A multi-pipe once-through boiler having at least one row of a plurality of circumferentially arranged pipes on which a plurality of fins are arranged in such a manner that the fins are in contact with the flow of the combustion gas in a substantially parallel maner. Elements are provided for increasing the heat transfer effect, such as slits in the fins, or an inclined arrangement of the fins, or pipes without fins at the region near to the inlet of the combustion gas passageway, are provided. Furthermore, a heat insulating member for decreasing operational noise as well as a cleaner device for blow-cleaning the combustion gas passageway are provided.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a multi-pipe once-through type boiler 
having a small volume, and used, for example, in a domestic heating 
device. 
2. Description of the Related Art 
Known in the prior art is an multi-pipe once-through type boiler provided 
with an inner row of circumferentially arranged pipes, an outer row of 
circumferentially arranged pipes, a combustion chamber inside the inner 
pipes, and a tubular combustion gas passageway. The inner row of pipes is 
provided with an inlet opening to the combustion chamber, and the outer 
row of pipes is provided with an outlet for connecting the tubular 
combustion gas passageway with a flue pipe. 
The most closely related prior art is Japanese Unexamined Patent 
Publication (Kokai) No. 58-88502 published on May, 1983. In this prior 
art, the inner and outer pipes are provided with a plurality of fins to 
increase the efficiency, of the heat transfer from the combustion gas to 
water in the pipes. 
Other related prior arts belonging the same applicant are Japanese Examined 
Utility Model Publication No. 59-41361, Japanese Unexamined Patent 
Publication No. 57-29000, and Japanese Unexamined Patent Publication No. 
58-11303. Further related arts other than those of the applicant are; 
Japanese Examined Utility Model Publication No. 56-54401, Japanese 
Unexamined Utility Model Publication No. 52-133801, Japanese Examined 
Patent Publication No. 44-9161, German Patent Publication No. 2248223, and 
Austrian Patent Specification No. 308771. However, in view of the present 
demand for energy and cost saving devices, the heat transfer efficiency of 
the above prior arts is not satisfactory and should be improved. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a multi-pipe 
once-through type of boiler having the above mentioned kind of 
construction, but capable of enhancing the efficiency of the heat transfer 
between the combustion gas in the tubular combustion gas passageway and 
the fluid to be heated in the pipes. 
According to the present invention, a multi-pipe once-through type boiler 
is provided, comprising: a casing having a substantially tubular shape 
with a longitudinal axis; at least one row of a plurality of 
circumferentially arranged pipes about the longitudinal axis, each tube 
extending substantially in parallel to and along the axis in such a manner 
that a pair of spaced ends is provided, this row defining, at the inside 
thereof, a combustion chamber extending along the axis, the chamber being 
open at one end; means for generating a flow of a combustible mixture to 
be directed into the combustion chamber via the open end thereof, closure 
means for closing the other end of the combustion chamber facing the 
generating means; a tubular combustion gas passageway, extending along the 
axis and formed around the combustion chamber in such a manner that the 
flow of gas in the combustion chamber is in contact with at least one row 
of the pipes. 
The tubular combustion gas passageway has an inlet extending along 
substantially the entire length thereof along the axis for introduction of 
the combustion gas from the combustion chamber and the tubular combustion 
gas passageway also has an outlet extending along substantially the entire 
length thereof along the axis for removing the combustion gas from the 
passageway. 
A flue means is connected to the outlet for removing the combustion gas to 
the outside; a first header is connected to the casing, the pipes in the 
casing being, at their first end, connected to the first header so that 
communication of a fluid to be heated is attained therebetween; a second 
header is axially spaced from the first header, the pipes in the casing 
being, at their second ends, connected to the second header so that 
communication of the fluid to be heated is attained therebetween. 
A plurality of fins is formed on substantially the entire portions of the 
outer surfaces of the pipes in such a manner that the fins are in contact 
with the flow of the combustion gas in the combustion gas passageway, each 
of the fins extending substantially in the direction of the flow of the 
heating gas, and a means for improving the heat transfer efficiency 
between the heating gas in the combustion gas passageway and the tubes 
through which the fluid to be heated is passed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a vertical cross-sectional view of an embodiment of an 
once-through type boiler according to a first embodiment of the present 
invention. In FIG. 1, reference numeral 10 denotes a cylindrical casing, 
and an upper annular header 12 and lower annular header 14 are arranged on 
the upper and the lower ends of the casing 10, respectively. A row of 
inner pipes 16 are arranged circumferentially in the casing 10, each of 
the inner pipes 16 extending in parallel to the axis of the tubular casing 
10 and providing an upper end 16A and lower end 16B, respectively, having 
a reduced diameter A second row of outer pipes 18 are also arranged 
circumferentially in the casing 10, so that the first row of inner pipes 
16 and the second row of outer pipes 18 are coaxial with respect to the 
axis of the casing 10. 
Each of the outer pipes 18 extends in parallel to the axis of the casing, 
and provides an upper and a lower end 18A and 18B, respectively, having a 
reduced diameter. The upper ends 16A and 18A of the rows of inner pipes 16 
and outer pipes 18 are connected, via a filler layer of heat resistant 
material 19, to the upper header 12 in such a manner that the header 12 is 
in communication with the pipes 16 and 18. The lower ends 16B and 18B of 
the inner pipes 16 and outer pipes 18 are connected, via filler layer of 
heat resistant material 21, to the bottom header 14 in such a manner that 
the header 14 is in communication with the pipes 16 and 18. As shown in 
FIG. 2, the inner pipes 16 are arranged in such a manner that two adjacent 
inner pipes 16 are in contact with each other at their geometric lines, 
and the outer pipes 18 are arranged so that two adjacent pipes 18 are 
circumferentially spaced. As a result, two adjacent pipes in both of the 
inner and outer rows 16 and 18 are arranged at the same angular interval 
in such a manner that each adjacent pair of the inner and outer pipes 16 
and 18 is located on the same radial plane. In order to obtain the 
angularly spaced arrangement of the outer pipes 18, the inner surface of 
the casing 10 is formed with a plurality of axially extending and 
circumferentially spaced grooves in which the corresponding outer pipes 18 
are arranged 
As is well known, water is supplied to the bottom header 14 by way of a 
water supply system (not shown) in such a manner that water in the inner 
and outer pipes 16 and 18 is maintained at a predetermined level 
Therefore, vapor generated in the pipes 16 and 18 due to the heat exchange 
occurring between the combustion gas and water is directed upward so as to 
fill the upper header 12. 
A combustion chamber 24 is formed inside the row of inner pipes 16 and 
extends axially thereto The combustion chamber 24 is open at the upper end 
thereof, and the bottom end thereof is closed by the heat resistant 
material layer 21. A burner 25 is arranged in the upper open end of the 
combustion chamber 24 opposite the closed bottom 21 of the combustion 
chamber 24 The burner 25 per se is known, and is constructed, for example, 
by a fuel nozzle (not shown) encircled by an air tube connected to a 
forced air flow source, so that the burner 25 produces a flow of 
combustible mixture which is injected into the combustion chamber 24 and 
burnt therein Alternately, the burner 25 may be arranged in a space formed 
inward of the lower header 14 In this case, the upper end of the 
combustion chamber 24 is, of course, closed. 
A combustion gas passageway 26 having an annular shape is formed between 
the inner row of pipes 16 and the outer row of pipes 18 so that the 
chamber 24 extends along substantially the entire length of the pipes 16 
and 18. As shown in FIG. 2, the arrangement of the inner pipes 16 is 
interrupted at a position in the circumference of the row of pipes 16 in 
such a manner that an inlet 28 extending along the entire length of the 
combustion chamber 24 is formed, to allow communication between the 
combustion chamber 24 and the combustion gas passageway 26. Similarly, the 
arrangement of the outer pipes 18 is interrupted at a position in the 
circumference of the row in such a manner that an outlet 30 extending 
along the entire length of the combustion chamber 24 is formed, to allow 
the gas in the passageway 26 combustion gas to be exhausted therefrom. A 
flue pipe 32 is connected to the casing 10 and communicates with the 
outlet 30 for exhausting the combustion gas to the atmosphere. 
As shown in FIGS. 1 and 2, each of the inner and outer pipes 16 and 18 has 
a plurality of axially spaced fins 34 welded to the pipes 16 and 18. Each 
of the fins 34 extends outwardly and radially from the tubular surface of 
the corresponding pipe 16 or 18, so that the heating gas or combustion gas 
in the combustion passageway 26 flows as shown by an arrow 38 in parallel 
to the fins 34. The fins 34 on the inner pipes 16 are arranged in such a 
manner that they face the corresponding fins 34 of the corresponding outer 
pipes 18 in the same place. 
As shown in FIG. 5, each of the fins 34 forms a plate extending vertically 
from the cylindrical surface of the corresponding pipe 16 (18) in a 
cantilever fashion, so that arc shaped peripheral 34a and straight side 
edges 34b connected therewith are formed. Furthermore, the fin 34 forms 
slits 34c extending inward and substantially radially from the arc shaped 
peripheral edge 34a in a direction which is substantially transverse to 
the direction of flow of the heating gas, as shown by the arrow 38, in the 
combustion gas passageway 26. 
During the operation of the first embodiment of the present invention, the 
combustible mixture from the burner 25 is injected into the combustion 
chamber 24 to be burnt therein. Water in the inner pipes 16 is heated due 
to a heat exchange based on the radiation heat transmission principle. 
Then, the resultant combustion gas or heating gas in the combustion 
chamber 24 is, via the inlet 28, introduced into the combustion gas 
passageway 26 as shown by an arrow 42 in FIG. 2, to generate a flow of the 
combustion gas in the combustion gas passageway 26 in the direction 
substantially transverse to the longitudinal direction of the inner pipes 
16 and outer pipes 18. As a result, a heat exchange takes place between 
the water in the pipes 16 and 18 and the combustion gas in the combustion 
gas passageway 16 via the walls of the pipes 16 and 18 and the fins 34, 
mainly under the convection heat transmission principle. The combustion 
gas is then exhausted into the flue pipe 32 via the outlet 30 as shown by 
arrows 44. Viewed from above, as shown in FIG. 2, the flow of the 
combustion gas in the combustion gas passageway 26 from the inlet 28 to 
the outlet 30, as shown by the arrows 42 and 44, depicts a shape which is 
similar to the Greek letter .delta. Accordingly, this system is often 
called the "omega" flow type system. 
The multiplicity of the fins 34 on the pipes 16 and 18 does not cause any 
substantial increase in pressure drop when the combustion gas passes 
through the combustion gas passageway 26 from the inlet 28 to the outlet 
30, since the fins 34 are arranged so that they are in parallel to the 
flow, shown by the arrow 38, of the combustion gas, as shown in FIG. 4. In 
addition, as shown in FIG. 5, each of the fins 34 is provided with slits 
34c extending radially inwardly from the edges 34a in the direction 
transverse to the direction of the flow of the combustion gas. Thus, a 
so-called front edge effect in a convection heat transfer is attained, and 
thus the efficiency of the heat transfer is enhanced Furthermore, the 
difference in the degree of heat expansion between the fins 34 and the 
pipes 16 and 18 due to the temperature difference therebetween is 
compensated by the slits 34c in the fins 34, so that the generation of 
thermal stress in the welded regions of the fins 34 to the pipes 16 and 
18, which would otherwise generate cracks or deformation, is eased. 
FIG. 6 shows a modification of the arrangement of the fins 134. In this 
modification, each of the fins 34 is constructed by a plate having a pair 
of axially spaced parallel planes 134a and 134b which are more or less 
inclined with respect to the direction of the flow of the combustion gas 
in the combustion gas passageway 6, as shown by the arrow 38. 
Due to the fact that the inclination of the fins 34 with respect to the 
direction of the flow of the combustion gas in the combustion gas 
passageway 26 is small, the pressure loss occurring across the combustion 
gas passageway 26 becomes small even if a multiplicity of fins 134 are 
employed Furthermore, since the fins 134 are inclined with respect to the 
direction of the flow of the combustion gas, the gas flow along the fins 
134 serves to strip the temperature boundary layers of the combustion gas 
formed in the proximity of the surfaces of the fins 134 by the viscosity 
of the gas, so that turbulence is generated in the temperature boundary 
layers, enhancing the heat transfer efficiency. 
In a modification shown in FIGS. 8 and 9, fins 234 of different shapes are 
provided on the pipes 16 and 18. Each of the fins 234 forms a plate having 
a pair of axially spaced parallel first planes 234a and 234b which are 
inclined toward the direction of the flow of the combustion gas (arrow 38) 
and a plate having pair of axially spaced parallel second planes 234c and 
234d which are inclined toward the opposite direction of that of the first 
planes 234a and 234b. 
In the embodiment shown in FIGS. 8 and 9, as the combustion gas passes each 
of the fins 234, turbulence of the temperature boundary layers formed in 
the proximity of the fins 234 is repeatedly attained due to the provision 
of the inclined first planes 234a and 234b and the oppositely inclined 
second planes 234c and 234d. As a result, an increased heat transfer 
efficiency is attained. 
FIG. 10 shows another modification of the shape of a fin. In this 
modification, the fin 334 forms a plate provided with a plurality of 
parallel closed end slits 334c, each of which extends in a direction 
traverse to the direction of the flow of the heating gas, as shown by an 
arrow 38. Similar to the embodiment shown in FIG. 5, the provision of the 
slits 234c serves to generate the so-called front edge effect in a 
convection heat transfer, enhancing the efficiency of the heat 
transmission. 
FIG. 11 shows a further embodiment of the shape of a fin. In this 
modification, the fin 434 is located at regions near the outer edge 
thereof and has a plurality of portions 434' having a triangular shape 
which are bent slightly from the general plane of the fin 434 so that a 
plurality of slits 434c open outwardly to extend in the direction 
transverse to the direction of the flow of the heating gas in the 
combustion gas passageway 26, as shown by the arrows 38, are formed 
between the bent portions and the facing edges. The "front edge effect" is 
provided due to the provision of the slits in the boundary layers formed 
in the regions near to the surfaces of the pipes 16 and 18, so that heat 
transfer efficiency is enhanced. Furthermore, the portions 434' are only 
slightly bent, and therefore, a substantial pressure loss can not occur. 
FIGS. 12 and 13 show two alternatives in obtaining the arrangement of the 
inner row of the pipes 16 and the outer row of the pipes 18. In FIG. 12, 
in both the inner and outer rows of pipes, the adjacent pipes 16 and 18 
are arranged so that they are in direct contact with each other at their 
geometric lines, so that the combustion gas passageway 26 is formed 
between the inner pipes 16 and the outer pipes 18. In this case, the angle 
.theta..sub.1 between the adjacent two outer pipes 18 in the outer row 
becomes smaller than the angle .theta..sub.2 between the adjacent two 
inner pipes 16 in the inner row. 
In FIG. 13, spacers 51 are arranged between every two adjacent pipes 16 in 
the inner row while spacers 52 are arranged between every two adjacent 
pipes 18 in the outer row, in such a manner that an angle .theta..sub.1 ' 
between the adjacent outer pipes 18 is equal to an angle .theta..sub.2 ' 
between the adjacent pipes 16, and in such a manner that the inner and 
outer pipes 16 and 18 are alternately arranged in the circumferential 
direction. In this arrangement, the length of the outer spacer 52 is, of 
course, larger than the length of the inner spacer 51. This alternate 
arrangement of the inner and outer pipes 16 and 18 with the same angular 
distance (.theta..sub.1 =.theta..sub.2) is also attained without the use 
of the inner spacers, as shown in FIG. 14. In the alternate arrangement of 
the inner and the outer pipes 16 and 18 as shown in FIGS. 13 and 14, the 
combustion gas passageway 26 is provided with a substantially uniform 
width, and a flow of combustion gas having a direction which is slightly 
and periodically changed along the circumferential direction is obtained. 
This periodical change in the flow direction gives an increase in the 
efficiency in the heat transfer, without increasing the pressure loss 
generated when the combustion gas passes through combustion gas passageway 
26. 
The provision of the spacers 51 and 52 between adjacent pipes allows a 
smooth flow of the combustion gas in the combustion gas passageway 26, 
preventing the generation of dead spaces between the adjacent pipes, in 
which the combustion gas is apt linger out of the general flowstream, 
giving a higher interior efficiency in the heat transfer. 
FIG. 15 shows another embodiment, wherein the inner row of pipes 16 and 
outer row of pipes 18 are provided with fins 34 except at regions near the 
inlet 28. The temperature of the combustion gas just after it is 
introduced from the combustion chamber 24 into the combustion gas 
passageway 26 via the inlet 28, is high enough to cause a large 
temperature difference between the fins 34 and the corresponding pipes, 
which would generate cracks in the pipes 16 and 18 if the fins 34 were 
provided. In this embodiment, the pipes 16-1 and 18-1 near the inlet 28 
are not provided with fins, and thus the temperature difference is 
decreased which decreases the chance of crack generation Furthermore, 
because the fins 34 are not provided on the pipes 16-1 and 18-1 near the 
inlet 28, a predetermined heat transfer efficiency is maintained without 
change even over a prolonged period of operation, since there are no fins 
34 on the pipes 16-1 and 18-1 to be eroded by the high temperature of the 
combustion gas. It should be noted that, at the regions where the 
combustion gas passageway 26 is spaced from the inlet 28, the temperature 
of the combustion gas is decreased due to the heat convection at the 
regions where the pipes 16 and 18 are not provided with fins 34, i.e., 
near the inlet, and therefore, the problem of thermal corrosion of the 
fins 34 does not arise. 
In the embodiment shown in FIG. 16, the inner row of pipes 16 and the outer 
row of pipes 18 are slightly eccentrically arranged in such a manner that 
the distance .delta..sub.1 between the facing inner and outer pipes 16 and 
18 at the inlet 28 is smaller than the distance .delta..sub.2 between the 
facing inner and outer pipes 16 and 18 at the outlet 30. As a result, the 
combustion gas passageway 26 is throttled at the inlet 28 to increase the 
speed of the combustion gas and thus increase the heat transfer rate at 
the region near the inlet 28. Thus, the heat transfer effect attained by 
the pipes 16-1 and 18-1 without fins can be equal to the heat transfer 
effect attained by the pipes 16-1 and 18-1 with fins. The pressure drop 
generated when the combustion gas passes through the combustion gas 
passageway 26 at the region where the pipes are without fins is, of 
course, correspondingly increased. However, the pipes 16-1 and 18-1 extend 
along a limited area near the inlet 28, so that there is no adverse effect 
caused by the throttling of the combustion gas passageway 26, and a 
sufficient flow of the combustion gas is obtained to maintain a stable 
operation of the boiler. Furthermore, due to the throttling of the 
combustion gas passageway 26, at the inlet 28, uniform distribution of the 
combustion gas along the entire axial length of the combustion gas 
passageway 26 is obtained, allowing the combustion gas to evenly come into 
contact with the entire surface of fins 34, to increase the heat transfer 
efficiency and to prevent the pipes from local overheating for preventing 
the generation of cracks in the pipes 16-1 and 18-1. 
In the arrangement of FIGS. 15 and 16, the outer pipes 18-1 without fins 34 
extend further downstream in the direction of the flow of the combustion 
gas in the combustion gas passageway 26 than the inner pipes 16-1 without 
fins 34. Since they are submitted to the heat from the combustion chamber 
24, the temperature difference between the combustion gas and the inner 
pipes 16-1 is smaller than the temperature difference between the 
combustion gas and the outer pipes 18-1 which are spaced further from the 
combustion chamber 24. Therefore, the limited extension arrangement of the 
pipes 16-1 without fins 34 maintains a lower possibility of the generation 
of cracks in the welded regions of the fins 34 to the pipes 16-1, and the 
heat transfer efficiency is increased by this arrangement. 
FIG. 17 shows a modification wherein the pipes 16-1 and 18-1 without fins 
34 in FIGS. 15 and 16 are combined with the alternate arrangement of the 
inner and the outer pipes shown in FIG. 13, by the provision of the 
spacers 51 and 52. 
In FIG. 18, in addition to the essential constituent members as already 
described, the boiler is provided with elements for maintaining the 
temperature therein and for decreasing the operational noise. In this 
embodiment, a layer 60, made from a heat resisting material such as glass 
wool, is arranged outside the casing 10. The heat resisting layer 60 is 
held in place by an outer tubular cover 62 made from a thin metal plate. 
The provision of the heat resisting material layer 60 covered by the plate 
62 prevents heat loss and suppresses operational noise 
In this embodiment shown in FIG. 18, the bottom plate 21 made of a heat 
resistant filler material is spaced from the lower header 24 in such a 
manner that the plate 21 is in contact with the inner periphery of the 
inner row of pipes 16. 
In an embodiment shown in FIGS. 19 and 20, the boiler is provided with a 
single row of pipes 16 arranged circumferentially in such a manner that 
the combustion chamber 24 is formed inside the row of pipes 16. The 
combustion chamber 24 is open at the upper end and a burner 25 is arranged 
therein. The bottom end of the combustion chamber 24 is formed by the 
layer 21 of the heat resistant filler material. A combustion gas 
passageway 26 is formed between the row of pipes 16 and the casing 10. The 
row of pipes 16 is provided with an inlet 28 extending axially, through 
which the combustion gas from the combustion chamber 24 is introduced into 
the combustion gas passageway 26. The combustion gas from the combustion 
gas passageway 26 is introduced into a space 66 formed below the plate 21 
via a slit 65 formed at the bottom between the pipes 16 and exhausted from 
the flue pipe 32. 
In this embodiment, the pipes 16 also are provided with fins 34 on 
substantially the entire length and width thereof. Each of the fins 34 is 
arranged substantially parallel to the flow of the combustion gas in the 
combustion gas passageway 26. Furthermore, each of the fins 34 is provided 
with means for increasing the heat transfer efficiency, similar to the 
slits shown in FIG. 5, or have an inclined arrangement as shown in FIGS. 6 
to 9, or are provided with slits as shown in FIGS. 10 or 11. 
Furthermore, in the embodiment shown in FIG. 19, the heat resisting layer 
60 is arranged inside the casing 10, and an inner plate 70 made of a 
perforated plate, such as a punched plate, is arranged inside the layer 60 
of heat resisting material. Due to the provision of the perforated plate 
64, a high noise suppression effect is obtained. Furthermore, since the 
combustion gas can act directly on the heat resisting material layer 60, 
via the perforated plate, the temperature in the boiler is effectively 
maintained without heat loss. The casing 10 is covered by an outer cover 
72. It should be noted that this construction of the heat resisting 
coating can be also applied to the two row arrangement of pipes as 
explained with reference to FIGS. 1 to 18. 
In FIG. 21, the two row type boiler, in addition to the essential 
constituent members as already explained, is provided with guide pipes 75 
(one of which is shown in the drawing), to allow the introduction of a 
nozzle for blowing-out the boiler for cleaning purposes. Each of the guide 
pipes 75 is connected, in this embodiment, to the upper header 12 so that 
it passes through the header 12 and the heat resistant material layer 19 
attached to the bottom surface thereof. The pipe 75 is opened at its 
bottom end to the combustion gas passageway 26. 
When the boiler is to be cleaned, high pressure cleaner devices provided 
with tip nozzles are inserted in the upper ends of the respective guide 
pipes 75, and high pressure water is ejected from the nozzles while they 
are moved downward or upward. It should be noted that the nozzles are 
arranged so that horizontal jets of water are ejected therefrom. 
FIG. 22 shows a modification of the arrangement of the guide pipe 75, 
wherein a portion 75' of the guide pipe 75 surrounded by the heat 
resistant material layer 19 is thin-walled. The high temperature of the 
combustion gas acts mainly on the thin wall portion so that it is 
gradually burnt. Therefore, the portion of the guide pipe 70 in the upper 
tank 12 is maintained free from thermal damage. 
It should be noted that this cleaner nozzle arrangement can be also applied 
to the single row arrangement shown in FIG. 19. 
Although embodiments and modifications of the present invention are 
described with reference to the attached drawings, many other changes may 
be made by those skilled in this art without departing from the scope and 
spirit of the present invention.