Note: Descriptions are shown in the official language in which they were submitted.
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PARALLEL FLOW HEAT EXCHANGERS INCORPORATING
POROUS INSERTS
Cross-Reference to Related Application
[0001] Reference is made to and this application claims priority from and the
benefit of U.S. Provisional Application Serial No. 60/649,425, filed February
2,
2005, and entitled PARALLEL FLOW EVAPORATOR INCORPORATING
POROUS CHANNEL INSERTS, which application is incorporated herein in its
entirety by reference.
Background of the Invention
[0002] This invention relates generally to air conditioning, heat pump and
refrigeration systems and, more particularly, to parallel flow evaporators
thereof.
[0003] A definition of a so-called parallel flow heat exchanger is widely
used in the air conditioning and refrigeration industry and designates a heat
exchanger with a plurality of parallel passages, among which refrigerant is
distributed and flown in the orientation generally substantially perpendicular
to the
refrigerant flow direction in the inlet and outlet manifolds. This definition
is well
adapted within the technical community and will be used throughout the text.
[0004] Refrigerant maldistribution in refrigerant system evaporators is a
well-known phenomenon. It causes significant evaporator and overall system
performance degradation over a wide range of operating conditions.
Maldistribution
of refrigerant may occur due to differences in flow impedances within
evaporator
channels, non-uniform airflow distribution over external heat transfer
surfaces,
improper heat exchanger orientation or poor manifold and distribution system
design. Maldistribution is particularly pronounced in parallel flow
evaporators due
to their specific design with respect to refrigerant routing to each
refrigerant circuit.
Attempts to eliminate or reduce the effects of this phenomenon on the
performance
of parallel flow evaporators have been made with little or no success. The
primary
reasons for such failures have generally been related to complexity and
inefficiency
of the proposed technique or prohibitively high cost of the solution.
[0005] In recent years, parallel flow heat exchangers, and furnace-brazed
aluminum heat exchangers in particular, have received much attention and
interest,
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not just in the automotive field but also in the heating, ventilation, air
conditioning
and refrigeration (HVAC&R) industry. The primary reasons for the employment of
the parallel flow technology are related to its superior performance, high
degree of
compactness and enhanced resistance to corrosion. Parallel flow heat
exchangers
are now utilized in both condenser and evaporator applications for multiple
products
and system designs and configurations. The evaporator applications, although
promising greater benefits and rewards, are more challenging and problematic.
Refrigerant maldistribution is one of the primary concerns and obstacles for
the
implementation of this technology in the evaporator applications.
[0006] As known, refrigerant maldistribution in parallel flow heat
exchangers occurs because of unequal pressure drop inside the channels and in
the
inlet and outlet manifolds, as well as poor manifold and distribution system
design.
In the manifolds, the difference in length of refrigerant paths, phase
separation and
gravity are the primary factors responsible for maldistribution. Inside the
heat
exchanger channels, variations in the heat transfer rate, airflow
distribution,
manufacturing tolerances, and gravity are the dominant factors. Furthermore,
the
recent trend of the heat exchanger performance enhancement promoted
miniaturization of its channels (so-called minichannels and microchannels),
which in
turn negatively impacted refrigerant distribution. Since it is extremely
difficult to
control all these factors, many of the previous attempts to manage refrigerant
distribution, especially in parallel flow evaporators, have failed.
[0007] In the refrigerant systems utilizing parallel flow heat exchangers, the
inlet and outlet manifolds or headers (these terms will be used
interchangeably
throughout the text) usually have a conventional cylindrical shape. When the
two-
phase flow enters the header, the vapor phase is usually separated from the
liquid
phase. Since both phases flow independently, refrigerant maldistribution tends
to
occur.
[0008] If the two-phase flow enters the inlet manifold at a relatively high
velocity, the liquid phase (droplets of liquid) is carried by the momentum of
the flow
further away from the manifold entrance to the remote portion of the header.
Hence,
the channels closest to the manifold entrance receive predominantly the vapor
phase
and the channels remote from the manifold entrance receive mostly the liquid
phase.
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If, on the other hand, the velocity of the two-phase flow entering the
manifold is
low, there is not enough momentum to carry the liquid phase along the header.
As a
result, the liquid phase enters the channels closest to the inlet and the
vapor phase
proceeds to the most remote ones. Also, the liquid and vapor phases in the
inlet
manifold can be separated by the gravity forces, causing similar
maldistribution
consequences. In either case, maldistribution phenomenon quickly surfaces and
manifests itself in evaporator and overall system performance degradation.
[0009] Moreover, maldistribution phenomenon may cause the two-phase
(zero superheat) conditions at the exit of some channels, promoting potential
flooding at the compressor suction that may quickly translate into the
compressor
damage.
Summary of the Invention
[0010] It is therefore an object of the present invention to provide for a
system and method which overcome the problems of the prior art described
above.
[0011] The objective of the present invention is to introduce a pressure drop
control for the parallel flow (microchannel or minichannel) evaporator that
will
essentially equalize pressure drop through the heat exchanger circuits and
therefore
eliminate refrigerant maldistribution and the problems associated with it.
Further, it
is the objective of the present invention to provide refrigerant expansion at
the
entrance of each channel, thus eliminating a predominantly two-phase flow in
the
inlet manifold, which is one of the main causes for refrigerant
maldistribution. It
has been found that the introduction of a porous media inserted in each
parallel flow
evaporator channel, or at the entrance of each parallel flow evaporator
channel,
accomplishes these objectives. For instance, these porous media inserts can be
brazed in each channel during furnace brazing of the entire heat exchanger,
chemically bonded or mechanically fixed in place. Furthermore, these inserts
can be
used as primary (and the only) expansion devices for low-cost applications or
as
secondary expansion devices, in case precise superheat control is required and
a
thermostatic expansion valve (TXV) or an electronic expansion valve (EXV) is
employed as a primary expansion device.
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[0012] Any suitable porous insert which accomplishes the above objectives
may be used. Suitable and inexpensive porous inserts may be made of sintered
metal, compressed metal, such as steel wool, specialty designed porous
ceramics,
etc. When inexpensive porous media insert is placed in each channel of the
parallel
flow evaporator, or at the entrance of each parallel flow evaporator channel,
it
represents a major resistance to the refrigerant flow within the evaporator.
In such
circumstances, the main pressure drop region will be across these inserts and
the
variations in the pressure drop in the channels or in the manifolds of the
parallel
flow evaporators will play a minor (insignificant) role. Further, since
refrigerant
expansion is taking place at the entrance to each channel, a predominantly
single-
phase liquid refrigerant is flown through the inlet manifold, especially in
the case
when the porous inserts are utilized as the primary and the only expansion
devices.
Hence, uniform refrigerant distribution is achieved, evaporator and system
performance is enhanced and, at the same time, precise superheat control is
not lost
(whenever required). Furthermore, low extra cost for the proposed method makes
this invention very attractive.
Brief Description of the Drawings
[0013] For a further understanding of the objects of the invention, reference
will be made to the following detailed description of the invention which is
to be
read in connection with the accompanying drawing, where:
[0014] Fig. 1 is a schematic illustration of a parallel flow heat exchanger in
accordance with the prior art.
[0015] Fig. 2 is a partial side sectional view of one embodiment of the
present invention.
[0016] Fig. 3 is an end view of a porous insert positioned at the entrance to
a
channel of the present invention.
[0017] Fig. 4 is a perspective view of the porous insert illustrated in Fig.
3.
[0018] Fig. 5a is a side sectional view illustrating a further embodiment of
the present invention.
[0019] Fig. 5b is a side sectional view illustrating yet a further embodiment
of the present invention.
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[0020] Fig. 6 is an end view of a plurality of channels in one embodiment of
the invention.
[0021] Fig. 7a is a perspective view which illustrates a porous cap
embodiment of the invention.
[0022] Fig. 7b is a perspective view which illustrates a second porous cap
embodiment.
[0023] Fig. 7c is a perspective view which illustrates a third porous cap
embodiment.
Description of the Preferred Embodiment
[0024] Referring now to Fig. 1, a parallel flow (minichannel or
microchannel) heat exchanger 10 is shown which includes an inlet header or
manifold 12, an outlet header or manifold 14 and a plurality of parallel
disposed
channels 16 fluidly interconnecting the inlet manifold 12 to the outlet
manifold 14.
Typically, the inlet and outlet headers 12 and 14 are cylindrical in shape,
and the
channels 16 are tubes (or extrusions) of flattened or round cross-section.
Channels
16 normally have a plurality of internal and external heat transfer
enhancement
elements, such as fins. For instance, external fins 18, uniformly disposed
therebetween for the enhancement of the heat exchange process and structural
rigidity, are typically furnace-brazed. Channels 16 may have internal heat
transfer
enhancements and structural elements as well.
[0025] In operation, refrigerant flows into the inlet opening 20 and into the
internal cavity 22 of the inlet header 12. From the internal cavity 22, the
refrigerant,
in the form of a liquid, a vapor or a mixture of liquid and vapor (the most
typical
scenario in the case of an evaporator with an expansion device located
upstream)
enters the channel openings 24 to pass through the channels 16 to the internal
cavity
26 of the outlet header 14. From there, the refrigerant, which is now usually
in the
form of a vapor, in the case of evaporator applications, flows out of the
outlet
opening 28 and then to the compressor (not shown). Externally to the channels
16,
air is circulated preferably uniformly over the channels 16 and associated
fins 18 by
an air-moving device, such as fan (not shown), so that heat transfer
interaction
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occurs between the air flowing outside the channels and refrigerant within the
channels.
[0026] According to one embodiment of the present invention, a porous
insert 30 is inserted at the entrance of each channel 16. When the channels 16
have
internal structural elements such as support members 16a (Fig. 3), usually
included
for structural rigidity and/or heat transfer enhancement purposes, the porous
inserts
30 incorporate slots 32 to accommodate the support members 16a when in
position
at the channel entrance (See Fig. 4). Further, in case a various degree of
expansion
and/or hydraulic impedance are desired to be provided by the inserts 30 or 32,
for
instance, to counter-balance other abovementioned factors effecting
refrigerant
distribution amongst the channels 16, characteristics such as porosity values
or
geometric dimensions (insert depth, insertion depth, etc.) of the inserts can
be altered
to achieved the desired result for each channel 16.
[0027] Fig. 5a illustrates another embodiment in which all the entrances to
the channels 16 are covered by a single porous member 34 positioned within a
manifold 40. Further, a support member 36 may be used to assist in setting up
a
relative position of the porous member 34 and the channels 16 within the
manifold
40. It should be noted that an assembly of the porous member 34 and support
member 36 can be manufactured from and combined in a single member made from
porous material.
[0028] Fig. 5b is a further embodiment of the structure of Fig. 5a in which
the porous member is a composite of two different porous materials 34 and 34a.
Obviously, a number of composite materials within the porous member can be
more
than two.
[0029] Fig. 6 illustrates a side view of Fig. 5a.
[0030] Fig. 7a illustrates a unitized elongated porous member 34b which
seals multiple channels 16 at a predetermined distance from the channel
entrance.
[0031] Fig. 7b illustrates an elongated porous member 34c which caps the
ends of multiple channels 16.
[0032] Fig. 7c a modification of the structure of Fig. 7b in which the porous
member 34d is accurate in shape and caps the ends of the channels 16. The
shape of
the porous member 34d can be of any suitable configuration, rather than a
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rectangular in cross-section. Further, the porous member 34d is preferably
positioned within the manifold 40 in such way that there is a gap between the
inner
wall of the manifold 40 and the porous member 34a allowing for more uniform
refrigerant distribution prior to entering the porous member 34d and channels
16.
[0033] It should be understood that any type of porous member and/or
material which accomplishes the objectives of the present invention may be
used.
Similarly, as illustrated by Figs. 2-7, any design or configuration which
accomplishes the objectives of the invention may be employed in the use of the
present invention.
[0034] Also, it has to be noted that the porous inserts can be used in the
condenser and evaporator applications within intermediate manifolds as well.
For
instance, if a heat exchanger has more than one refrigerant pass, an
intermediate
manifold (between inlet and outlet manifolds) is incorporated in the heat
exchanger
design. In the intermediate manifold, refrigerant is typically in a two-phase
state,
and such heat exchanger configurations can similarly benefit from the present
invention by incorporating the porous inserts into such intermediate
manifolds.
Further, the porous inserts can be placed into an inlet manifold of the
condenser and
an outlet manifold of the evaporator for providing only hydraulic resistance
uniformity and pressure drop control and with less effect on overall heat
exchanger
performance.
[0035] Since, for particular applications, the various factors that cause the
maldistribution of refrigerant to the channels are generally known at the
design
stage, the inventors have found it feasible to introduce the design features
that will
counter-balance them in order to eliminate the detrimental effects on the
evaporator
and overall system performance as well as potential compressor flooding and
damage. For instance, in many cases, it is generally known whether the
refrigerant
flows into the inlet manifold at a high or low velocity and how the
maldistribution
phenomenon is affected by the velocity values. A person of ordinarily skill in
the art
will recognize how to apply the teachings of this invention to other system
characteristics.
[0036] While the present invention has been particularly shown and
described with reference to the preferred embodiments as illustrated in the
drawing,
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it will be understood by one skilled in the art that various changes in detail
may be
effected therein without departing from the spirit and scope of the invention
as
defined by the claims.
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