GB2381571A

GB2381571A - Oil-fired boiler with two heat exchangers

Info

Publication number: GB2381571A
Application number: GB0126356A
Authority: GB
Inventors: Hedley Roland Mickleburgh
Original assignee: HRM BOILERS Ltd
Current assignee: HRM BOILERS Ltd
Priority date: 2001-11-02
Filing date: 2001-11-02
Publication date: 2003-05-07
Anticipated expiration: 2021-11-02
Also published as: GB0126356D0; GB2381571B

Abstract

A boiler (10) suitable for supplying hot water to a domestic hot water or heating system. The boiler (10) has two heat exchange chambers to extract thermal energy from a combusted fuel. The second chamber (19) is located between an outlet flue (20) for combustion gases and the first chamber (13). The second chamber (19) is substantially co-extensive with an outer wall of the boiler and located adjacent the outer wall. Both the first and the second heat-exchange chambers (13,19) include a number of heat exchange surfaces or baffles (15) to remove heat from the boiler (10). The combustion products are primarily directed towards the inlet for the carrier fluid. The first heat-exchange chamber (13) has a frusto-conical cross-section to improve mixing of the combustion gases from the fuel. At least one baffle may be formed from steel with a fibreboard outer covering of low thermal conductivity.

Description

AN IMPROVED OIL-FIRED BOILER Field of the Invention The present invention relates to an oil-fired boiler In particular, the invention relates to a non-condensing boiler, primarily for use in a domestic environment Background to the Invention In spite of recent increases in the price of domestic fuel oil, the use of oil to provide the source of energy for hot water and central heating systems remains common For many people, for example in rural communities, where there is no gas supply, oil is the only viable and economic option In standard heating systems, the thermal energy from the combustion of the oil is used to heat water, the water subsequently being pumped around the radiators of a building or to a hot water system.

Oil fired systems have been developed which suit most needs with regard to the location of the boiler and venting of the fumes produced For example, boilers can be free standing within a room, the waste gases from the combustion of the oil being vented from the building via a flue An increasingly popular alternative is to mount the boiler on an external wall. In this case, the gases are vented directly outside through the external wall to which the boiler is mounted. Such wallmounted boilers are typically more compact that free standing ones in order to minimise the amount of wall space taken up They also tend to be of a more lightweight construction, to ease installation and reduce strain on the wall and the boiler mountings.

A further advantage of wall-mounted boilers is that, if they are appropriately constructed, the majority of maintenance and service work can be carried out from outside the building This not only minimises disturbance to the occupier of the building, but also enables the engineer carrying out the work to plan a schedule without necessarily taking into account when he can obtain access to the building Increasingly however, the regulations governing such boilers are becoming more demanding with regard to the efficiency of energy usage. In particular, the efficiency in converting the heat from the combustion of the oil to thermal energy of the water must be high Transfer of the combustion energy of the oil to the water is effected by heat exchangers within the boiler. Usually, such heat exchangers comprise a series of metal baffles whose interior contains the water The baffles ensure that the hot gases formed during combustion must travel a lengthy path and repeatedly impact the cold surface of the baffle prior to exiting the boiler The baffles thereby remove the majority of the thermal combustion energy of the gases Condensing boilers, in a final step, remove also the heat of condensation in the gases The energy is transferred through the wall of the baffles and then to the water Circulation of water through the baffles carries heat away from the boiler and introduces colder water into the boiler to maintain the temperature differential

between the gas and the water and thereby maintain a high rate of energy transfer 1. 9 Unfortunately, the baffles, particularly in non-condensing boilers are often prone to corrosion, particularly as they normally contain many sharp or highly curved surfaces-such surfaces being particularly vulnerable to chemical attack by the hot gases Moreover, the baffles in the regions at the bottom of the boiler where cold water is introduced are also prone to corrosion as they contain water below the dew point temperature of water. Acidic water from the gases can readily therefore form on their surfaces Although a number of materials, such as steel or ceramics have been tried and used to form the outer walls of baffles in order to increase the

corrosion resistance of the baffles, they frequently need to be replaced It would be advantageous therefore to include within a boiler features which enable heat to be transferred to the heating water without the gases falling below the dewpoint, even when the temperature of the water is substantially below the dewpoint The reduction in corrosion would reduce maintenance costs and prolong the working life of the boiler. Moreover, the transfer of energy through a corroded baffle is less efficient than through a non-corroded one It is an object of the invention to provide a boiler which addresses the problems described above.

Summary of the Invention According to the invention there is provided a boiler suitable for use in supplying hot water to a domestic hot water or heating supply, the boiler comprising a fuel combustion unit for converting a fuel into thermal energy, a first heat exchange chamber operably connected to the fuel combustion unit, a flue to enable combustion products from a fuel to exit the boiler, and a second heat exchange chamber operably connected, between the first heat exchange chamber and the flue, the second heat exchange chamber being substantially coextensive with an outer wall of the boiler and located adjacent said outer wall The first and/or second chamber includes a path for directing the combustion product over a plurality of heat exchange surfaces and/or baffles By increasing the length of the path which the combustion products need to traverse before exiting the boiler, the efficiency of the boiler is increased and corrosion reduced The or each baffle and wall are advantageously hollow to enable a carrier fluid to flow through the or each baffle and wall, the carrier fluid entering the boiler at an

inlet region and exiting the boiler at an outlet region Thermal energy is transferred to the carrier fluid which then transports the energy to the required location In the general domestic use considered in the preferred embodiment, the fluid is water Advantageously, the combustion products are directed from the first heat exchanger to the carrier fluid inlet region. The combustion products therefore encounter the carrier fluid whilst the carrier fluid is still at its coldest and the combustion products relatively hot this increases the rate of heat transfer and also minimises the risk of condensation.

The second heat exchange chamber is preferably located adjacent the front wall to facilitate maintenance of the boiler For clarification, the term"front"is here defined, as the surface including the fuel combustion unit Preferably, the first heat exchange chamber includes one or more baffles The baffles facilitate the extraction of heat to be extracted from the hot combustion products.

Optionally, an intermediate baffle chamber is located intermediate the second heat exchange chamber and the flue. The intermediate baffle chamber enables further thermal energy to be removed from the combustion products and increases the efficiency of the boiler Preferably, the first heat exchange chamber has a frusto-conical cross-section to improve mixing of the combustion products and increase the transfer of heat from the combustion products A drain can advantageously be included in the boiler to enable any condensed water vapour to run off from the boiler Advantageously, any removable baffles are formed from steel, with a fibreboard outer covering, having low thermal conductivity, to provide a baffle with good thermal conductivity able to withstand the corrosive conditions within the boiler

Brief Description of the Drawings The invention will now be described with reference to the accompanying drawings which show by way of example only, one embodiment of an oil fuelled boiler In the drawings Figure I is a side view of the internal structure of a boiler, Figure 2 is a front view of the internal structure of a boiler, Figure 3 is a further side view of the internal structure of a boiler, and Figure 4 is a further front view of the internal structure of a boiler.

Detailed Description of the Invention Referring to Figures I and 2, a boiler 10 comprises an oil burner unit 11 in which the combustion of oil takes place The oil burner unit 11 is mounted to the outside of the boiler 10 A vent 12 passes through the front walls of the boiler 10 and links the oil burner unit 11 to a first chamber 13 within the boiler 10 As can be seen from Figure 3, the chamber 13 has a frusto-conical section An insulating ring 14 passes around

the vent 12 and prevents, firstly, heat being conducted from the oil burner unit 1 to the front wall of the boiler 10 Secondly, the ring 14 prevents the escape of combustion gases from the chamber 13 A baffle 15 forms the lower surface of the chamber 13 and, as can be seen from Figure 4, extends across a substantial area of the chamber 13. The baffle 15 is typically formed from steel and has an exterior surface formed of fibre board or other material having a low thermal conductivity.

A gap 16 links the chamber 13 with a connecting cavity 17, which in turn is connected to the first end of a further cavity 18 adjacent to the front wall of the boiler 10. The further cavity 18 is connected at a second end to a baffle chamber

19 An exit flue 20 provides a vent for the waste gases to exit the boiler 10 The walls 21 of the boiler 10, along with the baffles 15,22 are hollow to enable water to circulate within them. The water transfers heat from the combustion to a hot water or heating system. The boiler 10 is provided with an entry valve 23 and an exit valve 24 to control the flow of water within the boiler 10 In use, oil is oxidised within the oil burner unit 11, thereby producing heat in the form of hot waste gases consisting predominantly of carbon dioxide and water gas /vapour The carbon dioxide and water exit the burner unit 11 entering the first chamber 13 via the vent 12. As the gases impact the wall 21 and the baffles 15 of the first chamber 13, heat is transferred from the gases to the water within the walls 21 and the baffles 15 The gases pass along the route shown by arrow A in Figure 1 to the baffle chamber 19 En route to the baffle chamber 19, the gases impact the water inlet region 25 This configuration is particularly advantageous as it results in the still hot gases encountering the water whilst the water is at its coldest The difference in heat results in rapid heat transfer, but also reduces the risk of condensation of water from the gases As the gases pass through to the baffle chamber 19, and whilst in the baffle chamber 19, further thermal energy is transferred from the gases to the water contained within the walls and the baffles.

In the above described embodiment it should be noted that although a substantial amount of thermal energy is removed from the gases, their temperature remains such that at the flow rate of the gases produced, the water remains as a gas or vapour and does not condense out However, in a further embodiment of the invention which is not illustrated, the boiler can be so configured that the heat of condensation is also removed In this further embodiment the boiler is mounted essentially, the other way up to that already described with the exit flue 20 at the bottom of the boiler unit 10 A drain is fitted to the boiler unit 10 remove the condensed water Further removal of such heat may at first sight appear to produce energy savings, the additional corrosion problem caused by the acidic nature (due to the presence of large amounts of dissolved carbon dioxide in the water) of the water produced do not necessarily result in cost savings during prolonged use

A comparison between the efficiency of an oil fuelled boiler constructed according to the present invention and one constructed according to the prior art was made The results are presented in Table 1 Table I-Efficiency Comparison

Boiler I (According to Boiler 2 (According to the Present Invention) the Prior Art) Efficiency % 935 888 NO Emissions (ppm) 69 81 NOx Emissions (ppm) 73 86 CO Emissions (ppm) 8 8 O2% 60 61 COz % 110 110

From Table I it can be seen that the efficiency of boiler I is greater than that of boiler 2 With regard to emissions of polluted gases such as NO,, carbon monoxide and carbon dioxide, the two boilers are comparable and well within current statutory guide lines.

The internal volumes of the walls and baffles shown in the above diagrams are empty, giving hollow walls and baffles The internal volumes can however be reduced by the inclusion of solid volumes The amount of water flowing through the boiler is thereby reduced, which means that the water flowing through can reach a higher temperature than would otherwise be the case before being passed through and into the heating system

It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention.

Claims

Claims A boiler suitable for use in the supply of hot water to a domestic hot \\ater or heating system, the boiler comprising a fuel combustion unit for converting a fuel into thermal energy, a flue to enable fuel combustion products to exit the boiler, a first heat exchange chamber operably adjacent to the fuel combustion unit, and a second heat exchange chamber operably connected between the first heat exchange chamber and the flue, the second heat exchange chamber being substantially coextensive with and adjacent to an outer housing wall of the boiler
2. A boiler according to Claim 1, wherein one or both of the first and second heat exchange chambers includes a path for directing the combustion product over a plurality of heat exchange surfaces or baffles
3 A boiler according to Claim 2, wherein the or each baffle or surface is I hollow to enable water to flow through the or each baffle or surface, the boiler including inlet and outlet coupling to enable water to enter and exit the boiler
4 A boiler according to Claim 3. wherein the combustion products are I directed from the first heat exchanger to the carrier fluid inlet coupling
5 A boiler according to any preceding claim, wherein the second heat exchange chamber is located adjacent an in-use front housing-wall to facilitate maintenance of the boiler

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6, A boiler according to any preceding claim, wherein the boiler includes a baffle chamber located intermediate the second heat exchange chamber and the flue
7 A boiler according to any preceding claim, wherein the first heat exchange chamber has a frusto-conical cross-section to improve mixing of combustion products and to increase the transfer of heat from the combustion products
8 A boiler according to any preceding claim, which includes a drain to I' z enable any condensed water vapour to run off from the boiler 9 A boiler according to any of Claims 2 to 8. wherein at least one baffle is I formed from steel, with a fibreboard outer covering of low thermal conductivity 10 A boiler substantially as herein described with reference to and as illustrated in the accompanying drawings