GB2113366A

GB2113366A - Boiler

Info

Publication number: GB2113366A
Application number: GB08300424A
Authority: GB
Inventors: Kenneth Herbert Haywood; Paul Christopher Haywood
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1982-01-09
Filing date: 1983-01-07
Publication date: 1983-08-03
Also published as: GB2113366B; GB8300424D0

Abstract

A boiler has a downwardly fired burner 1 and a heat exchanger 2 for extracting heat from the flue gases and transferring it to a working fluid. A downstream secondary heat exchanger 3 enables evaporation of water into the inlet air supply by having a wicking arrangement capable of transferring condensate from the flue gases and evaporating it into the inlet air side of the burner <IMAGE>

Description

SPECIFICATION Boiler The present invention relates to boilers and more particularly to boilers for domestic or industrial heating systems.

Conventional water heating boilers using gas or oil fired burners generally have efficiencies of up to 75%. A significant amount of heat loss occurs in the flue gases as the boilers are designed to operate at a sufficiently high temperature to prevent condensation of the flue gas water content in the heat exchanger which could otherwise cause operating and corrosion problems.

Thus, according to the present invention there is provided a boiler comprising a burner having an air supply and a fuel supply, the burner being downward firing, and a heat exchanger for extracting heat from the flue gases from the burner and transferring it to a fluid, there also being means for condensing water from the flue gases and evaporating water into the inlet air supply to the burner.

The flat short uniform flames typical of the burners claimed in UK Patent No. 1205432 (a) allow downward firing of the burner, (b) facilitate construction of a compact boiler and (c) give a more uniform temperature across the inlet face of the heat exchanger. The downward firing avoids flue gas condensation running back onto the burner as in the case of upward firing burners. This tends to reduce corrosion, malfunctioning of the burner and allows easier drain off of condensation.

The fuel supply is usually a fuel gas e.g. natural gas, however, a liquid fuel, e.g. kerosene may possibly be used.

It has been found that by use of secondary heat exchanger system for evaporating water into the inlet air with subsequent recondensation from the flue gases, significantly improved boiler efficiences may be obtained even with boiler hot water return.

The means for evaporating water into the inlet air supply is preferably a secondary heat exchanger most preferably using water vapour condensate from the flue gases. A suitable device is a reservoir of water or the like communicating with a capillary wicking arrangement in the inlet air flow, the reservoir being replenished by condensation of water vapour from the flue gases. The capillary wick can be constructed of any suitable material, such as woven glass or metal fibre, sintered metal, porous coatings formed of ceramic or metal by spraying or electroplating. The capillary wicking arrangement may communicate with the water in the reservoir or the like for example by use of a wick in the inlet air flow which absorbs water passing through apertures from the reservoir or the like to the inlet air side of the secondary heat exchanger.Alternatively, it is envisaged that the capillary wicking arrangement may comprise walls of the secondary heat exchanger (or a part of them) fabricated from porous materials, e.g. sintered metal, which allow controlled passage of water from the flue gas side to the inlet air side of the secondary heat exchanger. The relatively large surface area of the capillary wicking arrangement facilitates evaporation of water into the inlet air.

The invention also includes a method of heating a fluid of a heating system comprising the steps of (a) supplying fuel and oxygen or air to a burner and igniting the resultant mixture (b) passing the flue gases into a primary heat exchanger adapted to allow transfer of heat from the flue gas to the fluid (c) transferring heat from the flue gas to the inlet air supply and condensing the water from the flue gases and evaporating water into the inlet air supply to the burner. The step (c) is preferably achieved by the use of a secondary heat exchanger which is designed to be large enough to raise the temperature and water content of the inlet air such that the dew point at the exit of the primary heat exchanger is above the flue gas temperature at that point, i.e. so that condensation occurs in the primary heat exchanger.It is this feature that significantly increases the boiler efficiency relative to that of a conventional boiler when operated with hot water return.

The invention also includes secondary heat exchangers (as hereinbefore described) per se. It is also envisaged that the aforementioned means for evaporating water into the inlet air may be used with other heating means, e.g. geothermal heating, in order to obtain more efficient heat transfer.

The invention will now be described by way of example only and with reference to the accompanying drawing.

A boiler for supplying hot water to a heating system has been developed to operate at up to 35,000 Btu/hour, with an efficiency of approximately 90% on a hot water return. The boiler includes a burner 1, a finned heat exchanger 2 and a secondary heat exchanger 3 that utilizes a maximum amount of exhaust heat, even with a hot water return to the heat exchanger.

The burner 1 of the matrix type comprises a fuel chamber through which a plurality of combustion air tubes or passage ways 8 pass. The fuel is supplied to the burner 1 by an inlet line 6. The air tubes 8 are of rectangular or square cross-section. The burner 1 is of the diffusion flame type and the fuel emerges into the combustion zone 9 through fuel outlet holes 10. The fuel outlet holes 10 are of circular cross section and they are arranged so that there is one fuel outlet per side of the square cross section air passageway.

Downstream of the burner is a finned heat exchanger unit 2 carrying pipes 11 for hot water outlet. The heat exchanger 2 is heated by the hot flue gases passing downwards from the burner 1.

Beneath the finned heat exchanger unit 2 is a secondary heat exchanger 3 comprising a number of parallel openings 1 2 through which the flue gases can pass and which causes condensation of water from the flue gases. The condensed water runs into a reservoir 1 3 at the base of the secondary heat exchanger 3 and excess water in the reservoir is lead off by a weir duct arrangement (not shown). The reservoir 1 3 further contains a wicking system or capillary wicks which connect with the inlet air side of the secondary heat exchanger 3 thereby enabling the water content of the inlet air to the burner 1 to be increased.

The various sections are surrounded by a main housing 14 which contains a viewing window 4 for observing the performance of the burner flame.

A fan in a housing 1 5 is bolted onto the side of the main housing 1 4 for supplying inlet air to the burner 1.

The table shows test results and resultant efficiency details for natural gas as fuel. It will be noted that this operates from 15,000 to 37,000 Btu/hour at high efficiency.

In use of the boiler, fuel gas is fed to the burner 1 va inlet 6 and mixes with air from fan 5.

The gas is ignited to form a diffusion flame on the lower face of burner 1. The flue gases pass into heat exchanger 2 thereby heating an external water load system.

The flue gas then passes into secondary heat exchanger 3 where some of its water content is condensed out before the flue gas passes out through exhaust 7. The condensed water runs to a reservoir in the base of the recuperator 3 fitted with capillary wicks which pass out into the cold air inlet stream from the fan 5. In this way the condensed water from the flue gas is used to increase the water content of the inlet air to the burner 1.

The inlet air subsequent to its passage through the secondary heat exchanger 3 is raised to a temperature and water vapour content sufficient to cause condensation of water from the flue gases in the finned heat exchanger 2 thereby transferring heat to the external water load system. This feature results in an increase in boiler efficiency.

Table Water Temperature Thermal Input ------- Gas-to-Water (Gross) In Out Efficiency (Btu/hour) ( C) ( C) (%) 15,900 63 75 87.7 25,500 55 75 91.8 31,800 55 80 88.6 37,300 52 82 90.7

Claims

1. A boiler comprising a burner having an air supply and a fuel supply, the burner being downward firing, and a heat exchanger for extracting heat from the flue gases from the burner and transferring it to a fluid, there also being means for condensing water from the flue gases and evaporating water into the inlet air supply to the burner.

2. A boiler according to claim 1 in which the downward firing burner is a diffusion flame burner.

3. A boiler according to claim 2 in which the downward firing burner comprises a plurality of air passageways through a fuel chamber, the fuel chamber having fuel outlets adapted to convey fuel and air passing through the air passageways to a combustion zone adjacent to the lower surface of the burner.

4. A boiler according to claim 3 in which the bore of each combustion air passageway has a cross sectional area of 0.01-1.0 cm2 where the passageway opens into the combustion zone and the bores of the combustion air passageways account for at least 25% of the surface area of the burner adjacent to the combustion zone.

5. A boiler according to claims 3 or 4 in which the combustion air passageways have a square or circular cross section.

6. A boiler according to any one of claims 3 to 5 in which the combustion air passageways are arranged with their axes parallel to one another.

7. A boiler according to any of the preceding claims in which the means for evaporating water into the inlet air supply is a secondary heat exchanger.

8. A boiler according to claim 7 in which the secondary heat exchanger evaporates condensate from the flue gases.

9. A boiler according to claim 7 or 8 in which the secondary heat exchanger comprises a capillary wicking arrangement and a water container in the inlet air flow.

10. A boiler according to claim 9 in which the water container is replenished by condensation of water vapour from the flue gases.

11. A boiler according to claim 9 or claim 10 in which the capillary wicking arrangement takes the form of a wick which allows transfer of water through the wick from the water container to the inlet air side of the secondary heat exchanger.

1 2. A boiler according to claim 9 or claim 10 in which the capillary wicking arrangement comprises walls of the secondary heat exchanger fabricated from porous material which allows controlled passage of water from the flue gas to the inlet air sides of the secondary heat exchanger.

1 3. A boiler according to any of claims 9 to 1 2 in which the wick material is woven glass, metal fibre, sintered metal or a porous coating formed. of ceramic or metal by spraying or electroplating.

14. A boiler as hereinbefore described and with reference to the accompanying drawings.

1 5. A method of heating a fluid of a heating system comprising the steps of (a) supplying fuel and oxygen or air to a burner and igniting the resultant mixture (b) passing the flue gases into a primary heat exchanger adapted to allow transfer of heat from the flue gas to the fluid (c) transferring heat from the flue gas to the inlet air supply and condensing the water from the flue gases and evaporating water into the inlet air supply to the burner.