GB2373841A

GB2373841A - Secondary heat exchange unit

Info

Publication number: GB2373841A
Application number: GB0207077A
Authority: GB
Inventors: Andrew John Robertson
Original assignee: AQUAFLAME Ltd
Current assignee: AQUAFLAME Ltd
Priority date: 2001-03-30
Filing date: 2002-03-26
Publication date: 2002-10-02
Anticipated expiration: 2022-03-26
Also published as: GB2373841B; GB0207077D0

Abstract

The heat exchange unit may be connected to a fossil-fuel fired boiler 10 that generates exhaust combustion gases and supplies them via a connector 18. The unit contains a water-filled hollow core 11 that receives water from a pipe 12. The combustion gasses from the boiler enter casing 16 and circulate around the core before leaving via a flue gas exit 20. The water is heated by the gases and returned to the boiler by an outlet pipe 14. A triangular end face 19 and plate (19a, figure 1A) on the core prevent direct communication between the incoming combustion gases and the flue exit. The temperature of the gases as they enter the unit from the boiler may be in the range of 75{ - 500{C and as they exit to the flue may be between 50{ and 110{C. The casing also has a drain 22 for any condensation that is produced. A removable panel 24 is provided for cleaning and maintenance of the unit.

Description

Title: Heat Exchanger for Fossil Fuel Fired Boiler Field of the Invention This invention relates to a boiler, especially but not exclusively a domestic boiler for providing hot water for central heating systems.

Background to the invention Over many years, domestic boiler design, whether gas or oil or solid fuel, has been directed to the manufacture of boilers. The design changes have to a great extent involved the variety and types of baffle plates, with the aim of causing the hot exhaust gases of combustion to move through constricted passages in a generally outward and upward direction in contact with the inner surfaces of the walls of a hollow water containing jacket. The jacket constitutes a heat exchanger to provide a source of hot water. The baffle plates are usually solid steel or ceramic in construction and offer no cooling facility to the hot exhaust gases, they merely direct the hot exhaust gases towards the outer water-cooled jacket. This has resulted in the efficiency of boilers increasing over time such that by 2000 many designs of boilers regularly achieve a net efficiency in the range of 87 to 92%.

Further attempts to increase efficiency have met increasing difficulty.

Some of the difficulties are due to tightening of laws aimed at reducing atmospheric pollution by imposing strict limits on the emission of soot particles and noxious gases such as carbon monoxide and nitrous oxide.

Also, as the number and size of baffles increases, so also does the internal back pressure in the combustion chamber and there is some evidence to suggest that this back pressure slightly reduces combustion efficiency. It is well known in the industry that the principal gas of combustion, namely carbon

dioxide, if increased by reducing the airflow rate to the burner, increases boiler efficiency iency which appears to be at a maximum when the C02 is fixed at around 12. 5 %. If the C02 is permitted to rise much beyond 12. 5% formation of undesirable soot particles occurs and

the production of the dangerous gas carbon monoxide rises very sharply.

ZD Ideally, if the percentage C02 can be increased to say 14-15% with very little carbon monoxide, then such a boiler will be more efficient, probably by several full percentage points. Moreover, the effluent gas temperature can be reduced to about 60oC, then

additional energy will be captured by not only extracting energy from the cooled gas, but in addition from the latent heat of water vaporisation.

It h believed that the latent heat described amounts to about 3 % of the total combustion L IUU L i I L, L energy when burning oil, and as much as 5% when burning gas.

As a general rule, the latent heat of water vaporisation is totally lost up the flue.

It has for long been known that as hot combustion gases are cooled, the volume of the gas is considerably reduced by as much as 50%-60% depending on the original combustion temperature, and the present invention makes use of this fact. Previous boiler designs make use of a variety of types of baffles which by"squeezing"the hot gas through many small spaces, causes the gas to transit the boiler very rapidly and with an appreciable pressure, such pressure preventing to a large extent the falling temperature of gas. The result of this is a high effluent gas temperature to the Hue. The higher the flue gas temperature, the less efficient is the boiler.

Present practice of setting up a central heating system is to so arrange matters that the hot

water outlet from the boiler is about 72-7SOC and the return cooler water enters the boiler at about 10-12OC lower, i. e. about 62Oc.

It is not possible any longer to achieve large efficiency gains. There is an upper limit to the efficiency of any heat device. However, the present invention aims to achieve an improvement in efficiency which is significant and is not costly to put into practice.

Summary of the invention According to one aspect of the invention, there is provided a secondary heat exchanger for fitting to a fossil-fuel fired boiler in which water to be heated is passed through a water jacket in heat exchange relationship with combustion gases generated by a burner unit, wherein the secondary heat exchanger comprises an inner core adapted to receive return water from a hot water system through which the said return water will flow and further adapted to transfer the return water after passing therethrough to a return water inlet of the main boiler, and wherein the secondary heat exchanger is adapted to receive and coerce exhaust combustion gases from the main boiler to flow around the inner core to a flue gas exit.

Preferably the secondary heat exchanger comprises an inner water cooled core which is surrounded by an outer skin.

Exhaust gases enter the secondary heat exchanger and travel down, underneath and up the other side of the core, passing between the outer skin and over the entire water cooled surface of the inner core.

The temperature of the exhaust gases as they enter the secondary heat exchanger from the main boiler is in the range 115 -130 C and the temperature of the exhaust gases when they exit to the flue is around 50 -65 C.

During the cooling process of the exhaust gases in the secondary heat exchanger, condensation takes place and the condensate leaves via a drain. The secondary heat exchanger is preferably contained in a housing and for reasons of maintenance a service door is provided in the housing allowing the device to be cleaned out if required.

According to another aspect of the invention, there is provided a fossil-fuel fired boiler comprising a main boiler wherein water to be heated is passed through a water jacket in

heat exchange relationship with combustion gases generated by a burner unit, the exhaust z) b combustion gases from the main boiler being fed to a second heat exchanger comprising an L, L inner core through which return water from a hot water system flows and then exits to a return water inlet of me main boiler, the exhaust combustion gases from the main boiler being coerced to flow around the inner core to a flue gas exit.

In such a combination the secondary hear exchanger preferably comprises an inner water cooled core which is surrounded by an outer skin.

Preferably exhaust gases from the main boiler enter the secondary heat exchanger and travel down, underneath and up the other side of the core, passing between the outer skin and over the entire water cooled surface of the inner core.

In such a combination the temperature of the exhaust gases as they enter the secondary heat exchanger from the main boiler is in the range 115 -130"C and the temperature of the exhaust gases when they exit to the flue is around 50"-65"C.

Where condensation takes place during the cooling process of the exhaust gases, the l condensate can leave via a drain in the secondary heat exchanger.

For reasons of maintenance a service door may be provided to allow the main boiler and/or the secondary heat exchanger to be cleaned out as required, The addition of this device to a primary boiler such as described in my copending UK Patent Application 0107960.7 will result in an increase in overall net efficiency to at least 98% and in some circumstances to more than 99%.

The present invention is preferably employed in combination with a boiler as described and claimed in our corresponding patent application claiming priority from UK Patent Application No. 0107960.7, according to which such a boiler comprises a water filled jacket containing a plurality of water cooled tubes in one region thereof, at least one of which communicates at one end with a combustion chamber in another region of the jacket wherein gaseous products of combustion (exhaust gases) are generated by a burner unit in the combustion chamber, and at least one other of which communicates at an opposite end with an exit to a flue for conveying exhaust gases to atmosphere, and gas deflecting means at opposite ends of the plurality of tubes whereby the exhaust gases are forced to follow a labyrinth path from one end to the other of the plurality of water cooled tubes in passage from the combustion chamber to the exhaust exit, the exhaust gases giving up heat in so doing to the water surrounding the tubes as well as to the water in the jacket.

Description of embodiment The present invention will be further described with reference to the accompanying drawings, in which :- Fig 1 shows the second heat exchanger in diagrammatic perspective view from the front and one end, Fig la shows the internal water filled core, also in perspective, but from the other end, Fig 2 is an end elevation of the casing with the near end panel (service door) of the casing removed, Fig 3 is a side elevation of the casing, from the front, Fig 4 is a perspective view of a primary boiler having the secondary heat exchanger of Fig 1 attached thereto, partly cut away and showing the gaseous passage through the two units, and Fig 5 is a similar view, this time showing the water path through the two units.

Referring to Fig 1, the illustrated second heat exchanger is relatively thin and is mounted to the side (or to the rear, front or even above or below if desired) of the water jacket 10 of a main boiler. The secondary heat exchanger comprises a water-filled hollow core I I receiving water at 12 returning from a hot water system such as the return from a hot water central heating system, and exiting water at 14 to the water inlet of the main boiler.

Exhaust combustion gases from the main boiler enter the casing 16 via a connecting duct 18 and leave via a flue gas exit connection 20. A triangular end face 19 and plate 19A (see

Fig lA) prevent direct communication between the incoming combustion gases and the Hue c, as exit, as wi t, gas exit, as will also be seen from Fig 2, so that the gases can only flow around the core 11 between it and the interior of the casing 16. In addition to the water outlet 14, the casing of the second heat exchanger has a condensation drain 22. It also has a removable 0 service door or panel 24 enabling cleaning and maintenance (see Fig 1).

Thus, the present invention provides an additional separate heat exchanger conveniently attached to a face of the main boiler housing, which latter of course includes the primary heat exchanger. The additional (or second) heat exchanger comprises an inner core into which the cooler return water flows from the heating system, and an outer case which allows the flue gases to pass completely around the water filled inner core 11 that the exhaust gases will pass over the whole surface of the inner core. The water, which is pumped from the main boiler to the radiators forming a central heating system, is cooled as it passes through radiators and it is usually in the range 40 C to 65 C by the time it reenters the primary heat exchanger in the main boiler, i. e. up to lye cooler than the water leaving the main boiler water outlet. By passing the cool return water through the inner core of the second heat exchanger, it will gain heat from the exhaust gases. As a result the latter are cooled, and the water is warmed before it enters the primary heat exchanger of the main boiler.

In operation the outer casing and core are arranged so that the flue gas enters the gap between the inner core and the surrounding case at the top of the heat exchanger (by means of connection 18). The flue gas moves directly down one side of the core to the bottom and then returns up the other side of the core, finally to exit to a flue which may be a conventional flue or a balanced flue. As the flue gases proceed to pass over the core which is filled with the cooler return water, the flue gas is considerably cooled, typically to a temperature in the range 50-70 C. The cool water in the core is heated somewhat by the hot exhaust gases and this water temperature is increased by a few'C before returning to the main boiler.

The additional (or second) heat exchanger is possible of modification in several ways. For example, the outer casing, being plain as described, can itself be water-cooled or comprise a water jacket. Also the outer casing can be widened to accommodate more than one core, which is almost certainly necessary in the case of boilers having an output greater than 100,000 BThU.

In the second heat exchanger water vapour contained within the flue gases from the boiler may well condense and if so, gives up this further energy to the second heat exchanger. It is also necessary to provide a drain 22 at the bottom of the casing 16 to permit condensed water to escape.

It is also to be noted that the second heat exchanger may be subjected to a condition of severe corrosion because oil fuels contain a small percentage of sulphur, perhaps around 0.3%. During combustion, the sulphur is oxidised to sulphur dioxide which reacts with some of the water in the combusted gas to create sulphurous acid which will attack many metals. Therefore the second heat exchanger must be able to resist corrosion. The materials of construction proposed for the second heat exchanger are therefore stainless steel (all grades), PTFE, plastics, nylon, ceramic or as an alternative mild steel, copper or other cheaper materials, which are coated with a protective layer of plastics or resin, or electroplated with lead or nickel.

In a further variation of the second heat exchanger which can result in a flue gas temperature as low as 40"C, a fan/convector heat exchanger is incorporated to further cool the returning water.

Thus in a domestic heating system for heating a home, a fan/convector driven heat exchanger, similar in operation to a motor car cooling system may be included, possibly attached to the main boiler casing. As the return water flows to the main boiler, some or all of the water is fed first through the fan convector heat exchanger which further reduces the return water temperature. The exhaust air from the fan/convector, will normally be

heated to around 20-25OC, and this can be directed into the house, via suitable ducting thereby to heat part of the house, possibly the kitchen, if heat is required in that room.

The inlet air to the fan is preferably outside air, which assuming only that the time of year is winter, is likely to be at a temperature in the range 0 to IO'C. Typically the cooled water from the fan/convector exchanger is led to the water inlet of the second exchanger above described. As the gases from the main boiler are led into the second heat exchanger, the cooled water (from le fan/convector exchanger) at about 35-40"C cools the gases to a temperature typically in the range 35-45"C. Because the flue gas temperature is further reduced the overall efficiency of the main boiler is thereby increased, and an additional amount of heat energy is available for heating the house via the fan convector exhaust, if desired.

This last described concept utilises the principle of conservation of energy and takes advantage of the often significant temperature differential between a house interior and outdoors. Finally it is important that each and every part of the boiler and accessories, such as the additional heat exchanger and where provided the fan/convector exchanger, is thermally insulated as far as is practicable.

The invention, especially when used in combination with the invention of the corresponding application above mentioned, makes it possible to achieve an overall boiler net efficiency of at least 98% and in some circumstances greater than 99%.

Figs 4 and 5 show the combination of the primary and secondary heat exchangers, the construction of the former being as described in my copending Application, and more particularly as follows.

Referring to Figs 4 and 5, the main (primary heat exchanger) comprises a water filled jacket 10 having a combustion chamber 112 at the bottom wherein combustion gases are generated by a burner unit 114, typically a gas or oil fired burner unit.

The jacket 10, with water inlet IIOA and water exit 110B, contains a plurality of water cooled square cross-section tubes generally designated 116. In this example twenty tubes arranged in five rows of four tubes in each row are fixed in position above the combustion chamber 112 and below the exhaust gas manifold 118 at the top, the latter communicating with the secondary heat exchanger casing 16 via connecting duct 18.

Two plates, 122 and 124 respectively, are fixed in position above and below the tubes. The effect of these plates is to deflect the combustion gases so that they are forced to enter only the right hand row of tubes 125 etc. , and then to travel down the next row of tubes 127, then up the next row of tubes 129, down the next row 131 and finally up the last row of four tubes 133 into the exhaust gas manifold 118.

Deflector plate 122 includes an end wall 126 engaging the division between rows of tubes 131 and 133 and an intermediate wall 128 engaging the division between rows of tubes 127 and 129.

In a similar way plate 124 includes an end wall 130 and an intermediate wall 132.

It is to be understood that the number of water-cooled tubes is by way of example only. A smaller boiler may have a lesser number of tubes and a larger boiler a greater number. The bottom limit is a single tube for upward flow, followed by a single tube for downward flow and a final single tube for upward flow. The only upper limit is practicality.

To assist in seeing the interior of the main boiler, the near side wall of the water jacket 10 ZD i is cut away at A to reveal the interior.

In addition the near side end wall of upper flue gas chamber 118 has been cut away to I reveal the plate 122 and the upper ends of the tubes, and the front wall of chamber 112 has been cut away to reveal plate 124 and the interior of Hue gas combustion chamber 112.

Water returning from the radiators of a central heating system Hows into the core 11 of the secondary exchanger via inlet pipe 12 and leaves after gaining heat and rising in temperature, via pipe 14 to enter the water jacket 10 of the main boiler via inlet 110A.

The water flow from 110A spreads out in the gap between the wall of water jacket 10 and the opposite wall of chamber 112 to flow upwardly and laterally around chamber 112 and in a generally upward circulating movement around the rows of tubes 116, and then around chamber 118 to leave via HOB.

Net boiler efficiency is typically expressed as the ratio between the heat output transmitted to the boiler water and the product of the net calorific value at constant fuel pressure and the consumption expressed as a quantity of fuel per unit time, expressed as a percentage.

The secondary heat exchanger could be retro-fitted to a main fossil-fuel boiler by connecting the flue inlet (18) of the secondary heat exchanger to the flue of an existing boiler. The secondary heat exchanger could be either attached to the old boiler or the wall of a property or dwelling. The flue of the secondary heat exchanger (20) would then exit outside the property thus reducing the older boiler emissions and flue gas temperature. The flow and return water pipes to the old boiler would be redirected via the secondary heat exchanger to ensure the correct operation of the unit.

Claims

C1076/A Claims 1. A secondary heat exchanger for fitting to a fossil-fuel fired boiler in which water to be heated is passed through a water jacket in heat exchange relationship with combustion gases generated by a burner unit, wherein the secondary heat exchanger comprises an inner core adapted to receive return water from a hot water system through which the said return water will flow and further adapted to transfer the return water after passing therethrough

to a return water inlet of the main boiler, and wherein the secondary heat exchanger is adapted to receive and coerce the exhaust gases from the main boiler to flow around the inner core to a Hue gas exit.
2. A secondary heat exchanger as claimed in claim 1 in which the core is water cooled and is surrounded by an outer skin or a water jacket in heat exchange relationship with exhaust gases.
3. A secondary heat exchanger as claimed in claim 2 wherein the exhaust gases enter the secondary heat exchanger and travel down, underneath and up the other side of the core, passing between the outer skin and over the entire water cooled surface of the inner core.
4. A secondary heat exchanger as claimed in any of claims 1 to 3 wherein during the cooling process of the exhaust gases, condensation takes place and the heat exchanger includes a drain to allow condensate to pass therefrom.
5. A secondary heat exchanger as claimed in any of claims 1 to 4 when enclosed within a housing which for reasons of maintenance includes a service door.
6. A fossil-fuel fired boiler comprising a main boiler wherein water to be heated is passed through a water jacket in heat exchange relationship with combustion gases generated by a burner unit, the exhaust combustion gases from the main boiler being fed to a second heat exchanger comprising an inner core through which return water from a hot water system flows and then exits to a return water inlet of the main boiler, the exhaust gases from the main boiler being coerced to flow around the inner core to a flue gas exit.
7. A fossil-fuel fired boiler as claimed in claim 6 wherein the secondary heat exchanger comprises an inner water cooled core which is surrounded by an outer skin.
8. A fossil-fuel fired boiler as claimed in claim 7 wherein the exhaust gases enter the secondary heat exchanger and travel down, underneath and up the other side of the core, passing between the outer skin and over the entire water cooled surface of the inner core.
9. A fossil-fuel fired boiler as claimed in any of claims 6 to 8 wherein the temperature of the exhaust gases as they enter the secondary heat exchanger from the main boiler is in the range 75 -500 C and the temperature of the exhaust gases when they exit to the flue is in the range 30 -110 C.
10. A fossil-fuel fired boiler as claimed in any of claims 6 to 9 wherein during the cooling process of the exhaust gases, condensation takes place and the condensate leaves via a drain.
11. A fossil-fuel fired boiler as claimed in any of claims 6 to 10 wherein for reasons of maintenance a service door is provided in the secondary heat exchanger allowing the device to be cleaned out if required.
12. A fossil-fuel fired boiler as claimed in any of claimed 6 to 11 wherein the main boiler comprises a water filled jacket containing a plurality of water cooled tubes in one region thereof, at least one of which communicates at one end with a combustion chamber in another region of the jacket wherein exhaust combustion gases are generated by a burner unit in the combustion chamber, and at least one other of which communicates at an opposite end with an exit to the secondary heat exchanger, and gas deflecting means at opposite ends of the plurality of tubes whereby the exhaust gases are forced to follow a labyrinth path from one end to the other of the plurality of water cooled tubes in passage from the combustion chamber to an exit leading to the secondary heat exchanger, the exhaust gases giving up heat in so doing to the water surrounding the tubes as well as to the water in the jacket.
13. A secondary heat exchanger for use with a main boiler, wherein the secondary heat exchanger is constructed arranged and adapted to operate substantially as herein described or with reference to the accompanying drawings.
14. The combination of a main boiler and secondary heat exchanger constructed arranged and adapted to operate substantially as herein described or with reference to the accompanying drawings.
15. A water heating system including a fossil-fuel fired boiler as claimed in any of claims 1 Z-1 I : > to 9, which includes a flow path to the system from the hot water outlet of the main boiler and a return path from the system for returning cooled water from the system to the inlet to the secondary heat exchanger for pre-heating the return water before it is supplied to the inlet to the main boiler.