ESL-IE-09-05-25
DESIGN OF HEAT EXCHANGER FOR HEAT RECOVERY IN CHP SYSTEMS
Theodore A. Kozman
Bimaldeep Kaur
Associate Professor
Research Assistant
Department of Mechanical Engineering
P. O. Box 44170, Room 244 CLR Hall
University of Louisiana at Lafayette
Lafayette, LA 70504-2250, USA
ABSTRACT
The objective of this research is to review issues
related to the design of heat recovery unit in
Combined Heat and Power (CHP) systems. To meet
specific needs of CHP systems, configurations can be
altered to affect different factors of the design. Before
the design process can begin, product specifications,
such as steam or water pressures and temperatures,
and equipment, such as absorption chillers and heat
exchangers, need to be identified and defined. The
Energy Engineering Laboratory of the Mechanical
Engineering Department of the University of
Louisiana at Lafayette and the Louisiana Industrial
Assessment Center has been donated an 800kW
diesel turbine and a 100 ton absorption chiller from
industries. This equipment needs to be integrated
with a heat exchanger to work as a Combined Heat
and Power system for the University which will
supplement the chilled water supply and electricity.
The design constraints of the heat recovery unit are
the specifications of the turbine and the chiller which
cannot be altered.
INTRODUCTION
Combined Heat and Power (CHP), also known
as cogeneration, is a way to generate power and heat
simultaneously and use the heat generated in the
process for various purposes. While the cogenerated
power in mechanical or electrical energy can be
either totally consumed in an industrial plant or
exported to a utility grid, the recovered heat obtained
from the thermal energy in exhaust streams of power
generating equipment is used to operate equipment
such as absorption chillers, desiccant dehumidifiers,
or heat recovery equipment for producing steam or
hot water or for space and/or process cooling,
heating, or controlling humidity. Based on the
equipment used, CHP is also known by other
acronyms such as CHPB (Cooling Heating and
Power for Buildings), CCHP (Combined Cooling
Heating and Power), BCHP (Building Cooling
Heating and Power) and IES (Integrated Energy
Systems). CHP systems are much more efficient than
producing electric and thermal power separately.
Jim Lee
Professor
According to the Commercial Buildings Energy
Consumption Survey, 1995 [14], there were 4.6
million commercial buildings in the United States.
These buildings consumed 5.3 quads of energy, about
half of which was in the form of electricity. Analysis
of survey data shows that CHP meets only 3.8% of
the total energy needs of the commercial sector.
Despite the growing energy needs, the average
efficiency of power generation has remained 33%
since 1960 and the average overall efficiency of
generating heat and electricity using conventional
methods is around 47%. And with the increase in
prices in both electricity and natural gas, the need for
setting up more CHP plants remains a pressing issue.
CHP is known to reduce fuel costs by about 27% [15]
and significantly reduce emissions of NOx, SOx, and
CO released into the atmosphere.
The objective of this research is to review issues
related to the design of heat recovery unit in
Combined Heat and Power (CHP) systems. To meet
specific needs of CHP systems, configurations can be
altered to affect different factors of the design. Before
the design process can begin, product specifications,
such as steam or water pressures and temperatures,
and equipment, such as absorption chillers and heat
exchangers, need to be identified and defined.
The Mechanical Engineering Department and the
Industrial Assessment Center at the University of
Louisiana Lafayette has been donated an 800kW
diesel turbine and a 100 ton absorption chiller from
industries. This equipment needs to be integrated to
work as a Combined Heat and Power system for the
University which will supplement the chilled water
supply and electricity. The design constraints of the
heat recovery unit are the specifications of the turbine
and the chiller which cannot be altered.
Integrating equipment to form a CHP system
generally does not always present the best solution.
In our case study, the absorption chiller is not able to
utilize all of the waste heat from the turbine exhaust.
Proceedings of the Thirty-First Industrial Energy Technology Conference, New Orleans, LA, May 12-15, 2009
ESL-IE-09-05-25
This is because the capacity of the chiller is too small
as compared to the turbine capacity. However, the
need for extra space conditioning in the buildings
considered remains an issue which can be resolved
through the use of this CHP system.
BACKGROUND LITERATURE
The decision of setting up a CHP system
involves a huge investment. Before plunging into
one, any industry, commercial building or facility
owner weighs it against the option of conventional
generation. A dynamic stochastic model has been
developed that compares the decision of an
irreversible investment in a cogeneration system with
that of investing in a conventional heat generation
system such as steam boiler combined with the
option of purchasing all the electricity from the grid
[21]. This model is applied theoretically based on
realistic fuel costs and accounting for CO2 tax
exempts. Keeping in mind factors such as rising
electricity and gas costs, the NOx, SOx and COx
emissions, and the availability and security of
electricity supply, the benefits of a combined heat
and power system are many.
CHP systems demand that the performance of
the system be well tested. The effects of various
parameters such as the ambient temperature, inlet
turbine temperature, compressor pressure ratio and
gas turbine combustion efficiency are investigated on
the performance of the CHP system and determines
of each of these parameters [1]. Five major areas
where CHP systems can be optimized in order to
maximize profits have been identified as optimization
of heat to power ratio, equipment selection, economic
dispatch, intelligent performance monitoring and
maintenance optimization [6].
Many commercial buildings such as universities
and hospitals have installed CHP systems for meeting
their growing energy needs. Before the University of
Dundee installed a 3 MW CHP system, first the
objectives for setting up a cogeneration system in the
university were laid and then accordingly the
equipment was selected. Considerations for
compatibility of the new CHP setup with the existing
district heating plant were taken care by some
alterations in pipe work so that neither system could
impose any operational constraints on the other [5].
Louisiana State University installed a CHP system by
contracting it to Sempra Energy Services to meet the
increase in chilled water and steam demands. The
new cogeneration system was linked with the existing
central power plant to supplement chilled water and
steam supply. This project saves the university $ 4.7
million each year in energy costs alone and 2,200
tons less of CO2 every year. The reduced NOx
emissions are equivalent to 530 annual vehicular
emissions.
Another example of a commercial CHP set-up is
the Mississippi Baptist Medical Center. First the
energy requirement of the hospital was assessed and
the potential savings that a CHP system would
generate [10]. CHP applications are not limited to the
industrial and commercial sector alone. CHP systems
on a micro-scale have been studied for use in
residential applications. The cost of UK residential
energy demand is calculated and a study is performed
that compares the operating cost for the following
three micro CHP technologies: Sterling engine, gas
engine, and solid oxide fuel cell (SOFC) for use in
homes [9].
The search for different types of fuel cells in
residential homes finds that a dominant cost effective
design of fuel cell use in micro – CHP exists that is
quickly emerging [3]. However fuel cells face
competition from alternate energy products that are
already in the market. Use of alternate energy such as
biomass combined with natural gas has been tested
for CHP applications where biomass is used as an
external combustor by providing heat to partially
reform the natural gas feed [16]. A similar study was
preformed where solid municipal waste is integrated
with natural gas fired combustion cycle for use in a
waste-to-energy system which is coupled with a heat
recovery steam generator that drives a steam turbine
[4].
SYSTEM DESIGN CONSIDERATIONS
Integration of a CHP system is generally at two
levels: the system level and the component level.
Certain trade-offs between the component level
metrics and system level metrics are required to
achieve optimal integrated cooling, heating and
power performance [18]. All CHP systems comprise
mainly of three components, a power generating
equipment or a turbine, a heat recovery unit and a
cooling device such as an absorption chiller.
There are various parameters that need to be
considered at the design stage of a CHP project. For
instance, the chiller efficiency together with the plant
size and the electric consumption of cooling towers
and condenser water pumps are analyzed to achieve
the overall system design [20]. Absorption chillers
work great with micro turbines. A good example is
the Rolex Reality building in New York, where a 150
kW unit is hooked up with an absorption chiller that
Proceedings of the Thirty-First Industrial Energy Technology Conference, New Orleans, LA, May 12-15, 2009
ESL-IE-09-05-25
provides chilled water. An advantage of absorption
chillers is that they don’t require any permits or
emission treatment [2]
desiccant wheel thereby regenerating it for further
dehumidification. This makes them useful in CHP
systems as they utilize the waste heat.
Exhaust gas at 800°F comes out of the turbine at
a flow rate of 48,880 lbs/h [7]. One important
constraint during the design of the CHP system was
to control the final temperature of this exhaust gas.
This meant utilizing as much heat as required from
the exhaust gas and subsequently bringing down the
exit temperature. After running different iterations on
temperature calculations, it was decided to divert
35% of the exhaust air to the heat exchanger while
the remaining 65% is directed to go up the stack. This
is achieved by using a diverter damper. In addition,
diverting 35% of the gas relieves the problem of back
pressure build-up at the end of the turbine.
An absorption chiller is mechanical equipment
that provides cooling to buildings through chilled
water. The main underlying principle behind the
working of an absorption chiller is that it uses heat
energy as input, instead of mechanical energy.
A diverter valve can also used at the inlet side of
the heat exchanger which would direct the exhaust
gas either to the heat exchanger or out of the bypass
stack. This takes care of variable loads requirement.
Inside the heat exchanger, exhaust gas enter the shell
side and heats up water running in the tubes which
then goes to the absorption chiller. These chillers run
on either steam or hot water.
The absorption chiller donated to the University
runs on hot water and supplies chilled water. A
continuous water circuit is made to run through the
chiller to take away heat from the heat input source
and also from the chilled water. The chilled water
from the absorption chiller is then transferred to the
existing University chilling system unit or for another
use.
Thermally Activated Devices
Thermally activated technologies (TATs) are
devices that transform heat energy for useful
purposed such as heating, cooling, humidity control
etc. The commonly used TATs in CHP systems are
absorption chillers and desiccant dehumidifiers.
Absorption chiller is a highly efficient technology
that uses less energy than conventional chilling
equipment, and also cools buildings without the use
of ozone-depleting chlorofluorocarbons (CFCs).
These chillers can be powered by natural gas, steam,
or waste heat.
Desiccant dehumidifiers are used in space
conditioning
by
removing
humidity.
By
dehumidifying the air, the chilling load on the AC
equipment is reduced and the atmosphere becomes
much more comfortable. Hot air coming from an airto-air heat exchanger removes water from the
Though the idea of using heat energy to obtain
chilled water seems to be highly paradoxical, the
absorption chiller is a highly efficient technology and
cost effective in facilities which have significant
heating loads. Moreover, unlike electrical chillers,
absorption chillers cool buildings without using
ozone-depleting chlorofluorocarbons (CFCs). These
chillers can be powered by natural gas, steam or
waste heat.
Absorption chiller systems are classified in the
following two ways:
1. By the number of generators.
i) Single effect chiller – this type of chiller, as
the name suggests, uses one generator and
the heat released during the absorption of
the refrigerant back into the solution is
rejected to the environment.
ii) Double effect chiller – this chiller uses two
generators paired with a single condenser,
evaporator and absorber. Some of the heat
released during the absorption process is
used to generate more refrigerant vapor
thereby increasing the chiller’s efficiency as
more vapor is generated per unit heat or fuel
input. A double effect chiller requires a
higher temperature heat input to operate and
therefore its use in CHP systems is limited
by the type of electrical generation
equipment it can be used with.
iii) Triple effect chiller – this has three
generators and even higher efficiency than a
double effect chiller. As they require even
higher heat input temperatures, the material
choice and the absorbent/refrigerant
combination is limited.
2. By type of input:
i) Indirect-fired absorption chillers – they use
steam, hot water, or hot gases from a boiler,
turbine, engine generator or fuel cell as a
primary
power
input.
Indirect-fired
absorption chillers fit well into the CHP
schemes where they increase the efficiency
by utilizing the otherwise waste heat and
producing chilled water from it.
Proceedings of the Thirty-First Industrial Energy Technology Conference, New Orleans, LA, May 12-15, 2009
ESL-IE-09-05-25
ii) Direct-fired absorption chillers – they
contain burners which use fuel such as
natural gas. Heat rejected from these chillers
is used to provide hot water or dehumidify
air by regenerating the desiccant wheel.
An absorption cycle is a process which uses two
fluids and some heat input to produce the
refrigeration effect as compared to electrical input in
a vapor compression cycle in the more familiar
electrical chiller. Although both the absorption cycle
and the vapor compression cycle accomplish heat
removal by the evaporation of a refrigerant at a low
pressure and the rejection of heat by the condensation
of refrigerant at a higher pressure, the method of
creating the pressure difference and circulating the
refrigerant remains the primary difference between
the two. The vapor compression cycle uses a
mechanical compressor that creates the pressure
difference necessary to circulate the refrigerant, while
the same is achieved by using a secondary fluid or an
absorbent in the absorption cycle [11].
The primary working fluids ammonia and water
in the vapor compression cycle with ammonia acting
as the refrigerant and water as the absorbent are
replaced by lithium bromide (LiBr) as the absorbent
and water (H2O) as the refrigerant in the absorption
cycle. The process occurs in two shells - the upper
shell consisting of the generator and the condenser
and the lower shell consisting of the evaporator and
the absorber.
Heat is supplied to the LiBr/H2O solution
through the generator causing the refrigerant (water)
to be boiled out of the solution, as in a distillation
process. The resulting water vapor passes into the
condenser where it is condensed back into the liquid
state using a condensing medium. The water then
enters the evaporator where actual cooling takes
place as water is passes over tubes containing the
fluid to be cooled.
A very low pressure is maintained in the
absorber-evaporator shell, causing the water to boil at
a very low temperature. This results in water
absorbing heat from the medium to be cooled and
thereby lowering its temperature. The heated low
pressure vapor then returns to the absorber where it
mixes with the LiBr/H2O solution low in water
content. Due to the solution’s low water content,
vapor gets easily absorbed resulting in a weaker
LiBr/H2O solution. This weak solution is pumped
back to the generator where the process repeats itself.
Heat Exchanger
The heat recovery steam generator (HRSG) is
primarily a boiler which generates steam from the
waste heat of a turbine to drive a steam turbine. The
heat recovery boiler design for cogeneration process
applications covers many parameters. The boiler
could be designed as a fire-tube, water tube or
combination type. Further for each of these
parameters, there is a variety of tube sizes and fin
configurations. For a given boiler, a simplified
method that determines the boiler performance has
been developed [8].
The shell and tube heat exchanger is the most
common and widely used heat exchanger in different
industrial applications [13]. It is compared to a
classic instrument in a concert playing all the
important nodes in different complex system set-ups
and can be improved by using helical baffles. There
are other ways to augment the heat transfer in a shell
and tube exchanger such as through the use of wallradiation [25].
The design of a shell and tube heat exchanger for
a combined heat and power system basically involves
determining its size or geometry by predicting the
overall heat transfer coefficient (U). The process of
obtaining the heat transfer coefficient values is
obtained from literature by correlating results from
previous findings in the determination of heat
exchanger designs.
This involves listing assumptions at the
beginning of the procedure, obtaining fluid
properties, calculation of Reynolds number and the
flow area to obtain the shell and tube sizes. Once U is
calculated, the heat balances are calculated. This
study also compares the theoretical U values with the
actual experimental ones to prove the theoretical
assumptions and to obtain the optimum design model
[18].
A mathematical simulation for the transient heat
exchange of a shell and tube heat exchanger based on
energy conservation and mass balance can be used to
measure the performance. The design of the heat
exchanger is optimized with the objective function
being the total entropy generation rate considering
the heat transfer and the flow resistance [20].
Once a heat exchanger is designed, a total cost
equation for the heat exchanger operation is deduced.
Based on this, a program is developed for the optimal
selection of shell-tube heat exchanger [24].
Proceedings of the Thirty-First Industrial Energy Technology Conference, New Orleans, LA, May 12-15, 2009
ESL-IE-09-05-25
The heat exchanger to be used in the CHP
system in the end needs to be tested for its
performance. A heat recovery module for
cogeneration is tested before use for CHP application
through a microprocessor based control system to
present the system design and performance data [19].
The basis of a CHP system lies in efficiently
capturing thermal energy and using it effectively.
Generally in CHP systems, the exhaust gas from the
prime mover is ducted to a heat exchanger to recover
the thermal energy in the gas. The commonly used
heat recovery systems are heat exchangers and Heat
Recovery Steam Generators depending on whether
hot water or steam is required.
1.
2.
3.
The heat exchanger is typically an air-to-water
kind where the exhaust gas flows over some form of
tube and fin heat exchange surface and the heat from
the exhaust gas is transferred to make hot water.
Sometimes, a diverter or a flapper damper is used to
maintain a specific design temperature of the hot
water or steam generation rate by regulating the
exhaust flow through the heat exchanger.
4.
5.
Diesel Oxidation Catalyst (DOC) – They are
know to reduce emissions of carbon monoxide
by 70 percent, hydrocarbons by 60 percent, and
particulate matter by 25 percent (Emissions
Control : CHP Technologies Gulf Coast CHP
2007) when used with the ultra-low sulfur diesel
(ULSD) fuel. Reductions are also significant
with the use of regular diesel fuel.
Diesel Particulate Filter (DPF) - DPF can reduce
emissions of carbon monoxide, hydrocarbons,
and particulate matter by approximately 90 to 95
percent (Emissions Control : CHP Technologies
Gulf Coast CHP 2007). However, DPF are used
only in conjunction with ultra-low sulfur diesel
(ULSD) fuel.
Exhaust Gas Recirculation (EGR) – They have a
great potential for reducing NOx emissions.
Selective Catalytic Reduction (SCR) – SCR cuts
down high levels of NOx by reducing NOx to
nitrogen (N2) and oxygen (O2).
NOx absorbers – catalysts are used which adsorb
NOx in the exhaust gas and dissociates it to
nitrogen.
The HRSG is essentially a boiler that captures
the heat from the exhaust of a prime mover such as a
combustion turbine, gas or diesel engine to make
steam. Water is pumped and circulated through the
tubes which are heated by exhaust gases at
temperatures ranging from 800°F to 1200°F. The
water can then be held under high pressure to
temperatures of 370°F or higher to produce high
pressure steam [21].
CONCLUSIONS
The various components needed in a CHP
system have been presented. Important parameters
such as the mass flow rates of the exhaust gas and
water can then be defined. The CHP system has been
integrated by the use of a heat recovery unit, the
design of which has been discussed. A shell and tube
configuration is commonly selected based on
literature survey. The pressure drops at both the shell
and the tube side can be calculated after the
exchanger has been sized.
The Delaware method is a rating method
regarded as the most suitable open-literature
available for evaluating shell side performance and
involves the calculation of the overall heat transfer
coefficient and the pressure drops on both the shell
and tube side for single-phase fluids [12]. This
method can be used only when the flow rates, inlet
and outlet temperatures, pressures and other physical
properties of both the fluids and a minimum set of
geometrical properties of the shell and tube are
known.
Integrating equipment to form a CHP system
generally does not always present the best solution.
In our case study, the absorption chiller is not able to
utilize all of the waste heat from the turbine exhaust.
Approximately 65% goes is left to go out the stack.
This is because the capacity of the chiller is too small
as compared to the turbine capacity. However, the
need for extra space conditioning in the buildings
considered remains an issue which can be resolved
through the use of this CHP system.
Emission Control
Emission control technologies are used in the
CHP systems to remove SO2 (sulphur dioxide), SO3
(sulphur trioxide) NOx (nitrous oxide) and other
particulate matter present in the exhaust of a prime
mover. Some common emission control technologies
are:
The heat exchanger designed can either be
constructed following the TEMA standards or it can
be built and purchased from an industrial facility. The
design that is used is based on the methodology of
the Bell-Delaware method and the approach is purely
theoretical, so the sizing may be slightly different in
industrial design. Also the manufacturing feasibility
needs to be checked.
Proceedings of the Thirty-First Industrial Energy Technology Conference, New Orleans, LA, May 12-15, 2009
ESL-IE-09-05-25
After the heat exchanger is constructed, the CHP
equipment can be hooked together. Again since the
available equipment is integrated to work as a
system, the efficiency of the CHP system needs to be
calculated. Some kind of control module needs to be
developed that can monitor the performance of the
entire system. Finally, the cost of running the set-up
needs to be determined along with the airconditioning requirements.
[10] Hodge, B.K, and Louay Chamra. CHP (Cooling,
Heating, and Power)at the Mississippi Baptist
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[11] Kevin D. Rafferty. "Absorption Refrigeration."
Geo-Heat Bulletin, 1996-2207.
[12] Kuppan, T. Heat Exchanger Design Handbook,
New York: Marcel and Dekker Inc., 2000.
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Proceedings of the Thirty-First Industrial Energy Technology Conference, New Orleans, LA, May 12-15, 2009