EP0359306A2

EP0359306A2 - Boiler

Info

Publication number: EP0359306A2
Application number: EP89202169A
Authority: EP
Inventors: Marcel Castermans
Original assignee: "STELRAD IDEAL"
Current assignee: "STELRAD IDEAL"
Priority date: 1988-09-13
Filing date: 1989-08-28
Publication date: 1990-03-21
Also published as: EP0359306A3; BE1002487A6

Abstract

Boiler with heat transfer by convection between a heat source and a profiled heat exchanger (3), characterized in that the surface of the profile applied to the heat exchanger and over which the heat transfer takes place, increases with increasing distance from the heat source. Preferably, the profile applied to the heat exchanger is formed by fins, the surface of which increases the further the fins are remote from the heat source.

Description

The invention relates to a boiler with heat transfer by convection between a heat source and a profiled heat exchanger.
Such boilers are generally used in central heating installations. The heat source, usually a gas or oil burner, generates heat which warms up a transfer medium, generally the flue gases of the burned gas or oil. That transfer medium then transfers its absorbed heat to a liquid, usually water, by means of a heat exchanger. The flue gases flow along the outer surface of the heat exchanger and transfer their heat by convection to the metal, usually cast iron, of the heat exchanger which transfers then, on its turn, the absorbed heat by convection to the water. The surface of the heat exchanger is usually profiled, for instance provided with fins if the boiler is made of cast iron, in order to increase the effective surface of the heat exchanger.
A drawback of the known boilers consists in that the profiling of the heat exchanger is substantially identical over its whole surface. The temperature gradient of the transfer medium over the heat exchanger forms an exponentially decreasing function as a result of which the part of the heat exchanger, which is first brought into contact with the transfer medium, will absorb the largest quantity of heat whereas the part the furthest remote from the heat source, will absorb the least quantity of heat. All this results in an unequal heat transfer over the whole surface of the heat exchanger. That unequal heat transfer on its turn causes stresses in the material of the heat exchanger. Furthermore, there also arise larger losses in the heat exchange as a result of said unequal heat transfer.
The object of the invention is to realize a boiler wherein a substantially equal heat transfer takes place over the surface of the heat exchanger.
To this end, a boiler according to the invention is characterized in that the surface of the profile applied to the heat exchanger and over which the heat transfer takes place, increases with increasing distance from the heat source. Since the temperature of the transfer medium is higher in the vicinity of the heat source then remote from the heat source, the smaller surface near the heat source will absorb less heat at a higher temperature and the larger transfer surface remote from the heat source will absorb more heat at a lower temperature. As a result thereof, the heat transfer per surface unit is substantially constant when further parameters, such as for example the speed of the transfer medium, remain constant and no stresses are created in the material of the heat exchanger. Due to the equal heat transfer, the maximum thermal load of the heat exchanger is lowered and a smaller heat exchanger can be used for a transfer of an equal amount of heat which constitutes an economic value.
A first preferred embodiment of a boiler according to the invention wherein the surface of the heat exchanger is provided with fins, is characterized in that the surface of the fins increases the further the fins are remote from the heat source. As a result thereof, the fins which are situated nearby the heat source have a smaller surface than the ones which are situated further from the heat source and the heat transfer per fin is substantially constant if other parameters remain of course constant.
A second preferred embodiment of a boiler according to the invention is characterized in that the surface increase of the fins is mainly realized by an increase in length of the fins. In this way, the increase in surface can be realized easily.
A third preferred embodiment of a boiler according to the invention is characterized in that the fins are serially disposed in series each time of substantially parallel fins,wherein each of the fins of a same series has substantially a same surface, and in that the surface of the fins of successive series increases with increasing distance from the heat source. The heat transfer is hereby optimized.
Advantageously, the increase in length between two successive fins in a direction away from the heat source, is mainly determined by the total number of fins of a different length on the same heat exchanger. Hereby, constructional limitations are taken into account and manufacturing costs are reduced.
A further preferred embodiment of a boiler according to the invention is characterized in that the profile of the fin is mainly determined by the length of the fin. This results in a simple determination of the profile taking into account the length of the fin.
The invention will now be further described on the hand of the drawing. The invention is however not limited to the examples shown in the drawing. It will be clear that several variants are possible within the scope of the invention. In the drawing :

Figure 1 shows a section of a boiler according to the invention.
Figure 2 shows a graph of the temperature gradient of the transfer medium over the surface of the heat exchanger.
Figure 3 shows an example of a profile of a fin.
Figure 4 shows a longitudinal section of a further type of heat exchanger according to the invention.

The boiler shown in Figure 1, comprises a connection 1 for supplying a fuel, in the chosen example natural gas, which connection is connected to an atmospheric burner 2 operating as a heat source.
It will be clear that the invention is also applicable to boilers provided with non-atmospheric burners such as pressurized boilers. Furthermore, fuel oil is also suitable as fuel.
The boiler is further provided with a heat exchanger 3 the outer surface of which is provided with series of fins (4-10). A liquid, for example water, circulates along the inner surface of the heat exchanger. The cold water is supplied through a pipe coupling 11 and the heated water is outputted through a pipe coupling 12.
Besides the primary air, which is supplied in addition to the fuel, there is also supplied from underneath secondary air 13 to incinerate the fuel. Due to the combustion of the fuel there is formed a transfer medium, for example flue gases which flow along the fins towards the chimney. The heat stored in the flue gases is transferred to the fins by convection, which fins transfer the thus absorbed heat, again by convection, to the water.
The heat capacity of a fin, that is the amount of heat which can be absorbed by a fin, is mainly determined by the dimension of the fin and the material of which the fin is made at least if other parameters of the transfer medium remain constant. The larger the surface of the fin, the more heat the fin can absorb, respectively supply, at least within certain limits. In the heat exchanger as shown in Figure 1, the fins 4 of the first series which is situated the nearest to the heat source, have the smallest surface. The surface of the further series of fins (5-10) increases the further the fins are remote from the heat source. The fins which are situated more closely to the heat source thus have a smaller heat capacity than the ones which are remotely situated since they have a smaller surface.
The temperature of the flue gases is however the highest near the heat source and this temperature decreases due to heat transfer to the fins and due to further losses the more the flue gases are remote from the heat source. The curve 14, shown in Figure 2, illustrates the decrease in temperature of the flue gases as a function of the distance over the heat exchanger (vertical y-direction: temperature of flue gas; horizontal x-direction : distance over the heat exchanger).
The curve shows for example that, at the height of the first series of fins 4, the temperature is 1400°C whereas the temperature is for example 270°C at the height of the last series of fins 10.
The geometry of the heat exchanger is as such that its cross-section decreases with increasing distance from the heat source. This geometry is thus chosen because a volume change caused by the temperature decrease of the transfer medium, occurs in the transfer medium. The density of the transfer medium increases as the temperature and the volume of the latter decreases. However, since the cross-section of the heat exchanger decreases with increasing distance from the heat source, the speed of the transfer medium remains substantially the same as a result of which no disturbing of the heat transfer is caused.
The combination of the temperature fall of the flue gases and the increase in heat capacity of the fins the further they are remote from the heat source causes now that the heat transfer between flue gases and fins is approximately constant over the whole surface of the heat exchanger. Indeed, the fins 4 with a small heat capacity absorb heat only during a short time, at a high temperature whereas the fins 10 with a larger heat capacity are absorbing heat during a longer time, at a lower temperature. As a result thereof, the total amount of heat supplied to each of the fins is substantially constant (ΔQ = constant) so that also the heat transferred to the water is substantially constant.
The heat exchanger is now submitted to a temperature gradient the absolute value of which is, on the one hand, much smaller than the maximum temperature gradient in a conventional heat exchanger and which remains, on the other hand, approximately constant over the whole length of the heat exchanger. Stresses in the material of the heat exchanger are thereby avoided. Even if there are some changes over the surface, the fluctuations remain nevertheless limited. The fact that stresses in the material are avoided results in that less material is needed since it is no longer necessary to compensate those stresses with additional material.
The fin length as well as the total number of different fin lengths, is mainly determined by :
- the temperature difference between the flue gases on the hot and on the cold side of the heat exchanger.
- the length of the heat exchanger as determined in the flow direction of the flue gases.
- the desired optimalization degree.
- the constructional limitations.
In order to determine for each series of fins each time the length of the fins from that series, there is firstly determined, in function of the optimalization degree, how many series of fins there will be applied to the heat exchanger. Suppose now, as an example, that one wishes to apply four series of fins on the outer surface of the heat exchanger. In order to determine the length of the fins from each series, there is started from the curve 14 represented in Figure 2. Now four points A, B, C and D are determined on this curve since four series of fins are needed. These are determined by dividing the surface delimited by the curve 14 and the coordinate system, into four approximately equal parts. Point A, is then chosen in such a manner that the surface of the rectangle OVTR is approximately equal to the surface of the plane OVSP. The temperature belonging to this point A, in this example 1250°C, corresponds then approximately to the average temperature of the flue gases over the part OV of the heat exchanger. The length of the fin is now given by the distance OV. An analogous procedure is now followed to determine the points B, C and D and in this way to determine the length of the fins WL, MN and EF. The surfaces OVTR, WLZK, MNJI and EFHG are all approximately equal to each other so that the heat transfer for each of the fins is approximately the same.
It will be clear that if one wants to refine the optimalization degree, the number of fins increases. However, this number cannot increase unlimited and is subjected to constructional limitations.
When the length of the fins has been determined for each series, then the profile of the fin is determined. Care is taken that the flue gases experience a minimum resistance and that they thus have an aerodynamic profile. In order to design an aerodynamic profile, use is for example made of studies of the NACA (National Advisory Committee for Aeronautics). Figure 3 shows an example of a profile of a fin. The length (c) is indicated in the x-direction whereas the y-direction indicates the height. Starting from the predetermined total length of the fin, the profile is calculated by determining each time for a given x-value (= fraction of the total length of the fin) a corresponding y-value.
Preferably, the fins from the successive series are disposed shifted with respect to each other so that an optimal heat transfer between flue gases and fins is realized at a minimum aerodynamic resistance.
Besides the use of fins to optimalize the heat transfer it is also possible to choose alternative solutions. So it is for instance possible to profile the surface of the heat exchanger as illustrated in Figure 4 wherein a longitudinal section of a part of a further type of heat exchanger is shown. The surface of the heat exchanger illustrated at that place is profiled in such a manner that the surface of the cames 15 to 19 applied thereto increases with increasing distance from the heat source. The effective surface over which the heat transfer takes place increases thus also with increasing distance from the heat source.

Claims

1. A boiler with heat transfer by convection between a heat source and a profiled heat exchanger (3), characterized in that the surface of the profile applied to the heat exchanger and over which the heat transfer takes place, increases with increasing distance from the heat source.

2. A boiler as claimed in claim 1 wherein the profile applied to the heat exchanger is formed by fins (4-10), characterized in that the surface of the fins increases the further the fins are remote from the heat source.

3. A boiler as claimed in claim 2, characterized in that the surface increase of the fins is mainly realized by an increase in length of the fins.

4. A boiler as claimed in claim 3, characterized in that the increase in length between two successive fins in a direction away from the heat source, is mainly determined by the total number of fins of a different length on the same heat exchanger.

5. A boiler as claimed in claim 4, characterized in that the length of a fin is each time further determined by the temperature decrease of a transfer medium through which the heat is transferred between the heat source and the heat exchanger over the contact surface of heat exchanger covered by that fin.

6. A boiler as claimed in claim 4, characterized in that said total number of fins of different length is predetermined.

7. A boiler as claimed in any one of claims 2 to 6, characterized in that the fins are serially disposed in series each time of substantially parallel fins, and wherein each of the fins of a same series has substantially a same surface, and that the surface of the fins of successive series increases with increasing distance from the heat source.

8. A boiler as claimed in claim 7, characterized in that the fins of successive series are each time shifted with respect to each other.

9. A boiler as claimed in any one of claims 2 to 8, characterized in that the profile of the fin is mainly determined by the length of the fin.

10.A heat exchanger for a boiler as claimed in any one of claims 1 to 9, characterized in that the surface of the profile applied to the heat exchanger and over which surface the heat transfer takes place, increases with increasing distance from the heat source.