with refrigerant R134a working fluid for energy recovery in HVAC systems.
• Aluminium envelope with ammonia working fluid for Spacecraft Thermal Control.
• Super alloy envelope with alkali metal (cesium, potassium, sodium) working fluid for high temperature heat
pipes, most commonly used for calibrating primary temperature measurement devices.
• Other pairs include stainless steel envelopes with nitrogen, oxygen, neon, hydrogen, or helium working fluids
at temperatures below 100 K, copper/methanol heat pipes for electronics cooling when the heat pipe must
operate below the water range, aluminium/ethane heat pipes for spacecraft thermal control in environments
when ammonia can freeze, and refractory metal envelope/lithium working fluid for high temperature (above
1,050 °C (1,920 °F)) applications.

Different Types of Heat Pipes
• In addition to standard, Constant Conductance Heat Pipes (CCHPs), there are a number of other types of heat
pipes, including:
• Vapour Chambers (planar heat pipes), which are used for heat flux transformation, and isothermalization of
surfaces
• Variable Conductance Heat Pipes (VCHPs), which use a Non-Condensable Gas (NCG) to change the heat pipe
effective thermal conductivity as power or the heat sink conditions change
• Pressure Controlled Heat Pipes (PCHPs), which are a VCHP where the volume of the reservoir, or the NCG
mass can be changed, to give more precise temperature control
• Diode Heat Pipes, which have a high thermal conductivity in the forward direction, and a low thermal
conductivity in the reverse direction
• Thermosyphons, which are heat pipes where the liquid is returned to the evaporator by
gravitational/accelerational forces,
• Rotating heat pipes, where the liquid is returned to the evaporator by centrifugal forces

Applications
• Spacecraft
• The spacecraft thermal control system has the function to keep all components on the spacecraft within their
acceptable temperature range. This is complicated by the following:
• Widely varying external conditions, such as eclipses Micro-g environment Heat removal from the spacecraft
by thermal radiation only Limited electrical power available, favouring passive solutions Long lifetimes,
with no possibility of maintenance Some spacecraft are designed to last for 20 years, so heat transport
without electrical power or moving parts is desirable. Rejecting the heat by thermal radiation means that
large radiator panes (multiple square meters) are required. Heat pipes and loop heat pipes are used
extensively in spacecraft, since they don’t require any power to operate, operate nearly isothermally, and can
transport heat over long distances.
• Grooved wicks are used in spacecraft heat pipes, as shown in the first photograph in this section. The heat
pipes are formed by extruding aluminium, and typically have an integral flange to increase the heat transfer
area, which lowers the temperature drop. Grooved wicks are used in spacecraft, instead of the screen or
sintered wicks used for terrestrial heat pipes, since the heat pipes don’t have to operate against gravity in
space. This allows spacecraft heat pipes to be several meters long, in contrast to the roughly 25 cm maximum
length for a water heat pipe operating on Earth. Ammonia is the most common working fluid for spacecraft
heat pipes. Ethane is used when the heat pipe must operate at temperatures below the ammonia freezing
temperature.
• The second figure shows a typical grooved aluminium/ammonia Variable Conductance Heat Pipe (VCHP)
for spacecraft thermal control. The heat pipe is an aluminium extrusion, similar to that shown in the first
figure. The bottom flanged area is the evaporator. Above the evaporator, the flange is machined off to allow
the adiabatic section to be bent. The condenser is shown above the adiabatic section. The Non-Condensable
Gas (NCG) reservoir is located above the main heat pipe. The valve is removed after filling and sealing the
heat pipe. When electric heaters are used on the reservoir, the evaporator temperature can be controlled
within ±2 K of the set point.
• Computer systems
• Heat pipes began to be used in computer systems in the late 1990s, when increased power requirements and
subsequent increases in heat emission resulted in greater demands on cooling systems. They are now
extensively used in many modern computer systems, typically to move heat away from components such
as CPUs and GPUs to heat sinks where thermal energy may be dissipated into the environment.

Solar thermal
Heat pipes are also widely used in solar thermal water heating applications in combination
with evacuated tube solar collector arrays. In these applications, distilled water is commonly
used as the heat transfer fluid inside a sealed length of copper tubing that is located within an
evacuated glass tube and oriented towards the sun. In connecting pipes, the heat transport
occurs in the liquid steam phase because the thermal transfer medium is converted into steam
in a large section of the collecting pipeline.
Permafrost cooling
Alaska pipeline support legs cooled by heat pipe thermosyphons to keep permafrost frozen
Building on permafrost is difficult because heat from the structure can thaw the permafrost.
Heat pipes are used in some cases to avoid the risk of destabilization. For example, in
the Trans-Alaska Pipeline System residual ground heat remaining in the oil as well as heat
produced by friction and turbulence in the moving oil could conduct down the pipe's support
legs and melt the permafrost on which the supports are anchored. This would cause the
pipeline to sink and possibly be damaged. To prevent this, each vertical support member has
been mounted with four vertical heat pipe thermosyphons.
Applications

Cooking
The first commercial heat pipe product was the "Thermal Magic Cooking Pin" developed
by Energy Conversion Systems, Inc. and first sold in 1966. The cooking pins used water
as the working fluid. The envelope was stainless steel, with an inner copper layer for
compatibility. During operation, one end of the heat pipe is poked through the roast. The
other end extends into the oven where it draws heat to the middle of the roast. The high
effective conductivity of the heat pipe reduces the cooking time for large pieces of meat
by one-half.
Alaska pipeline support
legs cooled by heat pipe
thermosyphons to
keep permafrost frozen
Ventilation heat recovery
In heating, ventilation and air-conditioning systems, HVAC, heat pipes are positioned
within the supply and exhaust air streams of an air handling system or in the exhaust
gases of an industrial process, in order to recover the heat energy.
Nuclear power conversion
Grover and his colleagues were working on cooling systems for nuclear power
cells for space craft, where extreme thermal conditions are encountered. These alkali
metal heat pipes transferred heat from the heat source to a thermionic or thermoelectric
converter to generate electricity.
Nuclear power conversion
Grover and his colleagues were working on cooling systems for nuclear power cells for space craft, where extreme
thermal conditions are encountered. These alkali metal heat pipes transferred heat from the heat source to
a thermionic or thermoelectric converter to generate electricity.
Applications

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.
The other applications in industries are:
• Preheating of boiler combustion air
• Recovery of Waste heat from furnaces
• Reheating of fresh air for hot air driers
• Recovery of waste heat from catalytic deodorizing equipment
• Reuse of Furnace waste heat as heat source for other oven
• Cooling of closed rooms with outside air
• Preheating of boiler feed water with waste heat recovery from flue gases in the heat pipe economizers.
• Drying, curing and baking ovens
• Waste steam reclamation
• Brick kilns (secondary recovery)
• Reverberator furnaces (secondary recovery)
• Heating, ventilating and air-conditioning systems
Applications

Volume
Recovered heat
Plant capacity reduction
Electricity cost (operation)
Plant capacity reduction cost
(Capital)
Capital cost savings
Payback period
140 m3/min Exhaust
28225 kCal/hr
9.33 Tons of Refrigeration
Rs. 268/Million kCal (based on 0.8
kW/TR)
Rs.12,000/TR
Rs. 1,12,000/-
16570 hours
Case Example of Heat Pipe
Savings in Hospital Cooling Systems

Economizer
In boilers, economizers are heat exchange devices that heat fluids, usually water, up to but not normally beyond
the boiling point of that fluid. Economizers are so named because they can make use of the enthalpy in fluid streams that
are hot, but not hot enough to be used in a boiler, thereby recovering more useful enthalpy and improving the boiler's
efficiency. They are a device fitted to a boiler which saves energy by using the exhaust gases from the boiler to preheat the
cold water used to fill it (the feed water
An economizer serves a similar purpose to a feed water heater, but is technically different as it does not use cycle steam
for heating. In fossil-fuel plants, the economizer uses the lowest-temperature flue gas from the furnace to heat the water
before it enters the boiler proper. This allows for the heat transfer between the furnace and the feed water to occur across a
smaller average temperature gradient (for the steam generator as a whole). System efficiency is therefore further increased
when viewed with respect to actual energy content of the fuel.
Most nuclear power plants do not have an economizer. However, the Combustion Engineering System 80+ nuclear plant
design and its evolutionary successors, (e.g. Korea Electric Power Corporation's APR-1400) incorporate an integral feed
water economizer. This economizer preheats the steam generator feed water at the steam generator inlet using the lowest-
temperature primary coolant.

Shell and Tube Heat Exchanger
When the medium containing waste heat is a liquid or a vapour 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
vapour contains the waste heat, it usually condenses, giving up its latent heat to the liquid being heated. In
this application, the vapour is almost invariably contained within the shell. If the reverse is attempted, the
condensation of vapours 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 o materials for the
tubes and shells. A shell and tube heat exchanger is illustrated in Figure.
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:
– Pasteurisation section in milk packaging plant.
– Evaporation plants in food industry.

Feed Water Heater
• A feed water heater is a power plant component
used to pre-heat water delivered to
a steam generating boiler Preheating the feed
water reduces the irreversibilities involved in
steam generation and therefore improves
the thermodynamic efficiency of the system. This
reduces plant operating costs and also helps to
avoid thermal shock to the boiler metal when the
feed water is introduced back into the steam cycle.
• In a steam power plant (usually modelled as a
modified Rankine cycle), feed water heaters allow
the feed water to be brought up to the saturation
temperature very gradually. This minimizes the
inevitable irreversibilities associated with heat
transfer to the working fluid (water). See the article
on the second law of thermodynamics for a further
discussion of such irreversibilities.
Fig: A Rankine cycle with two steam turbines and
a single open feed water heater.

Heat Pump
• A heat pump is a device that transfers heat energy from a source of heat to what is called
a heat sink. Heat pumps move thermal energy in the opposite direction of spontaneous
heat transfer, by absorbing heat from a cold space and releasing it to a warmer one. A heat
pump uses a small amount of external power to accomplish the work of transferring
energy from the heat source to the heat sink. The most common design of a heat pump
involves four main components – a condenser, an expansion valve, an evaporator and
a compressor. The heat transfer medium circulated through these components is
called refrigerant.
• While air conditioners and freezers are familiar examples of heat pumps, the term "heat
pump" is more general and applies to many HVAC (heating, ventilating, and air
conditioning) devices used for space heating or space cooling. When a heat pump is used
for heating, it employs the same basic refrigeration-type cycle used by an air conditioner
or a refrigerator, but in the opposite direction – releasing heat into the conditioned space
rather than the surrounding environment. In this use, heat pumps generally draw heat from
the cooler external air or from the ground.
• In heating mode, heat pumps are three to four times more effective at heating than
simple electrical resistance heaters using the same amount of electricity. However, the
typical cost of installing a heat pump is also higher than that of a resistance heater.
• When discussing about and comparing energy efficiency of heat pumping applications
some different factors are mainly used, COP (Coefficient of Performance), SCOP
(Seasonal Coefficient of Performance) and SPF (Seasonal Performance Factor). The more
efficient a heat pump is the less energy consuming it will be and the more cost-effective it
can be to operate. There are several factors that will affect the efficiency of a heat pump
such as climate, temperature, auxiliary equipment, technology, size and control system.

Applications
• There are millions of domestic installations using air source heat pumps. They are used in climates
with moderate space heating and cooling needs (HVAC) and may also provide domestic hot water.
The purchase costs are supported in various countries by consumer rebates.
• HVAC
• In HVAC applications, a heat pump is typically a vapuor-compression refrigeration device that includes a
reversing valve and optimized heat exchangers so that the direction of heat flow (thermal energy movement)
may be reversed. The reversing valve switches the direction of refrigerant through the cycle and therefore the
heat pump may deliver either heating or cooling to a building. In cooler climates, the default setting of the
reversing valve is heating.
• Water heating
• In water heating applications, a heat pump may be used to heat or preheat water for swimming pools or
heating potable water for use by homes and industry. Usually heat is extracted from outdoor air and
transferred to an indoor water tank, another variety extracts heat from indoor air to assist in cooling the
space.
• District heating
• Heat pumps can be integrated in district heating systems, especially if these are operated with low
temperatures.
• Heat pumps can also be used as heat supplier for district heating. Possible heat sources for such applications
are sewage water, ambient water (like sea, lake and river water), industrial waste heat, geothermal
energy, flue gas, waste heat from district cooling and heat from solar heat storage.
• Industrial heating
• There is a great potential to reduce the energy consumption and related greenhouse gas emissions in the
industry by application of industrial heat pumps. An international collaboration project completed in 2015
collected totally 39 examples of R&D-projects and 115 case studies worldwide.

Run-around coil
• A run-around coil is a type of energy recovery heat exchanger most often positioned
within the supply and exhaust air streams of an air handling system, or in the exhaust
gases of an industrial process, to recover the heat energy. Generally, it refers to any
intermediate stream used to transfer heat between two streams that are not directly
connected for reasons of safety or practicality. It may also be referred to as a run-around
loop, a pump-around coil or a liquid coupled heat 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 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.

Thermo-Compressor
• In many cases, very low pressure steam are reused as water after condensation for lack of any better
option of reuse. In many cases it becomes feasible to compress this low pressure steam by very high
pressure steam and reuse it as a medium pressure steam. The major energy in steam, is in its latent heat
value and thus thermo-compressing would give a large improvement in waste heat recovery.
The thermo-compressor is a simple equipment with a nozzle where HP steam is accelerated into a high
velocity fluid. This entrains the LP steam by momentum transfer and then recompresses in a divergent
venturi. A figure of thermo-compressor is shown in Figure 8.13.
It is typically used in evaporators where the boiling steam is recompressed and used as heating steam.

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 consists 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 8.14. Typical application is in the Deaerator of a steam generation station.

Energy Recovery Ventilation
• Energy recovery ventilation (ERV) is the energy recovery process of exchanging the energy contained in normally
exhausted building or space air and using it to treat (precondition) the incoming outdoor ventilation air in residential
and commercial HVAC systems. During the warmer seasons, the system pre-cools and dehumidifies while
humidifying and pre-heating in the cooler seasons. The benefit of using energy recovery is the ability to meet
the ASHRAE ventilation & energy standards, while improving indoor air quality and reducing total HVAC
equipment capacity.
• This technology has not only demonstrated an effective means of reducing energy cost and heating and cooling
loads, but has allowed for the scaling down of equipment. Additionally, this system will allow for the indoor
environment to maintain a relative humidity of 40% to 50%. This range can be maintained under essentially all
conditions. The only energy penalty is the power needed for the blower to overcome the pressure drop in the system.
Energy recovery device Type of transfer
Rotary enthalpy wheel Total & sensible
Fixed plate Total** & sensible
Heat pipe Sensible
Run around coil Sensible
Thermosiphon Sensible
Twin towers Sensible
Types Of Energy Recovery Devices

Advantages and Limitations of WHRD’s
• Advantages:
• These systems have many benefits which could be direct or indirect.
• Direct benefits: The recovery process will add to the efficiency of the process and thus decrease the
costs of fuel and energy consumption needed for that process.
• Indirect benefits:
• Reduction in Pollution: Thermal and air pollution will dramatically decrease since less flue gases of
high temperature are emitted from the plant since most of the energy is recycled.
• Reduction in the equipment sizes: As Fuel consumption reduces so the control and security
equipment for handling the fuel decreases. Also, filtering equipment for the gas is no longer needed
in large sizes.
• Reduction in auxiliary energy consumption: Reduction in equipment sizes means another reduction
in the energy fed to those systems like pumps, filters, fans,...etc.
• Limitations :
• Capital cost: The capital cost to implement a waste heat recovery system may outweigh the benefit
gained in heat recovered. It is necessary to put a cost to the heat being offset.
• Quality of heat: Often waste heat is of low quality (temperature). It can be difficult to efficiently
utilize the quantity of low quality heat contained in a waste heat medium. Heat exchangers tend to
be larger to recover significant quantities which increases capital cost.
• Maintenance of Equipment: Additional equipment requires additional maintenance cost.
• Units add addition size and mass to overall power unit. Especially a consideration on power units
which are on vehicles.

Waste Heat Recovery Devices by Varun Pratap Singh

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