FI119891B - Hearth - Google Patents

Description

HEARTH

The invention relates to a fireplace, which includes a firebox for burning solid fuel and generating fire gases, and a heat accumulator for storing the heat of the flue gases in the fireplace.

Wood-burning fireplaces, such as stoves, fireplaces and fireplaces, generally have a problem with poor thermal efficiency. For example, in a continuous-burning stove, the heat energy released from burning wood is primarily transmitted by irradiating the stove walls to the stove stones. In other words, the flue gases first heat the walls of the furnace and the flue gases have no direct contact with the heater stones. Thus, in the transfer of thermal-15 energy, the convection rate is low, especially when the contact surfaces of the heater stones to the furnace walls are small and the heat conductivity of the heater stones is low. It is mainly for these reasons that the thermal efficiency of a continuously heated stove is very low. The efficiency is estimated to be less than 20%.

20

There are also so-called disposable heaters, where the efficiency of heat transfer is substantially better than that of continuous heaters. In a single-heated heater, the flue gases pass through the heater stones, whereby the flue gases are in direct contact with the heater stones. The heat energy is then transferred to the sauna stones by both convection and radiation. However, the thermal efficiency remains low, estimated at less than 40%, for a variety of reasons. First, the flue gas flow is unevenly distributed across the heater rocks. In addition, the area in contact with the flue gases in the sauna stones is relatively small. The thermal conductivity of sauna stones is also poor. Further, the thermal efficiency is reduced by the way the disposable heater is used. The charcoal must be burned down before starting the sauna to avoid the formation and spread of carbon monoxide gases in the sauna-35 space. As the charcoal fades, the temperature of the flue gases passing through the single-heated heater drops and at the same time the heater begins to cool down. Similar problems occur in other wood fireplaces 2. In addition, due to the low thermal efficiency, a large amount of fuel must be used to heat the fireplace. This causes unnecessarily high emissions of carbon dioxide, carbon monoxide, hydrocarbons, nitrogen oxides and particulates into the air. In one known single-fired heater, the manufacturer recommends using 10 to 20 kg of dry firewood per heating cycle.

The object of the invention is to provide a newcomer. Characteristic features of this invention are apparent from the appended claims. The fireplace according to the invention employs a new type of heat storage structure which utilizes heat more efficiently than before. In practice, the fireplace becomes warmer faster than before with less firewood. At the same time, emissions are significantly reduced.

The invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional and side view of a fireplace according to the invention,

Figure 2 is a front elevation view of the fireplace according to the invention, Figure 3 is a top view of a portion of the front heat accumulator according to the invention,

Figure 4 shows a part of the front heat accumulator according to the invention in three-dimensional view.

Figures 1 and 2 show an example of a fireplace according to the invention as a disposable heater. Generally, the hearth includes a furnace 10 for burning solid fuel and for generating combustion gases. Usually firewood is used as fuel. In addition, the fireplace includes a heat accumulator 11 for storing the heat of the combustion gases 35 in the fireplace. According to the invention, the fireplace 3 further comprises a front heat accumulator 13 which is arranged between the furnace 10 and the heat accumulator 11 in the direction of flue gas flow. In addition, the front heat accumulator consists of a metallic cell 14 having a heat storage capacity of 11 to 5 pi. In this way, the heat energy of the hot flue gases can be trapped as efficiently as possible in the pre-heater, whereupon the heat is released into the heater. At the same time, the thermal efficiency of the fireplace is substantially improved, thereby reducing carbon dioxide emissions as the fuel required is reduced. In other words, the energy contained in the fuel can be utilized more efficiently. This gives more heat with the same amount of fuel. Alternatively, the same heat can be obtained with less fuel.

15 The sample heater is rectangular and 450 mm x 450 mm. The height of the heater is 800 mm. Below the furnace 10 is a grate 15 which is wide and long enough to provide sufficient oxygen for combustion. Underneath the grate 15 is an ashtray 16. According to the invention, the volume of the furnace is substantially equal to or greater than the volume of solid fuel needed to charge the fireplace. In this case, the amount of firewood needed to heat the fireplace can be freely contained in the furnace at one time. The exemplary stove has a spacious furnace 10 having a width and height of 300 mm and a depth of 450 mm. In addition, the furnace edges 25 also have side panels 17 limiting the width of the combustion chamber, which prevent the spread of firewood. From the outside of the side panels and below the grate, air can flow under the firewood and into the sites. The air can also get above the firewood to provide the 30 oxygen needed to burn the hydrocarbons that are evaporated from the firewood.

The spacious furnace allows efficient combustion of the fuel. Further, the bottom surface area of the furnace 10 is preferably larger than the surface area of the heat accumulator 11 and the front heat accumulator 13. In this case, the combustion gases 35 are uniformly distributed and at the same time the front heat accumulator is heated throughout. The exemplary heater also has a combustion chamber 19 confined between the furnace 10 and the front heat accumulator 13 in the flow direction of the combustion gases 4. The wall 18 directs the combustion gases into the combustion chamber 19, which may be fed through a perforated plate 20

5

The sauna heater has a closed damper (not shown) during bathing. To speed up the initiation of the sauna, the heater includes shut-off means 21 for closing the flow connection from the furnace 10 to the combustion chamber 19. The flow connection ίο from the furnace can then be closed. In addition, the fireplace includes a by-pass channel 22 starting from the furnace 10 and adapted to bypass the heat accumulator 11 and the front heat accumulator 13. At the same time, unnecessary cooling of heat reservoirs is avoided. In the embodiment shown in Figure 1, the bypass passage 22 is open with the closing means 21 closed.

In the heating step, the closing means 21 shuts off the flow of combustion gases into the bypass passage 22 and discharges the combustion gases through the combustion chamber 20 19 into the front heat accumulator 13 and thereafter through the heat accumulator 11 and the upper valve 26 to the flue 23. When there is no more charcoal left in the furnace, the passage through the stove is closed and the by-pass from the furnace to the chimney is opened. In this case, the 25 charcoal may burn off completely without cooling the heat reservoir. At the same time, bathing can be started immediately without the risk of diffusion of carbon monoxide gases into the sauna areas. When the thermometer 25 indicates that the heater is in bathing condition, the closing means 21 are turned to a position whereby they close the flue gas flow 30 into the combustion chamber 19 and at the same time open the flow to the bypass duct 22. The heater is now bath ready. At the beginning of the sauna, the steam door 24 is opened and at the same time the upper valve 26 is closed, which prevents the flue gases from flowing into the flue 23. It takes about half an hour to heat the stove to ready for bathing.

35 5

The structure of the cell may vary from application to application. In any case, the cell 14 is formed by a plurality of flow channels 27, whereby the heat transfer area can be clearly increased. Preferably, the flow passages are criss-crossed, which enhances the mixing of the combustion gases 5 and contributes to the improvement of the thermal efficiency. The hearth according to the invention contains an efficient and compact frontal heat accumulator. A preheater is, for example, a cell stacked or wrapped in corrugated steel or cast iron plates placed immediately after the furnace. The cell can be made of a heat-resistant sheet of steel sheet which is corrugated, for example, at an angle of 70 degrees to the longitudinal direction of the web. Generally, each flow channel 27 is delimited by one or more plate-shaped profiled bodies 28, whereby the relationship between the flow cross-sectional area and the heat-retaining mass is achieved. Figure 3 15 shows, in part, four cast iron profile pieces 28, which restrict a plurality of flow channels 27. The cell may also be made of curved web shaped molded profile pieces. The profile pieces are positioned so that the ridge angles of the waves are in opposite directions. This results in a structure having a ductwork intersecting with each other. In this case, the flue gas flow is efficient and the flue gases are effectively mixed. Figure 4 shows two overlapping profile pieces 28 with ridge angles of profiles in different directions.

If necessary, the profile pieces can be coated with, for example, an oxidizing catalyst. In this case, the cell acts as a catalyst, which further significantly reduces toxic emissions such as carbon monoxide and hydrocarbons.

It is also important for the operation of the fireplace according to the invention that the furnace is properly designed. The furnace should be large enough to accommodate the amount of wood needed for heating at one time. It is also essential to ensure that the combustion oxygen is sufficiently available in the combustion chamber 35 so that the hydrocarbons vaporizing from the firewood can burn properly. This further raises the temperature of the combustion gases, which is necessary to minimize hydrocarbon and carbon monoxide emissions. Due to the space limitations of the heaters, the residence time of the combustion gases in the combustion chamber is short, about 0.2 seconds. In this case, a complete combustion requires temperatures above 800 ° C. If necessary, the combustion chamber can be insulated from the inside with a heat-resistant insulation, such as mineral wool insulation, to achieve a sufficient temperature 5. At the same time, the insulation reduces the temperature of the outer surface of the fireplace.

ίο Efficient combustion reduces hydrocarbon, carbon monoxide and particle emissions. The fireplace according to the invention also reduces carbon dioxide emissions by improving the thermal efficiency of the fireplace with a preheater and by-pass. In this case, considerably less firewood is needed to heat the fireplace. For example, in the stove application, as an additional advantage, a significantly faster warm-up is also achieved.

Preferably, the fireplace according to the invention has a preheater stored or wrapped in corrugated steel sheets immediately after the furnace 20. However, a sufficient combustion space is provided before the preheater. The aforementioned ducts have a free gas passage between the flow ducts. This allows the flue gases to pass sideways if the gas pressure is unevenly distributed within the front heat accumulator. If any part of the cell has a flow greater than 25, a higher back pressure is created which automatically presses the flue gases to travel partially sideways. In this case, the flow and at the same time the temperature distribution in the preheater is equalized. This improves the efficiency of the preheater and reduces the pressure loss caused by the cell.

30

The ducts of the cells typically have a laminar flow. In this case, the flow velocity in the middle of the flow passage is highest and decreases to zero at the wall surface. A so-called immovable boundary layer is also known, which forms an insulator 35 which impairs heat transfer. In the cell according to the invention, the flow channels are criss-crossed and as a result, each flow channel is continuously shrinking and expanding 7. This prevents the formation of a stationary boundary layer and substantially improves heat transfer from the combustion gas to the front heat reservoir.

5 The relative difference between the shapes of different flow channels is illustrated by the so-called Nusselt number (Nu). Nusselt's figure can be used to describe surface heat transfer relative to a situation where only heat conduction would occur. For example, the Nusselt number for a square flow channel is about 3.0 at a flow rate of 2 m / s. Correspondingly, for the mixing flow channel of the invention, the Nusselt number is about 12. In other words, heat is transferred to the heat reservoir four times more efficiently than in a square flow channel.

The above example compares well-structured structures with fairly uniform flow distributions. In a sauna stove application, the size of the flow channels between the sauna stones varies greatly. Then the flue gases flow through the smallest resistor. In other words, the flue gases flow from the largest openings. The back resistance of the Vir-20 is proportional to the other power of the orifice hydraulic diameter, i.e., the orifice cross-section. In this case, the flow of flue gases through the smallest gaps is negligible and heat transfer to the sauna stones is inefficient. At the same time, the heater stones are heated unevenly.

25 Another important factor for heat transfer is the heat transfer area. The heat transfer is directly proportional to the heat transfer surface. In a plate-like structure, this area is large compared to, for example, charging furnaces. Typically, it may be from 0.05 to 1.5 m 2 for each charge cubic decimeter. It is much more than the heater stones and the walls of the charging furnaces, typically 0.005 to 0.05 m2 / dm3.

The thermal conductivity of the stones used in the furnaces is about 6.0 - 6.5 35 W / mK. The cast iron has a thermal conductivity of about 40 - 45 W / mK. In other words, the thermal conductivity of cast iron is 6-7 times that of stone 8. In the case of heat transfer primarily by convection, the heat transfer is proportional to the temperature difference between the gas and the heat transfer surface. When heat is efficiently transferred from the surface to the inside of the accumulator material, the temperature difference remains as large as possible.

The exemplary metallic cell heats up quickly above the flame to very high temperatures, for example 700 ° C due to its good heat transfer properties, ίο This is a large energy charge, even though the iron's specific heat capacity is relatively low, about 0.5 kJ / kgK. The heat can be utilized to continuously heat the sauna stones above the front heater during bathing. In heaters, heat is used to heat convection air or accumulators. 15

One advantage of the metallic honeycomb preheater is the ability to collect combustion soot due to mixing flow and thermophoresis. The soot adhering to the surface of the flow channels burns when the surface temperature reaches 600 ° C.

20

The advantages of stone, brick or other ceramic heat storage material are its cheap price, good heat storage capacity (about 1 kJ / kgK) and, on the other hand, relatively poor thermal conductivity and transfer ability compared to the metal structure. Thus, the fireplace according to the invention 25 unexpectedly combines two different heat accumulators and thus takes advantage of both. High heat transfer capacity of the metal front heat accumulator and high storage capacity and slow heat release of the ceramic heat accumulator. The heater according to the invention is also advantageously shallow, whereby it also heats the lower parts of the sauna room.

The amount of wood needed to heat the stove is as follows: 35 1. Cast iron accumulator (size 150 * 450 * 450 mm) - Volume = 1.5 dmx 4.5 dmx 4.5 dm = 30.4 dm3 9 - Weight = 78 kg

- specific heat capacity = 0,5 kJ / kg ° K

- charge energy 0 - 700 ° C (steam range 100 - 700 ° C)

- Qkok = 700 x 0.5 X 78 = 27.3 MJ

5 - energy for steam in the accumulator:

Qiöyiyv = 600 x 0.5 x 78 = 23.4 MJ

2. Stoves - stone volume = 2.5 dm x 4.5 dm x 4.5 dm = 50.6 dm3 ίο - bulk density of soapstone = 2.98 kg / dm3 - fill density = 50% - amount of stones = 0.5 x 2, 98 x 50.6 = 75 kg

- specific heat capacity = 1 kJ / kgK

- heating of sauna stones 0 - 300 ° C

15 - charge energy = 300 x 1 x 75 = 22.5 MJ

- Energy used for steam in sauna heaters:

Qlöylyk = 200 X 1 X 75 = 15 MJ

3. Energy available for steam recovery:

20 - Qlöyly = 23.4 MJ + 15 MJ = 38.4 MJ

4. Energy required to heat the stove

Qito ityB = 27.3 MJ + 22.5 MJ = 49.8 MJ

25 5. The amount of wood needed for heating - the heat value of the wood is about 19 MJ / kg - the thermal efficiency is about 70% - the amount of wood = 49.8 MJ / (0.7 x 19 MJ / kg) = 3.7 kg of dry wood 30 6. Steam capacity for water

- specific heat capacity of water = 4,2 kJ / kgK

- water evaporative heat = 2260 kJ / kg - energy needed to heat water at 10-100 ° C and evaporation = 90 ° C x 4.2 kJ / kgK + 2260 kJ / kg = 2638 kJ / kg 35 - loss = energy consumption other than water for steaming = 20% 10 - steaming capacity of the stove = 0.8 x 38400/2638 kJ / kg = 11.5 kg water

Based on the above calculation, 5 kilns of dry wood (3.7 kg) can be used to heat the sauna stove to a sauna. In addition, 11.5 liters of water can be used for steam extraction. The sauna room is ready for bathing in about half an hour, while heating in known one-time heaters takes from one to two hours. In addition, firewood consumes only one fifth of the known heaters ίο. Fast burning can be ensured by using smaller and dry firewood than usual. For example, the size of the firewood could be 4 x 4 x 35 cm3 with a large firing area. In this case, the flame temperature is high, which further contributes to the complete combustion of the flue gases.

15

As an alternative to the traditional furnace, a new type of movable fire chamber, consisting of a grate with sides, can be used. The firebox is pulled out of the furnace's firebox along the sliding rail and firewood and lighters are loaded into the firebox. After this, the firewood is ignited and the firebox pushed back into place. Ignition can be done similarly on the sides and on the top. The lower part of the fire chamber acts as a grate with transverse ribs at the vertical supports, which allow a corresponding gap between the grate surface and the trees to ensure good air circulation. In the stove application, the heater stones forming the heat accumulator are stacked on the front heat accumulator. After the heating step, the lower metal front heater is significantly hotter than the upper heater. When bathing, this means a soft steam, because the heater stones 30 are not too hot. On the other hand, the heater stones will stay suitably warm for a long time, because the water-cooled heater stones receive additional energy from the hot metal preheater.

Claims (9)

1. A fireplace, which comprises - a combustion chamber (10) for combustion of solid fuel and 5 for the generation of fire gases, and - a heat accumulator (11) for storing the heat of the fire gases in the fireplace, characterized in that the fireplace further comprises a preheat accumulator ( 13), which is arranged in the direction of flow of the fire gases between the combustion chamber (10) and the heat accumulator (11), and which consists of a metal package (14) of metal, whose heat storage capacity is greater than the thermal efficiency of the fireplace (11). and thus, carbon dioxide emissions are reduced as the required amount of fuel is reduced. 15 Fireplace according to claim 1, characterized in that the cell package (14) consists of several flow channels (27) which intersect. Fireplace according to claim 2, characterized in that each flow channel (27) is limited by one or more disc-shaped profile pieces (28). Fireplace according to claim 3, characterized in that a catalytic coating is provided on the profile pieces (28). 25 5. Fireplace according to any of claims 1-4, characterized in that the bottom surface of the combustion chamber (10) is larger than the surface of the heat accumulator (11) and the preheat accumulator (13). 6. A fireplace according to any of claims 1-5, characterized in that in the direction of flow of the flue gases between the combustion chamber (10) and the preheat accumulator (13) there is a combustion chamber (19) bounded by a wall (18). The fireplace according to claim 6, characterized in that the fireplace comprises shut-off devices (21) for closing the flow connection from the combustion chamber (10) to the combustion chamber (19). 5 8. Fireplace according to claim 7, characterized in that the fireplace comprises an overflow duct (22) starting in the combustion chamber (10) and is arranged to pass both heat accumulator (11) and preheat accumulator (13), which duct is open when the shut-off devices (21) are closed. 9. A fireplace according to any one of claims 1-8, characterized in that the fireplace is a sauna oven, in which the heater blocks (12) constituting heat accumulator (11) have been placed on top of the preheat accumulator 15 (13).