Background to the invention A condensing boiler system will cause the steam of combustion of a fuel to condense to liquid water and will also collect the latent heat of vaporisation of the steam and recycle this heat into the boiler system and thus increase the thermal efficiency of the boiler system. See for example UK Patent Application 0107963. 1.

Such a condensing boiler and a heating system using such a boiler is described in UK Patent Application 0223369.0 filed 9 October 2002.

In order to recover the heat from the water vapour content of the exhaust gases, it is necessary to cool these gases to below the dew point of the water vapour component therein. In a typical domestic boiler this dew point is about 50°-55°C. It is quite impossible to fully condense this water and to capture the energy of condensation unless the medium used to cool the gases is below, and ideally considerably below, the dew point. The temperature of the return water from the radiators previously proposed to cool the flue gases is at about 60°C, and therefore is unavailable to cool the flue gases below the dew point.

As stated in the earlier Application it is not considered sensible to reduce the water flow through the radiators so that the return temperature is sufficiently low (e. g. 25°-30°C) as to allow this water to be employed in the heat exchanger to effect full condensation of the water vapour in the hot flue gases.

Instead other techniques have been proposed in that earlier Application to obtain water in the range 25°-30°C, to achieve the required cooling of the flue gases.

Object of the invention It is an object of the present invention to provide further improvements to the condensing boiler and associated heating system described in UK Patent Application 0223369.0 so as to provide a more efficient method and apparatus for recovering the latent heat in the water vapour present in the flue gases,-and an improved condensing heat exchanger for such a boiler.

Summary of the invention According to one aspect of the present invention in a method of recovering latent heat from water vapour in the flue gases from a hydrocarbon fuel-burning boiler adapted to heat water to be circulated through radiators to heat a building, in which hot flue gases from the boiler are cooled first by a heat exchanger through which water returning from the radiators is caused to flow and then by a second heat exchanger through which water at a temperature typically in the range 20° to 40°C so as to be below the dew point of the water vapour in the flue gases is caused to flow in direct contact with the flue gases, so that not only is heat extracted from the latter but soluble products of combustion (such as S02) are dissolved in the water and are thereby separated from the flue gases before they exit to atmosphere, and wherein before the water returning from the radiators is supplied to the first heat exchanger a small proportion of the returning water is diverted through an additional heat exchange device located in a relatively cool region in the building being heated, and thereafter the diverted water is further cooled as it passes through an air to water heat exchanger through which air which is to support combustion in the burner of the boiler passes en route to the combustion chamber, such that the exit water temperature therefrom will be in the range 20°-40°C so as to be below the said dew point and the exiting water is supplied to cool water in the second heat exchanger.

The second heat exchanger may include a subsidiary water to water heat exchanger through which the said exiting water flows to cool water which flows around a closed circuit defined by the second heat exchanger, and after passing through the subsidiary heat exchanger the water is returned to the boiler.

Preferably the water leaving the subsidiary heat exchanger is mixed with water returning to the boiler from the first heat exchanger before the latter is returned to the boiler.

The invention also lies in a boiler for supplying hot water to radiators in a building having first and second heat exchange means for cooling the exhaust gases through the first of which water returning from the radiators is caused to flow before returning to the boiler, which includes a water path by which a fraction of the water returning from the radiators to the first heat exchanger is first caused to flow direct to an additional heat exchange device instead of to the first heat exchanger, and after passing therethrough to pass through an air to water heat exchanger in an air inlet to the boiler supplying air to support combustion therein, thereafter to pass through a water to water heat exchanger adapted to cool water circulating in the said second heat exchanger, and lastly to return the water to a water inlet to the boiler, where it is mixed with water passing thereto from the first heat exchanger.

Preferably the said water path includes valve means for adjusting the proportion of the water returning from the radiators which travels therealong and the proportion which proceeds to the said first heat exchanger.

The said additional heat exchange device may be a conventional panel radiator or a fan assisted radiator.

The said first and second heat exchangers may be located in a common housing with the first above the second and spaced vertically therefrom, and the hot flue gases from the boiler flue are directed into an opening at the top of the housing to pass over and around an enclosure forming the first heat exchanger through which water returning from the radiators is caused to pass, thereby to cool the gases, before they proceed to be further cooled by the said second heat exchanger, wherein baffle means is provided below the said opening to direct some of the incoming gases sideways.

It has been found advantageous if the first heat exchanger is comprised of two enclosures arranged side by side with a gap between them and between their outer faces and the internal walls of the housing, to allow for the passage of hot gases therearound as they pass in a downward direction through the housing.

Preferably the interior of the or each enclosure is divided into at least two regions by means of at least one baffle plate.

Conveniently the or each baffle plate extends partly across the interior of the or each enclosure so as to leave a gap at one end between it at the internal surface of a wall of the enclosure, but sealingly engages with the internal surface of each of the other three walls of the enclosure, the gap allowing water to pass from one region to the other.

Alternatively the or each baffle plate extends completely across the interior of the or each enclosure and an opening is provided at one end in the baffle plate to allow water to pass from one region to the next. Typically at least two baffle plates are provided, spaced apart within the or each enclosure, and the gaps or openings are located alternately at different sides of the enclosure, so as to create a tortuous path for water therethrough.

In a preferred arrangement there are four baffle plates defining five regions through which the water flows.

Typically the water flows in parallel through the two enclosures.

The second heat exchanger typically comprises an enclosure through which cool water flows after leaving the air to water heat exchanger in the burner air inlet, which is spaced from the walls of the housing near the base thereof, the latter forming a reservoir for water which has cascaded down through a space between the first and second heat exchangers in the housing to mix with and cool the flue gases as they descend through the housing.

The enclosure of the second heat exchanger may include at least one baffle plate to divide the interior into two regions, with a gap or opening at or near the end of the or each baffle plate to allow water to flow from one region to the next. Typically at least two such baffle plates are arranged so that the gaps or openings are alternately at different sides of the enclosure, to create a tortuous water path therethrough.

Conveniently the or each baffle plate includes tongues which extend laterally of edges thereof and the enclosure walls include slots therein through which the tongues pass when the plates are fitted therein and in which they are welded or bonded by an adhesive.

Preferably the baffle plates are of thermally conductive material and make good thermally conductive connection to the walls of the enclosure (s).

Advantageously the baffle plates additionally serve to brace the enclosures.

The invention also lies in a heating system comprising a boiler embodying the invention in combination with radiators connected by pipe means to each other and to the boiler for heating a building in which the radiators and the boiler are located.

The invention also lies in a heating system comprising a boiler embodying the invention and radiators connected to each other and to the boiler for heating two or more separate buildings in each of which some of the radiators are located and in one of which the boiler is located, or in which the boiler is located in a separate building not containing radiators.

A heating system operating according to the invention thus allows the transfer of heat energy from the flue gases to the circulating water, to the air supply to the boiler burner, and via the additional heat exchange device to the building by full condensation of the boiler flue gas water content, collecting as it does the heat energy in the gases and the latent energy from the water content of the flue gases, therefore reducing fuel consumption and/or increasing available energy for heating the building.

The invention also lies in a boiler operating as aforesaid and in such a boiler in combination with a pumped radiator system through which hot water from the boiler circulates to heat the building so as to provide a system which not only recovers the latent energy in the water vapour content of the exhaust gases but recycles this recovered energy back to the boiler and to the building thus reducing the boiler fuel consumption.

The additional heat exchange device may for example be located in a stair-well, entrance lobby or utility room in which a high temperature of the order of 68-72°F would not be called for, but instead a temperature of the order of 55-60°F for example.

According to a preferred feature of the invention a secondary heat exchanger and connections to an additional heat exchange device as aforesaid, may be fitted to an existing hydro-carbon fuel burning boiler installation, in particular so as to fit inside a casing for such a boiler.

The invention also lies in a building having a heating system as aforesaid installed therein.

The invention also lies in a local area heating system powered by a condensing boiler as aforesaid in which the radiator system extends to two or more buildings and the additional heat exchange device is located in one of the buildings and the boiler is also located in one of the buildings or in a separate building.

The advantages of the invention may be seen by considering a domestic dwelling in the UK, in winter, when full boiler heat output will be required.

In winter the temperature of the air drawn in to enable combustion to occur in the boiler is typically in the range 0°-5°C. Most domestic boiler flue systems are of the so-called balanced flue type, so that cold air is drawn straight into the boiler burner system. It has been calculated that in the case of an 80, OOOBTHu boiler, the mass of this cold air drawn in per hour is about 34kg. The hourly aspiration of this mass of air at 0°-5°C affects the boiler heat output since it represents a cooling effect in the combustion chamber and it has been calculated that in such a boiler, the heat output is reduced by approximately 0.5- 0. 75%.

Another significant heat loss is directly due to the fact that a typical household consumes about 250 litres daily of water. In winter, this water is supplied to the house at a temperature of about 6°-8°C, and this cold water has to be warmed or even boiled for many household uses. The energy required to do this in winter will therefore be higher than in summer when the cold water (especially if stored in the house before use) can be in the range 15°-20°C. Frequently the cold water is stored in a tank in a loft or attic, and unless carefully insulated such tanks can freeze in winter. The invention envisages a system which can help to avoid this problem by providing a water to water heat exchanger in the cold water storage tank and circulating thereto some of the return flow water from the radiator system within the building either directly from the radiators or after passing through the said additional heat exchange device after passing through the air-cooled heat exchanger in the air inlet to the burner of the boiler, and before the cooled water is supplied to the second heat exchanger so as to further cool water to be used to cool the flue gases while simultaneously warming the water stored in the tank, thereby reducing energy needed to heat that water and also reducing the possibility of the reservoir of water freezing in winter. A thermostatically controlled valve can restrict the return flow if the temperature of the water in the storage reservoir exceeds a preset temperature.

It has been calculated that the overall efficiency of an installation comprising a boiler operating at say 15kw in combination with a closed loop water/hot gas (CLW/HG) system will be enhanced by typically from 9% to 15%. Put another way, the boiler fuel consumption will fall by a similar amount (i. e. typically from 9% to 15%), thus reducing C02 emissions by the same amount.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figs 1 and 2 are end and side elevation diagrammatic views of an improved secondary heat exchanger for use with a water heating boiler, which allows for direct water/flue gas contact; Figs 3 and 4 illustrate alternative arrangements for direct flue gas/water contact.

Fig 5 is a side elevation diagrammatic view of another heat exchanger, Fig 6 is a diagram of a system using a condensing boiler and heat exchange device, Figs 7 and 8 illustrate a modified form of construction for the heat exchanger of Fig 6, and Fig 9 illustrates the form and construction of baffle plate used in Figs 7 and 8.

Calculations of Specific latent heat of steam According to published data, the specific latent heat of steam at 100°C = 2. 26x10 kilojoules per kilogram or, more suitable for our calculations = 2.26 megajoules per kilogram, (Mj/Kg) Now in the case of a boiler consuming fuel at the rate of 1 US gallon/hour, this equates to 3.64 litres of fuel/hour.

The weight of fuel consumed per hour is obtained by multiplying the volume by the density of the fuel. The density of one typical fuel is 0.7945. Therefore the weight of such a fuel burnt (per hour) = 2.892 kilograms per hour.

If the gross fuel calorific value = 46.08 Mj/Kg, then the total calorific value per hour is 2.892 x 46.08 = 133.2 Mj per hour.

If, as is typical, the percentage of hydrogen in the fuel = 13. 75%, then the weight of hydrogen in 2.892 Kg of fuel = 2.892 x 0.1374 = 0.4 Kg, i. e. 0.4 Kg of hydrogen is oxidised each hour.

Hydrogen reacts with 8 times its weight of oxygen to produce water. Thus the weight of water produced per hour is 0.4 x 9 = 3.6 Kg of water per hour, albeit produced in the form of steam.

If the latent heat of steam = 2.26 Mj/Kg, the latent heat of 3.6 Kg of steam is 3.6 x 2.26 = 8.136 Mj. This is the heat per hour contained in the steam in the flue gases of such a boiler burning this"typical"fuel.

If the total energy of the burnt fuel is 133.2 Mj per hour, then the percentage of the latent heat in the steam to the total energy is 8.136/133. 2 = 6% of the total energy from the fuel.

This figure should be seen in relation to the commonly held belief that the latent heat of steam in the boiler flue gases is approximately 3 % of the total calorific value of the fuel burnt.

Also the total weight of steam per hour is 3.6 Kg, which is equivalent approximately to 3.6 litres of water per hour, which by coincidence is similar to the volume of the fossil fuel consumed. This is about 60cc/mimute.

Modifications to the secondary heat exchanger proposed by UK Patent Application No.

0107963. 1 As has been described so far the previously proposed secondary heat exchanger extracts only a small percentage of the latent heat from condensing steam and the purpose of the present invention is to substantially increase the extraction rate of this latent heat.

Reference is hereby made to the drawings and related description in UK Patent Application No. 0107963.1 for details of the design and functionality of the previously proposed secondary heat exchanger.

In the embodiment shown in Figs 1 and 2, direct flue gas cooling is achieved if condensate water is pumped from a reservoir 10 via a pump 12 and pipe 14 to a perforated pipe 16 located above, centred and parallel to the upper face of a core 18, through which water at 60°C or less flows as it returns from a heating system to the main boiler heat exchanger.

The design of core 18 may be such as is described for the secondary heat exchanger in UK 0107963.1. Water from 16 will flow down the external surfaces of the core as a film or sheet of water eventually to fall into and entirely fill by overflowing a shallow tray 20 which may comprise the reservoir 10, but is more preferably the top one of a plurality of similar trays 22,24, 26 each of which overflows to feed the one below and the lowest of which feeds the reservoir 10. The core 18, perforated pipe 16, trays 20 to 26 and reservoir 10 are located within or formed by a housing 28.

In parallel the flue gases from the main boiler are fed into the housing 28 via pipe 80 to mix with the water escaping from 16 and to travel therewith as it passes over the core 18 and via trays 20 to 26 to the reservoir 10.

As the film of water cascades down the outside of the core, the hot gases will be cooled by contact therewith, and will continue to be cooled as they traverse the surface of the water in the trays 20 et seq. In cooling, steam in the flue gases will be cooled below the steam/water transition temperature and in doing so its latent heat will transfer into the water in the trays, and the condensing steam will also be absorbed into the water. Thus simultaneously the water temperature and volume will increase. The water flows into the reservoir 10 from which it will be drawn by the pump 12 and recirculated via the perforated pipe 16 to be cooled as it again makes contact with the surface of the core 18 which is internally maintained at 60°C by the return flow to the main boiler. As the process proceeds, the increase in volume of the circulating condensate can be drawn off by overflow pipe 32, and this excess water could amount to several litres per hour depending on boiler heat output.

The path of the hot gases from inlet 30 to the final flue outlet 31 (see Fig 1) is denoted by arrows such as 33,35.

If significant condensation is to be achieved the temperature of the cascading water needs to be below the dew point of the water vapour in the flue gases. Typically this temperature should be below 40°C, preferably below 30°C.

Second embodiment In the arrangement shown in Figs 3-5 the core is replaced by five water filled trays 34,26, 38,40, 42, each tray being individually cooled by one or more heat exchange tubes 44, 46,48, 50,52, arranged to extend across each of the trays, and typically will be immersed by the water in each tray when the system is operating. This design is more compact than that of Figs 1,2 and has a larger flue gas cooling area contained within any given size of housing 28. Water overflowing from 52 is collected in the well 54 at the bottom of the housing.

Hot gas entering the top of the housing from the pipe 30 encounters the water surface of the first tray 34. This water is cooled as before by the water returning to the main boiler.

As the hot gases pass over and under the trays, the gases are cooled first by contact with the water surface and then with the underside of the tray.

It is possible that insufficient cooling of the gases will occur by tray 34 to condense steam present in the gases into tray 34. In that event a small pump (not shown) can be provided to circulate condensate from the well 54 to tray 34, from which it will overflow and, by cascade, all the trays will be constantly replenished.

The cooling tubes 44 et seq. of Fig 5 may be of circular or square or rectangular cross section. Thus Fig 3 shows a circular section pipe and Fig 4 a rectangular cross section pipe. By careful selection of height and width dimensions of the latter, an optimum pipe surface area can be achieved for any given size of tray, given that the larger the proportion of the tray cross section occupied by a pipe the smaller will be the volume available for water in the tray.' An overflow pipe 32 is shown in Fig 5 showing how if water in the well 54 exceeds the height of the overflow pipe, water will leave 54 and can be recovered for re-use elsewhere or simply drained to waste.

In a complete system, the"excess"water can be simply run to waste, or be stored for use as low temperature hot water in a domestic, office or factory environment, or allowed to cool naturally to external ambient temperature for use as"cold"water, perhaps for irrigation or return to water reserves underground for subsequent recovery and use.

The passage of the hot gases from inlet pipe 30 to the final flue outlet 56 is denoted by a series of arrows such as 58,60.

As with the Figs 1,2 embodiment, the temperature of the cascading water should be below the dew point of the water in the flue gases, and in practice should be below 40°C, and preferably below 30°C to achieve a high level of condensation of the steam content of the flue gases.

Comparison of cooling capabilities The total cooling surface of the original core 18 is typically 3500 square centimetres.

Considering now the alternative arrangement of Fig 5. If there are 5 trays in the Fig 5 embodiment, each 11 cms wide and 52 cms long, then the total surface area of the trays is 2860 sq. cms.

The exhaust gases also traverse the water in the well. If the width of the casing is greater than the width of the trays by 1 cm, the water surface area in the well is 12 x 52 = 624 sq. cms.

Therefore the total gas/water interface is (2860+624) =3484 sq. cms.

The gas/metal interface is made up of the total surface area of the walls of the trays. If each tray is 11 cms wide by 3 cms high, the wall area per tray exposed to the passing exhaust gases is (3+3+11) x 52 = 884 sq. cms.

If there are 5 trays the total metal surface area will be 5 x 884 = 4420 sq. cms.

The total surface available for cooling the gases is given by adding the areas of the gas/water and gas/metal interfaces.

Therefore the total surface area for cooling is 3484+4420 = 7904 sq. cms.

The water in the trays is of course itself cooled by the return flow to the boiler through the pipes, and if a small gap exists below each pipe (such as 44) the total external surface of each pipe will be in contact with water. If the rectangular cross-section pipes have a cross- section of 7 x 2 cms, the surface area of the 5 pipes (each 52 cms long) will be 5 x (7+7+2+2) x 52 = 4680 sq cms.

The area of 7904 sq. cms available in a Fig 5 arrangement compares very favourably with the 3500 sq. cms of exhaust gas cooling surface available in the original core 18 of Figs 1 and 2. Furthermore this larger area can be packed into a height of less than 30 cms, typically 28 cms.

As with the earlier embodiments, the temperature of the water passing through the heat exchange tubes (or pipes) should be below the dew point of the water vapour content of the flue gases, typically below 40°C and preferably below 30°C.

Conventionally hot water leaving the boiler for the radiators is at 72°C and typically the flow rate through the radiators is adjusted so that the return water temperature is of the order of 60°C. It is possible to arrange matters so that this return water is at a lower temperature such as 30°C or lower, so as to achieve useful condensation of the steam/water vapour in the flue gases. This will of course raise the temperature of the water before is passes to the boiler to be reheated, which is to advantage since it is normally considered desirable for the return water not to be too low in temperature.

Comparison with other fuels The foregoing has assumed the fuel to be kerosene or the like. However similar advantages are obtained if the fuel is a gas such as natural gas, Propane or Butane. These gases all contain hydrogen as follows:- Natural gas 23. 18% by weight Propane 17. 98 % by weight Butane 17. 21% by weight.

The recoverable latent heat from these fuels will be directly proportional to the percentage of hydrogen present in the fuels.

Turning now to Fig 6, the system shown will now be described in detail.

1. Commencing with the boiler 100, hot water at about 72°C is pumped around a number of heating devices, such as conventional radiators 96. In the case of an ordinary heating system this hot water is returned directly to the boiler for re-heating and continuous recycling. As indicated above, the usual return temperature of the water is about 60°C. The fall in temperature of 12°C relates to the heat given out by the radiators into the building.

In the arrangement shown in Fig 6 the 60°C return water is directed via pipe 98 towards the upper core 102 of a secondary heat exchanger 104.

2. In accordance with the invention a small proportion of this return water is directed via a pipe 99 to the input of an additional heat exchange device 110. A valve 101 allows the proportion to be controlled. Typically 5% of the flow along 98 will be diverted along 99, leaving 95 % to pass along pipe 103 to the upper core 102.

3. At the same time flue gases from the boiler, typically at a temperature in the range 150°-250°C, are led into the top of the secondary heat exchanger via duct 106 and are free to flow downward, over and around the external surface of the upper core 102. As this takes place the flue gases give up energy to the core 102. The gas temperature falls to a temperature in the range 75°-95°C, and the water flowing through core 102 rises in temperature, typically by a few degrees centigrade. This water, now typically at a temperature of 62°C instead of 60°C, is returned to the main boiler 100 via pipe 108. It should be noted that this apparently small temperature rise should be seen in relation to the 12°C temperature drop across the radiators mentioned above. This would give a probable boiler efficiency of about 92% if no other steps were taken to improve efficiency. In general there will be no condensation of water vapour in the flue gases at temperatures of 75°C or higher, since dew point is most probably around 60°C.

4. The low flow of water along pipe 99 at approximately 60°C will in a domestic installation be typically about 1/2 litre per minute. The additional exchange device may be a household radiator 110. To achieve maximum overall efficiency this radiator must be contained inside the building which is heated by the other radiators 96 for reasons which will be explained later. The radiator 110 may be an ordinary panel radiator with or without fins, and may be fan assisted, but should be of such a size and be positioned in the building so that water flowing therethrough at the rate governed by valve 101 (typically 1/2 litre per minute), and at a temperature of 60°C on entry, will drop to a much lower temperature before it leaves the radiator. Typically an exit temperature of 35°-40°C can be achieved in a domestic dwelling but an even lower temperature would be better still.

On leaving the radiator 110 the water, still flowing at about l/2 litre per minute and at a temperature of say 35°C, is directed via pipe 112 to a further heat exchanger 114 (typically a so-called honeycomb air to water heat exchanger) located in the air intake to the burner of the boiler 100. The heat exchanger 114 is therefore subjected to an inflow of air, the temperature of which in the winter will typically lie in the range 0°-10°C, typically 5°C, and on very cold days may even be lower than 0°C.

The aim of heat exchanger 114 is to cool the water leaving the radiator 110 to as low a temperature as possible, while still keeping this water part of the constant volume of water contained within the closed system made up of the boiler 100 and the radiators 96.

This cool water from 114 is now directed into the lower heat exchanger core 116 via pipe 118.

The lower part of 104 comprises a reservoir 120 containing water 122 which surrounds and just covers the central core 116. The level is governed by a weir 124. Alternatively or in addition a syphon may be employed to maintain the level.

The water 122 is cooled by the water in 116, and in winter it is possible to lower it to a temperature which will be below the dew point of water vapour contained in the flue gases entering 104 via 106.

Water 122 from around 116 is drawn out by a pump 124 and delivered to a manifold 126 containing jet orifices providing a cascade of water for mixing with descending flue gases.

A tortuous path may be presented to the water and gases by a plurality of horizontally spaced apart baffles 128.

After passing down and around core 102, the flue gases will now be at a temperature in the range 75°C-95°C and these gases are mixed with the water cascading from the manifold 126 and if provided around the baffles in 128. The gases are thereby cooled before exiting to the flue 130 and the area and number of the baffles 128 and volume of water flow from the manifold 126 are selected so as to achieve the desired temperature drop in the gases, prior to exit, so that the exit temperature is typically in the range 40°C.

In the case of space heating systems, boilers such as 100 will normally only be operated in winter, and in the UK the external winter average ambient temperature is 5°C (about 41°F). It is at such times that the system should operate at the highest overall efficiency in terms of fuel/energy conversion and with the least C02/S02 loss to atmosphere. Of course, if the ambient temperature is much lower than 5°C, the probability is that total efficiency of the system will be even higher. If the ambient temperature is higher, the amount of energy needed to heat the building will be less and therefore the quantity of fuel burnt will be less and the quantities of Co2 and So2 will be proportionately less.

The need to obtain cold water is to enable water vapour in the flue gases to be condensed so as to recover the latent heat energy in that"low temperature steam".

It can be shown that, in the case of a boiler operating at a rated output of 80,000 BTUs per hour, the mass of air demanded by the burner is about 34 kilograms per hour. When this mass of air per hour is passed through the heat exchanger 114 at 5°C, water at 35°C is found to be cooled typically to 15°-20°C, and the air entering the burner will itself be heated to a temperature typically in the range 20°-30°C.

In a domestic situation, the total area of the baffles is typically in the range of 1-2 square metres, and since the water will cool both faces of the baffles the total cooling area will be twice that, i. e. typically 2-4 square metres. Moreover, since the cooling of the gases is by direct contact with the cooled water, it is not difficult to ensure that the gas temperature falls to below the dew point of water vapour in the gas stream, so causing condensation and loss of heat from the vapour and simultaneous cooling of the hot gases.

In general the baffles 128 should be packed as tightly as possible in the space available so as to present a tortuous path for the exhaust gases and the water. In general the more baffles the better.

In the case of an 80,000 BTHu boiler, the expectation is that about 2 litres per hour of water will condense out from the flue gases, and a flue gas temperature of 30°C-40°C should be achieved.

Calculations show that the energy recovered from the condensation of water vapour to produce 2 litres of water per hour, is 4.5 mega joules which is equivalent to 1.3 kilowatts/hour. This energy will raise the temperature of the water 122 circulating around core 116 unless it is cooled, and it is for this reason that it is necessary to remove each hour slightly more than 1.3 kilowatts/hour from the 1/2 litre/minute water flow. This energy of approximately 1.3 kilowatts/hour is equivalent to about 4% of the energy provided by the radiators 96, where the water flow, and the flow and return temperatures are as specified above. Therefore if radiator 110 (which dissipates this heat in order to cool the return water to 35-40°C) is located within the building which also contains the radiators, its heat output can be added to that from the radiators 96, so as to produce a theoretical efficiency of the order of 96% for the system taken as a whole.

In addition however the transfer of heat to the incoming air to the boiler can increase the energy available from the boiler. For an 80,000 BTHu boiler, this increase can be just over 1/2 kilowatt per hour. This means that less fuel is needed to generate the rated heat output from the boiler, and a slightly increased overall efficiency obtained. Typically the further increase in efficiency can be of the order of li %.

Total system energy efficiency can therefore be of the order of 96. 5%.

The only loss is the overflow of water from 124.

Sulphur dioxide can be removed as follows: From published information, sulphur dioxide is very soluble in water. At NTP, a given volume of water will absorb 39 times that volume of sulphur dioxide. Thus if 30 litres of water per minute is cascading down the baffle plates at 128 this volume of water will readily absorb far more sulphur dioxide than could possibly be present in the volume of exhaust gases passing through and present in the baffle containing region of the heat exchanger housing 104 in the same period of time.

As the water vapour condenses it will increase the volume of the water 122 in the reservoir 120. This rising volume is controlled by weir 124. Water draining over the weir carries with it absorbed sulphur dioxide which can be neutralised by using the alkaline materials such as are used in domestic laundry facilities. Since the circulating water 122 will also become acidic, it may be necessary to remove this acidity by a neutralising cartridge 132 which may be in the form of a replaceable or rechargeable unit. The pipework 134, pump 124, manifold 126, baffles 128, core 116 and interior of the housing of the heat exchanger 104 should if possible be formed from or coated with a material not significantly chemically affected by sulphurous acid, or where appropriate be constructed in such a way as to be readily replaceable such as at annual servicing intervals.

Whether spaced apart baffles 128 are employed, or the exchanger relies on mixing the gases with fine water sprays, one thing is common to all variations, namely that the hot exhaust gases are cooled by direct contact with cold water, and the recovered sensible and latent heat of condensation thereby recovered from the flue gases, is incorporated into the heat supplied to the building and/or to the boiler heating radiators within the building so as to increase the overall efficiency of the installation.

The closed loop system described herein is aimed principally at the heating of one building such as a domestic dwelling or single office building. Nevertheless any circulating hot water heating system can be improved by the invention, which could be applied to a town or city district heating system, or to a multiple occupation high rise building or office block, provided in each case the system includes a boiler house from which hot water is circulated for heating and to which cooler water is returned. The CLW/HG procedure will function equally well, whether the heat output is 30KW or 300, OOOKW, the only difference being that of scale.

Thus with reference to Fig 6 (i), item 96 can be a set of radiators in one house, or all the radiators in all the houses in a district heating system.

In Fig 6 (ii) the heat exchanger is shown as having an upper heat exchange core 102 (which will be referred to as A) and a lower heat exchanger core 116 (which will be referred to as C) inside the one casing. The efficiency of these exchangers can be improved. Thus in the example shown there is one of A and one of C. In practice and for the sake of saving space, multiples of A or C are possible.

A preferred arrangement is shown in Figs 7 and 8 in which there are two A-units Al, A2, arranged in parallel and a single C-unit below. Additionally a problem has been noted with the layout as shown in Fig 6 (ii).

Fig 7 shows the nature of the problem. Note first the hot exhaust gas inflow M. This hot gas does not uniformly spread over and around the enclosure forming core A, and will probably never reach corners x and y of the enclosure.

Secondly the water flow in and out of core A will tend to take the shorter path as shown by P. It is therefore probable that there will be little if any water flow to region Z of the enclosure forming core A. Even if water were introduced and removed at opposite ends of the enclosure, the result would be similar, with some regions not seeing any water flow at all.

The core C in the lower heat exchanger, which is surrounded by water, suffers a similar problem, in that internal flow of water will tend not to reach region ZZ of the enclosure forming core C. The net result is a marked reduction of effective heat exchange area and therefore of total efficiency.

The solution is shown in Fig 8.

Firstly curved baffle plates E, F deflect the incoming gases causing them to change direction left and right. This causes exhaust gas to flow more evenly over and around the two enclosures of core A, thus improving its contact with regions x and y of each enclosure.

Secondly, internal baffles J and K are provided in enclosures A and C, five in A and three in C.

The baffles create a tortuous zig-zag path for water through the enclosures, and by connecting the baffles to the walls of the enclosures water circulating through A and C is forced into all the corners, thus reducing or eliminating the low efficiency areas at Z and ZZ identified in Fig 7.

By making them of thermally conductive material the effective surface area of material in contact with the water is increased if the baffles also make good thermally transmittive contact with the walls of the enclosures forming the cores of the heat exchangers.

By making the baffles of relatively rigid material such as a thermally conductive metal, they will not only create the tortuous path and increase the effective surface area, but will also internally brace the enclosures of cores A and C.