GB2403790A

GB2403790A - Secondary heat exchanger for a boiler assembly

Info

Publication number: GB2403790A
Application number: GB0315900A
Authority: GB
Inventors: Martin Hodgson
Original assignee: Worcester Heat Systems Ltd
Current assignee: Bosch Thermotechnology Ltd
Priority date: 2003-07-08
Filing date: 2003-07-08
Publication date: 2005-01-12
Also published as: GB0315900D0

Abstract

The secondary heat exchanger 24 has a first fluid flow path for liquid to be heated and a second flow path transverse to the first for hot flue gas to flow. An inlet port 30 is connected to a primary liquid system outlet 32, such as the outlet of a central heating system. Plates 40 with longitudinal passages 42 are held in spaced relationship by heat exchanger body 44. The secondary heat exchanger is used in conjunction with a primary heat exchanger 16 which heats liquid flowing in a boiler body 12. An outlet port 34 is in fluid communication with an inlet of the primary heat exchanger 16. The inlet and outlet ports of the secondary heat exchanger are in fluid communication with each passage 42 between the plates. A burner 14, fan 20, and flue housings 18, 22, and 26 are also mounted in the boiler assembly 10.

Description

Improvements in or relating to a boiler assembly This invention relates to

a boiler assembly of the type which is used for heating water or other liquids, and to a secondary heat exchanger for the boiler assembly.

It is an on going process to improve the efficiency (SEDBUK) ratings of gas- fired and oil-fired boilers. Many condenser boilers achieve a SEDBUCK C efficiency rating, but a SEDBUK A rating is the best efficiency rating, and corresponds to 90% efficiency or higher.

It is also desirable that, whilst improving the efficiency of a boiler assembly, the components remain, as far as possible, standard and uncomplicated, thereby reducing manufacturing and maintenance costs, and increasing life expectancy.

It is known to have a boiler assembly which includes a secondary heat exchanger to pre-heat primary liquid, for example central heating liquid. See Figure 1. This type of heat exchanger is a plate heat exchanger having ports for hot flue gas and for the primary liquid. The plates are brazed copper and spaced, thereby defining parallel passages for the flow of the respective fluids. To improve the heat exchange, the fluid flows are contra-oriented.

However, this type of secondary heat exchanger, due to the use of ports, provides constrictions to the flow, resulting in the rate of heat exchange being limited, and therefore limiting efficiency.

Furthermore, corrosion of the copper plates occurs due to the acidic nature of the condensate formed as a by-product of the heat exchange process.

The present invention seeks to overcome these problems while attaining the above-mentioned desirable criteria.

According to a first aspect of the present invention, there is provided a boiler assembly comprising a boiler body, a primary heat exchanger for heating liquid flowing into the boiler body, a burner for heating the primary heat exchanger, a secondary heat exchanger for pre-heating the said liquid flowing into the boiler body, and a fan for directing resulting hot flue gas from the primary heat exchanger to the secondary heat exchanger, the secondary heat exchanger having a first flow path along which the liquid to be heated can flow and a second flow path transverse to the first flow path along which the hot flue gas can flow.

Preferable and/or optional features of the first aspect of the invention are set forth in claims 2 to 9, inclusive.

According to a second aspect of the present invention, there is provided a secondary boiler heat exchanger for internal use in conjunction with a primary boiler heat exchanger, the secondary heat exchanger comprising a plurality of heat exchanger plates having one or more passageways deeming a first flow path for a first fluid, the plates being spaced from each other to define a second flow path for a second fluid, and the first and second flow paths being transverse to each other..

According to a third aspect of the present invention, there is provided a boiler assembly having a secondary boiler heat exchanger in accordance with the second aspect of the invention.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a prior art secondary heat exchanger for a boiler assembly; Figure 2 is one embodiment of a boiler assembly in accordance with the first and third aspects of the invention and showing one embodiment of a secondary heat exchanger in accordance with the second aspect of the invention, Figure 3 is an enlarged view of the secondary heat exchanger shown in Figure Figure 4 is a perspective view of the of the secondary heat exchanger removed from the boiler assembly; and Figure 5 is a top plan view of the secondary heat exchanger shown in Figure 4.

Referring firstly to Figures 2 and 3, there is shown a condenser-type boiler assembly 10 (with front removed for clarity) which comprises a boiler body 12 in which is housed an up-fired gas burner 14, a primary heat exchanger 16 extending over the burner 14, a first flue housing 18 extending over the primary heat exchanger 16, an electric fan 20 mounted on the first flue housing 18, a second flue housing 22 leading from the fan 20 to a secondary heat exchanger 24, and a third flue housing 26 supported by the first flue housing 18 and on which the secondary heat exchanger 24 and flue pipe 28 are mounted.

The arrangement of the boiler assembly 10 is essentially a traditional well known arrangement, but with the addition of the second and third flue housings 22,26 and the secondary heat exchanger 24. As such, a majority of the components of the boiler assembly 10 are stock parts which require little, if any, modification.

The secondary heat exchanger 24 has an inlet port 30 which is connected to a primary liquid system outlet 32, such as the outlet of a central heating system. The primary liquid enters the boiler body 12 from outside the boiler assembly 10. An outlet port 34 of the secondary heat exchanger 24 is in fluid communication with an inlet of the primary heat exchanger 16, allowing flow of the primary liquid from the secondary heat exchanger 24 to the primary heat exchanger 16. An outlet of the primary heat exchanger 16 is connected to a primary liquid system inlet (not shown), such as the inlet of the central heating system, via an electric pump (not shown). The primary and secondary heat exchangers 16,24 are spaced from each other within the boiler body 12.

The burner 14 is spaced from the primary heat exchanger 16 and is connected to a mains gas supply in the usual fashion. A heating chamber 36 is thus formed between the burner 14 and the primary heat exchanger 16, and the sides of this heating chamber are insulated with known heatresistant insulating material 38 to further improve the efficiency of the boiler assembly 10.

The fan 20 draws hot flue gas through the primary heat exchanger 16, from the burner 14, through the first flue housing 18 and directs it into the second flue housing 22 adjoining the secondary heat exchanger 24. The hot flue gas then enters the secondary heat exchanger 24 at one longitudinal side, pre-heating the primary liquid as it flows therethrough, before exiting the secondary heat exchanger 24 at the other longitudinal side and passing into the third flue housing 26. The flue gas then leaves the boiler body 12 through the flue pipe 28.

Referring now to Figures 4 and 5, the secondary heat exchanger 24 will be described in more detail. The secondary heat exchanger 24 is an open plate heat exchanger and comprises a plurality of plates 40, each of which is formed with a passage or passages 42 extending in the longitudinal direction of the secondary heat exchanger 24. The plates 40 are held in spaced relationship by a heat exchanger body 44, so that the hot flue gas can flow in the space between adjacent plates 40.

The heat exchanger body 44 extends over the front and rear surfaces 46,48 and two sides 50 of the secondary heat exchanger 24, leaving unrestricted access to the two longitudinal sides 52 of each plate 40. The inlet and outlet ports 30,34 are mounted in spaced relationship on the front surface 46 of the heat exchanger body 44, and are in fluid communication with each passage 42 of each plate 40.

The secondary heat exchanger 24 is formed from brazed stainless steel, which inhibits corrosion due to the acidity of the condensate formed when in use. Since the heat exchanger plates 40 extend vertically or substantially vertically when the secondary heat exchanger 24 is housed in the boiler body 12, condensate can fall under gravity into the third flue housing 26, which thus acts as a catch-pan.

The secondary heat exchanger 24 therefore provides two flow paths, a first flow path, being the passages 42 in the plates 40, along which the primary liquid to be heated flows and which extends in the direction of the longitudinal extent of the secondary heat exchanger 24; and a second flow path which has a width corresponding to the length of the secondary heat exchanger 24 and which extends along the gaps between the heat exchanger plates 40. The second flow path is thus transverse to the first flow path. Preferably, the second flow path is perpendicular or substantially perpendicular to the first flow path.

In a modification to the above embodiment, the secondary heat exchanger plates may include a further plurality of passages (not shown) to enable independent pre-heating of domestic water, such as for showers, baths and washing, as well as pre-heating the primary liquid. In this case, a second pair of ports are provided on the heat exchanger body for inlet and outlet of the domestic water. These ports are connected to the domestic water passages in the heat exchanger plates in a similar manner to the ports for the primary liquid. The primary heat exchanger is also provided with an extra flow path for the domestic water.

The further plurality of passages defines a third flow path which flows along the longitudinal extent of the secondary heat exchanger, similarly to the first flow path. As a result, the second flow path of the hot flue gas flows transversely to both the first and third flow paths.

By use of a secondary heat exchanger which has transverse flow paths and which dispenses with the need for inlet and outlet ports for the flue gas, restrictions which have heretobefore limited the rate of flow of the flue gas through the secondary heat exchanger are significantly reduced, thereby allowing greater heat transfer between the flue gas and the primary / domestic liquids.

In tests using the above-described secondary heat exchanger and boiler assembly, the SEDBUCK A efficiency rating was obtained. It is therefore possible to provide a generally standard boiler assembly having a secondary heat exchanger which enables attainment of the highest efficiency rating. It is also possible to provide a compact heat exchanger formed from brazed stainless steel, thereby inhibiting corrosion and prolonging operational life.

The embodiments described above are given by way of example only and modifications will be apparent to persons skilled in the art without departing from the scope of the invention as defined by the appended claims. For example, the burner could be an oil-fired burner; and the arrangement of the components within the boiler assembly could different.

Claims

1. A boiler assembly comprising a boiler body, a primary heat exchanger for heating liquid flowing into the boiler body, a burner for heating the primary heat exchanger, a secondary heat exchanger for pre-heating the said liquid flowing into the boiler body, and a fan for directing resulting hot flue gas from the primary heat exchanger to the secondary heat exchanger, the secondary heat exchanger having a first flow path along which the liquid to be heated can flow and a second flow path transverse to the first flow path along which the hot flue gas can flow.

2. A boiler assembly as claimed in claim 1, wherein the primary and secondary heat exchangers are housed together within the boiler body.

3. A boiler assembly as claimed in claim 1 or claim 2, wherein the primary and secondary heat exchangers are spaced from each other.

4. A boiler assembly as claimed in any one of claims 1 to 3, wherein the secondary heat exchanger is an open plate heat exchanger.

5. A boiler assembly as claimed in claim 4, wherein each plate of the secondary heat exchanger includes one or more passages through which the liquid to be heated can pass.

6. A boiler assembly as claimed in claim 5, wherein inflowing flue gas enters the secondary heat exchanger along the longitudinal extent of each plate.

7. A boiler assembly as claimed in any one of the preceding claims, wherein the secondary heat exchanger is fommed from brazed stainless steel.

8. A boiler assembly as claimed in any one of the preceding claims, wherein the primary and secondary heat exchangers are adapted to heat first and second liquids.

9. A boiler assembly as claimed in claim 8, wherein the said first liquid is a heating system liquid and the said second liquid is a domestic use liquid.

10. A secondary boiler heat exchanger for internal use in conjunction with a primary boiler heat exchanger, the secondary heat exchanger comprising a plurality of heat exchanger plates having one or more passageways defining a first flow path for a first fluid, the plates being spaced from each other to define a second flow path for a second fluid, and the first and second flow paths being transverse to each other.

11. A boiler assembly having a secondary boiler heat exchanger as claimed in claim 10.

12. A boiler assembly as claimed in any one of the claims 1 to 9 and 11, wherein the boiler assembly is a condensing boiler assembly.

13. A boiler assembly substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

14. A secondary boiler heat exchanger substantially as hereinbefore described with reference to Figures 4 and 5 of the accompanying drawings. s