I J M E M S, 5(1) January-June 2012, pp. 37-42
Optimization and Design of Heat Exchanger with Different Materials
VEKARIYAMUKESH V., G. R. SELOKAR AND AMITESH PAUL
Student (M. Tech), SSSIST, Sehore, M.P.
2
Principal, SSSIST, Sehore, M.P.
3
HOD Department of Mechanical Engg., SSSIST, Sehore, M.P.
1
ABSTRACT
Heat Exchanger is a device built for efficient heat transfer
from one fluid to another in which either a wall to stop
them from mixing separates the fluids or the fluids are
allowed to be in direct contact with each other. The HVAC
industry is searching for ways to increase performance,
energy efficiency, and durability of HVAC equipment in
suitable way, while reducing the cost of manufacturing.
This search has spotlighted on the substitution of extruded
aluminum tubes and other aluminum components with
the use of decades old copper tube. Design of HEAT
EXCHANGERS carried out in this work is by considering
the comparison of different materials for different types
of Heat Exchangers, which are used for its construction
along with the thermal performance, durability, and cost
of the material.
Keywords: Aluminum, Copper, Brass, Thermal
Performance, Durability and Cost.
1. INTRODUCTION
The HVAC industry is searching for ways to
increase performance, energy efficiency, and
durability of HVAC equipment in a suitable way,
while reducing the cost of manufacturing these.
This search has put the spotlight on substituting
the copper tubes that have been in use for nearly a
century with extruded aluminum tubes and other
aluminum components. Aluminum, in its various
forms, provides clear paths to achieving all of these
goals right now, and positions adopters to meet the
challenges of the rapid pace of change in the
market’s demand for cost effective, energy-efficient
products. This is made possible by the large variety
of aluminum based materials and tubular product
forms that empower systemdesigners and
manufactures with multiple options for
significantrefrigeration equipment design
enhancements and cost reduction.
The state of the art in HVAC equipment design
is such that the majority of heat exchangers are
based on round tube plate fin (RTPF) designs. This
basic design has been enhanced in various ways to
Corresponding author: *vekariyamukesh84@gmail.com
achieve higher heat transfer performance; there
have not been any significant recent advances in
increasing heat transfer performance.
The quickest and most easily executed design
change is to substitute round aluminum tube for
round copper tube. This like for like substitution
provides adopter with the immediate advantage of
material cost savings and additional specific
benefits such as immunity to formicary corrosion.
Corrosion attack of copper tube DX evaporators has
become a significant problem with substantial cost
impact. It is estimated that formicary corrosion is
the root cause of approximately 10% of all cooper
tube evaporator coil failures. Cooper refrigeration
tube alloys are highly susceptible to this failure
mode. When the necessary ingredients are available
in the air being cooled, formic acid forms in the
evaporator condensate and attacks the copper tube
causing corrosion failure of the copper tube, and,
eventually, system failure. Aluminum is immune
to this failure mode (formicary corrosion) and has
provided the motivation for major HVAC OEM
manufacturers to eliminate copper tube from their
evaporators in favor of aluminum tube.
The second design alternative, the preferred
approach, is to substitute extruded aluminummicro
channel tube (or, multiport extrusion - MPE) for
round copper tube. The term “brazed aluminum
heat exchanger” is almost universally used
synonymously with micro channel (MPE) tube heat
exchangers to describe this type of heat exchanger.
In this document, “brazed aluminum heat
exchangers” refers exclusively to MPE tube heat
exchangers, unless otherwise stated. Substituting a
copper tube based RTPF heat exchanger with a
brazed aluminum MPE heat exchanger does not
represent like for like substitution; however, as in
the case of the like for like substitution, copper is
being replaces by a less expensive material,
aluminum. The chart below provides an overview
of the historical LME price of copper and aluminum.
38
Vekariyamukesh V., G. R. Selokar and Amitesh Paul
The impact of this substitution on cost is clear and
it becomes even more dramatic when considering
that, as a rule thumb, an aluminum tube based heat
exchanger only half as much as its copper
equivalent.
From several points of view, the RTPF to MPE
substitution path results in a considerable
performance upgrade that provides a substantial
reduction of weight, size, system, refrigeration
charge, reduced logistics costs and increased
durability. While these changes have direct cost
benefits with obvious appeal to the OEM
manufactures, these are additional benefits that
appeal to distributers, dealer/installers and
consumers. For example, reduced size and weight
will appeal to distributors and dealers since less
warehouse space and labor is required to store,
move, and install the equipment. Consumers will
like the smaller footprint of the brazed MPE design,
making this equipment less obtrusive in the
landscape.
2. SUBSTITUTING COPPER TUBE BASED
HEAT EXCHANGERS WITH ALLALUMINUM HEAT EXCHANGER
Below, aspects of manufacturing heat
exchangers with aluminum are discussed. This
includes a brief comparison of select physical
properties of copper, brass, and aluminum. The
various forms of aluminum heat exchanger
materials and the manufacturing process used to
manufacture all-aluminum heat exchangers is
included.
Table 1
Comments on Different Materials
Material
Comment
High conductivity
coppers
Have the best performance index,
but relatively poor corrosion
resistance
Brasses
Again, relatively poor corrosion
resistance
Aluminum Bronzes
An economical and practical choice
2.1. Comparison of Copper and Aluminum Heat
Exchanger Materials
The energy required to heat a quantity of metal is
dependent on the weight of metal being heated, its
specific heat and the temperature rise, according to
the equation:
Q = W × cp × ∆T
(1)
Table 2
Comparison of Material Properties
Material
Coefficient
of Thermal
Conductivity
(W/m-k)
Specific Heat
(kj/kgK)
Density
(kg/m3)
Brass
111
385
8520
Copper
386
383
8950
Aluminum
204
896
2707
The HVAC industry commonly uses filler
metals with liquids temperature approaching 800oC
to braze copper tubes. Good braze quality requires
that this temperature be exceeded by a safe margin
to ensure good filler metal flow and gap filling.
High quality aluminum brazing is achieved by
heating to 605-615 °C. Assuming an equivalent
Aluminum tube joint weighs only half that of the
copper tube joint and brazing is heated by only 595
°C while the copper tube joint must be heated by
795 °C, we can estimate that the copper tube joint
requires 14% more energy than the equivalent
Aluminum tube joint. Joining of aluminum by
brazing is more energy efficient than joining
Copper. Aluminum’s energy efficient than joining
copper. Aluminum’s energy efficiency is
multifaceted and is showcased by its recyclability,
superior manufacturing processes, and the product
designs enabled by its unique attributes.
2.2. Aluminum Heat Exchanger Components
Aluminum is available commercially in a variety
of forms. The forms which are of greatest interest
for HVAC applications are flat rolled products used
as fin stock and formed heat exchanger components,
and extruded tubes shapes. Aluminum heat
exchanger tubes are typically extruded and include
round tubes and MPE. Round tubes are either
smooth walled or may be enhanced with axial micro
fins on the inner diameter surface; this micro fin
geometry can be specified by the end-user to
provide a range of enhancement effects.
MPE tubes are manufactured to meet
customer’s specific requirements with respect to
alloy, outside dimensions, wall and web thickness,
hydraulic diameter and other attributes. Extrusion
is near net shape forming process that produces
Aluminum shapes that are not easily achievable
with any other metal. The rule of thumb regarding
what is possible is simply, “if a shape can be drawn
in two dimensions, it can be extruded”
Optimization and Design of Heat Exchanger with Different Materials
Aluminum rolled products is used in very high
volumes already in HVAC heat exchangers. Even
the majority of current copper RTPF heat
exchangers use Aluminum alloy fins due to
Aluminum’s ability to be formed into complex
enhanced fin geometries and its excellent thermal
conductivity, strength to weight ratio and corrosion
resistance. Aluminum rolled products is also used
in conjunction with brazed Aluminum heat
exchangers. In this case rolled products are clad
with layers of Al-Si, acting as the source for braze
filler metal during the braze process.
Aluminum brazing sheet is a highly engineered
material consisting of multilayer composite
materials of varying complexity. Depending on the
requirements of a specific application, these
materials can comprise 2, 3, 4 and even 5 layers.
Each layer either serves a specific purpose during
the heat exchanger production process or is used
to meet a heat exchanger functional requirement
while in service. For example, a 3-layer material can
consist of a modified 3XXX alloy layer for corrosion
protection, another 3XXX core alloy layer for postbraze strength and a 4XXX alloy layer to provide
the filler metal needed for joining the hundreds of
joints that typically make up a heat exchanger.
Both the extrusion and rolled products
production flow paths enable the manufacturing of
highly engineered heat exchanger materials that
meet a wide spectrum of application and customer
demands. The table on top of next page presents a
summary of commercially available forms of
Aluminum heat exchanger materials.
2.3. Manufacturing
Exchangers
All-Aluminum
manufacturing process, enabling overall cost
reduction. Aluminum solutions provide clear
pathways to achieving this cost reduction goal
along with many other benefits. Micro channel
tubes have certain inherent technical advantages
that result in the highest thermal performance gains
possible. The discussion that follows will focus
primarily on the performance benefits of brazed
Aluminum MPE solutions.
3.1. Thermal Performance
A number of studies have been presented showing
the clear thermal performance advantage of
switching away from copper RTPF heat exchangers
to brazed Aluminum MPE heat exchangers. The
specific results achieved very and will depend on
factors such as refrigerant, the degree to which the
system design was optimized, and others. One
study compared two commercially available split
ducted system-condensing units to evaluate the
impact of brazed Aluminum MPE heat exchangers
on system performance. The condensing unit with
the MPE heat exchanger had an ARI rating of 16
SEER, while the condensing unit with the MPE heat
exchanger had an ARI rating of 16 SEER, while the
condensing unit with the RTPF heat exchanger had
a 14SEER rating, per the manufacturer. Both had
scroll compressors and were charged with R-22.
Nominal capacities were very similar with 8.73 kW
and 8.79 kW for the MPE and RTPF condensing
units, respectively. In summary, that study found
the following:
•
Heat
The majority of heat exchangers used in HVAC
applications are of either mechanically expanded
RTPF design or brazed Aluminum design. The
manufacturing technologies used to make these two
different types of heat exchangers are distinct,
requiring different processes, equipment and often
requiring different alloys. Sapa heat transfer either
supplies all of the needed materials and alloys to
make the mechanically expanded RTPF or brazed
Aluminum heat exchangers.
3. BENEFITS OF SUBSTITUTING COPPER
TUBE BASED DESIGNS WITH ALL
ALUMINUM DESIGNS
The overarching goal for the HVAC industry is to
achieve higher performance of both product and
39
·
MPE condenser has
- Up to 45% better capacity per core volume
ratio
- Up to 25% lower refrigerant side pressure
drop at the same airside capacity and same
operating conditions
- Up to 40% lower air side pressure drop at
equal face velocity
- Up to 5% higher COP
MPE based unit has
- More compact outdoor from
- 39% lower refrigerant charge
A plate fin heat exchanger is a form of compact
heat exchanger consisting of a block of alternating
layers of corrugated fins and flat separators known
as parting sheets. A schematic view of such an
exchanger is given in Fig. 1. The corrugations serve
both as secondary heat transfer surface and as
40
Vekariyamukesh V., G. R. Selokar and Amitesh Paul
mechanical support against the internal pressure
between layers.
Table 3
Parameter of Design Consideration
Parameters
Equation
Fin Spacing (s)
(l/f)-t
Plate Thickness (b)
h+t
Free flow area (Aff)
(s-t) x h
Frontal Area (A)
(h+t) (s+t)
Heat Transfer Area (As)
2xhx 1.5+2xsx 1.5+2xhx 0.2
Eq. Dia (Dh)
(2*l*h(s-t))(|s+h|+ht)
Bulk Temperature
Inlet Temp. + Outlet Temp.
Mean Film Temperature
(Wall Temp. + Bulk Temp.)/2
Core Mass Velocity (G)
Mff / Aff
Fin Area (Af)
2xhx 1.5+2xhx 0.2
Fin Area / Total Surface Area (Af / As)
Reynolds no. (Re)
Pressure Drop
H
GD/ µ (Ref *)
Rej *
(0.5fG2) Deq
jcpl/pr (2/3)
M
√2h/kft
Mlf
Mb/2
nf
Tan h (ml)/ml
Total Area/Separating
Wall Area (A0 /A w)
(1-ft)/(1-Af /A w)
Overall Efficiency
1-(Af/As) (1-nf)
Overall Thermal
Resistance (Uo)
(nc wc/nhwh(Nohhh)) +
(aAo/KwAw)+(1-Nochc)
Pressure drop
(fG2/2pDeq)
Core height
(nc+nh) x b+ (nc +nh) x a
free flow area of cold side
core width x total height –
free flow area of hot side
Figure 1: Plate Fin Heat Exchanger Cross Section
3.2. Durability
The question of durability was touched upon
during the introductory remarks. It is a very
important issue for HVAC OE manufactures.
Consumers have expectations of long life from their
HVAC equipment, and the HVAC industry’s
manufactures have been able to supply equipment
with an average life exceeding 10years in North
America.
Many factors can contribute to HVAC
equipment failure. Looking at a residential HVAC
condensing unit, two potential catastrophic failure
modes are refrigerant leakage (due to corrosion of
the heat exchanger), and compressor failure (due
to a high duty cycle and high head pressure
resulting from the loss of condenser heat transfer
capacity). Both failure modes involve corrosion to
a large degree.
In the case of refrigerant leakage, this failure
mode is dealt with very effectively through a
combination of tube, fin and manifold material
selection, heat exchanger design and manufacturing
process design. It has been shown that Aluminum
heat exchangers are very durable in a variety of
environments. Sapa has developed a number of
long life tube, fin and manifold alloys along with
guidelines on how to combine those alloys to
achieve extremely high corrosion resistance.
Sapa’s Aluminum heat exchanger materials and
expertise also provide a way of mitigating the
second failure mode, the gradual loss of condenser
capacity due to the detachment of fin from the tube.
In the case of RTPF heat exchanger, this failure
mode is common. In copper RTPF heat exchangers,
the Aluminum fin provides sacrificial corrosion in
order to prevent the tubes from corroding; however,
as the fins are corroding, the condenser’s ability to
reject heat deteriorates.
Although aggressive environments accelerate
the effects of corrosion, these processes also take
place in more benign atmospheric conditions. The
loss of performance is more gradual, but it does take
place. The equipment owner is not able to achieve
the rated energy efficiency and capacity, except
during the earliest portion of the service life of that
equipment.
These issues manifest themselves in RTPF heat
exchangers with the greatest frequency and degree
of severity when the heat exchanger is based on
copper tube and Aluminum tube and matched fin
alloys, the severity and frequency of this failure
mode is vastly reduced. It cannot, however, be
completely eliminated since the nature of the
mechanical bond between fin and tube in RTPF heat
exchangers is such that the contact resistance will
increase over time. All-Sapa Aluminum RTPF will
slow down this loss of performance to the point
Optimization and Design of Heat Exchanger with Different Materials
where catastrophic unit failures will not occur by
this failure mode.
Eliminating the mechanical fin to tube bond
assures that HVAC equipment continues to provide
the owner with the rated equipment capacity at the
rated energy efficiency. This is an additional benefit
of brazed Aluminum MPE heat exchangers. The
points raised above, concerning the use of long life
SapaAluminum materials, along with due attention
given to implementing heat exchanger design
features that are known to enhance corrosion
resistance, produce very durable heat exchangers,
assuring long service life in a variety of operating
environments.
3.3. Cost
The potential for material cost savings comes from
several sources and these have already been
discussed. The table below provides a summary of
the identified sources for system cost savings as well
as an estimate of the value of that savings.
The table below shows directional cost savings
potential. There is additional upside potential due
to opportunities for energy savings, scrap cost
savings due to scrap reduction and increased scrap
reduction and increased scrap value and, other
benefits. The scrap would not comprise any mixed
metal scrap.
Graph 1. Price Comparison of Different Material over a
Decade
The impact of capital investments required to
begin manufacturing brazed Aluminum MPE heat
exchangers is not included in the estimation above.
This cost may reduce the benefit for some
manufactures, while other manufactures will not
feel the impact at all. Those manufactures
41
experiencing demand growth at a level that
requires an expansion of manufacturing demand
growth at a level that requires an expansion of
manufacturing capacity must support the
expansion with new capital investments,
regardless of heat exchanger design, or,
technologies that provide extremely high
performance for a variety of applications, now and
for the future, namely brazed Aluminum MPE heat
exchangers.
4. THE FUTURE WITH ALL-ALUMINUM
HEAT EXCHANGER DESIGNS IN HVAC
The value proposition presented in the discussion
above is based on two main components, namely,
the cost savings realized by substituting an
expensive raw material, copper, with a less
expensive raw material, Aluminum, and, cost
savings that are made possible by implementing
higher performance products and processes. These
options are available right now.
Aluminum heat exchangers materials are
available in a broad range of product forms to be
incorporated into high performance heat solutions.
These forms to be incorporated into high
performance heat solutions. These products are
highly customizable through a variety of forming
and fabrication processes used individually or
in combination, allowing users a great deal
of freedom in developing products and
applications.
Sapa heat transfer has a long history of
developing alloys and materials to meet everchanging market requirements. The drivers for
change come from different sources, including but
not limited to the need to improve performance,
reduce cost, and achieve compliance with evolving
regulations, more demanding thermal
management systems, and new refrigerants. Sapa
heat transfer is committed to the heat exchanger
materials market and to continuous innovation
that will provide customers with a number of tools
that will keep them at the leading edge of
developing Aluminum based heat exchanger
solutions.
5. RESULTS AND DISCUSSION
We can reach at design and required length of
different parameter of aluminum, copper and brass
it related cost and performance as well as its
durability and cost.
42
Material
Vekariyamukesh V., G. R. Selokar and Amitesh Paul
Coefficient
of Thermal
Conductivity
(W/m-k)
Specific Density
Heat Required
Heat (kg/m3) exchanger
length
(kJ/kg K)
area
(m)
(m2 )
Brass
111
385
8520
14.000
1.900
Copper
386
383
8950
14.005
1.922
Aluminum
204
896
2707
15.605
2.022
6. CONCLUSION
Adopters of Aluminum heat exchanger materials
and especially all-Aluminum heat exchanger
solutions will position themselves to reduce raw
material costs by direct substitution of copper with
Aluminum and to improve product performance
in a way that further reduces costs. The
manufacturing technologies for both Aluminum
RTPF and brazed Aluminum MPE heat exchangers
are very well established, having been in use for
decades to meet the needs of HVAC and automotive
applications.
Aluminum solutions offer many opportunities
for OE manufactures to design differentiated
products due to the flexibility afforded by the MPE
heat exchanger concept. The available Aluminum
heat exchanger material product forms provide a
high degree of compatibility with almost all
commercially relevant refrigerants. Aluminum
MPE can be tailored to meet the requirements of
both low-pressure refrigerants, like ammonia and
hydrocarbons, and high-pressure refrigerants, like
CO2.
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