U.S. patent application number 11/597908 was filed with the patent office on 2008-01-17 for microchannel-type evaporator and system using the same.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. Invention is credited to Yutaka Tasaki.
Application Number | 20080011462 11/597908 |
Document ID | / |
Family ID | 35450976 |
Filed Date | 2008-01-17 |
United States Patent
Application |
20080011462 |
Kind Code |
A1 |
Tasaki; Yutaka |
January 17, 2008 |
Microchannel-Type Evaporator and System Using the Same
Abstract
In an evaporator 1, space between two heat transfer plates 2
opposite to each other serves as a liquid path 3, and the outsides
of the heat transfer plates 2 serve as a gas path 4. At the lower
end of the liquid path 3, a liquid inlet, through which liquid to
be evaporated is supplied to the evaporator 1, is provided, and at
the upper end of the liquid path 3, a vapor outlet is provided. The
liquid to be evaporated vaporizes while flowing from bottom to top.
The heating gas is supplied form a gas inlet 7, which is provided
at the upper end of the evaporator, and discharged from a gas
outlet 8, which is provided at the lower end of the evaporator.
Size of space S of the liquid path 3 gradually increases from
bottom to top in a gas-liquid two phase region 11.
Inventors: |
Tasaki; Yutaka;
(Kanagawa-ken, JP) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
NISSAN MOTOR CO., LTD.
|
Family ID: |
35450976 |
Appl. No.: |
11/597908 |
Filed: |
April 19, 2005 |
PCT Filed: |
April 19, 2005 |
PCT NO: |
PCT/JP05/07428 |
371 Date: |
November 29, 2006 |
Current U.S.
Class: |
165/147 ;
165/170 |
Current CPC
Class: |
F28D 2021/0071 20130101;
F28F 13/08 20130101; F28D 2021/0064 20130101 |
Class at
Publication: |
165/147 ;
165/170 |
International
Class: |
F28F 3/12 20060101
F28F003/12 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2004 |
JP |
2004-162011 |
Sep 1, 2004 |
JP |
2004-254611 |
Claims
1. A microchannel-type evaporator, comprising: a path provided
substantially vertically, through which a liquid to be evaporated
passes, wherein a space size of the path is smaller than diameters
of bubbles departing from a heat transfer surface of the path, and
the space size of the path in a gas-liquid two phase region is a
minimum size satisfying that a heat flux is not more than a
critical heat flux with respect to a quality.
2. A microchannel-type evaporator according to claim 1, wherein the
space size of the path varies depending on a position in the path
in the evaporator.
3. A microchannel-type evaporator according to claim 2, wherein the
space size of the path varies along a direction of a flow of the
liquid to be evaporated.
4. A microchannel-type evaporator according to claim 2, wherein the
space size of the path in a gas phase region gradually decreases in
a direction of a flow of the liquid to be evaporated.
5. A microchannel-type evaporator according to claim 3, wherein the
flow of the liquid to be evaporated is substantially opposite to a
flow of a heating gas, and the space size of the path in the
gas-liquid two phase region gradually increases in the direction of
the flow of the liquid to be evaporated.
6. A microchannel-type evaporator according to claim 3, wherein the
flow direction of the liquid to be evaporated and a flow direction
of a heating gas are substantially in a same direction, and a
channel space of the path in the gas-liquid two phase region
gradually decreases in the direction of the flow of the liquid to
be evaporated.
7. A microchannel-type evaporator according to claim 2, wherein the
flow of the liquid to be evaporated is substantially orthogonal to
a flow of a heating gas, and a channel space of the liquid to be
evaporated varies in a direction of a high temperature gas.
8. A microchannel-type evaporator according to claim 7, further
comprising: a plurality of turning sections in a heating gas
path.
9. A microchannel-type evaporator according to claim 1, wherein
flow directions of the liquid to be evaporated and a heating gas
are substantially in a same direction, and a thin liquid film is
formed on the heat transfer surface by controlling a mass flow rate
and/or a temperature of a heating gas by use of a map of the mass
flow rate and a temperature of the heating gas with respect to a
mass flow rate and a temperature of the liquid to be
evaporated.
10. A system using a microchannel evaporator, comprising: a
microchannel-type evaporator comprising a path provided
substantially vertically, through which a liquid to be evaporated
passes, wherein a space size of the path is smaller than diameters
of bubbles departing from a heat transfer surface of the path, and
the space size of the path in a gas-liquid two phase region is a
minimum size satisfying that a heat flux is not more than a
critical heat flux with respect to a quality; and a superheater
which further heats vapor from the microchannel-type evaporator to
generate superheated vapor.
11. A system using a microchannel evaporator according to claim 10,
wherein, in the microchannel-type evaporator, flow directions of
the liquid to be evaporated and a heating gas are substantially in
a same direction, and when a mass flow rate of the heating gas is
not less than a prescribed value, the heating gas is supplied to
the superheater and the microchannel-type evaporator in
parallel.
12. A system using a microchannel evaporator according to claim 10,
wherein, in the microchannel-type evaporator, flow directions of
the liquid to be evaporated and a heating gas are substantially in
a same direction, and when a temperature of the heating gas is not
less than a prescribed value, the heating gas is supplied to the
superheater and then supplied to the microchannel-type
evaporator.
13. A system using a microchannel evaporator according to claim 10,
wherein, in the microchannel-type evaporator, flow directions of
the liquid to be evaporated and a heating gas are substantially in
a same direction, and when a mass flow rate and a temperature of
the heating gas are not less than respective prescribed values, the
heating gas is supplied to the superheater and the
microchannel-type evaporator in parallel, and the heating gas
discharged from the superheater is further supplied to the
microchannel-type evaporator.
Description
TECHNICAL FIELD
[0001] The present invention relates to a microchannel-type
evaporator in which a path for a liquid to be evaporated is
narrower than diameter of departing bubbles and to a system using
the same.
BACKGROUND ART
[0002] In a fuel cell, fuel gas such as hydrogen and oxidant gas
containing oxygen are electrochemically reacted through
electrolyte, and electric energy is directly extracted from
electrodes provided on both sides of the electrolyte. A polymer
electrolyte fuel cell using a solid polymer electrolyte operates at
low temperature and is easy to use. Accordingly, the polymer
electrolyte fuel cell has attracted attention as a power supply for
vehicles.
[0003] As a method of supplying hydrogen to the fuel cell, there
are a method of directly supplying hydrogen from a hydrogen storage
unit such as high-pressure hydrogen tank or a hydrogen storage
alloy tank and a fuel reforming method of extracting hydrogen from
fuel such as methanol or hydrocarbon and supplying the same. In the
fuel reforming method, when the fuel is liquid, fuel or water is
evaporated by an evaporator and then introduced to a fuel reformer,
in which hydrogen is generated by a fuel reforming reaction.
[0004] As a small evaporator with high efficiency which is suitable
for vehicles, an evaporator for air conditioning which evaporates
refrigerant has been known (see Japanese Patent No. 2786728).
DISCLOSURE OF INVENTION
[0005] However, the conventional evaporator has a problem of
reduction of heat exchange capability in a region of high heat
flux. Hereinafter, such a problem is described.
(Definition of Microchannel)
[0006] In the present invention, the microchannel is defined as
follows. A channel whose space is smaller than diameter of bubbles
departing from heat transfer surface is defined as a microchannel.
In other words, when bubbles which are yet smaller than departure
diameter are crushed by walls of a channel to form microlayers
between the bubbles and heating surfaces, such a channel is defined
as a microchannel.
[0007] The diameter of bubbles having departed depends on a type of
liquid to be evaporated, surface properties of heat transfer
plates, and degree of superheat. Specifically, as shown in FIG. 19,
when the liquid to be evaporated is water, the diameter is about
0.8 mm in the case of hydrophilic titanium oxide coating, and the
diameter is about 2.5 mm in the case of hydrophobic silicone
coating. Accordingly, in the case of titanium oxide coating, the
space size of the microchannel in the heating surfaces is set to
not more than 0.8 mm. Note that the relation between degree of
surface superheat and bubble diameter shown in FIG. 19 is for the
case where the heat transfer surfaces are planar. Moreover, in
polishing in the drawing, green carborundum of #2000 is used.
(Need for Formation of Thin Liquid Film on Heat Transfer
Surface)
[0008] As for a parallel plate-type evaporator, FIG. 20A shows a
relation between degree of superheat in the heat transfer surfaces
and heat flux in cases where the space between the heat transfer
surfaces is small and large. The case of small space between the
heat transfer surfaces is indicated by a solid line, and the case
of large space is indicated by a dashed line.
[0009] As shown in FIG. 20A, the two lines intersect with each
other at a degree of superheat and a heat flux. When the space
between the heat transfer surfaces is small, the evaporator
exhibits good heat transfer characteristics in a region of low heat
flux. However, the heat transfer characteristics are degraded in a
region of high heat flux, and accordingly the critical heat flux
(CHF) is reduced. On the contrary, when the space between the heat
transfer surfaces is large, the heat transfer characteristics are
good in the region of high heat flux but are degraded in the region
of low heat flux.
[0010] The mechanism of such characteristics is conceived to be
reduction of the heat transfer coefficient due to dryout of the
heat transfer surfaces. FIG. 20B shows variations in flow and ways
of heat transfer in an evaporation tube. Upper part in the drawing
shows downstream of the evaporation tube, and lower part in the
drawing shows upstream of the same. Liquid in the evaporation tube
is substantially 100% in a liquid phase in the upstream but is in a
two-phase state in the downstream in which the content of bubbles
gradually increases in the liquid phase toward the downstream. In
the downstream of the dryout position (post-dryout), the liquid to
be evaporated does not exist on the heat transfer surfaces, and the
heat transfer surfaces are completely dry. In a post-dryout
dispersed flow region, since there is no liquid always in contact
with the heat transfer surfaces, significant degradation of the
heat transfer characteristics is shown.
[0011] FIG. 20C shows a relation between the heating heat flux and
the heat transfer coefficient in the evaporation tube. In the
drawing, the heat flux is higher in the order of A, B, C, and D.
The drawing shows that as the heat flux increases, the region of
the heat transfer coefficient reduced, or the dispersed flow
region, moves to the upstream. In order to improve the reduction of
the heat transfer coefficient, it is necessary to supply the liquid
to be evaporated to the dispersed flow region and maintain the
transfer heat surfaces to be wet.
[0012] FIG. 21 shows a boiling state of a microchannel in the high
heat flux region. Lower part in the drawing shows the upstream, and
upper part shows the downstream. Liquid 106 to be evaporated is
supplied from the upstream of a heat transfer surface 101, and
vapor 107 is discharged to the downstream. The liquid 106 to be
evaporated is heated by the heat transfer surface 101 into
dispersed flow 105 in the downstream of a gas-liquid interface
104.
[0013] In the heat transfer surface 101, the gas-liquid interface
104 as an interface between a wet region in the upstream of the
dryout position and a dispersed flow region 103 moves toward the
upstream as the heat flux increases. Accordingly, when the heat
flux is increased, the wet region 102, where heat transfer is
effectively performed, is reduced, so that the heat exchange
efficiency of the entire heat transfer surfaces is reduced. In
order to increase the heat exchange efficiency, it is necessary to
increase the proportion of the wet region 102 having high heat
transfer efficiency in the heat transfer surface.
[0014] FIG. 22 shows states of boiling at heat fluxes of 9, 14, and
19 kW/m.sup.2. As shown in FIG. 22, along with an increase in heat
flux, the wet area in the heat transfer surfaces decreases. Arrows
in the drawing indicate the downstream direction.
[0015] FIG. 23A shows the heat transfer coefficient to a quality of
the liquid to be evaporated, and FIG. 23B shows the critical heat
flux to the quality of the liquid to be evaporated. As shown in
FIG. 23A, in order to increase the heat transfer coefficient of the
evaporator, the heat transfer surfaces need to maintain a wet
region with a quality smaller than that at a dryout point A
(quality X.sub.a). As shown in FIG. 23B, the heat flux at the
boundary between the pre-dryout region and the post-dryout region
slopes down from left to right with respect to an increase in a
quality. A heat flux corresponding to the quality X.sub.a is
q.sub.ca, and the heat flux needs to be controlled so as to be
smaller than this value.
[0016] The present invention was made in the light of the
aforementioned problem, and an object of the invention is to
increase the heat exchange efficiency of a microchannel-type
evaporator and reduce the size thereof.
[0017] To achieve aforementioned object, a microchannel-type
evaporator according to an aspect of the present invention includes
a path provided substantially vertically, through which a liquid to
be evaporated passes, and is characterized in that a space size of
the path is smaller than diameters of bubbles departing from a heat
transfer surface of the path, and the space size of the path in a
gas-liquid two phase region is a minimum size satisfying that a
heat flux is not more than a critical heat flux with respect to a
quality.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1(a) is a cross-sectional view showing a configuration
of Embodiment 1 of a microchannel-type evaporator according to the
present invention. FIG. 1(b) is a conceptual view showing a liquid
path of the microchannel-type evaporator of Embodiment 1. FIG. 1(c)
is a conceptual view showing a heating gas path of the
microchannel-type evaporator of Embodiment 1.
[0019] FIG. 2 is a graph for explaining a critical heat flux and a
heat flux with respect to a quality in Embodiment 1.
[0020] FIG. 3(a) is a cross-sectional view showing a configuration
of Embodiment 2 of the microchannel-type evaporator according to
the present invention. FIG. 3(b) is a conceptual view showing a
liquid path of the microchannel-type evaporator of Embodiment 2.
FIG. 3(c) is a conceptual view showing a heating gas path of the
microchannel-type evaporator of Embodiment 2.
[0021] FIG. 4 is a graph for explaining a critical heat flux and a
heat flux with respect to a quality in Embodiment 2.
[0022] FIG. 5(a) is a cross-sectional view showing a configuration
of Embodiment 3 of the microchannel-type evaporator according to
the present invention. FIG. 5(b) is a conceptual view showing a
liquid path of the microchannel-type evaporator of Embodiment 3.
FIG. 5(c) is a conceptual view showing a heating gas path of the
microchannel-type evaporator of Embodiment 3.
[0023] FIG. 6 is a graph for explaining a critical heat flux and a
heat flux with respect to a quality in Embodiment 3.
[0024] FIG. 7(a) is a cross-sectional view showing a configuration
of Embodiment 4 of the microchannel-type evaporator according to
the present invention. FIG. 7(b) is a conceptual view showing a
liquid path of the microchannel-type evaporator of Embodiment 4.
FIG. 7(c) is a conceptual view showing a heating gas path of the
microchannel-type evaporator of Embodiment 4. FIG. 7(d) is a
cross-sectional view taken along a line VIId-VIId in FIG. 7(b).
[0025] FIG. 8 is a graph for explaining a critical heat flux and a
heat flux with respect to a quality in Embodiment 4.
[0026] FIG. 9(a) is a cross-sectional view showing a configuration
of Embodiment 5 of the microchannel-type evaporator according to
the present invention. FIG. 9(b) is a conceptual view showing a
liquid path of the microchannel-type evaporator of Embodiment 5.
FIG. 9(c) is a conceptual view showing a heating gas path of the
microchannel-type evaporator of Embodiment 5.
[0027] FIG. 10(a) is a cross-sectional view showing a configuration
of Embodiment 6 of the microchannel-type evaporator according to
the present invention. FIG. 10(b) is a conceptual view showing a
liquid path of the microchannel-type evaporator of Embodiment 6.
FIG. 10(c) is a conceptual view showing a heating gas path of the
microchannel-type evaporator of Embodiment 6.
[0028] FIG. 11(a) is a cross-sectional view showing a configuration
of Embodiment 7 of the microchannel-type evaporator according to
the present invention. FIG. 11(b) is a conceptual view showing a
liquid path of the microchannel-type evaporator of Embodiment 7.
FIG. 11(c) is a conceptual view showing a heating gas path of the
microchannel-type evaporator of Embodiment 7.
[0029] FIG. 12(a) is a cross-sectional view showing a configuration
of Embodiment 8 of the microchannel-type evaporator according to
the present invention. FIG. 12(b) is a conceptual view showing a
liquid path of the microchannel-type evaporator of Embodiment 8.
FIG. 12(c) is a conceptual view showing a heating gas path of the
microchannel-type evaporator of Embodiment 8.
[0030] FIG. 13 is a graph for explaining a critical heat flux and a
heat flux with respect to quality in Embodiment 8.
[0031] FIG. 14 is a graph showing a relation between an inlet
condition of pure water and a boiling region based on the
characteristics of FIG. 13.
[0032] FIG. 15 is a schematic view showing a heating gas flow
pattern of Case A in a system of Embodiment 8.
[0033] FIG. 16 is a schematic view showing a heating gas flow
pattern of Case B in the system of Embodiment 8.
[0034] FIG. 17 is a schematic view showing a heating gas flow
pattern of Case C in the system of Embodiment 8.
[0035] FIG. 18(a) is a schematic view showing the heating gas flow
pattern of Case A. FIG. 18(b) is a schematic view showing the
heating gas flow pattern of Case B. FIG. 18(c) is a schematic view
showing a heating gas flow pattern of Case C.
[0036] FIG. 19 is a graph showing diameter of departing bubbles
depending on surface properties of a heat transfer surface and
degree of surface superheat.
[0037] FIG. 20A is a graph for explaining a relation between degree
of superheat and heat flux in the microchannel-type evaporator.
[0038] FIG. 20B is a view for explaining variations in flow and
heat transfer ways in an evaporation tube.
[0039] FIG. 20C is a graph for explaining a relation between a
heating heat flux and a heat transfer coefficient.
[0040] FIG. 21 is a view showing a boiling state of the
microchannel in a high heat flux region.
[0041] FIG. 22 graphically shows photographs of boiling states at
heat fluxes of 9, 14, and 19 kW/m.sup.2.
[0042] FIG. 23A is a graph showing the heat transfer coefficient
with respect to a quality.
[0043] FIG. 23B is a graph showing a critical heat flux with
respect to a quality.
BEST MODE FOR CARRYING OUT THE INVENTION
[0044] With reference to the drawings, a description is given of
embodiments of a microchannel-type evaporator according to the
present invention in detail. The evaporator according to the
present invention, which is not particularly limited, is suitable
for an evaporator which evaporates water or hydrocarbon type fuel
and supplies the same to a fuel reformer for a fuel cell.
Embodiment 1
[0045] FIGS. 1(a), 1(b), and 1(c) show a basic constituent unit of
Embodiment 1 of the evaporator according to the present
invention.
[0046] An evaporator 1 in this embodiment includes two heat
transfer plates 2 opposite to each other. Between the heat transfer
plates 2, a path 3 through which liquid to be evaporated passes is
provided. The path 3 through which liquid to be evaporated passes
is placed vertically, that is, along the direction G of gravity
force. In the present invention, vertically setting the liquid path
means setting the liquid path at such an angle that heat transfer
properties in the right and left heat transfer surfaces which form
a microchannel do not significantly lose symmetry because of the
inclination thereof and, for example, includes setting the same at
an angle of .+-. about 20 degrees from the vertical.
[0047] Furthermore, in the outside of the heat transfer plates 2,
paths 4 through which heating gas passes, is provided. To form a
real evaporator, it is preferable that the evaporator has a
structure in which a plurality of the basic constituent units shown
in the drawing are arranged in parallel. In this description, the
path 3 through which the liquid to be evaporated passes is
abbreviated as the liquid path 3, and the path 4 through which the
heating gas passes is abbreviated as the gas path 4.
[0048] At the lower end of the liquid path 3, a liquid inlet 5,
through which the liquid to be evaporated is supplied to the
evaporator 1, is provided. At the upper end of the liquid path 3, a
vapor outlet 6 is provided. The liquid to be evaporated evaporates
as flowing from the bottom to the top of the evaporator 1. On the
other hand, the heating gas is supplied from gas inlets 7, which
are provided at the upper end of the evaporator, and discharged
from gas outlets 8, which are provided at the lower end of
evaporator. The evaporator of Embodiment 1 is therefore a
countercurrent flow type evaporator in which the flow directions of
the liquid to be evaporated and the heating gas are opposite to
each other. During operation of the evaporator 1, the inside of the
liquid path 3 is composed of a liquid phase region 10, a gas-liquid
two phase region 11, and a gas phase region 12 sequentially from
the bottom.
[0049] Size of space S of the liquid path 3 gradually increases
from the bottom to the top in the gas-liquid two phase region
11.
[0050] Material of the heat transfer plates 2 can be a
corrosion-resistant metal, for example, stainless steel, titanium,
and titanium alloy.
[0051] Furthermore, the surface of each heat transfer plate 2 is
coated with titanium oxide or the like for hydrophilic treatment.
This can provide higher capillary pressure and more penetration
into the heating surfaces, thus further increasing the critical
heat flux.
[0052] In this embodiment, the mass flow rate of the heating gas is
sufficiently higher than that of the liquid to be evaporated, and
there is almost no change in temperature of the heating gas between
the gas inlets 7 and the gas outlets 8.
[0053] FIG. 2 shows relations between the quality and the critical
heat flux or heat flux according to variations in the space S of
the liquid path 3 in the gas-liquid two phase region 11. The
horizontal axis of FIG. 2 represents the quality as a mass ratio of
vapor and a mixture of gas and liquid which exist in the
evaporation tube. The vertical axis of FIG. 2 represents heat flux
q and critical heat flux q.sub.c (kW/m.sup.2). A quality of 0
indicates that all the liquid to be evaporated is in a liquid
state, and a quality of 1 indicates that all the liquid to be
evaporated is in a gas state.
[0054] The gas-liquid two phase region 11 of FIGS. 1(a) and 1(b)
can be considered to be a region in which the quality is 0 and 1 at
the lower and upper ends, respectively, and gradually increases
therebetween.
[0055] When such quality and critical heat flux are represented by
the horizontal and vertical axes, respectively, the correlation
between the quality X and the critical heat flux q.sub.c in cases
of spaces S.sub.A, S.sub.B, and S.sub.C
(S.sub.A>S.sub.B>S.sub.C) of the path of the liquid to be
evaporated in the parallel plate microchannel-type evaporator is
shown as FIG. 2.
[0056] When the entire evaporation tube has a same space, in part
of smaller quality, the ratio of liquid is high, and a larger
amount of heat is transferred from the heat transfer plates.
Accordingly, the critical heat flux tends to be higher. When the
space size of the evaporation tube is increased, a larger amount of
the liquid to be evaporated is held in the space, and the critical
heat flux tends to increase.
[0057] Herein, when the path space is increased with respect to the
path space S.sub.B as a criterion, the critical heat flux
increases, and the rate of the increase is higher when the quality
is smaller. On the contrary, when the path space is reduced, the
critical heat flux is reduced, and the rate of the reduction is
higher when the quality is smaller.
[0058] In this embodiment, it is assumed that the mass flow rate of
the heating gas is sufficiently higher than that of the liquid to
be evaporated and there is almost no change in temperature of the
heating gas between the gas inlets 7 and the gas outlets 8. The
heat flux given from the heating gas through the heat transfer
plates to the liquid to be evaporated is therefore constant
regardless of the quality and is indicated by a dashed line of FIG.
2. The dashed line indicating the heat flux and a line indicating
the critical heat flux of the path space S.sub.B intersect with
each other at a quality X.sub.B. In other words, the graph shows
that the critical heat flux is supplied at the position of the
quality X.sub.B of the evaporator with the path space S.sub.B. In a
region with a quality of not less than the quality X.sub.B,
supplied heat flux exceeds the critical heat flux, so that the heat
transfer surfaces change from the wet state to the dry state. When
the heat transfer surfaces change to the dry state, the heat
transfer coefficient is reduced. Accordingly, the heat flux is
drastically reduced as indicated by a chain line of FIG. 2.
[0059] In this embodiment, to avoid such reduction of heat flux, in
the region with a quality X of not less than X.sub.B, the path
space is increased to S.sub.B+1.sub.--.sub.case 1,
S.sub.B+2.sub.--.sub.case 1, and S.sub.B+3.sub.--.sub.case 1 as the
quality increases to X.sub.B+1, X.sub.B+2, and X.sub.B+3. This can
prevent the heat flux from exceeding the critical heat flux even if
the quality becomes larger than X.sub.B, thus providing a maximum
heat transfer coefficient.
[0060] In a similar way, in a region with a quality X of not more
than X.sub.B, even if the path space is reduced to
S.sub.B-1.sub.--.sub.case 1 and S.sub.B-2.sub.--.sub.case 1 as the
quality decreases to X.sub.B-1 and X.sub.B-2, the heat transfer
surfaces can be maintained to be wet. The heat flux can be
therefore within the critical heat flux even if the quality becomes
smaller than X.sub.B, thus providing a maximum heat transfer
coefficient.
[0061] As described above, in the microchannel-type evaporator of
countercurrent flow type of the present invention, the space S of
the liquid path is set to a minimum space size satisfying that heat
flux is not more than the critical heat flux according to the
quality X, so that thin liquid films of the liquid to be evaporated
is formed on part of the heat transfer surfaces which comes into
contact with the gas-liquid two phase region of the liquid path.
The part of the heat transfer surfaces which comes into contact
with the gas-liquid two phase region can be therefore always
maintained to have a high transfer coefficient. Accordingly, the
heat exchange efficiency of the microchannel-type evaporator can be
increased, and moreover the microchannel-type evaporator can be
reduced in size.
Embodiment 2
[0062] FIGS. 3(a), 3(b), and 3(c) show a basic constituent unit of
Embodiment 2 of the evaporator according to the present
invention.
[0063] The configuration of the evaporator 1 itself of Embodiment 2
is, similar to the configuration of the evaporator of Embodiment 1,
of the countercurrent flow type in which the flow directions of the
liquid to be evaporated and the heating gas are opposite to each
other. This embodiment is an embodiment in the case where the mass
flow rate of the heating gas is not negligible compared to that of
the liquid to be evaporated and the temperature of the heating gas
decreases from the gas inlets 7 to the gas outlets 8.
[0064] FIG. 4 shows a change (a dashed line) in heat flux from the
heating gas to the liquid to be evaporated and critical heat flux
of each path space in this embodiment. As indicated by the dashed
line of FIG. 4, the heat flux is low where the quality is small,
and the heat flux is high where the quality is large. This is
because the following reason: where the quality is large, the
temperature of the heating gas is high and the difference in
temperature between the heating gas and the liquid to be evaporated
is large, so that the heat flux is high; and where the quality is
small, the temperature of the heating gas decreases and the
difference in temperature between the heating gas and the liquid to
be evaporated is small, so that the heat flux is low.
[0065] In FIG. 4, the dashed line indicating the heat flux q given
from the heating gas to the liquid to be evaporated and a line
indicating the critical heat flux q.sub.c of the path space S.sub.B
intersect with each other at a quality X.sub.B. The graph shows
that the critical heat flux is supplied at the position of the
quality X.sub.B of the evaporator with the path space S.sub.B. In
the region with a quality of not less than the quality X.sub.B,
supplied heat flux exceeds the critical heat flux, so that the heat
transfer surfaces change from the wet state to the dry state. When
the heat transfer surfaces change to the dry state, the heat
transfer coefficient is reduced. Accordingly, the heat flux is
drastically reduced as indicated by a chain line of FIG. 4.
[0066] In this embodiment, to avoid such reduction of heat flux, in
the region with a quality X of not less than X.sub.B, the path
space is increased to S.sub.B+1.sub.--.sub.case 2,
S.sub.B+2.sub.--.sub.case 2, and S.sub.B+3.sub.--.sub.case 2 as the
quality increases to X.sub.B+1, X.sub.B+2, and X.sub.B+3. This can
prevent the heat flux from exceeding the critical heat flux even if
the quality becomes more than X.sub.B, thus providing a maximum
heat transfer coefficient.
[0067] In a similar manner, in a region with a quality X of not
more than X.sub.B, even if the path space is reduced to
S.sub.B-1.sub.--.sub.case 2 and S.sub.B-2.sub.--.sub.case 2 as the
quality decreases to X.sub.B-1 and X.sub.B-2, the heat transfer
surfaces can be maintained to be wet. The heat flux can be
therefore at the critical heat flux even if the quality becomes
less than X.sub.B, thus providing a maximum heat transfer
coefficient.
[0068] As described above, in the microchannel-type evaporator of
countercurrent flow-type of the present invention, the space S of
the liquid path is set to a minimum space size satisfying that heat
flux is not more than the critical heat flux according to the
quality X, so that thin liquid films of the liquid to be evaporated
are formed on part of the heat transfer surfaces which comes into
contact with the gas-liquid two phase region of the liquid path.
The part of the heat transfer surfaces which comes into contact
with the gas-liquid two phase region can be therefore always
maintained to have a high transfer coefficient. Accordingly, the
heat exchange efficiency of the microchannel-type evaporator can be
increased, and moreover the microchannel-type evaporator can be
reduced in size.
Embodiment 3
[0069] FIGS. 5(a), 5(b), and 5(c) show a basic constituent unit of
Embodiment 3 of the evaporator according to the present
invention.
[0070] As shown in FIG. 5(a), an evaporator 1 in this embodiment is
an embodiment in which space between two opposite heat transfer
plates 2 serves as a liquid path 3. Sections outside of the heat
transfer plates 2 serve as gas paths 4. To form a real evaporator,
it is preferable that the evaporator has a structure in which a
plurality of the basic constituent units shown in the drawing are
arranged in parallel.
[0071] At the lower end of the liquid path 3, a liquid inlet 5,
through which the liquid to be evaporated is supplied to the
evaporator 1, is provided. At the upper end of the liquid path, a
vapor outlet 6 is provided. The liquid to be evaporated evaporates
as flowing from the bottom to the top of the evaporator 1. On the
other hand, the heating gas is supplied from gas inlets 7, which
are provided at the lower end of the evaporator, and discharged
from gas outlets 8, which are provided at the upper end of
evaporator. Accordingly, the evaporator of this embodiment is a
same flow direction type evaporator in which the flow directions of
the liquid to be evaporated and the heating gas are substantially
in a same direction. During operation of the evaporator 1, the
inside of the liquid path 3 is composed of a liquid phase region
10, a gas-liquid two phase region 11, and a gas phase region 12
sequentially from the bottom.
[0072] Size of space S of the liquid path 3 gradually decreases
from the bottom to the top in the gas-liquid two phase region
11.
[0073] Material of the heat transfer plates 2 can be a
corrosion-resistant metal, for example, stainless steel, titanium,
and titanium alloy.
[0074] Furthermore, the surface of each heat transfer plate 2 which
comes into contact with the liquid to be evaporated is coated with
titanium oxide or the like for hydrophilic treatment. This can
provide higher capillary pressure and more penetration into the
heating surfaces, thus further increasing the critical heat
flux.
[0075] This embodiment is an embodiment in the case where the mass
flow rate m (g/s) of the heating gas is not negligible compared to
that of the liquid to be evaporated and the temperature of the
heating gas decreases from the gas inlets 7 toward the gas outlets
8.
[0076] FIG. 6 shows a change (a dashed line) in heat flux q from
the heating gas to the liquid to be evaporated and critical heat
flux q.sub.c of each path space in this embodiment. As indicated by
the dashed line of FIG. 6, the heat flux is high where the quality
is small, and the heat flux is low where the quality is large. This
is because the following reason: where the quality is small, the
temperature of the heating gas is high and the difference in
temperature between the heating gas and the liquid to be evaporated
is large, so that the heat flux is high; and where the quality is
small, the temperature of the heating gas is reduced and the
difference in temperature between the heating gas and the liquid to
be evaporated is small, so that the heat flux is small.
[0077] In FIG. 6, the dashed line indicating the heat flux q given
from the heating gas to the liquid to be evaporated and a line
indicating the critical heat flux q.sub.c of the path space S.sub.B
intersect with each other at a quality X.sub.B. The graph shows
that the critical heat flux is supplied at the position of the
quality X.sub.B of the evaporator with the path space S.sub.B. In
the region with a quality of not less than the quality X.sub.B,
heat flux supplied exceeds the critical heat flux, so that the heat
transfer surfaces change from the wet state to the dry state. When
the heat transfer surfaces change to the dry state, the heat
transfer coefficient is reduced. Accordingly, the heat flux is
drastically reduced as indicated by a chain line of FIG. 6.
[0078] To avoid such reduction of heat flux, in the region with a
quality X of not more than X.sub.B, the path space is increased to
S.sub.B-1.sub.--.sub.case 3 and S.sub.B-2.sub.--.sub.case 3 as the
quality decreases to X.sub.B-1 and X.sub.B-2. This can prevent the
heat flux from exceeding the critical heat flux even if the quality
becomes smaller than X.sub.B, thus providing a maximum heat
transfer coefficient.
[0079] In a similar manner, in the region with a quality X of not
less than X.sub.B, the path space is reduced to
S.sub.B+1.sub.--.sub.case 3, S.sub.B+2.sub.--.sub.case 3, and
S.sub.B+3.sub.--.sub.case 3 as the quality increases to X.sub.B+1,
X.sub.B+2, and X.sub.B+3, the heat transfer surfaces can be
maintained to be wet. Accordingly, the heat flux can be maintained
within the critical heat flux even if the quality becomes smaller
than X.sub.B, thus providing a maximum heat transfer
coefficient.
[0080] As described above, in the microchannel-type evaporator of
the same flow direction type of the present invention, the space S
of the liquid path is set to a minimum space size satisfying that
heat flux is not more than the critical heat flux according to the
quality X, so that thin liquid films of the liquid to be evaporated
are formed on part of the heat transfer surfaces which comes into
contact with the gas-liquid two phase region of the liquid path.
The part of the heat transfer surfaces which comes into contact
with the gas-liquid two phase region can be therefore always
maintained to have a high transfer coefficient. Accordingly, the
heat exchange efficiency of the microchannel-type evaporator can be
increased, and moreover the microchannel-type evaporator can be
reduced in size.
Embodiment 4
[0081] FIGS. 7(a), 7(b), and 7(c) show a basic constituent unit of
Embodiment 4 of the evaporator according to the present
invention.
[0082] As shown in FIG. 7(a), an evaporator 1 in this embodiment is
an embodiment in which space between two opposite heat transfer
plates 2 serves as a liquid path 3. Sections outside of the heat
transfer plates 2 serve as gas paths 4. To form a real evaporator,
it is preferable that the evaporator has a structure in which a
plurality of the basic constituent units shown in the drawing are
arranged in parallel.
[0083] At the lower end of the liquid path 3, a liquid inlet 5,
through which the liquid to be evaporated is supplied to the
evaporator 1, is provided. At the upper end of the liquid path, a
vapor outlet 6 is provided. The liquid to be evaporated evaporates
as flowing from the bottom to the top of the evaporator 1. On the
other hand, the heating gas is supplied from gas inlets 7, which
are provided at the right end of the evaporator, and discharged
from gas outlets 8, which are provided at the left end of the
evaporator. The evaporator of this embodiment is therefore a cross
flow type evaporator in which the flow directions of the liquid to
be evaporated and the heating gas are orthogonal to each other.
During operation of the evaporator 1, the inside of the liquid path
3 is composed of a liquid phase region 10, a gas-liquid two phase
region 11, and a gas phase region 12 sequentially from the
bottom.
[0084] Size of space S of the liquid path 3 changes three
dimensionally so as to gradually decrease from the bottom to the
top in the gas-liquid two phase region 11 and gradually decrease
from the right to the left.
[0085] Material of the heat transfer plates 2 can be a
corrosion-resistant metal, for example, stainless steel, titanium,
and titanium alloy.
[0086] Furthermore, the surface of each heat transfer plate 2 which
comes into contact with the liquid to be evaporated is coated with
titanium oxide or the like for hydrophilic treatment. This can
provide higher capillary pressure and more penetration into the
heating surfaces, thus further increasing the critical heat
flux.
[0087] FIG. 8 shows changes (dashed lines) in heat flux from the
heating gas to the liquid to be evaporated at a position L-L in the
upstream of the heating gas, a position M-M in the center of the
evaporator 1, and a position N-N in the downstream of the heating
gas and critical heat flux of each path space.
[0088] In this embodiment, the heat flux transferred from the
heating gas to the liquid to be evaporated in the evaporator is the
highest at the position L-L in the upstream of the heating gas and
decreases along with a decrease in temperature of the heating gas
toward the position M-M in the middle of the stream and the
position N-N in the downstream. In this embodiment, similar to
Embodiment 1, it is assumed that the mass flow rate of the heating
gas is sufficiently higher than that of the liquid to be evaporated
and there is almost no change in temperature of the heating gas
between the gas inlets 7 and the gas outlets 8. The heat flux given
from the heating gas through the heat transfer plates to the liquid
to be evaporated is therefore constant regardless of the quality
and is indicated by a dashed line of FIG. 8.
[0089] In FIG. 8, the dashed line indicating the heat flux q given
from the heating gas to the liquid to be evaporated at the position
M-M and a line indicating the critical heat flux q.sub.c of the
path space S.sub.B intersect with each other at a quality X.sub.B.
In other words, the graph shows that the critical heat flux is
supplied at the position of the quality X.sub.B of the evaporator
with the path space S.sub.B. In the region with a quality of not
less than the quality X.sub.B, supplied heat flux exceeds the
critical heat flux. The heat transfer surfaces therefore change
from the wet state to the dry state, and the heat transfer
coefficient is reduced. Accordingly, the heat flux is drastically
decreased in a similar manner to Embodiment 2, which is not shown
in the drawing.
[0090] In this embodiment, to avoid such reduction of heat flux, in
a region with a quality X of not less than X.sub.B at the position
M-M, the path space is increased to S.sub.B+1, S.sub.B+2, and
S.sub.B+3 as the quality increases to X.sub.B+1, X.sub.B+2, and
X.sub.B+3. The heat flux can be therefore prevented from exceeding
the critical heat flux even if the quality becomes larger than
X.sub.B, thus providing a maximum heat transfer coefficient. In the
region of a quality X of not more than X.sub.B, even if the path
space is reduced to S.sub.B-1.sub.--.sub.case 3 and
S.sub.B-2.sub.--.sub.case 3 as the quality decreases to X.sub.B-1
and X.sub.B-2, the heat transfer surfaces can be maintained to be
wet. Accordingly, the heat flux can be maintained within the
critical heat flux even if the quality becomes smaller than
X.sub.B, thus providing a maximum heat transfer coefficient.
[0091] In a similar manner, the dashed line indicating the heat
flux given from the heating gas to the liquid to be evaporated at
the position L-L of the evaporator 1 and a line indicating the
critical heat flux of the path space S.sub.A intersect with each
other at a quality X.sub.A. In other words, the graph shows that
the critical heat flux is supplied at the position of the quality
X.sub.A of the evaporator with the path space S.sub.A. The critical
heat flux is supplied at the position of the quality X.sub.A of the
path space S.sub.A, and in the region with a quality of not less
than the quality X.sub.A, supplied heat flux exceeds the critical
heat flux. The heat transfer surfaces therefore change from the wet
state to the dry state, and the heat transfer coefficient is
reduced. Accordingly, the heat flux is drastically reduced in a
similar manner to Embodiment 2, which is not shown in the
drawing.
[0092] To avoid such reduction of heat flux, in the region with a
quality X of not less than X.sub.A at the position L-L, the path
space is increased as the quality increases to X.sub.A+1,
X.sub.A+2, and X.sub.A+3. This can prevent the heat flux from
exceeding the critical heat flux even if the quality becomes larger
than X.sub.A, thus providing a maximum heat transfer coefficient.
In a region with a quality X of not more than X.sub.A, even if the
path space is reduced as the quality decreases to X.sub.A-1 and
X.sub.A-2, the heat transfer surfaces can be maintained to be wet.
The heat flux can be therefore within the critical heat flux even
if the quality becomes smaller than X.sub.A, thus providing a
maximum heat transfer coefficient.
[0093] In this embodiment, the same process as the processes
performed for the positions L-L and M-M is performed across the
entire region of the evaporator 1. The space size can be thus set
to a minimum space size that allows the heat flux to be not more
than the critical heat flux according to the quality X.
[0094] As described above, in the microchannel-type evaporator of
the cross flow type, the space S of the liquid path is set to the
minimum space size satisfying that the heat flux is not more than
the critical heat flux according to the quality X, so that thin
liquid films of the liquid to be evaporated are formed on part of
the heat transfer surfaces which comes into contact with the
gas-liquid two phase region of the liquid path. The part of the
heat transfer surfaces which comes into contact with the gas-liquid
two phase region can be therefore maintained to have a high
transfer coefficient. Accordingly, the heat exchange efficiency of
the microchannel-type evaporator can be increased, and moreover the
microchannel-type evaporator can be reduced in size.
Embodiment 5
[0095] FIGS. 9(a), 9(b), and 9(c) show a basic constituent unit of
Embodiment 5 as a modification of Embodiment 2.
[0096] A difference between the evaporator of this embodiment and
the evaporator of Embodiment 2 is that the path space S is
gradually reduced in the gas phase region 12 of the liquid path 3.
The other configuration is the same as that of Embodiment 2. Same
constituent components are given same numerals, and redundant
description is omitted. This embodiment has an effect on preventing
droplets of the liquid to be evaporated from being discharged from
the vapor outlet 6.
[0097] Gradually decreasing the path space S in the gas-phase
region 12 of the liquid path 3 like Embodiment 5 can be applied to
not only Embodiment 2 but also other embodiments.
Embodiment 6
[0098] FIGS. 10(a), 10(b), and 10(c) show a basic constituent unit
of Embodiment 6 of the evaporator according to the present
invention.
[0099] An evaporator 1 of Embodiment 6 includes, in addition to the
cross flow-type evaporator of Embodiment 4, a plurality of turning
sections provided with partitions 9 in a heating gas path 4 to
cause the heating gas to meander. The other configuration is the
same as that of Embodiment 4. The same constituent components are
given same numerals, and redundant description is omitted.
[0100] In Embodiment 4, the boundary line between the gas-phase
region 12 and the gas-liquid two phase region 11 and the boundary
line between the gas-liquid two phase region 11 and the
liquid-phase region 10 are sloped. However, according to this
embodiment, these boundary lines can be kept more parallel to the
flow direction of the heating gas than those of Embodiment 4.
Embodiment 6 therefore has an effect on further uniforming the
superheat of vapor generated from the end of the vapor outlet
6.
Embodiment 7
[0101] FIGS. 11(a), 11(b), and 11(c) show a basic constituent unit
of Embodiment 6 as a modification of Embodiment 2.
[0102] An evaporator 1 of Embodiment 7 includes, in addition to the
cross flow-type evaporator of Embodiment 4, a plurality of turning
sections provided with partitions 9 in a heating gas path 4 to
cause the heating gas to meander. Furthermore, a difference between
Embodiment 6 and Embodiment 7 is that the gas inlets 7 are in the
vicinity of the liquid inlet 5 and the gas outlets 8 are in the
vicinity of the vapor outlet 6. Accordingly, hottest part of the
heating gas heats the liquid phase region 10, whose critical heat
flux is highest, so that the space S of the liquid path 3 can be
configured to be narrower than that of Embodiment 6. Embodiment 7
therefore has an effect that the evaporator 1 thereof can be
smaller than that of Embodiment 6. The other configuration is the
same as that of Embodiment 4. The same constituent components are
given same numerals, and redundant description is omitted.
Embodiment 8
[0103] This embodiment is described using FIGS. 12 to 18. FIGS.
12(a), 12(b), and 12(c) show a basic constituent unit of Embodiment
8 of the evaporator according to the present invention. FIG. 13
shows a characteristic of the critical heat flux under inlet
conditions of pure water as the liquid to be evaporated and high
temperature gas in the evaporator shown in FIG. 12. FIG. 14 shows a
relation between the inlet condition of the pure water and the
boiling region based on the characteristic of FIG. 13. FIGS. 15 to
17 show flow patterns of the heating gas when the evaporator is
controlled based on a map of FIG. 14.
[0104] As shown in FIG. 12, an evaporator 1 is an evaporator in
which space between two opposite heat transfer plates 2 serves as a
liquid path 3. Sections outside of the heat transfer plates 2 serve
as the gas paths 4. To form a real evaporator, it is preferable
that the evaporator has a structure in which a plurality of the
basic constituent units shown in the drawing are arranged in
parallel.
[0105] At the lower end of the liquid path 3, a liquid inlet 5,
through which the liquid to be evaporated is supplied to the
evaporator 1, is provided, and at the upper end of the liquid path
3, a vapor outlet 6 is provided. The liquid to be evaporated
evaporates as flowing from the bottom to the top of the evaporator
1. On the other hand, the heating gas is supplied from gas inlets
7, which are provided at the lower end of the evaporator, and
discharged from gas outlets 8, which are provided at the upper end
of the evaporator. The evaporator of this embodiment is therefore a
same flow direction type evaporator in which the flow directions of
the liquid to be evaporated and the heating gas are substantially
in a same direction. During operation of the evaporator 1, the
inside of the liquid path 3 is composed of a liquid phase region
10, a gas-liquid two phase region 11, and a gas phase region 12
sequentially from the bottom. Size of space S of the liquid path 3
is substantially constant over the entire region of the evaporator
1.
[0106] Next, a description is given of the map of FIG. 14 using
FIGS. 12 and 13.
[0107] In Embodiment 8, as shown in FIG. 12, it is assumed that the
conditions at the inlet 5 of pure water are mass flow rate
m.sub.w(g/s) and temperature T.sub.w(K) and the conditions at the
gas inlets 7 are mass flow rate m.sub.g(g/s) and temperature
T.sub.g(K). Moreover, the critical heat flux characteristic shown
in FIG. 13 varies depending on the space S of the microchannel as
previously described, and as the larger the space S is, the higher
the critical heat flux is.
[0108] For example, a description is given of the following five
patterns of the relation between the quality and the heat flux when
the space S is set to S.sub.B. First, when the mass flow rate and
temperature at the gas inlet are m.sub.g2 and T.sub.g2,
respectively, the heat flux of the high temperature gas and the
liquid to be evaporated varies as indicated by a dashed line along
with changes in quality and is not more than the critical heat flux
of the space S.sub.B in a quality of 0 to 1. In a similar manner,
also in cases of (m.sub.g2, T.sub.g1) and (m.sub.g1, T.sub.g2), the
heat flux is not more than the critical heat flux of the space
S.sub.B. These show that a thin liquid film can be formed to enable
an efficient heat exchange.
[0109] On the other hand, in cases of (m.sub.g2, T.sub.g3) and
(m.sub.g3, T.sub.g2), as the quality increases, the heat flux of
the high temperature gas and the liquid to be evaporated intersects
with the critical heat flux characteristic and transits to the
dryout region to be considerably reduced. This causes the heat
exchange coefficient to be reduced.
[0110] The aforementioned results are put together as FIG. 14 in
terms of the relation between the (m.sub.g, T.sub.g) conditions at
the inlet of the high temperature gas and the boiling region. This
map is in the case where the inlet conditions of the pure water are
m.sub.w (g/s) and T.sub.w (K). When it is judged whether the
transition to the dryout region is performed and the transition to
the dryout region is judged to be performed, the map of FIG. 14 is
used to determine a quantitative .DELTA.T.sub.g and .DELTA.m.sub.g
to make a transition to the wet region. The line separating the
dryout region and the wet region in FIG. 14 is created by varying
the conditions m.sub.w (g/s) and T.sub.w (K) in a range of use and
plotting the conditions of m.sub.w (g/s) and T.sub.w (K) at which
the heat flux characteristic with respect to the quality obtained
by experiments or calculations is tangent to the critical heat flux
characteristic.
[0111] Next, using FIGS. 18(a) to 18(c), a description is given of
a configuration and an operation of systems using the evaporator of
this embodiment.
[0112] Each system show in FIGS. 18(a) to 18(c) includes the
evaporator 1, a superheater 21, a heating gas inlet 31 of the
system, valves 32, 33, and 34 each composed of a three-way valve to
switch paths of the heating gas, and a gas outlet 35 of the system.
The superheater 21 further heats vapor from the evaporator of the
present invention to generate superheated vapor.
[0113] The system of FIG. 18(a) is a system characterized by
supplying the heating gas to the superheater 21 and the evaporator
1 in parallel when the mass flow rate of the heating gas is not
less than a prescribed value.
[0114] The system of FIG. 18(b) is a system characterized by
supplying the heating gas to the superheater 21 and then to the
evaporator 1 when the temperature of the heating gas is not more
than a prescribed value.
[0115] The system of FIG. 18(c) is a system characterized by
supplying the heating gas to the superheater 21 and the evaporator
1 in parallel and then supplying the heating gas discharged from
the superheater 21 to the evaporator 1 when the mass flow rate and
temperature of the heating gas are not less than the respective
prescribed values.
[0116] Places indicated by thick lines in the drawing show flows of
the gas, and a black portion of each three-way valve indicates the
direction that the gas flow is stopped. The embodiment is composed
of two heat exchangers: one is the above described
microchannel-type evaporator 1 mainly used in the wet region; and
the other is the superheater 21 to generate superheated vapor. Gas
passing through these heat exchangers which are the evaporator 1
and superheater 21 is supplied to a hydrogen generation element
such as a not-shown auto thermal reactor (ATR). In a normal state
not described below (in the case of the wet region), the gas
bypasses the superheater 21 to be supplied. FIGS. 18(a) to 18(c)
correspond to FIGS. 15 to 17, respectively, and FIGS. 15 to 17 are
simplified and schematically shown. The configuration diagrams
shown in FIGS. 18(a) to 18(c) are just examples and do not limit
the present invention.
[0117] Next, a description is given of a specific control using
FIGS. 14 to 17. FIG. 15 shows a case of a change in a direction A
of FIG. 14, or a case where the condition is changed from the
condition (m.sub.g3, T.sub.g2) to the condition (m.sub.g2,
T.sub.g2). FIG. 16 shows a case of a change in a direction B of
FIG. 14, or a case where the condition is changed from the
condition (m.sub.g2, T.sub.g3) to the condition (m.sub.g2,
T.sub.g2). FIG. 17 shows a case of a change to a direction C of
FIG. 14, or a case where the condition is changed from the
condition (m.sub.g3, T.sub.g2) to the condition (m.sub.g2,
T.sub.g1).
[0118] In the system of FIG. 15, the heating gas supplied at the
condition (m.sub.g3, T.sub.g2) is supplied to the evaporator 1 and
the superheater 21 at the condition (m.sub.g2, T.sub.g2) and the
condition (.DELTA.m.sub.g, T.sub.g2), respectively. The heating gas
discharged from the evaporator 1 and the heating gas discharged
from the superheater 21 are joined and fed to the ATR. The heating
gas discharged from the evaporator 1 may be passed through the
superheater 21.
[0119] In the system of FIG. 16, the heating gas supplied at the
condition (m.sub.g2, T.sub.g3) is supplied to the superheater 21 at
the condition (m.sub.g2, T.sub.g3), and the temperature of the
heating gas is reduced at the superheater 21 by .DELTA.T.sub.g. The
heating gas is then supplied to the evaporator 1 at the condition
(m.sub.g2, T.sub.g2). The heating gas discharged from the
evaporator 1 is fed to the ATR. The heating gas discharged from the
evaporator 1 may be again passed through the superheater 21.
[0120] In the system of FIG. 17, the heating gas supplied at the
condition (m.sub.g3, T.sub.g2) is supplied to the evaporator 1 and
the superheater 21 at the conditions (.DELTA.m.sub.g, T.sub.g2) and
(m.sub.g3-.DELTA.m.sub.g, T.sub.g2), respectively. The heating gas
discharged from the superheater 21 is supplied to the evaporator 1
at a temperature (T.sub.g2-.DELTA.T.sub.g) and then fed to the ATR.
The heating gas discharged from the evaporator 1 may be passed
through the superheater 21. In FIGS. 15 to 17, reference numerals
25 to 28 denote a vapor inlet, a superheated vapor outlet, a
heating gas inlet, and a heating gas outlet, respectively.
[0121] As described above, in this embodiment, when the mass flow
rate of the heating gas is not less than the prescribed value, the
heating gas is supplied to the superheater and the
microchannel-type evaporator in parallel. When the temperature of
the heating gas is not lower than the prescribed value, the heating
gas is supplied to the superheater and then supplied to the
microchannel-type evaporator. The heat transfer surfaces can be
therefore maintained to be wet at narrow spaces with a constant
cross section. Accordingly, the evaporator can be reduced in size,
and the gas-liquid two phase region can be maintained to have a
high heat transfer coefficient. It is therefore possible to realize
a compact evaporator with high efficiency.
[0122] Note that the space S is configured to be constant in
Embodiment 8 but may be changed like Embodiments 1 to 7.
[0123] The entire contents of Japanese Patent Applications No.
2004-162011 (Filing Date: May 31, 2004) and No. 2002-254611 (Filing
Date: Sep. 1, 2004) are herein incorporated by reference.
[0124] Hereinabove, a description is given of the contents of the
present invention along the embodiments and examples. However, it
is obvious for those skilled in the art that the present invention
is not limited to the description about these embodiment and
examples and various modifications and improvements can be
made.
INDUSTRIAL APPLICABILITY
[0125] In the evaporator of the present invention, a thin liquid
film of a liquid to be evaporated is formed on part of heat
transfer surfaces coming into contact with a gas-liquid two phase
region. The part of the heat transfer surfaces coming into contact
with the gas-liquid two phase region can be therefore always
maintained to have a high transfer coefficient. Accordingly, the
heat exchange efficiency of the evaporator can be increased, and
the evaporator can be reduced in size.
* * * * *