EP1734303B1

EP1734303B1 - Pellet boiler with high energy recovery

Info

Publication number: EP1734303B1
Application number: EP06114199.0A
Authority: EP
Inventors: Mario Acquaro
Original assignee: FAMAR ITL (Innovation Technology Lab) Srl
Current assignee: FAMAR ITL (Innovation Technology Lab) Srl
Priority date: 2005-06-17
Filing date: 2006-05-19
Publication date: 2014-06-11
Anticipated expiration: 2026-05-19
Also published as: EP1734303A2; EP1734303A3; ITTV20050086A1

Description

The present invention relates to a boiler which is fed with elements obtained by compression of combustion-supporting material, also known as pellets, characterized by high energy recovery.
The use of pellets allows to limit the problems that occur when using ordinary firewood as solid fuel, such as the large volume occupied by the wood and the need to add frequently by hand new pieces of wood in order to feed the fire.
Since pellets are usually composed of pressed wood, with respect to ordinary firewood they have smaller volumes for an equal heating value, and the addition of new pellets to feed the fire, thanks to their limited dimensions, is fit for automation, therefore no longer requiring frequent intervention by an operator.
Stoves are in use which use pellets and usually comprise a hopper from which they are introduced periodically and automatically, for example by means of one or more screw feeders driven by electric motors, into a burner in which they burn.
The main drawback of these known types of pellet stoves is that their energy efficiency is rather low, since the heat of the gases generated by combustion is not utilized adequately.
As a partial solution to this drawback, boilers are also known in which the heat of the combustion chamber is transferred outside said chamber by means of a heat transfer medium, typically constituted by water, which is made to flow adjacent to the combustion chamber so as to come into contact with the hot parts thereof, becoming warm; the heat transfer medium is then sent to the heating system.
The main drawback of these known types of pellet boilers, which in any case always have a low capacity (8-10 kW), is that their energy efficiency is rather low; therefore, they cannot ensure the heating power required for example to heat simultaneously an enclosed space and water for hygienic and sanitary uses or to heat medium and large enclosed spaces.
Another drawback of these known types of pellet boiler is the difficulty in cleaning the heat exchange surfaces of the heat exchanger, which become clogged very rapidly, consequently requiring very frequent cleaning.
In order to make the most of the heat generated by burning the pellets, burners are known which partially recover the heat contained in the combustion gases; these known types of pellet boiler are present in two types, termed respectively fire tube and water tube.
In so-called fire tube boilers, the gases generated in the combustion of the pellets, before entering the flue from which they are expelled into the outside environment, are made to flow by forced draft inside tubes immersed in water; the gases thus transfer heat to the water, heating it, and said water is sent to the heating system.
In known types of so-called water tube boilers, the combustion gases are instead made to flow outside tubes in which water to be heated circulates, and by making contact with the surface of said tubes they transfer a certain amount of their heat to the water, which is sent to the heating system.
The main drawback of these known types of pellet boilers is that by burning plant-derived fuels they distill resinous substances, which due to the presence of water vapor and to the relatively low temperature reached in the combustion chamber cannot burn completely; these resinous substances, conveyed by the flow of the combustion gases, deposit on the walls, which are relatively cold (typically 60-80°C), of the fire tubes or water tubes, reducing the passage section of the former and the heat exchange surface of the latter, with the result of degrading the efficiency and capacity of the burner and further reducing its life.
In order to achieve efficient heating with acceptable performance with these known types of pellet boilers, it is necessary to perform frequent and onerous maintenance thereof, with very short intervals between one maintenance intervention and the other, which depend on the more or less prolonged use of the boiler and on the type of fuel used.
DE-A-19806428 discloses a pellet boiler having a combination of elements as set forth in the pre-characterizing portion of the appended claim 1.
The aim of the present invention is to solve the above-mentioned problems, eliminating the drawbacks of the cited background art, by providing a pellet boiler which has high energy efficiency and is therefore capable of heating even large enclosed spaces and optionally of heating simultaneously a sufficient amount of fluid for hygienic-sanitary uses and of having warm air in the surrounding environment,
Within this aim, an object of the present invention is to provide a pellet boiler which does not require frequent cleaning and maintenance interventions and has a long life.
Another object of the invention is to achieve high energy efficiency, utilizing optimally the heat generated by the combustion of the pellets.
Another object of the invention is to provide a pellet boiler which is structurally simple and has low manufacturing costs.
In accordance with the invention, there is provided a pellet boiler as defined in the appended claims.
Further characteristics and advantages of the invention will become better apparent from the following detailed description of a particular but not exclusive embodiment thereof, illustrated by way of non-limiting example in the accompanying drawings, wherein:

Figure 1 is a rear perspective view of a boiler, shown partially open on one side and to the rear;
Figure 2 is a rear view of the boiler of Figure 1;
Figure 3 is a sectional view, taken along the line III-III of Figure 2;
Figure 4 is a view of a detail of Figure 3;
Figure 5 is a partially sectional perspective view of the heat exchanger;
Figure 6 is a longitudinal sectional view of the heat exchanger;
Figure 7 is a sectional view, taken along the line VII-VII of Figure 6;
Figures 8 and 9 are two side perspective views of the boiler with the access for the pellet hopper;
Figure 10 is a detail perspective side view of some components of the boiler;
Figures 11 and 12 are respectively a plan view and a top view of the part of the fins exposed longitudinally to the channels of the heat exchanger inside which the combustion, gases are conveyed.

In the exemplary embodiments that follow, individual characteristics, given in reflation to specific examples, may actually be interchanged with other different characteristics that exist in other exemplary embodiments.
With reference to the figures, the reference numeral 1 designates a pellet boiler with high energy recovery.
The boiler I comprises a frame, which is composed of a footing 2, which has a rectangular base and from two sides of which there protrude two uprights 3, and a front wall 4, in which a first opening 5 is provided; a door 6 for accessing a combustion chamber 7 is hinged to said first opening, and said combustion chamber is delimited at the rear by a first wall 8, connected to an overlying heat exchanger 29, and at the front by a second wall 9.
A plurality of first through slots 10 is formed proximate to the upper end of the front wall 4.
A removable hatch 11 is formed proximate to the lower end of the front wall 4 and allows access to a control panel for activating several electrical and/or hydraulic devices of the boiler 1 (not shown in the accompanying figures).
A hollow hopper 12 for pellets is fixed between the upper ends of the uprights 3, in the rear region of the boiler 1, and can be closed in an upper region by means of a lid 100, which conveniently can be opened laterally in order to facilitate filling; said lid can be hidden from view by a door 101 pivoted along one side to the boiler.
Advantageously, the walls of the hopper 12 are thermally insulated in order to avoid the overheating and consequent possible spontaneous ignition of the pellets contained therein.
One or more screw feeders 13 are provided at the bottom of the hopper 12, are actuated for example by a corresponding number of electric motors, and are connected, at their upper end, to appropriate ducts 14, which are connected to the combustion chamber 7.
Below the combustion chamber 7, between the first wall 8 and the second wall 9, a box-like element 15 is provided, which is advantageously approximately shaped like a parallelepiped and hollow and is connected, by means of a sleeve 16, to the outside at the rear region of the boiler 1.
The box-like element 15 is open upward, toward the inside of the combustion chamber 7, and is closed thereat by a hearth 17, which is partially inserted therein and is advantageously approximately frustum-shaped and arranged so that its smaller end face is directed toward the bottom of the box-like element 15.
The hearth 17, made for example of steel or cast iron, has means for feeding primary air, which are constituted advantageously by first through holes 18 formed in the portion of the hearth 17 which lies inside the box-like element 15; the first holes 18 allow the passage of the primary combustion-supporting air required for the combustion of the pellets inside the hearth 17.
Behind the bottom of the hearth 17 there is an igniter 19, which is adapted to trigger the combustion of the pellets; said igniter is constituted advantageously by a tube 20 inside which an electric resistor 21 is arranged.
The boiler 1 is provided with means for feeding secondary combustion-supporting air into the combustion chamber 7, which advantageously comprises a first plate 22, preferably made of cast iron, which is arranged approximately vertically inside and to the rear of the combustion chamber 7 above the hearth 17 and is spaced from the first wall 8, so as to be struck directly by the flame generated by the combustion of the pellets.
The first plate 22 is approximately parallel and spaced with respect to the first wall 8 so as to form an interspace 23, which is open in a lower region and is connected to the outside of the combustion chamber 7.
The interspace 23 is connected to the inside of the combustion chamber 7 through second holes 24 formed in the first plate 22.
Ribs 8a are associated with the first wall 8 and with the plate 22, are directed toward the combustion chamber, at the interspace 23, form a finned structure, and are adapted to increase the heat exchange surface of the plate 22 in contact with the combustion gases and the flames in the combustion chamber, heating by conduction the air that is present in the interspace 23.
The ribs 8a are arranged radially and diverge upward at the plate 22.
A second plate 25 protrudes from the second wall 9 above the first opening 5 and toward the inside of the combustion chamber and is provided with a first inclined portion and a second vertical portion, which is arranged so as to affect approximately half of the space provided between the facing heat exchanger 29 and the second wall 9.
The second plate 25 delimits, within the combustion chamber 7, an inner chamber 7a, which is comprised between the heat exchanger 29 and the second plate 25, and an outer chamber 7b, which is comprised between the second plate 25 and the second wall 9.
The inner chamber 7a and the outer chamber 7b are insulated hermetically from each other by the second plate 25.
The outer chamber 7b is connected to the outside environment through second slots 26, which are formed in the second wall 9 and face the first slots 10.
Outer fins 27 and inner fins 28, which are conveniently welded, protrude approximately at right angles thereto from the two surfaces of the second plate 25, which are directed respectively toward the second wall 9 and toward the heat exchanger 29.
Advantageously, the outer fins 27 and the inner fins 28 are made of a material which is a good heat conductor, of the metallic type.
Above the first rear wall 8 of the combustion chamber 7 there is the heat exchanger 29, which faces the second plate 25 and into which the gases produced by the combustion of the pellets are conveyed.
The heat exchanger 29 is constituted by a hollow hermetic enclosure 30, within which there is a plurality of channels 31, which are approximately vertical and advantageously parallel and connected to each other so as to form a coiled or labyrinth-like path for the combustion gases.
The combustion gases are conveyed into the channels 31 through at least one upper intake 35 and at least one first lower outlet 36, with which it is possible to associate means for the forced draft of the combustion gases, which are constituted advantageously by one or more first fans 37, the delivery of which is connected to a discharge tube 38, through which the combustion gases are expelled outside the boiler 1.
The enclosure 30 is provided, in a lower region, with a second intake 33 and, in an upper region, with a second outlet 34, so as to form a labyrinth-like path 32 for a heat transfer medium which is adapted to be used in a heating and/or hygienic-sanitary system of a building.
Fins 31a protrude from one or more of the internal walls of the channels 31, approximately at right angles thereto, are arranged mutually side by side, and are aligned along axes which are transverse and/or longitudinal with respect to the channels 31, so as to form an alveolar structure.
The part of the fins 31a arranged longitudinally is riveted to the contact surface so as to ensure a suitable and expected heat exchange.
Advantageously, said fins, being made of a metallic material which does not have a very high heat conductivity, although continuing to transfer heat to the heat transfer medium by conduction, become hot and burn the unburnt substances which otherwise deposit thereon, generating additional energy.
Further, the boiler 1 comprises means for the forced input of warm air into the environment, which are constituted advantageously by one or more ducts 40, which are connected, at a first end thereof, to the outer chamber 7b and, at a second end, to the delivery of a second fan 41, which is adapted to force air from the outside environment into the outer chamber 7b.
Operation is therefore as follows: with reference to the accompanying figures, after loading the pellets into the hopper 12, by pressing an appropriate ignition button, which is not shown in the accompanying figures, and is connected to an electronic control board, also not shown, the process for igniting the boiler 1 begins.
The screw feeders 13 start turning, conveying the pellets from the hopper 12, through the ducts 14, into the hearth 17.
At the same time, the electric resistor 21 of the igniter 19 begins to warm, and when it reaches a preset temperature, which is sufficient to trigger the ignition of the pellets, the first fans 37 start to operate, drawing combustion gases from the combustion chamber 7 or forcing them to pass through the channels 31 of the heat exchanger 29, and then discharging them, through the discharge tube 38, into the outside environment, producing in the combustion chamber 7 a partial vacuum with respect to the outside environment.
The partial vacuum draws air through the first holes 18 of the hearth 17 and the tube 20 of the igniter 19; said air, by making contact with the electric resistor 21, is heated and enters the hearth 17 at a very high temperature (typically between 500 and 600°C), such as to trigger the combustion of the pellets.
An appropriately provided temperature sensor, not shown in the accompanying figures, detects the ignition of the pellets and interrupts the heating of the electric resistor 21.
The flame generated by the combustion of the pellets in the hearth 17 rises, striking the first plate 22.
Since the combustion chamber 7 is in partial vacuum, air is drawn from the outside environment through the interspace 23 and the second holes 24, thus making contact with the first plate 22 and entering the combustion chamber 7 at a high temperature; other air at a high temperature is thus introduced in the combustion chamber 7 and improves the combustion of the pellets, reducing the percentage of any unburnt substances and increasing the thermal efficiency of the boiler 1.
The combustion gases, both due to the natural draft and due to the action of the first fans 37, rise along the combustion chamber, striking the inner fins 28, which, being heated by the contact with said combustion gases, transfer part of the absorbed heat to the outer fins 27; it is therefore possible to start the second fan 41, which forces through the ducts 40 air so that it enters the outer chamber 7b and then exits into the outside environment through the second slots 26 and the first slots 10, after being heated by contact with the outer fins 27.
From the combustion chamber 7, the combustion gases then enter the channels 31 of the heat exchanger 29, flowing through it completely until they exit from the discharge tube 38.
In the channels 31, the combustion gases release by conduction part of the heat they contain to the heat transfer medium which circulates along the path 32.
The execution of the channels is such as to lead to a substantial overheating of the fins and therefore to very high temperatures, which cause the combustion of any particles of unburnt substances conveyed by the combustion gases; said particles are therefore burned, becoming ash, which can be removed periodically, with rather long periods between one cleaning intervention and the next.
The amount of heat sent to the heat transfer medium depends on the amount of fuel burned in the combustion chamber, and this entails a variation of the flow rate of the combustion gases through the channels 31.
If one wishes to transfer more heat to the heat transfer medium, it is sufficient to increase the flow rate of the fuel in the combustion chamber, by increasing the rotation rate of the screw feeders 13, and the speed of the first fans 37, so that the stoichiometric ratio between the fuel and the oxidizer remains unchanged.
The combustion gases exit from the discharge tube 38 completely free from unburnt particles and at relatively low temperatures (typically below 160°C), and therefore can be introduced directly into the atmosphere.
It has thus been found that the invention has achieved the intended aim and objects, a pellet boiler having been devised which, by recovering almost all the heat contained in the combustion gases generated by the combustion of the pellets, allows to achieve high energy efficiency and therefore can heat even large spaces and can also heat simultaneously a sufficient amount of water for hygienic-sanitary uses.
Further, the boiler according to the invention, thanks to the presence of the heat exchanger and to the particular arrangement of the channels 31 having a very specific heat exchange coefficient which reach high temperatures and burn any unburnt particles present in the combustion gases, allows to minimize the deposition of said particles on the walls of the path 32 affected by the heat transfer medium, thus reducing the required cleaning and maintenance interventions and increasing the lifespan of the boiler.
Moreover, the invention, in view of its structure, ensures the possibility of introducing greater or smaller quantities of heated air depending on the requirements of the user.
The invention is of course susceptible of numerous modifications and variations, all of which are within the scope of the same appended claims.
The materials used, as well as the dimensions which constitute the individual components of the invention, may of course be more pertinent according to specific requirements.
The various means for performing certain different functions need not certainly coexist only in the illustrated embodiment but can be present per se in many embodiments, including ones that are not illustrated.
The characteristics indicated as advantageous, convenient or the like might also be omitted or be replaced with equivalents.

Claims

A pellet boiler (1), comprising:
a combustion chamber (7) which is delimited at the rear by a first wall (8) and at the front by a second wall (9) and contains in a lower region a hearth (17),

means (16,18) for feeding primary combustion-supporting air from outside into said combustion chamber (7),

a heat exchanger (29), which is arranged above said combustion chamber (7) for the recovery of the combustion gases and for a heat transfer medium,

a hopper (12) for said pellets, said hopper (12) being provided in a lower region with means (13) for feeding said hearth (17) and, in an upper region, with a lid (100) which can be removed and opened laterally,

means (41) for the forced input of heated air into the environment,

a frame which is composed of a footing (2), from the sides of which there protrude two uprights (3) and a front wall (4) in which there is a first opening (5) to which a door (6) is hinged for accessing said combustion chamber (7) which is delimited at the rear by said first wall (8) and at the front by said second wall (9), between which there is a hollow box-like element (15), which is connected to the outside and is closed in an upper region by said hearth (17), which has, in a lower region, first holes (18) for the passage of the primary combustion-supporting air required for the combustion of said pellets,

the boiler (1) being characterized in that it further comprises:
means, (22,23,24) for feeding secondary combustion-supporting air from outside into said combustion chamber (7), which comprise an interspace (23) arranged above said hearth (17) and adjacent to said first wall (8), and a first plate (22), which is arranged approximately vertically inside and behind said combustion chamber (7) above said hearth (17) and is spaced from said first wall (8) so as to be struck directly by the flame generated by the combustion of said pellets, said first plate (22) being arranged approximately parallel and spaced with respect to said first wall (8) so as to form said interspace (23), which is open downward and is connected to the outside of said combustion chamber (7), said interspace (23) being connected to the inside of said combustion chamber (7) through second holes (24) formed in said first plate (22),

ribs (8a) which form a finned structure being associated with said first wall (8) and with said plate (22) and being directed toward said combustion chamber (7), at said interspace (23), said ribs (8a) being adapted to increase the heat exchange surface in contact with the combustion gases and to exit into said combustion chamber (7), heating by conduction the air that is present in said interspace (23), said ribs (8a) being arranged radially and diverging upward at said plate (22),

a second plate (25) protruding from said second wall (9) above said first opening (5) and toward the inside of said combustion chamber (7) and being provided with a first inclined portion and with a second vertical portion which is arranged so as to affect approximately half of the space provided between said facing heat exchanger (29) and said second wall (9), said second plate (25) delimiting, within said combustion chamber (7), an inner chamber (7a), which is comprised between said heat exchanger (29) and said second plate (25), and an outer chamber (7b), which is comprised between said second plate (25) and said second wall (9), said internal and external chambers (7a,7b) being hermetically insulated from each other by said second plate (25), said outer chamber (7b) being connected to the outside environment through second slots (26) which are formed in said second wall (9) and face first slots (10) which are formed proximate to the upper end of said front wall (4),

said heat exchanger (29) being arranged to the rear of said combustion chamber (7), in a region which lies above said first wall (8) of said combustion chamber (7) and faces said second plate (25), said heat exchanger (29) being constituted by a hollow hermetic enclosure (30), within which there is a plurality of channels (31) which are approximately vertical and advantageously parallel and connected to each other so as to form a coiled or labyrinth-like path for the combustion gases, said combustion gases being conveyed into said channels (31) through at least one first upper intake (35) and at least one first lower outlet (36) associated with means for the forced draft of the combustion gases, which are constituted by one or more first fans (37), the delivery of which is connected to a discharge tube (38) through which said combustion gases are expelled externally,

said hopper (12) being arranged adjacent and to the rear of said heat exchanger (29), and

said means for the forced input of warm air into the environment being constituted by one or more ducts (40), which are connected, at a first end, to said outer chamber (7b) and, at a second end, to the delivery of a second fan (41), which is adapted to force air from the outside environment into said outer chamber (7b).
The boiler (1) according to claim 1, characterized in that outer fins (27) and inner fins (28) protrude respectively, approximately at right angles thereto, from the two surfaces of said second plate (25) which are directed respectively toward said second wall (9) and toward said heat exchanger (29), said fins (27,28) being appropriately welded and made of a material which is a good heat conductor.
The boiler (1) according to claim 1, characterized in that said enclosure (30) is provided, in a lower region, with a second intake (33) and, in an upper region, with a second outlet (34) so as to form a labyrinth-like path for a heat transfer medium which is adapted to be used in a heating and/or hygienic-sanitary system of a building.
The boiler (1) according to claim 3, characterized in that fins (31a) protrude from one or more of the internal palls of said channels (31), approximately at right angles thereto, are arranged mutually side by side and are aligned along axes which are transverse and/or longitudinal with respect to said channels (31) so as to form an alveolar structure, the part of said fins (31a) which is arranged longitudinally being riveted to the contact surface so as to ensure a suitable and expected heat exchange.
The boiler (1) according to one or more of the preceding claims, characterized in that said hopper (12) for said pellets, is hollow and can be closed by means of said lid (100) which can be opened laterally, said lid (100) being hidden from sight by a door (101) which is hinged to said boiler (1), one or more screw feeders (13) being provided at the bottom of said hopper (12) and being connected, at their upper end, to appropriately provided ducts (14) which are connected to said combustion chamber (7).
The boiler (1) according to one or more of the preceding claims, characterized in that an igniter adapted to trigger the combustion of said pellets is arranged to the rear of the bottom of said hearth (17), said igniter being constituted by a tube (20) which protrudes externally with respect to said combustion chamber (7) and inside which there is an electric resistor (21).