This invention relates to heat exchangers and is mainly concerned
with heat exchangers for domestic gas boilers of the type providing separate fluid
flow paths for central heating water and domestic hot water for consumption, i.e.
so-called 'duplex' boilers.
A known form of heat exchanger used in gas boilers has an upright
cylindrical gas burner surrounded by an annular array of fluid carrying tubes extending
between upper and lower headers or manifolds, and a casing or housing which encloses
and is spaced from the tubes and upper header to define therebetween passages
for flow of flue gases, a lower end of the housing being sealed to the lower header.
Hot gases from the gas burner pass between the tubes underneath the upper header
and then pass upwardly around and across the top of the upper header before passing
out through a central flue outlet located above the upper header.
Heat exchangers of this construction are generally efficient and
are effective for boilers having a single fluid flow path, so that water in the
upper header is not static at any time during burner operation. In a duplex boiler,
however, there are two distinct flow paths through the heat exchanger and when
the burner is in operation for heating water flowing through one path, water filling
the other fluid path within the heat exchanger may be static, and under such conditions
it has been found that with the known heat exchanger construction overheating of
the static fluid may occur.
The invention addresses this problem and provides a heat exchanqer
for a boiler comprising upper and lower headers with tubes extending therebetween
in an annular array surrounding a combustion chamber for accommodating a gas burner,
the headers and tubes defining two distinct flow paths for liquid to be heated,
wherein the flow paths through the tubes are in heat exchange relationship with
each other, and baffle means are provided in the upper part of the combustion
chamber spaced below the upper header so that flue gases are constrained to pass
outwardly from the combustion chamber between the tubes before any heat exchange
with the upper header is possible.
It has been found that static water held in the upper header is at
most risk of becoming overheated during burner operation. By directing the flue
gases across the tubes before they can contact the upper header the temperature
of the flue gases is reduced sufficiently to considerably diminish the possibility
of overheating static water in the upper header. Because the flow paths through
the tubes are in heat exchange relationship, the flow of liquid through one path
during burner operation will prevent static liquid in the other path over heating.
Conveniently, each tube comprise an outer tube through which extends a coiled tube,
the coiled tube constituting one flow path and the spaces within the straight
tube not filled by the coiled tube defining the other flow path.
A clear understanding of the invention will be gained from the following
detailed description, reference being made to the accompanying drawings, in which:
- Figure 1 illustrates schematically a cross-section through a known heat exchanger;
- Figure 2 illustrates schematically a cross-section through a preferred embodiment
of a heat exchanger according to the present invention; and
- Figure 3 shows a section through one of the heat exchanger tubes employed in
the heat exchanger of Figure 2.
The known heat exchanger 1 illustrated in Figure 1 includes an upper
header 2 generally cylindrical in outer form, an annular lower header 3, and several
upright heat exchanger tubes 4 extending between the headers and arranged in an
annular array around a combustion chamber into which extends an upright gas burner
5 connected to a fan 6 for delivering a premix of gas, e.g. natural gas (mostly
methane), and air into the interior of the burner. A cylindrical casing or housing
7 has its lower end sealed to the lower header 3 and surrounds the upper header
2 and the tubes 4. The upper end wall 9 of the casing includes a central opening
leading to a flue duct 8 which extends upwardly from the casing.
The headers 2,3 include chambers which communicate with the tubes
4 to define a fluid path (not shown) for the passage of water through the heat
exchanger 1 from an inlet 10 to an outlet 11. The water makes several passes through
tubes 4 between the upper and lower headers in following this flow path. Hot combustion
gases from the gas burner 5 pass out between the tubes 4 for heating the water
in the tubes. Fins 12 are brazed to the tubes 4 to improve the efficiency of the
heat exchange. Hot combustion gases are prevented from rising directly to the flue
outlet from the burner 5 by the upper header 2, the lower surface of which is
exposed to the hot gases in the combustion chamber. The upper header constrains
gases to pass into a space formed outside of the tubes 4 and then to flow between
the upper header 2 and the casing 5, eventually departing the heat exchanger 1
via duct 8.
Figures 2 and 3 illustrate a preferred embodiment of a heat exchanger
20 according to the invention. The general construction of the heat exchanger
20 is similar to the known heat exchanger of Figure 1 in that it has tubes 4 extending
between upper and lower headers and arranged around a combustion chamber and within
a casing. However, the upper header 22 is annular and positioned around the flue
duct 23 at the top of the casing 24 to which it is sealed and a baffle in the
form of an imperforate plate 21 is mounted between the gas burner 5 and the upper
end wall of the casing formed by the underside of the upper header.
The headers 22,3 include chambers which communicate with the tubes
4 to define two separate fluid flow paths for flows of water to be heated, the
flow paths extending between respective inlets 25 and outlets 26 and each path
including several passes through the tubes between the upper and lower headers.
Conveniently, as shown in Figure 3, in the heat exchanger tubes 4 a helical tube
28 defines a first fluid flow path 30, the radially outmost surface of tube 28
abutting the internal surface of the tube 4. The spaces within tube 4 not filled
by tube 28 define a second fluid flow path 29. According to this convenient arrangement
the two fluid flow paths 29,30 are in direct heat exchange relationship both with
each other, as well as with hot combustion gases from the combustion chamber,
which prevents overheating of static fluid in either flow path through the tubes
4 during boiler operation.
The baffle plate may be brazed to the tubes 4 in the same way as
the fins 12.
The length of tubes 4 above the plate 21 is substantially smaller
than that below the plate and only the sections of the tubes below the baffle plate
are equipped with fins 12.
During operation of the heat exchanger 20, when water may be static
in one of the fluid paths, hot combustion gases from the burner 5 pass around tubes
4, between the baffle plate 21 and the casing 24, and subsequently pass between
the baffle plate and upper header to exit the heat exchanger through duct 23. Due
to the baffle plate, the upper header is not contacted by hot gases in the combustion
chamber and it is only contacted by the gases after they have been cooled by heat
exchange via the finned tube sections.