PARALLEL-FLOW EVAPORATORS WITH LIQUID TRAP FOR PROVIDING BETTER FLOW DISTRIBUTION

EVAPORATEURS A FLUX PARALLELE POURVUS D'UN PIEGE A LIQUIDES POUR UNE MEILLEURE DISTRIBUTION DE FLUX

English Abstract

A parallel-flow evaporator has a liquid trap for regulating velocity of
refrigerant delivered to an evaporator from an expansion device. In its
simplest configuration, the liquid trap is a u-shaped pipe positioned
vertically and connected to an inlet manifold of the evaporator. By providing
a liquid trap, a small amount of liquid refrigerant separates from the vapor
phase at certain conditions. This separated liquid will tend to collect in the
trap, and reduce a flow cross-sectional area of the line leading to the inlet
manifold of the evaporator. As this cross-sectional area decreases, the
velocity of the refrigerant passing through the line will increase. In this
sense, as a small amount of liquid phase separates out, it will ensure that
the velocity of the remaining refrigerant will increase such that further
separation will be significantly reduced or entirely avoided. As a result,
homogeneous refrigerant flow is provided to the evaporator, resulting in its
performance enhancement and system reliability improvement.

French Abstract

Selon l'invention, un évaporateur à flux parallèle présente un piège à liquides conçu pour réguler la vitesse d'un frigorigène distribué à un évaporateur provenant d'un dispositif d'expansion. Sous sa configuration la plus simple, le piège à liquides est un tuyau en forme de U positionné verticalement et relié à un collecteur d'admission de l'évaporateur. La mise en place d'un piège à liquides permet de séparer une petite quantité de frigorigène liquide de la phase vapeur dans certaines conditions. Le liquide séparé tend à s'accumuler dans le piège et à diminuer une zone transversale de flux de la conduite menant au collecteur d'admission de l'évaporateur. A mesure que cette zone transversale se réduit, la vitesse du frigorigène traversant la conduite augmente. Ainsi, comme une petite quantité de phase liquide se sépare, cela garantit que la vitesse du frigorigène restant augmente, de telle manière qu'une séparation ultérieure diminuera considérablement ou sera complètement évitée. Par conséquent, un flux de frigorigène homogène est acheminé jusqu'à l'évaporateur, ce qui débouche sur l'amélioration de son efficacité et de la fiabilité du système.

Claims

CLAIMS
We claim:
1. A refrigerant system comprising:
a compressor delivering a compressed refrigerant to a condenser, refrigerant
passing from said condenser to an expansion device, and from said expansion
device
to an evaporator, said evaporator comprising an inlet manifold, outlet
manifold, a
plurality of channels receiving refrigerant from said inlet manifold and
delivering it to
said outlet manifold, and fins disposed between said channels; and
a line connecting said expansion device and said evaporator, said line being
provided with a liquid trap to collect liquid separated out of a vapor
refrigerant
passing from said expansion device to said evaporator.
2. The refrigerant system as set forth in claim 1, wherein said liquid trap
extends
vertically beneath said inlet manifold.
3. The refrigerant system as set forth in claim 1, wherein said liquid trap is
generally provided by a u-shape downwardly extending portion of said line.
4. The refrigerant system as set forth in claim 1, wherein said liquid trap is
positioned within 5 inches from said inlet manifold.
5. The refrigerant system as set forth in claim 1, wherein said refrigerant
system
is. also provided with an economizer circuit, said economizer circuit having
an
economizer heat exchanger, and said economizer heat exchanger being provided
with
a tap line connecting a main flow line through an economizer expansion device,
and
then into said economizer heat exchanger, said tap line being returned to an
intermediate compression point in said compressor downstream of said
economizer
heat exchanger, and a liquid trap to collect liquid separated out of a vapor
refrigerant
passing from said economizer expansion device to said economizer heat
exchanger.
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6. The refrigerant system as set forth in claim 1, wherein said liquid trap
includes
a plurality of serially spaced u-shaped liquid trap portions.
7. A method of operating a refrigerant system comprising the steps of:
providing an evaporator having a plurality of tubes receiving refrigerant from
an inlet manifold, and delivering said refrigerant to an outlet manifold, and
from said
outlet manifold to compressor, said compressor delivering the refrigerant to a
condenser, and said refrigerant passing from said condenser to an expansion
device,
and then back to said evaporator, and providing a fluid line connecting said
expansion
device to said evaporator, said fluid line being provided with a liquid trap
to capture
liquid that has separated from a vapor refrigerant; and
passing refrigerant through said refrigerant system and such that liquid trap
self-regulates a velocity of refrigerant as the liquid separates from the
vapor
refrigerant to deliver refrigerant into said inlet manifold in a predominantly
homogeneous state.
8. The method as set forth in claim 7, wherein the refrigerant system further
being provided with an economizer circuit, said economizer circuit including
an
economizer heat exchanger, and tapping refrigerant and passing the tapped
refrigerant
through an economizer expansion device into said economizer heat exchanger,
and a
liquid trap provided to capture liquid that has separated from a vapor passing
from
said economizer expansion device into said economizer heat exchanger, and
further
including the steps of passing refrigerant through said economizer expansion
device,
and to said economizer heat exchanger, such that said liquid trap self-
regulates a
velocity of refrigerant as the liquid separates from the vapor refrigerant to
deliver
refrigerant into said economizer heat exchanger in a predominantly
homogeneously
state.
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9. A heat exchanger and fluid line system comprising:
a fluid line leading into an inlet manifold;
a liquid trap on said fluid line; and
a heat exchanger having a plurality of channels receiving a fluid from said
inlet manifold.
10. The heat exchanger and fluid line system as set forth in claim 9, wherein
said
heat exchanger is a refrigerant system evaporator.
11. The heat exchanger and fluid line system as set forth in claim 9, wherein
said
heat exchanger is a refrigerant system economizer heat exchanger.
12. The heat exchanger and fluid line system as set forth in claim 9, wherein
said
liquid trap extends vertically beneath said inlet manifold.
13. The heat exchanger and fluid line system as set forth in claim 9, wherein
said
liquid trap is generally provided by a u-shape downwardly extending portion of
said
line.
14. The heat exchanger and fluid line system as set forth in claim 9, wherein
said
liquid trap is positioned within 5 inches from said inlet manifold.
15. The heat exchanger and fluid line system as set forth in claim 9, wherein
said
liquid trap is provided by a plurality of serially spaced u-shaped structures.
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Description

CA 02604466 2007-10-10
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PARALLEL-FLOW EVAPORATORS WITH LIQUID TRAP
FOR PROVIDING BETTER FLOW DISTRIBUTION
BACKGROUND OF THE INVENTION
This invention relates to a parallel-flow evaporator wherein a liquid trap is
positioned upstream of an inlet manifold to provide better flow distribution
among
parallel channels, improved heat transfer and enhanced system reliability.
Refrigerant systems are utilized to control the temperature and humidity of
air
in various indoor environments to be conditioned. In a typical refrigerant
system
operating in the cooling mode, a refrigerant is compressed in a compressor and
delivered to a condenser (or an outdoor heat exchanger in this case). In the
condenser, heat is exchanged between outside ambient air and the refrigerant.
From
the condenser, the refrigerant passes to an expansion device, at which the
refrigerant
is expanded to a lower pressure and temperature, and then to an evaporator (or
an
indoor heat exchanger if the system operates in the cooling mode). In the
evaporator,
heat is exchanged between the refrigerant and the indoor air, to condition the
indoor
air. When the refrigerant system is operating in the cooling mode, the
evaporator
cools and typically dehumidifies the air that is being supplied to the indoor
environment.
One type of evaporator that could be utilized in refrigerant systems is a
parallel-flow evaporator. Such evaporators have several parallel channels for
communicating refrigerant between an inlet manifold and an outlet manifold.
Each
channel typically has numerous parallel internal paths of various cross-
sectional shape
separated by internal walls. Corrugated fins are disposed in between the
channels for
heat transfer enhancement and structural rigidity. Usually, the channels,
manifolds
and fins are constructed from similar materials such as aluminum and are
attached to
each other by furnace brazing. Recently, parallel-flow evaporators have
attracted a lot
of attention and interest in the air-conditioning field due to their superior
performance, compactness, rigid construction, and enhanced resistance to
corrosion.
However, one concern with parallel-flow evaporators is maldistribution of the
refrigerant among their channels. The maldistribution problem in the parallel-
flow
evaporators is typically caused by the liquid phase separating from the vapor
in the
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inlet manifold due to gravity combined with insufficient refrigerant velocity,
and thus
manifests itself in unequal amounts of vapor and liquid refrigerant passing
through the
evaporator channels. Additional phenomena effecting maldistribution can be
attributed to different distances the refrigerant must flow to reach various
channels
and to exit them, unequal pressure impedances and variations in the heat
transfer rates
between the channels, etc.
Known parallel-flow evaporators typically have inlet and outlet manifolds that
are cylindrical in shape. The channels are typically made of identical
aluminum
extrusions that form flat tubes. As the two-phase refrigerant enters the inlet
manifold,
the vapor phase is often separated from the liquid phase. Since the two phases
will
move independently from each other after separation, the problem of
refrigerant
maldistribution often arises.
When such maldistribution occurs, the heat exchanger performance drops
significantly, frequently resulting in liquid refrigerant leaving the outlet
manifold.
This liquid refrigerant can cause serious reliability problems and permanent
compressor damage. Obviously, this is undesirable.
SUMMARY OF THE INVENTION
In a disclosed embodiment of this invention, a parallel-flow evaporator is
provided with a liquid trap upstream of its inlet manifold. In this manner,
should the
refrigerant be moving at a speed such that the liquid phase will not separate
from the
vapor phase, it can flow through the trap, into the manifold, and into the
evaporator
channels in a generally equal distribution. However, should the refrigerant be
moving
at reduced speed, such that separation of liquid is likely to occur, then the
liquid will
tend to separate and accumulate in the liquid trap. As the liquid accumulates
in the
liquid trap, the flow cross-sectional area for the remainder of the
refrigerant will
become smaller. Since the flow cross-sectional area becomes smaller, then the
refrigerant velocity will increase, creating a jetting effect that will carry
droplets of
liquid into the inlet manifold and will limit further phase separation. This
phenomenon will be self-regulating, to ensure that an adequate refrigerant
velocity
will be maintained such that the refrigerant liquid will tend not to separate
from the
vapor.
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In one embodiment, rather than having a single u-shaped trap, a serpentine
path provides by a number of such u-shaped structures is utilized.
In another disclosed embodiment, the refrigerant system is provided with an
economizer circuit, and the liquid trap is utilized on a line directing the
tapped two-
phase refrigerant mixture into the economizer heat exchanger. This embodiment
will
provide the benefit and function as with regard to the first disclosed
embodiment.
These and other features of the present invention can be best understood from
the following specification and drawings, the following of which is a brief
description.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional view of an evaporator incorporating the present
invention.
Figure 2 shows the Figure 1 evaporator in a different flow condition.
Figure 3 shows another embodiment.
Figure 4 shows yet another embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A refrigerant system 20 is illustrated in Figure 1 having a parallel-flow
evaporator 22. As is known, refrigerant moves from the evaporator 22
downstream to
a compressor 24, a condenser 26, through an expansion device 28, and back to
the
evaporator 22. The refrigerant leaving the expansion device 28 is in a mixed
vapor
and liquid state. The evaporator 22 has a plurality of parallel channels 32
spaced
along an inlet manifold 34. The channels 32 and inlet manifold 34 are in fluid
communication with each other. Further, the channels 32 are similarly
positioned and
communicated with an outlet manifold 35. Fins 30 are disposed between the
channels
32. The channels 32, fins 30, inlet manifold 34, and outlet manifold 35 are
typically
attached to each other by furnace brazing. As is known, air is passed over the
fins 30
and channels 32 to be conditioned. Due to heat transfer interaction with air
supplied
to a conditioned space, refrigerant evaporates inside the channels 32.
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As mentioned above, should the velocity of the refrigerant approaching the
inlet manifold 34 be insufficiently low, it may cause liquid refrigerant to
separate
from the vapor. This can result in a poor distribution of the two refrigerant
phases
among the channels 32. As shown in Figure 1, the refrigerant is moving at an
adequate velocity, and little or no separation of refrigerant phases occurs.
A tube 36 leading into the inlet manifold 34 is positioned downstream of a
liquid trap 38. As illustrated, the liquid trap 38 generally extends
vertically in a u-
shape. Thus, any liquid that tends to separate will collect in the liquid trap
38.
As shown in Figure 2, the refrigerant velocity is insufficiently low in
comparison to the Figure 1 condition to prevent phase separation, and a
certain
quantity of liquid refrigerant 40 has collected in the trap 38. As a result,
the cross-
sectional area 42 remaining for the flow of refrigerant decreases
significantly. This in
turn increases the velocity of the refrigerant passing to the inlet manifold
34. As the
velocity of the refrigerant flow increases, the vapor refrigerant will tend to
carry its
liquid phase to the channels 32 in a homogeneous manner to ensure generally
equal
distribution. In effect, a jetting zone is created to increase velocity and
limit
additional phase separation. Thus, by including the liquid trap 38 upstream of
the
header 34, the present invention self-regulates the velocity of the
refrigerant and
ensures that other than the initial separation of a small quantity of liquid
refrigerant
40, the remaining liquid refrigerant will tend not to separate form the vapor
phase
resulting in homogeneous flow conditions in the inlet manifold 34. Of course,
the
inlet manifold 34 should be of an appropriate cross-sectional area and length
to
sustain this flow homogeneity. Also, the liquid trap 38 should be positioned
in close
proximity to the inlet manifold 34. Preferably, the liquid trap 38 should be
located
within 5 inches from the entrance to the inlet manifold 34 and extend
vertically
beneath it. Consequently, the evaporator performance is improved. This will
also
result in no liquid refrigerant in the evaporator outlet manifold 35 and
system
reliability enhancement.
While this invention is disclosed in a conventional evaporator, other heat
exchangers, for instance economizer heat exchangers (or so-called brazed plate
heat
exchangers) also performing an evaporator function, may equally benefit from
this
invention.
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Further, although the liquid trap 38 is shown in its simplest configuration,
other arrangements (such as multiple u-shape segments connected together,
local flow
impedances, etc.) are also feasible.
Another embodiment 100 shown in Figure 3 has a plurality of serial u-shaped
traps 102 upstream of the portion 104 leading into the inlet manifold 34. Each
liquid
trap 102 can collect small amount of liquid refrigerant, increasing velocity
of the
vapor phase and promoting homogeneous conditions at the entrance of the inlet
manifold 34.
Another refrigerant system embodiment 110 is illustrated in Figure 4. In this
embodiment, a compressor 112 delivers a compressed refrigerant to a condenser
114.
A line 116 is tapped off of a main refrigerant flow line 126, and passed
through an
economizer expansion device 118. A liquid trap 120 regulates the refrigerant
passing
through an inlet 122, to an economizer heat exchanger 124. The liquid trap 120
will
provide the function and will operate as described with regard to the Figures
1 and
Figure 2 embodiments. It should be understood that the economizer heat
exchanger
124 is structured to have adjacent channels such that heat is exchanged
between the
refrigerant in the tap line 116 and the refrigerant in the main flow line 126.
The main
flow line 126 delivers refrigerant to an outlet 128 and passes it through a
main
expansion device 130 to an evaporator 132. The present invention can utilize
the
liquid trap with both the economizer heat exchanger 124, and the evaporator
132. The
refrigerant returns from the evaporator 132 back to the compressor 112. A line
134
downstream of the economizer heat exchanger 124 returns the tapped refrigerant
back
to an intermediate compression point in the compressor 112.
It has to be pointed out that although all inlet manifolds are shown in a
horizontal configuration, the maldistribution phenomenon is more pronounced in
a
vertical orientation. In such circumstances, the benefits of the present
invention
become even more pronounced.
Although a preferred embodiment of this invention has been disclosed, a
worker of ordinary skill in this art would recognize that certain
modifications would
come within the scope of this invention. For that reason, the following claims
should
be studied to determine the true scope and content of this invention.
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Representative Drawing