Hot-water boiler, for instance a central heating boiler, and a metal casting therefor

A hot water boiler, particularly for central heating systems, has a double-walled structure containing water passages and bounding a combustion chamber, and a multi-walled structure outside the double-walled structure providing passages for the combustion gases and the incoming air for combustion. In order to increase thermal efficiency, while providing an economic and simple design, the double-wall structure is a pair of opposed hollow metal castings which bound not only the combustion chamber but also a combustion gas passage leading upwardly from that chamber and having in it projections integrally formed on the castings. Outside the castings are walls providing, in sequence, downward and upward passages for combustion gas and a downward passage for combustion air.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to hot water boilers, and particularly such boilers 
for use in domestic or other central heating systems. The invention also 
relates to hollow metal castings suitable for use in such boilers. 
2. Description of the Prior Art 
The object of the present invention is to provide a hot water boiler which 
can have a thermal efficiency exceeding 86% or even 90%, and at the same 
time can be manufactured at low cost and occupies a small amount of room. 
The invention as claimed is intended to solve this problem. It proposes 
improvement of the heat exchange between the combustion gas and the water 
by locating the water passage in double-walled metal castings which have 
integral projections projecting not only into a passage for combustion gas 
outside the casting but also into a combustion gas passage leading out of 
the upper end of the combustion chamber. Additionally, further passages 
are arranged, in a triple-wall casing structure, for heat exchange between 
the combustion gas and inflowing combustion air. 
In view of the rise in energy costs in recent years, there has been a 
distinct need for designs of hot water producing apparatus of higher 
thermal efficiency. In this connection there have been proposed central 
heating boilers which are provided with additional insulation, or the flue 
gas exhaust of which has an additional heat exchanger. This and similar 
designs have proved to result in only limited improvements in thermal 
efficiency, but they do tend to cause a steep rise in cost as well as in 
the space required by such boilers. It should be mentioned that a 
conventional method of expressing the thermal efficiency of gasfired 
boilers is based on the calorific top value of the fuel, ie. the accepted 
upper limit of the calorific value of the fuel. In conventional designs of 
hot water boilers, it is not possible in a simple way to achieve a thermal 
efficiency of over 80% on the water side, defined in this way. 
SUMMARY OF THE INVENTION 
The object of this invention is to provide a hot water boiler which has a 
thermal efficiency exceeding 86% and is even as high as 90%. 
It is another object of the invention to provide a hot water boiler which 
can be manufactured at low cost and which occupies a small amount of room. 
The hot water boiler of the invention has a double wall structure 
containing passages for flow of the water being heated and providing 
bounding walls on at least two opposite sides of a combustion chamber. 
Outside said double wall structure is a plurality of further walls 
providing passages for flow of combustion air and combustion gases whereby 
the combustion gases exchange heat with the water and the air for 
combustion. The said double-wall structure has vertically extending first 
projections on its outside face which project into a said passage for 
combustion gas. The said double-wall structure is provided, in a manner 
known per se, by opposed hollow metal castings which provide opposite 
walls of the combustion chamber and further provide opposed bounding walls 
of a combustion gas outlet passage extending upwardly from the combustion 
chamber. The castings have second projections extending into said 
combustion gas outlet passage. Outside each of the castings, three walls 
provide, in sequence in the outward direction, firstly a first passage for 
downward flow of the combustion gases from the said outlet passage over 
the surfaces of said first projections on the outside of the castings, 
secondly a second passage for upward flow of the combustion gases from 
said first passage and thirdly a passage for downward flow of combustion 
air which is connected into the bottom of the combustion chamber. 
In comparison with, for instance the hot water boiler of U.S. Pat. No. 
2,787,256, the boiler of the invention has--besides high thermal 
efficiency--the following advantages and differences. 
In the first place, with the invention there is no need for a second 
double-walled body with water channels acting as a recuperator, which 
leads to a much simpler design, in which much external ductwork can be 
omitted. In the second place, the prior art boiler has no flue gas 
exhaust, so that a power burner is required to force the combustion gases 
through the relatively narrow and long passages. 
Thirdly, in the boiler of the invention, combustion is in an upwards 
direction instead of a downwards direction. If the water in the water 
passages flows upwards, heat exchange on the combustion chamber side is 
consequently concurrent and on the exterior side of the water passages is 
countercurrent. 
Fourthly, in the prior art hot water boiler combustion air is conducted 
first downwardly and then upwardly while with the invention the combustion 
air is preferably conducted downwardly only. Also, the castings used in 
the present invention can be used side-by-side to provide a larger boiler, 
whereas the prior art boiler is cylindrical, and does not lend itself to 
adaptation using the same components. 
Finally, the use of castings for the body with water channels with the 
invention is much cheaper than the use of steel plate. 
In the boiler of the invention, because the castings are heated on both 
sides by the hot combustion gases, they are less liable to inequalities in 
thermal expansion, and consequently to the occurrence of thermal tensions 
within the system. 
Preferably the said wall separating said second and third passages outside 
the casting has vertically extending projections on both its sides. This 
improves heat transfer between the combustion gas to the incoming air. 
This wall, ribbed on both sides can for example be shaped as an extruded 
profile. Another improvement of thermal efficiency may be achieved if the 
wall separating said first and second passages outside the casting is 
covered over at least part of its height and on at least one side, by a 
layer of insulating material. 
It has appeared in tests that such intensive cooling of the combustion 
gases can be achieved in the boiler of the invention that the temperature 
in the gases finally is insufficient for an adequate natural chimney 
draught. For this reason it may be desirable to include a fan in the 
combustion gas exhaust system. 
By fastening the castings to the casing structure it is possible to fix 
them in the desired position in relation to each other. A sturdier 
structure can be obtained by welding the castings to each other by means 
of end plates and/or a bottom plate. It is preferred according to the 
invention, however, to achieve a simpler and yet effective connection in 
which the two opposed castings each have at least one side or bottom wall 
which extends towards the corresponding wall of the other casting, and 
H-section elements embrace the respective opposed edges of these walls in 
order to join them together. The H-section profiles can firmly grip the 
edges of the bottom or side walls. 
Although it is conceivable to manufacture the castings from a different 
metal, the use of light metal is greatly preferred for this purpose; on 
the one hand it can be cast very easily into complicated shapes, and on 
the other hand its use can result in marked reduction of weight. Besides, 
thermal efficiency can also be considerably improved in this way. Finally 
light metal, in particular aluminium, is very suitable on account of its 
resistance to corrosion by condensate. 
It should be remarked that in Dutch patent applications Nos. 7102691 and 
7606640, it is proposed to use light metal castings for a hot water 
boiler, but these proposals relate to massive castings, cast without 
cores, which are welded together to get hollow castings having passages 
within them. 
The boiler of the invention may comprise only two castings arranged 
opposite to each other, alternatively boilers of greater capacity may be 
made from the same castings by combining two or more pairs of castings 
side-by-side into larger units. Literature provides enough information on 
the design in such a case of the water passages in each of the castings 
and in the castings joined together in order to achieve optimum water 
circulation and heating. It is not necessary to provide more details on 
this subject here.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, the bottom 1 of the boiler consists of two 
bottom half plates, each forming an integral part of the two castings 5 
(see shaded portions of FIG. 4). The boiler is further enveloped by a 
composite outer casing which is shown in three parts 2, 3 and 4, having a 
bent or cranked form. This shape is not essential, and particularly in the 
case of a high-load combustion chamber it is possible to design the 
castings and the casings as generally flat. 
At its top the boiler is connected to a flue or combustion gas exhaust 6, 
which is connected to a chimney shaft 8 by a combustion gas fan 7 to 
provide draught for combustion, as described above. 
Around the exhaust pipe 6 there is an annular hole 9 in a top plate of the 
casing, through which combustion air is drawn into the boiler. In FIG. 2 
the fan 7 is omitted, for clarity. 
FIG. 4 which also does not show the fan 7 and the chimney shaft 8 shows 
that each of the castings 5 consists of an integrally cast double-wall 
structure 10 which contains two bends, as seen in vertical section. 
Between the double walls of each casting is a water passage 11, partitions 
causing this to have a folded or zig-zag path. The water passages 11 can 
be coupled to each other in series, but it is alternatively possible to 
have the two castings feed separate hot water circuits. The top end of 
each casting 5 has a large number or field or elongate finger-shaped 
projections 12 which point towards the corresponding projections of the 
other casting. In this manner a wide combustion chamber is provided 
between the castings 5 at the bottom of the boiler (with burners which are 
not shown), this chamber tapering off towards its top and joining 
immediately into a flue gas passage in which heat transfer to the water 
passages 11 is additionally effected by the finger-shaped projections 12. 
Further upflow of the hot gases is prevented by a plate 21, which deflects 
them horizontally between ribs 13 provided on the upper edge of the 
castings 5. Thereafter the gases pass into a downward flow passage 14 
bounded by one casting 5 and a plate 18 forming one of the three walls of 
the triple-walled casing. The plate 18 is coated with a layer of 
insulating material 26 on one or both sides. In this passage the flue 
gases pass between outwardly projecting longitudinally extending vertical 
ribs 17 of the casting, thus transferring heat via these ribs 17 to the 
water in the castings. 
The passage 14 bounded by the casting 5 and the casing plate 18 joins at 
the bottom of the boiler via a condensate trough into an upward passage 
which is bounded by casing plates 18 and 19 and which joins at its top end 
into the gas exhaust pipe 6. Between the plate 19 and the outer casing 2, 
3 and 4 is a third passage 15, for downflow of incoming combustion air. 
The casing plate 19 is provided on both sides with vertical ribs 24 and 25 
in order to improve heat transfer from the combustion gas to the 
combustion air, and with these ribs is formed as an aluminium extruded 
profile. 
The combustion air is sucked into the top of the boiler via an aperture 9 
and flows downwards in the passage 15 to enter the combustion chamber via 
gates 16. Burners (not shown) are supplied at the bottom of the combustion 
chamber. The design of these burners and the manner in which they are 
fitted in the boiler are conventional and need not be illustrated or 
described. 
FIGS. 5 and 6 show in more detail how the castings 5 are enclosed in the 
boiler between end plates 20. The castings 5 can for example be fixed to 
these end plates 20 by welding, although other method of construction are 
also feasible. FIG. 4 shows the manner in which the bottom plates of the 
castings 5 are connected by means of an aluminium H-section profile 22 
which embraces the opposed edges of these plates and grips them. If 
required this profile 22 can be secured to the bottom plates by means of 
screws. In an alternative embodiment, the end plates 20 are omitted, and 
the castings 5 may be designed with integrally cast side walls which can 
be connected to each other in a similar manner as the bottom plates, using 
an aluminium H-profile. 
Although the boiler shown in the figures has only two castings arranged 
opposite each other, it is also conceivable to join two or more pairs of 
opposed castings together side-by-side in line into a larger unit with a 
large combustion chamber. These and similar variants of the present 
structure will be self-evident to an expert and no further details are 
required. All such modifications are considered to come within the scope 
of the present invention.