BACKGROUND
This invention relates generally to heat pump and refrigeration applications and, more
particularly, to a microchannel heat exchanger configured for use in a heat pump or
refrigeration system5 .
Heating, ventilation, air conditioning and refrigeration (HVAC&R) systems include
heat exchangers to reject or accept heat between the refrigerant circulating within the system
and surroundings. One type of heat exchanger that has become increasingly popular due to
its compactness, structural rigidity, and superior performance, is a microchannel or
10 minichannel heat exchanger. A microchannel heat exchanger includes two or more
containment forms, such as tubes, through which a cooling or heating fluid (i.e. refrigerant or
a glycol solution) is circulated. The tubes typically have a flattened cross-section and
multiple parallel flow channels. Fins are typically arranged to extend between the tubes to
assist in the transfer of thermal energy between the heating/cooling fluid and the surrounding
15 environment. The fins have a corrugated pattern, incorporate louvers to boost heat transfer,
and are typically secured to the tubes via brazing.
Conventional microchannel heat exchangers commonly have substantially identical
fins throughout the heat exchanger core. In the heat pump and refrigeration applications,
when the microchannel heat exchanger is utilized as an evaporator, moisture present in the
20 airflow provided to the heat exchanger for cooling may condense and then freeze on the
external heat exchanger surfaces. The ice or frost formed may block the flow of air through
the heat exchanger, thereby reducing the efficiency and functionality of the heat exchanger
and HVAC&R system. Microchannel heat exchangers tend to freeze faster than the round
tube and plate fin heat exchangers and therefore require more frequent defrosts, reducing
25 useful heat exchanger utilization time and overall performance. Consequently, it is desirable
to construct the microchannel heat exchanger with improved frost tolerance and enhanced
performance.
SUMMARY OF THE INVENTION
30 A heat exchanger is provided including a first manifold, a second manifold, and a
plurality of heat exchange tube segments fluidly coupling the first and second manifold. The
heat exchange tube segments include a bend defining a first slab and a second arranged at an
angle to one another. Each of the heat exchange tube segments includes at least a first heat
3
exchange tube and a second heat exchange tube at least partially connected by a web
extending there between. The first heat exchange tube and the second heat exchange tube are
asymmetrical such that a cross-sectional flow area of the first heat exchange tube is different
than that of the second heat exchange tube. A fluid flows sequentially through the first heat
exchange tubes of the first slab and the second slab, and then through the second hea5 t
exchange tubes of the second slab and first slab.
In addition to one or more of the features described above, or as an alternative, in
further embodiments an airflow across the heat exchanger moves from the first slab toward
the second slab.
10 In addition to one or more of the features described above, or as an alternative, in
further embodiments an airflow across the heat exchanger moves from the second slab
toward the first slab.
In addition to one or more of the features described above, or as an alternative, in
further embodiments the cross-sectional flow area of the first heat exchange tubes is smaller
15 than the cross-sectional area of the second heat exchange tubes.
In addition to one or more of the features described above, or as an alternative, in
further embodiments the fluid within the first heat exchange tubes includes a liquid or liquidvapor
mixture including less than 20% vapor by mass.
In addition to one or more of the features described above, or as an alternative, in
20 further embodiments the fluid within the second heat exchange tubes includes a vapor or
liquid-vapor mixture including at least 50% vapor by mass.
In addition to one or more of the features described above, or as an alternative, in
further embodiments the cross-sectional flow area of the first heat exchange tubes is larger
than the cross-sectional area of the second heat exchange tubes.
25 In addition to one or more of the features described above, or as an alternative, in
further embodiments the fluid within the second heat exchange tubes includes a liquid or
liquid-vapor mixture including less than 20% vapor by mass.
In addition to one or more of the features described above, or as an alternative, in
further embodiments the fluid within the first heat exchange tubes includes a vapor or liquid30
vapor mixture including at least 50% vapor by mass.
According to yet another embodiment of the invention, a heat exchanger is provided
including a first manifold, a second manifold, and a plurality of heat exchange tube segments
fluidly coupling the first and second manifold. The heat exchange tube segments include a
bend defining a first slab and a second arranged at an angle to one another. Each of the heat
4
exchange tube segments includes at least a first heat exchange tube and a second heat
exchange tube at least partially connected by a web extending there between. A fluid flow
sequentially through the first heat exchange tubes and the second heat exchange tubes of the
heat exchanger such that the fluid within the first heat exchange tubes is a liquid and the fluid
within the second heat exchange tubes is a vapor5 .
In addition to one or more of the features described above, or as an alternative, in
further embodiments the first heat exchange tube and the second heat exchange tube are
asymmetrical such that a cross-sectional flow area of the first heat exchange tube is different
than a cross-sectional flow area of the second heat exchange tube.
10 In addition to one or more of the features described above, or as an alternative, in
further embodiments the cross-sectional flow area of the first heat exchange tubes is smaller
than the cross-sectional area of the second heat exchange tubes.
In addition to one or more of the features described above, or as an alternative, in
further embodiments an airflow across the heat exchanger moves from the first slab toward
15 the second slab.
In addition to one or more of the features described above, or as an alternative, in
further embodiments wherein an airflow across the heat exchanger moves from the second
slab toward the first slab.
20 BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter, which is regarded as the invention, is particularly pointed out and
distinctly claimed in the claims at the conclusion of the specification. The foregoing and
other features, and advantages of the invention are apparent from the following detailed
description taken in conjunction with the accompanying drawings in which:
25 FIG. 1 is a schematic diagram of an example of a vapor refrigeration cycle of a
refrigeration system;
FIG. 2 is a side view of a microchannel heat exchanger according to an embodiment
of the invention prior to a bending operation;
FIG. 3 is a cross-sectional view of a tube segment of a microchannel heat exchanger
30 according to an embodiment of the invention;
FIG. 4 is a cross-sectional view of a tube segment of a microchannel heat exchanger
according to an embodiment of the invention;
FIG. 5 is a perspective view of a microchannel heat exchanger according to an
embodiment of the invention;
5
FIG. 6 is a cross-sectional view of a microchannel heat exchanger according to
another embodiment of the invention;
FIG. 7 is a cross-sectional view of a microchannel heat exchanger according to yet an
embodiment of the invention; and
FIG. 8 is a cross-sectional view of a microchannel heat exchanger according to yet 5 an
embodiment of the invention.
The detailed description explains embodiments of the invention, together with
advantages and features, by way of example with reference to the drawings.
10 DETAILED DESCRIPTION
Referring now to FIG. 1, a vapor compression refrigerant cycle 20 of an air
conditioning or refrigeration system is schematically illustrated. Exemplary air conditioning
or refrigeration systems include, but are not limited to, split, packaged, chiller, rooftop,
supermarket, and transport refrigeration systems for example. A refrigerant R is configured
15 to circulate through the vapor compression cycle 20 such that the refrigerant R absorbs heat
when evaporated at a low temperature and pressure and releases heat when condensed at a
higher temperature and pressure. Within this cycle 20, the refrigerant R flows in a
counterclockwise direction as indicated by the arrow. The compressor 22 receives refrigerant
vapor from the evaporator 24 and compresses it to a higher temperature and pressure, with
20 the relatively hot vapor then passing to the condenser 26 where it is cooled and condensed to
a liquid state by a heat exchange relationship with a cooling medium (not shown) such as air.
The liquid refrigerant R then passes from the condenser 26 to an expansion device 28,
wherein the refrigerant R is expanded to a low temperature two-phase liquid/vapor state as it
passes to the evaporator 24. The low pressure vapor then returns to the compressor 22 where
25 the cycle is repeated. It has to be understood that the refrigeration cycle 20 depicted in FIG. 1
is a simplistic representation of an HVAC&R system, and many enhancements and features
known in the art may be included in the schematic. In particular, the heat pump refrigerant
cycle includes a four-way valve (not shown) disposed downstream of the compressor with
respect to the refrigerant flow that allows reversing the refrigerant flow direction throughout
30 the refrigerant cycle to switch between the cooling and heating mode of operation for the
environment to be conditioned.
Referring now to FIG. 2, an example of a heat exchanger 30 configured for use in the
vapor compression system 20 is illustrated in more detail. The heat exchanger 30 may be
used as either a condenser 24 or an evaporator 28 in the vapor compression system 20. The
6
heat exchanger 30 includes at least a first manifold or header 32, a second manifold or header
34 spaced apart from the first manifold 32, and a plurality of tube segments 36 extending in a
spaced, parallel relationship between and connecting the first manifold 32 and the second
manifold 34. In the illustrated, non-limiting embodiments, the first header 32 and the second
header 34 are oriented generally horizontally and the heat exchange tube segments 36 exte5 nd
generally vertically between the two headers 32, 34. However, other configurations, such as
where the first and second headers 32, 34 are arranged substantially vertically are also within
the scope of the invention.
As illustrated in the cross-sections of FIGS. 3 and 4, each of the plurality of tube
10 segments 36 extending between the first manifold 32 and the second manifold 34 is a
multiport extruded (MPE) tube segment 36 and includes at least a first heat exchange tube 38
and a second heat exchange tube 40 connected by a web 42 extending at least partially there
between. In one embodiment, the web 42 arranged at the outermost tube segments 36
includes a plurality of openings.
15 An interior flow passage of each heat exchange tube 38,40 may be divided by interior
walls into a plurality of discrete flow channels 44a, 44b that extend over the length of the
tube segments 36 and establish fluid communication between the respective first and second
manifolds 32, 34. The interior flow passages of the first heat exchange tubes 38 may be
divided into a different number of discrete flow channels 44 than the interior flow passages of
20 the second heat exchange tubes 40. The flow channels 44a, 44b may have any shape crosssection,
such as a circular cross-section, a rectangular cross-section, a trapezoidal crosssection,
a triangular cross-section, or another non-circular cross-section for example. The
plurality of heat exchange tube segments 36 including the discrete flow channels 44a, 44b
may be formed using known techniques, such as extrusion for example.
25 Each first heat exchange tube 38 and second heat exchange tube 40 has a respective
leading edge 46a, 46b, a trailing edge 48a, 48b, a first surface 50a, 50b, and a second surface
52a, 52b (FIG. 3). The leading edge 46a, 46b of each heat exchange tube 38, 40 is upstream
of its respective trailing edge 48a, 48b with respect to an airflow A through the heat
exchanger 30.
30 The first heat exchange tubes 38 and the second heat exchanger tubes 40 are
substantially different or asymmetric. In the illustrated, non-limiting embodiment, the second
heat exchange tubes 40 are wider and have a greater number of discrete flow channels 44
than the first heat exchange tube 38, resulting in a larger cross-sectional flow area. Although
the second heat exchange tube 40, as illustrated in FIG. 3, is wider than the first heat
7
exchange tube 38, other configurations, such as where the plurality of first heat exchange
tubes 38 have a greater cross-sectional flow area than the plurality of second heat exchange
tubes 40 for example, are within the scope of the invention. The ratio of asymmetry between
the first heat exchange tubes 38 and the second heat exchanger tubes 40 may depend on any
of a variety of parameters of the heat exchanger, such as capacity5 ,
Referring now to FIG. 5, each tube segment 36 of the heat exchanger 30 includes at
least one bend 60, such that the heat exchanger 30 has a multi-pass configuration relative to
the airflow A. The bend 60 is generally formed about an axis extending substantially
perpendicular to the longitudinal axis or the discrete flow channels 44a, 44b of the tube
10 segments 36. In the illustrated embodiment, the bend 60 is a ribbon fold; however other
types of bends are within the scope of the invention. In the illustrated, non-limiting
embodiment, the bend 60 is formed at an approximate midpoint of the tube segments 36
between the opposing first and second manifolds 32, 34.
The bend 60 at least partially defines a first section or slab 62 and a second section or
15 slab 64 of the plurality of tube segments 36. As shown in the FIG., the bend 60 can be
formed such that the first slab is positioned at an obtuse angle with respect to the second slab
64. Alternatively, or in addition, the bend 60 can also be formed such that the first slab 62 is
arranged at either an acute angle or substantially parallel to the second slab 64. The bend 60
allows for the formation of a heat exchanger 30 having a conventional A-coil or V-coil shape.
20 In embodiments where the first slab 62 and the second slab 64 are arranged substantially
parallel, the lengths of the first slab 62 and the second slab 64 may vary to offset the position
of the first manifold 32 relative to the second manifold 34. Alternatively, the free ends of the
first slab 62 and the second slab 64 may angle or flare away from one another to
accommodate the manifolds 32, 34.
25 As previously stated, the heat exchanger 30 includes a multi-pass configuration as a
result of the bend 60 formed therein. In one embodiment, illustrated in FIG. 6, the heat
exchanger 30 is configured such that both the first heat exchanger tube 38 and the second
heat exchanger tube 40 of a tube segment 36 within the first slab 62 define a first pass relative
to an airflow A. Similarly, both the first heat exchanger tube 38 and the second heat
30 exchanger tube 40 within the second slab 64 of the same tube segment 36 define a subsequent
pass relative to the airflow. Although in the illustrated FIG., the fluid or refrigerant has a
counter flow orientation relative to the direction of the airflow, other embodiments where the
refrigerant has a parallel flow orientation are also within the scope of the invention.
8
In another embodiment, as illustrated in FIGS. 7 and 8, the first heat exchanger tube
38 and the second heat exchanger tube 40 within the same first slab 62 or second slab 64 are
configured as different passes within the refrigerant flow path of the heat exchanger 30. For
example, as shown in FIG. 7, the heat exchanger 30 may be configured such that refrigerant
flows sequentially through the first heat exchanger tube 38 of both the first slab 62 and 5 nd the
second slab 64 prior to flowing through the second heat exchanger tube 40 of the second slab
64 and the first slab 62. However, other flow configurations such as where the refrigerant
flows through the second heat exchanger tubes 40 before flowing the first heat exchanger
tubes 38, as shown in FIG. 8, is within the scope of the invention. In addition, the refrigerant
10 may enter the heat exchanger 30 at the same slab as the airflow, as shown in the embodiments
of FIGS. 7 and 8, or alternatively, may enter the heat exchanger at a different slab as the
airflow.
Depending on the direction of the airflow A relative to the heat exchanger 30 and
which slab the refrigerant is configured as an inlet to the heat exchanger 30, the flow through
15 the first heat exchanger tube 38 has a first configuration and the flow through the second heat
exchanger tube 40 has a second configuration, different from the first configuration. As
shown in the illustrated, non-limiting embodiment of FIG. 7, with the airflow A flowing from
the first slab 62 toward the second slab 64, the flow within the first heat exchanger tube 38 is
parallel to the direction of the airflow A, and the flow within the second heat exchanger tube
20 40 is counter to the airflow A. In embodiments where the refrigerant is first provided to the
second heat exchanger tubes 40, as shown in FIG. 8, the flow within the second heat
exchanger tubes 40 is parallel to the direction of the airflow A, and the flow within the first
heat exchanger tubes 38 is counter to the airflow A.
To minimize the formation of frost on the heat exchanger 30, the flow path of the
25 refrigerant through the heat exchanger 30 may be configured such that the liquid or two phase
portion of the refrigerant flows through the heat exchanger tube having a smaller crosssectional
flow area and the vapor portion of the refrigerant flows through the heat exchanger
tube having a larger cross-sectional flow area. For example, in the embodiment illustrated in
FIG. 8, the second heat exchanger tube 40 has a smaller cross-sectional flow area than the
30 first heat exchanger tube 38. The airflow is configured to flow from the first slab 62 to the
second slab 64, and the liquid or two-phase refrigerant is input to the second heat exchanger
tubes 40 of the first slab 62. By the time the refrigerant reaches first heat exchange tubes 38
of the first slab 62, the refrigerant is a superheated vapor which is at a higher temperature
than the saturation temperature. As a result, the amount of heat transfer that occurs between
9
the airflow A and first heat exchange tubes 38 of the first slab 62 is limited. In such
embodiments, the liquid or liquid vapor mixture within the second heat exchange tubes 40 is
less than 20% vapor by mass and the vapor or liquid-vapor mixture within the first heat
exchanger tubes 38 is at least 50% vapor by mass.
In other embodiments, refrigerant may be provided to the first heat exchange tubes 5 s 38
then the second heat exchange tubes 40, as shown in FIG. 7. In such embodiments, first heat
exchanger tubes 38 may have a cross-sectional flow area smaller than that of the second heat
exchanger tubes 40 such that the liquid or liquid vapor mixture within the first heat exchange
tubes 38 is less than 20% vapor by mass and the vapor or liquid-vapor mixture within the
10 second heat exchanger tubes 40 is at least 50% vapor by mass.
Presence of superheated vapor and reducing the amount of heat transfer between an
airflow A and a fluid R in the pass of the refrigerant where the airflow initially contacts the
heat exchanger leads to reduced rate of frost accumulation and improved frost tolerance. .
As a result, the formation of frost, and therefore a number of defrost cycles required to
15 maintain the operational efficiency of the heat exchanger 30 are reduced. Because the
operational efficiency of the heat exchanger 30 is improved (due to a lower number of defrost
cycles and increased heat transfer in the second slab), the size of the heat exchanger 30
required for a desired application may also be reduced. Alternatively, size of other
components, such as a compressor may be reduced, which in turn would cause even higher
20 evaporation temperature and further reduction of defrost cycles as well as the system
performance boost.
While the present invention has been particularly shown and described with reference
to the exemplary embodiments as illustrated in the drawing, it will be recognized by those
skilled in the art that various modifications may be made without departing from the spirit
25 and scope of the invention. Therefore, it is intended that the present disclosure not be limited
to the particular embodiment(s) disclosed as, but that the disclosure will include all
embodiments falling within the scope of the appended claims. In particular, similar principals
and ratios may be extended to the rooftops applications and vertical package units.

We Claim:
1. A heat exchanger comprising:
a first manifold;
a second manifold separated from the first manifold;
a plurality of heat exchange tube segments arranged in spaced parallel relationshi5 p
and fluidly coupling the first manifold and the second manifold, the plurality of heat
exchange tube segments including a bend defining a first slab and a second slab of the heat
exchange tube segments, the first slab being arranged at an angle to the second slab, each of
the plurality of heat exchange tube segments including at least a first heat exchange tube and
10 a second heat exchange tube at least partially connected by a web extending there between,
the first heat exchange tube and the second heat exchange tube being asymmetrical such that
a cross-sectional flow area of the first heat exchange tube is different than a cross-sectional
flow area of the second heat exchange tube;
wherein a fluid is configured to flow sequentially through the first heat exchange
15 tubes of the first slab, the first heat exchanger tubes of the second slab, the second heat
exchange tubes of the second slab and the first heat exchange tubes of the first slab.
2. The heat exchanger according to claim 1, wherein an airflow across the heat
exchanger moves from the first slab toward the second slab.
3. The heat exchanger according to claim 1, wherein an airflow across the heat
20 exchanger moves from the second slab toward the first slab.
4. The heat exchanger according to claim 1, wherein the cross-sectional flow
area of the first heat exchange tubes is smaller than the cross-sectional area of the second heat
exchange tubes.
5. The heat exchanger according to claim 4, wherein the fluid within the first
25 heat exchange tubes includes a liquid or liquid-vapor mixture including less than 20% vapor
by mass.
6. The heat exchanger according to claim 4, wherein the fluid within the second
heat exchange tubes includes a vapor or liquid-vapor mixture including at least 50% vapor by
mass.
30 7. The heat exchanger according to claim 1, wherein the cross-sectional flow
area of the first heat exchange tubes is larger than the cross-sectional area of the second heat
exchange tubes.
11
8. The heat exchanger according to claim 7, wherein the fluid within the second
heat exchange tubes includes a liquid or liquid-vapor mixture including less than 20% vapor
by mass.
9. The heat exchanger according to claim 7, wherein the fluid within the first
heat exchange tubes includes a vapor or liquid-vapor mixture including at least 50% vapor 5 by
mass.
10. A heat exchanger comprising:
a first manifold;
a second manifold separated from the first manifold;
10 a plurality of heat exchange tube segments arranged in spaced parallel relationship
and fluidly coupling the first manifold and the second manifold, the plurality of heat
exchange tube segments including a bend defining a first section of the heat exchanger tube
segments and a second section of the heat exchange tube segments, the first section being
arranged at an angle to the second section, each of the plurality of tube segments including at
15 least a first heat exchange tube and a second heat exchange tube at least partially connected
by a web extending there between;
wherein a fluid is configured to flow sequentially through the first heat exchange
tubes and the second heat exchange tubes of the heat exchanger such that the fluid within the
first heat exchange tubes is a liquid and the fluid within the second heat exchange tubes is a
20 vapor.
11. The heat exchanger according to claim 10, wherein the first heat exchange
tube and the second heat exchange tube are asymmetrical such that a cross-sectional flow
area of the first heat exchange tube is different than a cross-sectional flow area of the second
heat exchange tube.
25 12. The heat exchanger according to claim 11, wherein the cross-sectional flow
area of the first heat exchange tubes is smaller than the cross-sectional area of the second heat
exchange tubes.
13. The heat exchanger according to claim 10, wherein an airflow across the heat
exchanger moves from the first slab toward the second slab.
30
12
14. The heat exchanger according to claim 10, wherein an airflow across the heat
exchanger moves from the second slab toward the first slab.