GB2348946A - Boiler with temperature controlled bypass in heat exchanger to prevent condensate formation - Google Patents

Abstract

In a boiler, a temperature controlled bypass 606 allows a portion 701 of the heated combustion products from the burner (not shown) to bypass the heat exchanger 605 and go straight to the flue, therefore reducing the formation of condensate in the flue. The bypass may be controlled indirectly by a temperature sensor (not shown) or directly by a bimetallic strip 607. If the temperature of the combustion products becomes too low, the bypass gap 606 widens to maintain the temperature of the flue products 705 emanating from the underside of the heat exchanger above the dew point. There may be two temperature controlled bypass gaps (513, 514, Fig 5), one at each side of the heat exchanger. The boiler may have a downwardly firing burner (Fig 4) or an upwardly firing burner (Fig 5).

Description

IMPROVEMENTS TO HEAT EXCHANGERS Field of the Invention The present invention relates to the field of heat exchangers and particularly although not exclusively to non-condensing heat exchangers and to boilers comprising a heat exchanger which derives heat from a pre-mix burner.

Background to the Invention Heating appliances for use in heating water and in providing central heating may be utilized to provide the required heating effect in both domestic buildings and commercial buildings. Such heating systems may be applied to a wide variety of premises including houses, factories, offices, schools, hospitals as well as a variety of other structures configured to accommodate people in various situations. The heat generating unit may be fuelled by oil or methane gas (CH4), the unit then being known as an oil burner or a gas boiler respectively. The basic chemical reaction involved is the buming of methane in air in a ratio of approximately 1 to 10 by volume. The products arising from the combustion of methane in air are substantially carbon dioxide gas, water vapour and residual air. Collectively the carbon dioxide gas, water vapor and any residual air and/or un-burnt methane gas are known as combustion products.

The temperature of the combustion products is such that heat may be transferred from the combustionproducts to another medium. Typically in a domestic context a substantial amount of the heat from the combustion products is desired to be transmitted to a circulating water system so as to create hot water which may be used from a tap for example, or in a building heating system comprising a bank of radiators distributed throughout the building.

Existing heating technology traditionally divides boilers into two broad categories as follows. Non-condensing boilers generally comprise a burner unit which is upward firing and these burners are frequently termed conventional atmospheric burners. By upward firing it is meant that the burner is configured within the boiler system such that the flames are directed in a substantially upwards direction. Having burnt the methane in air the combustion products arising from the burner unit are normally directed to a heat exchanger so as to extract the heat from the combustion product and thus heat a body of circulating fluid such as water. Typically the heat exchanger may be made out of cast iron or copper which are considered to be relatively low cost materials in this context. Following passage through the heat exchanger the combustion products are directed into a flue system which is typically configured to direct the combustion products to the outer environment. The conventional atmospheric burner mechanism is technically simple and is a commercially cheap arrangement to install due to the basic nature of the components employed. Furthermore, the methane gas may simply be directed into the burner with oxygen derived from air which is simply drawn in from the surrounding environment. Conventional atmospheric boilers of the type described are also known as standard efficiency boilers. Typically such boilers are limited in efficiency to typically approximately 80% since with higher efficiencies condensation may occur, which has a destructive effect on the components/material used.

The boilers described above may be termed non-condensing boilers in view of the fact that the temperature of the combustion products flowing through the flue system is considerably higher than the dew point. By dew point it is meant the temperature at which combustion product dew may condense from a humid combustion product atmosphere. In other words water vapor is not desired to condense in the boiler or flue system of a non-condensing boiler.

The non-condensing effect provides an advantage in that acidic solutions are not formed and thus the heat exchanger is not thereby damaged.

To improve efficiency of boilers a different kind of boiler system has been developed known as a condensing boiler which is normally characterized by having a substantially larger heat exchanger for the same load than the non condensing type described above. Alternatively, a condensing boiler may comprise a first heat exchanger and a second heat exchanger so as to further extract heat from the combustion products. Condensing boilers are generally fan assisted and may be configured to have a burner which fires downwards so as to prevent interference of the burner flames by condensing solutions dripping onto the flames. Condensing boilers are more expensive than conventional atmospheric boilers due to the provision of a more expensive heat exchanger arrangement. In a condensing boiler (commonly referred to in the art as a high efficiency boiler) the heat exchanger is typically made of cast aluminum or stainless steel and furthermore is usually fan assisted, particularly if the burner is configured to fire downwards. Due to the higher efficiency of heat transfer from the combustion products to the heat exchanger the temperature of the combustion products is substantially lower than in conventional boiler systems.

Thus, in a broad sense there is a problem with the use of premix burners in conjunction with non-condensing boilers in that there is generally a need for higher efficiency wherein long flue systems are provided as an option to the customer. A higher efficiency system having a longer flue system is associated with the problem of the flue system being prone to formation of condensate.

A simple method of dealing with condensate as described above is to use traps in the flue system which are configured to guide the condensate away from the boiler. This mechanism s only suitable for flues which can tolerate formation of condensate and thus is generally not an acceptable solution.

A method of preventing formation of condensate in the flue system is known which comprises use of an insert, (that is a"plug") in the heat exchanger so that on removing the insert a gap is effectively created in the heat exchanger enabling a greater proportion of the combustion products to pass by the heat exchanger substantially without transferring the heat contained therein. This mechanism therefore elevates the temperature of the combustion products in the flue system and thus may substantially reduce the formation of condensate in longer flue systems. This method represents a step change in the effective efficiency of the boiler which therefore suffers from reduced efficiency which may in certain circumstances not fall within legal requirements.

A further method of either removing formation of condensate in the flue system or limiting the amount of condensate formed may comprise a limitation of the degree to which the power/output of the boiler system can be changed.

By change in power/output it is meant the output adjustment according to the load demanded from the system. The above methods of reducing or eliminating the formation of condensate in the flue system each have disadvantages.

In view of the above there is a need for an apparatus and a method for substantially reducing or eliminating the formation of condensate in what are traditionally known as non-condensing boilers. Thus, in effect there is a need for a mechanism to guarantee operation of a non-condensing boiler to operate as a high efficiency non-condensing boiler.

Summary of the Invention One objective of the present invention is to provide a thermodynamic heat exchanger for use with non-condensing low temperature appliances and premixed burners.

A further objective of the present invention is to provide a dynamic heat exchanger which adjusts flue losses to a constant level from full to part load, thus significantly reducing the formation of condensate.

Yet another object of the present invention is to provide a dynamic heat exchanger which adjusts flue losses with a closed control loop. By a closed control loop it is meant a control system configured to continuously adjust to any operating conditions as a direct response to the environment comprising combustion products from a burner arrangement of a boiler unit.

Yet another object of the present invention is to provide a thermodynamic heat exchanger which solves the inherent design problems of conventional heat exchangers which utilise pre-mix burners.

Yet another object of the present invention is to provide a thermodynamic heat exchanger which is designed with an adverse operating bracket which can be used to increase and/or to maintain the efficiency of non-condensing boilers configured for use with conventional atmospheric burners.

According to a first aspect of the present invention, there is provided a heat exchanger apparatus comprising: a member containing a fluid for effecting transfer of heat from a heat conveying medium presented to said member; and a cross-sectional area presented to said heat conveying medium, said area comprising a first portion defined by said member and second portion comprising means for substantially automatically controlling the amount of said medium passing though said a second portion ; wherein said control means varies said amount in response to changes in the temperature of said heat conveying medium.

Suitably the heat conveying medium comprises combustion products derived from a premix burner.

Preferably the second portion comprises a gap for facilitating bypass of a proportion of said heat conveying medium past said first portion.

Preferably the gap is located at at least one side of said first portion.

Preferably the gap is located as an opening between said at least one side of said first portion and a wall of a container housing said apparatus.

Preferably the heat exchanger is configured such that said gap is located within the area defined by said first portion, said gap comprising a hole.

Preferably the heat exchanger is configured to substantially prevent formation of condensate in a flue system coupled to said heat exchanger apparatus.

Preferably the apparatus comprises a non-condensing heat exchanger.

Suitably the control means is located within said second portion.

Suitably the control means comprises means configurable to extend across said second portion.

Suitably the control means comprises a temperature responsive material.

Preferably the control means comprises a bi-metallic strip.

Preferably the bi-metallic strip comprises two metals or alloy based materials having different coefficients of expansion wherein the two materials are bonded together.

Furthermore, preferably the two materials each comprise an alloy having a nickel component.

Preferably the heat exchanger is substantially made of either copper or cast iron.

Suitably the heat exchanger is configurable for use in a heating system boiler of a building.

Preferably the control means is calibrated with respect to temperature.

According to a second aspect of the invention, there is provided, in a heat exchanger apparatus comprising: a member containing a fluid for effecting transfer of heat from a heat conveying medium presented to said member; and a cross sectional area presented to said heat conveying medium, said area comprising a first portion defined by said member and a second portion, a method of substantially automatically controlling the amount of said medium passing through said second portion, said method comprising the steps of: substantially configuring said second portion to facilitate by-pass of a proportion of said heat conveying medium past said first portion; incorporating control means for effecting control of flow of said heat conveying medium through said second portion; and in response to changes in the temperature of said heat conveying medium, adjusting said control means to either further restrict or de-restrict said flow.

Preferably said heat conveying medium comprises combustion products derived from a pre-mix burner.

Preferably said second portion comprises a gap configured for facilitating by-pass of a proportion of said heat conveying medium past said first portion.

Preferably the gap is substantially located at at least one side of said first portion.

Preferably the gap is configured as an opening between at least one side of said first portion and a wall of a container housing said apparatus.

Preferably the control means is located within said second portion, and suitably said heat exchanger apparatus is configured to substantially prevent formation of condensate in a flue system coupled to said heat exchanger apparatus.

Preferably the apparatus comprises a non-condensing heat exchanger.

Preferably the control means comprises means configurable to extend across the second portion and this may suitably comprise a temperature responsive bi-metallic strip.

Preferably the heat exchanger is made substantially of either copper or cast iron.

Suitably the heat exchanger apparatus may be configurable for use in a heating system boiler of a building.

Preferably the method additionally comprises the step of calibrating the control means with respect to temperature prior to installation of said control means.

According to a third aspect of the present invention, there is provided a thermodynamic non-condensing boiler comprising: a pre-mix bumer arrangement for mixing and burning a mixture of air and burner fuel resulting in heated combustion products; and means for directing said combustion products to a heat exchanger arrangement, said heat exchanger arrangement further comprising: a member containing a fluid for effecting transfer of heat from said heated combustion products to said member; and a cross-sectional area presented to said heated combustion products, said area comprising a first portion defined by said member and a second portion comprising means for substantially automatically controlling the amount of said medium passing through said second portion.

Preferably the control means is configured to vary the amount in response to changes in the temperature of the heat conveying medium.

Preferably the means for substantially automatically controlling the amount of said medium passing through said second portion comprises a direct mechanism. Alternatively the means for substantially automatically controlling the amount of said medium passing through said second portion may comprise an indirect ~-~ Preferably the second portion comprises a gap configurable to be opened and closed by said means for substantially automatically controlling the amount of said medium passing through said second portion.

Preferably the means for substantially automatically controlling the amount of said medium passing through said second portion comprises use of a bimetallic strip or similar device.

According to a fourth aspect of the present invention, there is provided apparatus configurable for use in conjunction with a heat exchanger, said apparatus being further configurable to substantially prevent formation of condensate from heated combustion products outlet from said heat exchanger, said apparatus comprising: means for facilitating a proportion of said combustion products to bypass said heat exchanger; and means for varying said proportion in response to a change in temperature of said combustion products.

Preferably said means for facilitating a proportion of said combustion products to by-pass said heat exchanger comprises a flow route created within the body of the heat exchanger's active heat exchange surface.

Preferably said means for facilitating a variable proportion of said combustion products to by-pass said heat exchanger comprises a flow route around the heat exchanger's active heat exchange surface.

Preferably said means for varying said proportion comprises a bi-metallic strip or similar device.

Preferably said means for varying said proportion acts by virtue of direct movement of said means in response to a said temperature change.

Preferably said means for varying said proportion acts by virtue of indirect movement of said means in response to a said temperature change.

Preferably said means for varying said proportion acts continuously and substantially automatically in response to said temperature changes.

Brief Description of the Drawings For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: Figure 1 schematically illustrates a boiler system of the type typically found in a domestic household, the embodiment detailed being configurable to operate with heat-exchanger apparatus configured in accordance with the present invention; Figure 2 relates to prior art gas boilers utilizing pre-mix burners and identify a first relationship between efficiency of a given heat exchanger and the output of the boiler unit and a second relationship between the temperature of the flue products and the boiler unit output; fig. 2 additionally illustrates 2 prior art methods of preventing formation of condensate in a flue system connected to a pre-mix burner arrangement ; Figure 3 schematically illustrates efficiency versus output and flue product temperature versus output for a heat exchanger configured in accordance with the present invention, that is a heat exchanger configured with an associated gap so as to decrease or increase the amount of by-pass of combustion products bypassing the heat exchanger active surface area; Figure 4 schematically illustrates in section a gas boiler utilizing pre-mix burner technology and which incorporates a heat exchanger arrangement configured in accordance with the present invention, the heat exchanger comprising a gap together with means for varying the amount of the combustion products by-passing the heat exchanger; Figure 5 schematically illustrates a conventional atmospheric gas boiler arrangement which utilizes apparatus configured in accordance with the present invention; Figure 6 further details in perspective view, the heat exchanger arrangement identified in figure 4; Figure 7 a sectional view, further details the heat exchanger arrangement identified in figures 4 and 6; and Figure 8 provides graphs of efficiency versus output and temperature of flue products versus output for both a conventional heat exchanger and a dynamic heat exchanger configured in accordance with the present invention, the graphs being derived from experimental data.

Detailed Description of the Best Mode for Carrying Out the Invention There will now be described by way of example the best mode contemplated by the inventors for carrying out the invention. In the following description numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the present invention.

Referring to Fig. 1 a domestic or commercial heating boiler system 101 is schematically illustrated for purposes of aiding the following description. Thus, boiler unit 101 as depicted, may comprise a conventional heat exchanger arrangement or an improved heat exchanger arrangement configured in accordance with the present invention. Boiler unit 101 is contained within a metal outer casing 102 which is affixed to a kitchen wall 103 as illustrated. In the particular example shown boiler unit 101 is located in a conventional domestic kitchen above a domestic kitchen unit 104. However, the boiler unit is not to be considered as restricted for locating in a kitchen-it may be located in any suitable room of domestic or commercial premises as appropriate. Boiler system 101 comprises a methane gas inlet pipe 105 which enables methane gas fuel to be inlet into boiler unit 101. The boiler unit 101 in the example shown may be considered to be a downward firing non-condensing boiler using pre-mix technology for improved combustion of the methane in air. Thus, a fan arrangement 106 is provided to provide air for buming the methane derived from inlet 105. Fan 106 derives air through air inlet 107 and both the methane gas and air are delivered to gas burner 108. Burner unit 108 may typically comprise ceramic burner elements through which, upon ignition, methane gas derived through pipe 105 is burnt with flames emanating in a downwards direction due to the air stream provided by fan 106. Upon combustion, the methane gas produces combustion products which again by virtue of fan 106 are directed in a downwards direction towards heat exchanger arrangement 109. In the particular example shown heat exchanger 109 may be made of copper or iron for example as a monobloc construction. Heat exchanger 109 is connected to a fluid supply inlet 110 and also to a fluid outlet 111 the usual fluid to be heated being water. Both water inlet 110 and water outlet 111 may be configured to circulate water around a bank of domestic radiators situated around a conventional house. In the example shown water inlet 110 and water outlet 111 are configured to be adjoined to further transport pipes upon passing through ceiling 112. Water circulation through heat exchanger 109 is effected by water pump 113 which receives water from heat exchanger 109 via inlet 114.

Similarly, water pump 113 returns water to heat exchanger 109 via pump outlet 115. Boiler unit 101 may be electrically powered in terms of ignition mechanisms and control mechanisms, receiving electrical power from the mains 116. Mains 116 is connected to boiler system 101 via electrical power lead 117.

Finally, as is the case with gas powered boiler systems in general, boiler unit 101 is configured to be operated by its owner using electric control panel 118 which may comprise a series of control buttons as illustrated.

Upon formation of combustion products which are directed to heat exchanges arrangement 109 the heated combustion products are directed to flue outlet means 119 which generally may be configured to deliver said combustion products to the outside environment surrounding the premises in which boiler unit 101 is located. The exact configuration of flue outlet means 119 is dependent upon several factors, including preferences of individual customers. Thus, for example. a longer flue outlet system 119 is often preferred since it provides more flexibility in terms of the positioning of boiler system 101 in a given room.

From the field of combustion engineering methane gas thoroughly combusted with air yields approximately 37.8 Mega Joules per meter cube (MJm-3). This value is known as the gross calorific value for the combustion.

Of the gross calorific value approximately 11% of the energy produced is contained in the steam produced from the combustion of the methane in air.

The remaining energy (approximately 34 MJm~3) is found to reside in the remaining combustion products which essentially comprise carbon dioxide gas.

The combustion product may also comprise additional components if the combustion is undertaken in air (air being approximately 70% nitrogen with various other components). The value 34 MJm~3 may be termed the net calorific value for the combustion of methane in air. In general, in the field of condensing boiler engineering it is desirable to maximize the use of the energy contained in the generated steam and the net energy. However, in a conventional (non-condensing boiler) arrangement only the net calorific value is utilisable since otherwise the steam would undergo a phase change to the liquid state and drip down onto the burners thus aiding corrosion. Atmospheric burners are thus generally engineered so that the temperature of the combustion products in the flue system associated with a given boiler are kept well above the dew point (this being typically around 160 C) and the associated heat exchanger arrangement being made of copper for example.

To increase efficiency boiler units are known to comprise larger heat exchanger arrangements made of cast aluminum for example or stainless steel.

In general, such heat exchanger arrangements are preferably made of a resistant material (not copper) and are supplied heat from a premix burner arrangement. Using the larger heat exchanger reduces the temperature of the combustion products, due to the increased efficiency of the heat exchanger arrangement. This in turn releases condensate into the boiler and flue system which therefore must be removed. Since the main combustion product is carbon dioxide gas then those skilled in the art frequently refer to running of a boiler unit in a range (that is a quantity of) of carbon dioxide gas. With pre-mix technology there are associated disadvantages. Thus, with decreasing air/gas ratio, less carbon dioxide gas is produced, this being effected by introducing either more air or less methane into the burner; in other words a drop in carbon dioxide production may effectively cause the boiler unit to extinguish its flames through the proportion of methane in the mixture of air/methane becoming too lean. Additional problems may arise such as noise being generated in response to poor combustion performance. Similarly, if the proportion of carbon dioxide gas produced is too high as compared with the air content then relatively poor combustion may result ; such a condition may arise form either too high a proportion of methane gas or too little air being supplie.

From the above combustion engineers have found that boilers utilizing premix technology are generalTy-best configured to operate within a range of 7.5%-10% of carbon dioxide gas by volume of the flue products.

As described in the introduction the lowest temperature a fluid may have in order that dew may condense on a surface from a humid atmosphere is known as the dew point of the humid atmosphere of concern. With the burning of methane in air it is found that the lower the carbon dioxide content of the combustion products, the lower the dew point of the combustion products.

Furthermore, within the band of 7.5%-10% carbon dioxide gas by volume of the flue products, the dew point of the flue products is found to be comparatively constant. At normal atmospheric pressure steam has a dew point of 100 C.

However, a combination of air, steam and carbon dioxide gas (8% carbon dioxide gas by volume for example) gives a dew point of approximately 55 C.

Thus, in this latter case the steam will condense into water at approximately 55 C, by virtue of the air and carbon dioxide gas content.

The amount of heat generated by a given boiler unit may be self-adjusting depending on the ambient weather conditions surrounding the building housing the boiler unit. Thus, in conditions of relatively coid weather the boiler unit effectively has to generate more energy to heat the given building as compared with the same boiler unit operating in conditions of warmer weather. The amount of work that a given boiler unit has to undergo to heat a given building (or a given body of water such as a hot water supply) is known as the load and thus in colder weather conditions the effective load is higher than in warmer conditions. Variation in output of a given boiler unit is known as modulation and in general it is preferable to enable control of the boiler unit to modulate the amount of output as required. A typical modulation range for boiler units comprising premix burners is between 100-40% of nominal output/load. With premix burners the amount of carbon dioxide gas produce stays approximately constant over the modulation range and thus in turn the dew point as a result remains approximately constant across the modulation range. However, for low output the efficiency in terms of heat exchange effected by the heat exchanger is considerably higher than for nominal loads.

Figure 2 relates to prior art gas boilers utilizing pre-mix burners in conjunction with a heat exchanger having a constant effective surface area.

The figure shows a first relationship 201 between efficiency of a given heat exchanger (expressed as a percentage of heat transferred from the combustion products to the fluid circulated through the heat exchanger against output of the boiler unit in kilowatts. The typical output range varies from 40% to 100%.

Relationship 201 identifies that the efficiency of the heat exchanger gradually increases as the output of the heat exchanger is reduced. This makes sense in that the lower the volume of heat carrying combustion products passing by the heat exchanger the more efficient that heat will be absorbed by the heat exchanger. Thus, the efficiency at lower output is greater than that at higher output. A second relationship is shown in figure 2, relationship 202 which represents of the flue products may be increased sufficiently above the dew point line so as to effectively maintain the temperature of the flue products above the dew temperature. In this way condensate is effectively prevented from forming in the modulation range.

Figure 2 also schematically illustrates the second method of preventing formation of condensate in the flue system of a given boiler system employing pre-mix burner technology with a prior art constant size heat exchanger. By restricting the modulation range the variation in the temperature of the flue products is constrained as indicated by the vertical broken line 206. In the example shown the allowable modulation range is restricted at about 80%.

Thus, a given boiler system may be configured to operate only between certain pre-defined more limited heat output values. In this way, curve 202 would effectively be terminated at the point where it crosses broken line 206, that is at point 207. As illustrated point 207 is some distance above the due point line 203 and thus condensate is prevented from forming in this restricted modulation range between 80% and 100% output.

In the case of a conventional boiler utilizing a conventional atmospheric burner the relationships between efficiency and output and between temperature of the flue products and output are such that the dew point falls gradually with reduced output. In this case the temperature of the flue products over the output range 40% to 100% for example remains above the dew point and thus problems with the formation of condensate substantially do not arise.

In accordance with the present invention the heat exchanger apparatus is provided which comprises a member containing a fluid for effecting transfer of heat from a heat conveying medium presented to the member. The heat exchanger apparatus comprises a cross-sectional area presented to the heat conveying medium which comprises a first cross-sectional area portion defined by the member and a second cross-sectional area portion which comprises means for substantially automatically controlling the amount of the medium passing though the second portion. The control means is configured to vary the amount of combustion products bypassing the first portion in response to changes in temperature of the heat conveying medium. In this way a proportion of the heat conveying medium (such as, for example, combustion products from a premix burner) are effectively directed through the second portion which may comprise a gap or another suitably configured channel for directing the combustion products. A heat exchanger arrangement of this type is suitable for use with premix burner technology. By varying the amount of combustion products bypassing the heat exchanger such that a proportion of the combustion products pass straight into the flue system on the downstream side of the heat exchanger then the temperature of the flue products can be automatically controlled to remain above the dew point of the combustion products.

Figure 3 schematically illustrates the general relationships in terms of efficiency of the heat exchanger versus output and temperature of the flue products versus output. In common with figure 2, the dew point of the combustion products in the flue is identified by a broken horizontal line 301 and is more or less constant within the output range. For comparison purposes the temperature relationship identified in figure 2 is shown in figure 3 by broken line 204. Efficiency relationship 201 is again identified on figure 3. The schematic relationships illustrated include the change in flue temperature versus output as represented by relationship 303. Relationship 303 schematically illustrates that the flue temperature remains constant with decreasing output. Relationship 303 results from the gap configured in the heat exchanger arrangement being gradually increased with decreasing output of the heat exchanger arrangement.

The opening of the gap is represented by linear broken line 304 for illustrative purposes only. Thus, in figure 3 increase in the size of the gap or orifice in a given heat exchanger arrangement is represented by the vertical distance between broken line 304 and the horizontal axis 305. With increasing gap size the temperature of the combustion products in the flue system is observed to be maintained more or less constant as represented by line 303, as is the efficiency (line 302) of such a set up. The means for increasing the gap size may be suitably calibrated so that an increase in the gap size corresponds to the required increased in the temperature of the flue products so as to prevent formation of condensate. In this way the mechanism for increasing the gap size and decreasing the gap size may be configured such that an automatic feed back control system oversees operation of the heat exchanger arrangement so as to ensure that the temperature of the flue products (relationship 303) remains above the dew point (line 301).

A preferred embodiment of the present invention comprises use of a heat exchanger arrangement wherein one or more heat exchangers of the heat exchanger arrangement are configured with a gap so as to vary the amount of combustion gases bypassing the heat exchanger heat exchange surface through a range of operational conditions. In the preferred embodiment such a heat exchanger arrangement is configured to be used with gas appliances operated with pre-mix burners which are designed for non-condensing and low temperature applications. In such apparatus, the primary property of a dynamic heat exchanger is an almost constant amount of flue losses from full to part load, thus keeping efficiency of such a heat exchanger more constant as schematically identified in figure 3 and described above. Figure 4, schematically illustrates in section, a gas boiler operated with pre-mix burner technology and which incorporates a heat exchanger arrangement configured in accordance with the present invention. Gas boiler 401 comprises an outer casing 402, which surrounds a downward firing heating system. The boiler unit 401 comprises a pre-mixing chamber 403 into which air 404 and methane gas 405 are provided via suitably configured inlets. The air and methane are mixed in chamber 403 whereafter they are delivered through a baffle arrangement 406 to ceramic burner 407. Thus, upon reaching burner 407 the air/methane gas mixture is ignited with downward firing flames arising from the lower surface of burner 407. In this downward firing configuration, the air 404 and methane gas 405 may be forced into mixing chamber 403 via use of suitably configured fan.

By using a fan is this way the flames arising from burner surface 407 extend in a downwards direction with heat emanating thereafter towards heat exchanger arrangement 408. Heat exchanger arrangement 408 may be substantially made of cast iron or copper for example and may be configured in a variety of ways. The simple example shown in figure 4 comprises heat exchanger 408 having a surface configured with a plurality of fins 409,410 forming a heat exchange member so as to enhance the efficiency of heat transfer from the combustion products derived from burner 407 passing the heat exchanger surface of heat exchanger 408. Heat exchanger 408 comprises a fluid inlet 411 through which a heat transfer medium such as water for example is transmitted.

Following passage of the combustion products through heat exchanger arrangement 408 the combustion products are directed to a suitably configured flue system 413 whereafter they are directed to the outer environment via outlet 414. Heat exchanger arrangement 408 is configured in accordance with the present invention wherein a gap 415 has been created in the heat exchanger arrangement 408. In the example shown the gap 415 is located towards an edge of the heat exchanger arrangement 408. The heat exchanger arrangement 408 comprising a heat transfer surface made up of fins 409,410, etc may be considered to comprise a first portion of a cross-sectional area presented to the heated combustion products derived from burner arrangement 407. The gap 415 may thus be considered to comprise a second crosssectional area portion also presented to the combustion products derived from burner 407. Thus, the first portion in the preferred embodiment is substantially defined by the heat transfer member 408. Furthermore, gap 415 is controlled, in terms of its size, via use of a control element 416 and which is configured to extend across gap 415. The control element 416 is held suitably in place by attachment means 417 which may be attached to outer casing 402 for example.

With decreasing output of heat exchanger arrangement 408 the control element 416 is configured to allow a greater proportion of the combustion products deriving from burner 407 to pass through heat exchanger gap 415. Similarly, with increased output of heat exchanger arrangement 408 the strip control element 416 is configured to reduce the size of gap 415 and thus restrict the proportion of the combustion products bypassing heat exchanger heat exchange member 408. Thermodynamic member 416 may be configured as a bracket to regulate the quantity of combustion products by-passing the crosssectioned area formed by heat exchanger member 408. In the preferred embodiment the control member 416 may comprise a suitably configured bimetallic strip or a strip of memory metal or any other suitably temperature responsive material which moves in response to a change in temperature of the combustion products. Suitably each of the metals and/or alloys comprising the bimetallic strip may have a nickel component. When gas boiler appliance 401 starts up in cold conditions or modulates down the bracket 416 is configured in the open position. In this configuration no heat is taken out of the by-passing combustion products and flue temperature is increased downstream of the heat exchanger in flue system 413. Bracket 416 is configured to close gap 415 when the appliance reaches the required flue temperature under full load. Flow control bracket 416 is therefore configured to continuously adjust the effective size of gap 415 for any condition between full and part load, so that the flue product temperature and efficiency of heat exchanger arrangement 408 remain constant. By bracket, any means to vary the amount of flue products bypassing the heat exchanger is to be understood by those skilled in the art. Such a bracket may be actuated by means of bimetal (as indicated above), memory metal or materials of similar nature and performance. Actuation can be direct, that is made from such a material or indirect. In the case of indirect actuation one of the above described materials may be used to guide a suitably configured bracket (or other member) into position as required.

The gap and bracket arrangement described above is merely intended to be construed as an example of carrying out the invention. A wide variety of different means may be employed to carry out the invention as will be understood by those skilled in the art. By having a bracket and gap there is effectively provided means for facilitating a proportion of the combustion products to bypass the heat exchanger and means for varying the proportion in response to a change in temperature of the combustion products.

A bimetallic strip is a device known to those skilled in the art which comprises two metals or alloy based materials having different coefficients of expansion wherein the 2 different materials are bonded together. An increase in temperature of the strip causes the strip to bend or curve, the metal having the greater coefficient of expansion being on the outside of the curve. One end is rigidly fixed and movement of the other end can serve to open or close a gap of the type described above. Thus, in itself a given bimetallic strip can be calibrated with respect to temperature so as to open a given gap or close a given gap by a predetermined amount such that the amount of combustion products by-passing the heat exchanger is continuously and accurately adjusted in respect of temperature changes associated with the combustion products.

A bimetallic strip or similar device is preferred to control the amount of combustion products bypassing the heat exchanger 408 in that it acts directly in response to changes of temperature. Indirect mechanisms may also be suitable wherein for example a temperature sensing device provides control signals to a moveable barrier, extendible across the gap by varying amounts, so as to effect further restriction or de-restriction of combustion products flowing through a given gap.

Figure 5 schematically illustrates a conventional atmospheric gas boiler arrangement 501, which in accordance with the present invention, comprises a thermodynamic heat exchanger 502 having a plurality of fins such as fins 503 and 504. The dynamic heat exchanger 502 is designed with an adverse operating bracket which can be used to increase the efficiency of conventional boiler 501 or which may be used to maintain the efficiency at an approximately constant value throughout a given output range. In conventional burner arrangement 501 methane gas is delivered to ceramic burner 505 via inlet 506.

Inlet 506 may simply comprise a cylindrical pipe arrangement. Boiler 501 has an outer casing 507 which may be configured in the usual way so as to enhance the appearance of the boiler. Burner 505 receives air as indicated by arrow 508 by simply"sucking"air up through the base of boiler arrangement 501. Thus, the base may simply comprise a grid like arrangement enabling air to pass into outer casing 507, however, the arrangement shown in figure 5 is not a conventional atmospheric burner in that, in accordance with the present invention, it comprises an improved heat exchanger arrangement 502. Heat exchanger 502 has inlet 509 and outlet 510. Inlet 509 and outlet 510 enable the passage of a fluid, such as water for example, which may thereafter be transported to and circulated around either a hot water tank system or a bank of radiators. Heat exchanger arrangement 502 is improved in that it comprises a gap associated with the heat exchanger active surface wherein the gap may be varied in size from fully open to fully closed so as to regulate the amount of excess air 508 passing though the heat exchanger active surface area. In the particular example shown a first gap 511 comprises means 512 for varying the effective size of the gap 512. Means 512 is held in place to outer casing 507 by attachment means 513. A similar gap to gap 511 is provided on the right hand side of the diagram as identified at 514, gap 514 being configured with means 515 for varying the size of the gap in the same way as for gap 511. By providing means for limiting the amount of excess air, conventional boiler arrangement 501 is enhanced in terms of its operational efficiency at a given output. Thus, when boiler 501 is required to modulate down its output, the amount of methane gas flowing into burner arrangement 505 is reduced.

Because the efficiency of this conventional atmospheric boiler arrangement is proportional to the amount of carbon dioxide gas produced as a combustion product then limiting the amount of excess air passing through gaps 511 and 514 increases the effect efficiency of boiler arrangement 501. In other words the air entering apparatus 501 is limited by a bracket (513 and 515) so as to increase the amount of carbon dioxide gas as a percentage of the combustion products produced and thereby increase the heat transfer efficiency of the apparatus as a whole. Following passage through heat exchanger arrangement 502 the combustion products and any remaining excess air are directed upwards towards flue system 516 whereafter they enter the outer environment.

In the case of figure 5, heat exchanger arrangement 502 may be configured around a more conventional heat exchanger arrangement, that is one not made of cast aluminum or steel for example.

The heat exchanger arrangement identified in figure 4 is further detailed in figure 6. Heat exchanger arrangement 401 comprises an outer casing 601 which may be made of a resilient material such as cast aluminum or steel or a less resistant material such as copper for example. Outer casing 601 comprises means for attaching heat exchanger arrangement 401 to further apparatus such as to aesthetic casing of a given boiler unit and/or to a kitchen wall for example. Such means of attachment may comprise orifices such as 602 and 603 respectively. Furthermore, heat exchanger arrangement 401 comprises heat exchange fluid inlet means and heat exchange fluid outlet means (a means to vent air) such as outlet air vent means 604. The active heat exchange surface area of the heat exchanger arrangement is identified at 605 wherein a plurality of heat exchange fins are configured to extract heat from combustion products arriving at and passing through surface 605 from a burner arrangement. The combustion products pass between the fins comprising surface 605 in a downwards direction (in accordance with the arrangement identified in figure 4) passing though the underside of arrangement 401.

Surface 605 comprises gap 606 formed to one side of surface 605. Surface 605 may be configured with one or more gaps. Gap 606 is fitted with a bracket 607, which in the preferred embodiment, is made or actuated by a bimetallic strip so as to effect further closing or further opening of gap 606. Bracket 607 is configured to mate with surface 605 in its closed position and to form the required gap in its full open position. Furthermore bracket 607 is configured to undertake movement in accordance with the temperature of the combustion products arriving at surface 605. Thus, upon calibrating bracket 607, the effective size of gap 606 may be responsive to combustion product temperature wherein if the combustion product temperature becomes too low then bracket 607 is configured to further open so as to increase the temperature of the flue products emanating from the underside of heat exchanger arrangement 401.

Similarly, if the temperature of the flue products is sensed to be too high then bracket 607 may effect further closure of gap 606 so as to further restrict the flow of combustion products through gap 606. Combustion products derived from a burner arrangement are directed into heat exchanger arrangement 401 through upper surface outer opening generally indicated at 608. Figure 6 thus may be considered to schematically illustrate, in perspective view, a heat exchanger arrangement configured as a preferred embodiment in accordance with the present invention.

A sectional view through the heat exchanger arrangement 401 identified in figure 6 is further detailed in figure 7. As with figure 6, figure 7 is intended to aid the understanding of the invention and is not to be construed as limiting the invention to any specific implementation. In relation to figure 6, figure 7 represents a section through heat exchanger arrangement 401 along the line A B. Double arrow 701 illustrates the entrance of combustion products into gap 606. Bracket 607 is identified as a bimetallic strip in the example shown and broken line 702 is provided in the illustration so as to identify bracket 607 in its position configured to close gap 606. Thus, in figure 7 closure of gap 606 is effected by a movement to the left of bracket 607 whereas opening of gap 606 is effected by movement of bracket 607 to the right. The active heat exchanger surface 605 comprises a plurality of fins such as fins 703 and 704. The heat exchanger tubes comprise heat transfer fluid such as water running within them, the water being circulated around a bank of domestic radiators for example and through the heat exchanger 408 via appropriate connecting pipes. The combustion products entering upper orifice 608 of heat exchanger arrangement 408 pass between the heat exchange fins in a downwards direction such that the heat contained within said combustion products is transferred to the water running within the fin arrangement. To automatically control the temperature of the flue products 705 emanating from the underside of heat exchanger arrangement 408, bracket 607 is adjusted automatically in response to the temperature of the combustion products in the immediate environment surrounding it. In this way the amount of combustion products 701 entering gap 606 and passing through gap 606 is controlled by bracket 607 in accordance with predefined calibration criteria. The predefined calibration criteria may be configured in accordance with individual requirements or may be pre-configured on a larger scale in accordance with a given manufacturers requirements. In the illustration a heat exchange fluid inlet means 706 is identified. It is to be understood by those skilled in the art that the given configuration of gap 606 and bracket 607 is not to be construed as limiting the invention to that identified in the figure. Thus, for example gap 606 may be configured towards the center of surface 605 either as a rectangular groove or as a circle or as a series of such shapes for example.

The use of pre-mix burners, developed mainly for use in condensing appliances, requires operation with a comparatively constant level of excess air and levels of carbon dioxide gas. As a result the efficiency increases from full load to part load with a constant size heat exchanger. Although desired in condensing boilers, this can be regarded as an inherent design problem with non-condensing boilers. Here, efficiencies can increase from nominal to minimum output by up to 5-7%, resulting in formation of condensate. This condensate is predominantly formed in the flue gas evacuation system downstream of the heat exchanger, and this problem is emphasized with long flue systems. In accordance with the invention, a specific example obtained from experimental data is now given in figure 8. Figure 8 represents an example based upon 24 kW output and a modulation range of 40% wherein the boiler unit operates at approximately 8.5% carbon dioxide gas (by volume of flue products). Thus, figure 8 may be considered to be a specific example of the graph identified in figure 4. The left hand axis of the graph comprises values of efficiency given as a net percentage of theoretical total heat output.

The right hand axis gives values of flue temperature in Degrees Celsius ranging from 0 C to 250 C. The horizontal axis corresponds to the output of a boiler unit comprising a heat exchanger arrangement of the type identified in figure 4.

A conventional heat exchanger arrangement (not having a variably adjusted gap) is compared with a thermodynamic heat exchanger arrangement configured in accordance with the present invention. Thus, a first relationship 801 identifies the efficiency of a conventional heat exchanger arrangement and a second relationship 802 identifies the efficiency of the thermodynamic heat exchanger arrangement. In this example the efficiency of the conventional heat exchanger is seen to be slightly higher than that of the invention throughout the modulation range 10kW-24 kW, the maximum difference occurring at 10kW where efficiency only differs by about 3.5%. However, two further relationships are identified in figure 8 as follows. Relationship 803 identifies the variation in flue temperature versus boiler unit output and relationship 804 identifies flue temperature of the thermodynamic heat exchanger versus output. Across the entire modulation range the flue temperature (line 804) of the combustion products which have passed through a heat exchanger configured in accordance with the present invention is higher than that of the conventional heat exchanger. Thus, the flue temperature of the combustion products associated with use of a heat exchanger in accordance with the present invention is further removed from the effective dew point of the combustion products (for a given carbon dioxide output) and similarly the efficiency relationships clearly show that the variation in efficiency across the modulation range identified is substantially reduced for the thermodynamic heat exchanger invention described herein. In other words the gradient of line 802 is less than that of a conventional heat exchanger relationship 801 thus providing a more constant efficiency across the modulation range from between 10 kW and 24 kW output.

Claims (46)

1. Claims : 1. A heat exchanger apparatus comprising: a member containing a fluid for effecting transfer of heat from a heat conveying medium presented to said member; and a cross-sectional area presented to said heat conveying medium, said area comprising a first portion defined by said member and a second portion comprising means for substantially automatically controlling the amount of said medium passing though said second portion; wherein said control means varies said amount in response to changes in the temperature of said heat conveying medium.
2. 2. A heat exchanger apparatus according to Claim 1, wherein said heat conveying medium comprises combustion products derived from a premix burner.
3. 3. A heat exchanger apparatus according to Claim 1 or Claim 2, wherein said second portion comprises a gap for facilitating bypass of a proportion of said heat conveying medium past said first portion.
4. 4. A heat exchanger apparatus according to Claim 3, wherein said gap is located at at least one side of said first portion.
5. 5. A heat exchanger apparatus according to Claim 4, wherein said gap is configured as an opening between said at least one side of said first portion and a wall of a container housing said apparatus.
6. 6. A heat exchanger apparatus according to Claim 3, wherein said gap is located within the area defined by said first portion, said gap comprising a hole.
7. 7. A heat exchanger apparatus according to any of claims 1-6, wherein said heat exchanger is configured to substantially prevent formation of condensate in a flue system coupled to said heat exchanger apparatus.
8. 8. A heat exchanger apparatus according to any of claims 1-6, wherein said apparatus comprises a non-condensing heat exchanger.
9. 9. A heat exchanger apparatus according to Claim 1, wherein said control means is located within said second portion.
10. 10. A heat exchanger apparatus according to Claim 1 or Claim 9, wherein said control means comprises means configurable to extend across said second portion.
11. 11. A heat exchanger apparatus according to any of claims 1,9 or 10, wherein said control means comprises a temperature responsive material.
12. 12. A heat exchanger apparatus according to any of claims 1,9,10 or 11, wherein said control means comprises a bi-metallic strip.
13. 13. A heat exchanger apparatus according to Claim 12, wherein said bimetallic strip comprises two metals or alloy based materials having different coefficients of expansion wherein said two materials are bonded together.
14. 14. A heat exchanger apparatus according to Claim 13, wherein said materials each comprise an alloy having a nickel component.
15. 15. A heat exchanger apparatus according to any preceding claim, wherein said heat exchanger is substantially made of either copper or cast iron.
16. 16. A heat exchanger apparatus according to any preceding claim, wherein said heat exchanger is configurable for use in a heating system boiler of a building.
17. 17. A heat exchanger apparatus according to any of claims 1,9,10 or 11, wherein said control means is calibrated with respect to temperature.
18. 18. In a heat exchanger apparatus comprising: a member containing a fluid for effecting transfer of heat from a heat conveying medium presented to said member; and a cross sectional area presented to said heat conveying medium, said area comprising a first portion defined by said member and a second portion, a method of substantially automatically controlling the amount of said medium passing through said second portion, said method comprising the steps of: substantially configuring said second portion to facilitate by-pass of a proportion of said heat conveying medium past said first portion; incorporating control means for effecting control of flow of said heat conveying medium through said second portion; and in response to changes in the temperature of said heat conveying medium, adjusting said control means to either further restrict or de-restrict said flow.
19. 19. A method according to Claim 18, wherein said heat conveying medium comprises combustion products derived from a pre-mix burner.
20. 20. A method according to Claim 19, wherein said second portion comprises a gap for facilitating by-pass of a proportion of said heat conveying medium past said first portion.
21. 21. A method according to Claim 20, wherein said gap is substantially located at at least one side of said first portion.
22. 22. A method according to Claim 21, wherein said gap is configured as an opening between at least one side of said first portion and a wall of a container housing said apparatus.
23. 23. A method according to any of Claims 18-22, wherein said heat exchanger apparatus is configured to substantially prevent formation of condensate in a flue system coupled to said heat exchanger apparatus.
24. 24. A method according to any of Claims 18-22, wherein said apparatus comprises a non-condensing heat exchanger.
25. 25. A method according to Claim 18, wherein said control means is located within said second portion.
26. 26. A method according to Claim 18 or Claim 25, wherein said control means comprises means configurable to extend across said second portion.
27. 27. A method according to any of Claims 18,25 or 26 wherein said control means comprises a temperature responsive bi-metallic strip.
28. 28. A method according to any of Claims 18-27, wherein said heat exchanger is made substantially of either copper or cast iron.
29. 29. A method according to any of Claims 18-28, wherein said heat exchanger apparatus is configurable for use in a heating system boiler of a building.
30. 30. A method according to Claim 18 additionally comprising the step of calibrating the control means with respect to temperature prior to installation of said control means.
31. 31. A thermodynamic non-condensing boiler comprising: a pre-mix bumer arrangement for mixing and buming a mixture of air and bumer fuel resulting in heated combustion products; and means for directing said combustion products to a heat exchanger arrangement, said heat exchanger arrangement further comprising: a member containing a fluid for effecting transfer of heat from said heated combustion products to said member ; and a cross-sectional area presented to said heated combustion products, said area comprising a first portion defined by said member and a second portion comprising means for substantially automatically controlling the amount of said medium passing through said second portion.
32. 32. A thermodynamic non-condensing boiler according to Claim 31, wherein said control means is configured to vary said amount in response to changes in the temperature of said heat conveying medium.
33. 33. A thermodynamic non-condensing boiler according to Claim 31, wherein said means for substantially automatically controlling the amount of said medium passing through said second portion comprises a direct mechanism.
34. 34. A thermodynamic non-condensing boiler according to Claim 31, wherein said means for substantially automatically controlling the amount of said medium passing through said second portion comprises an indirect mechanism.
35. 35. A thermodynamic non-condensing boiler according to any of Claims 31-34, wherein said second portion comprises a gap configurable to be opened and closed by said means for substantially automatically controlling the amount of said medium passing though said second portion.
36. 36. A thermodynamic non-condensing boiler according to any of Claims 31,33 or 34, wherein said means for substantially automatically controlling the amount of said medium passing though said second portion comprises use of a bi-metallic strip or similar device.
37. 37. Apparatus configurable for use in conjunction with a heat exchanger, said apparatus being further configurable to substantially prevent formation of condensate from heated combustion products outlet from said heat exchanger, said apparatus comprising: means for facilitating a proportion of said combustion products to by-pass said heat exchanger; and means for varying said proportion in response to a change in temperature of said combustion products.
38. 38. Apparatus according to Claim 37, wherein said means for facilitating a variable proportion of said combustion products to by-pass said heat exchanger comprises a flow route around the heat exchanger's active heat exchange surface.
39. 39. Apparatus according to Claim 37, wherein said means for facilitating a proportion of said combustion products to by-pass said heat exchanger comprises a flow route created within the body of the heat exchanger's active heat exchange surface.
40. 40. Apparatus according to Claim 37, wherein said means for varying said proportion comprises a bi-metallic strip or similar device.
41. 41. Apparatus according to Claim 37, wherein said means for varying said proportion acts by virtue of direct movement of said means in response to a said temperature change.
42. 42. Apparatus according to any of Claims 7,40 or 41, wherein said means for varying said proportion acts by virtue of indirect movement of said means in response to a said temperature change.
43. 43. Apparatus according to any of Claims 37 to 42, wherein said means for varying said proportion acts continuously and substantially automatically in response to said temperature changes.
44. 44. Apparatus according to any of Claims 1-17 and 37-43 as described herein and with reference to the accompanying drawings.
45. 45. The method according to any of Claims 18-30 as described herein and with reference to the accompanying drawings.
46. 46. A thermodynamic non-condensing boiler according to any of Claims 31-36 as described herein and with reference to the accompanying drawings.