GB2198506A

GB2198506A - Heating boiler with improved heat transfer

Info

Publication number: GB2198506A
Application number: GB08720641A
Authority: GB
Inventors: Kurt Bohm; Vage Dahl
Original assignee: ENERTECH VAERME AB
Current assignee: ENERTECH VAERME AB
Priority date: 1986-09-16
Filing date: 1987-09-02
Publication date: 1988-06-15
Anticipated expiration: 2007-09-02
Also published as: DE3730566A1; GB8720641D0; SE8603907L; GB2198506B; SE8603907D0; FI873975A0; DE8712344U1; DK480087A; DK480087D0; DE3730566C2; FI873975A; SE468365B

Description

1 HEATING BOILER WITH IMPROVED HEAT TRANSFER.

7n 1-06 219 8 f; The present invention relates to a heating boiler capable of burning a wide variety of gaseous, liquid or solid fuels, comprising a combustion chamber, the flue gas flow from which is in contact with at least one convection element for the transfer of heat from the flue gas to a heat absorbing medium, preferably a liquid, through a heat conducting wall over which, in use, flue gas flows.

Such heating boilers are manufactured in several embodi- ments. The convection elements can be formed of sections of cast iron or be made up from metal plates. In order to transfer heat from the hot flue gases to the heat absorbing liquid through a heat conducting wall, the heat must first be 1= transferred by convection to the wall. Technically it is a 2 matter of a heat transfer coefficient with the value W/mOC (watts/square metre/'C). The heat is then conducted through the wall, and is transferred by convection to a heat absorbing liQuid. This latter convection is characterized by another heat transfer coefficient, which is usually considerably larger than that on the flue gas side. This means that the heat transfer on the flue gas side of the wall is less than on the other side.

Several attempts have been made to increase the heat 2 transfer coerLfcient on the flue gas side in heating boilers of the prior art. Conventional elements can be designed in several different ways, for instance in the form of parallel sections with plane walls, which are spaced apart in such a way 2 1.3 tnat, taken two at a time, they form a flue gas duct. This design is quite usual. for convection elements made from, cast il-on. In such f lue turbulence inducers increase th e he-;: inducers layer of probably cas side, sa-d duct.

Tne convection elements can also be designed like tunes of diffe-rent forms, often with a circular cross section. They may De elongaEed, thalz is to say with a subsEantially straight lonci--udinal axis, or be designed in the form of toroids. The flue cas inlet into the tunes can be axial or tangential which, in la--ie- case, ineans that the flue gas flows in a helical through the convection tubes. This design aemands overpressure firing in order to do itself justice, and gives a thouah further irri)rovement is gas ducts it or flow retarders, the turbulence in the flue gas transfer on the flue gas side. Such turbulence known hitherto have been provided outside the boundary the flue gas flow, out in the free flow zone and have increased the heat transfer coefficient on the flue at the expense of an increased pressure drop through is common to provide the aim of which is to duct and thus increase D a:: n relatively good heat transfer, desirab-le.

-7 = Th e object of the present invention is to provide a hea-iinc boiler of the kind discussed above, which gives a su.,stanzially improved heat transfer coefficient on the flue gas sIde, which consequently reduces the temperature in the outgoing flue gas and thus improves the heat economy, whilst ion elements is noz t h e)ressure droQ t,hrouQh the convect 3 allowed to increase.

According to the invention such a heating boiler is characterised in that at least one turbulence creating means is provided on the flue gas side of said wall, with its height ZZ extending in a direction away from the wall, substantially within the boundary layer of the flue gas flow.

Suitably the turbulence creating means has a substantially larger elongation than height. It may be convenient, in certain embodiments, to provide a convection element with a single turbulence creating means, which in this case has a considerable elongation, but often several such means are spread over the wall of the convection element, on its flue gas side.

The turbulence creating means can have several different forms in cross section, but: it has proved to be especially advantageous to design the cross section with sharp corners, that is to say to let the cross section be limited by substantially straight lines, The cross section can be formed in the shape of a parallel trapezium, e.g. a rectangle, or a 20 quadrilateral, a suitable size being from 2 x 2 to 7 x 7 mm. In one suitable embodiment, several turbulence creating means are provided on the flue gas side of the wall of the convection element, such that their elongations form an angle of from 150 to 450 with the direction of the flue gas flow. With such convection elements, having plane walls, it is convenient to provide the turbulence creating means substantially evenly distributed over said wall with spaces in between, in zigzag fashion.

i- 4 convection circuiar toroidal creating exr-endinQ whicn is anale of direction of the longitudinal axis of the tube. In those cases where the f lue cas enters tangentially into such a convection t--e flue cas can be made to flow in a second helical path con':ecticn in Wnlcn C-Mce, rEf the loncitudinal axis of the turbulence creating means is arrangea so t'- a the second helical pat' runs counter to the f irst.

helical par-n. in one suitable embodiment tne angle of the -elica-, pan is about 10' ana the dimensions of the cross section are about 3 x 3 mm. Such an arrangement gives rise to no increased pressure drop in a toroidal convection element, at nor-.,,a I operation conditions.

Convection elements can be manufacturea from cast iron or from fne,:al sheets (plating). in both cases it is convenient to manufacture the tuzbulence creating means integral with the wall material, that is to say cast directly with the wall in the case of cast iron, or embossed in the case of sheet metal.

In suc,-, heating boilers, that are provided with elements in the forn., of tubes, preferably of a cross section, either straight or in the form of rings, it is advantageous to provide the turbulence means with its (or their) longitudinal axis (axes), in a nelical path along the inner wall of the tube, the flue gas side, this first helical path forming an f rom 51 to 451 with planes perpendicular to the c 1 = The invention will now be described in more detail, reference being made to the accompanying drawings, of which:

Figure 1 is a view in vertical cross section through a heating boiler with convection elements having plane walls, provided with turbulence creating means according to the invention; Figure 2 is a view of the turbulence creating means in Figure 1 to a larger scale (natural size); 1.0 Figure 3 is a cross section through a turbulence creating means, with underlying wall, along the line III - III in Figure 2; Figure 4 is a view in vertical longitudinal section through a heating boiler of the fine tube type, which is provided with turbulence creating means according to the invention; Figure 5 is a vertical cross section of the heating boiler in Figure 4, Figure 6 is a convecition element, turbulence creating larger scale than in Figure 7 shows view in longitudinal section through a that is to say a tube provided with means according to the invention, to a Figures 4 and 5, and a developed part of a wall of the flue gas side of a tubular convection element for a heating boiler according to the invention.

The heating boiler in Figure 1 has a combustion chamber formed by several cast sections, which is eccentrically arranged. A convection element 2 forms, together with a front 6 Z 1 = convec-:ion element (nor shown), a flue gas duct with plane walls, provided with several turbulence creating means 3. Said flue gas duct has an inler. 4 and an outlet 5, and the flow path of the flue gas is shown by arrows 6, 7. As clearly seen from the figure, the gas flows in a substantially vertical direction through the flue gas duct. In Figure 2 the turbulence creating means 3 are shown in normal size. They are arranged in zigzag fashion, juxtaposea both laterally and longitudinally and regularly spaced. They form the angles)61 142 with the flow direc, :iion of the flue gas, shown by the arrow 7. In the embodiment shown g 1 = 2 = 30'. From Figure 3 it is clearly sl-cwr, the crosS section of the turbulence creatinc means is bounded by subs-:antially straight lines and principally from a ouadralateral with a 4 mm side.

As also clearly shown in Figure 3, the turbulence creating means are manufactured in one piece with the underlying wall, that is to say cast integral with the wall.

In Figures 4 and 5 c 1-1 a m te -- 8 is provided a circular cylindrical combustion whicn is surrounced by convection elements with tubular flue gas ducts 9, tnat is to say in the form of toroids. These are provided with tangential flue gas inlets 10, ana outlets 11 in a joint outlet duct. 12. Water connecions to the war-er side of the tubes are referenced 13 and 14.

The tubes are provided with turbulence creating means 15 in the form of a helix extending along the inner side of the tubes, which forms an angle 153 witn planes perpendicular to the longitudinal axis of the tube. The turbulence creating means 7 C 1 = 7 = is provided in such a way in relation to the flue gas inlet 10, that the helical path which the flue gas flow follows is of the opposite hand (ie. runs counter) to the one that is formed by the turbulence creating means 15. The latter is of a quadrilateral cross section with the dimensions 3 x 3 mm, and the cross section of the tube in this case has a diameter of 75 MM. The provision of turbulence creating means 15 does not increase the pressure drop on the flue gas side in this heating 10 boiler, which is intended for over-pressure firing.

In Figure 7, a developed part of a wall of the flue gas side of a tubular convection element for a heating boiler is shown. An arrow 16 shows the direction of the longitudinal axis of the convection element. A number of turbulence creating means 17, 18 are distributed over the wall surface, one group 17 being provided with their longitudLnal axes perpendicular to the longitudinal axis of the convection element, and the other group 18 with their longitudinal axes parallel to the latter. The diameter of the convection element in this case is 80 mm and the length of the turbulence creating means is 25 mm and its height 2 mm tangentially describes a The f lue gas enters through an inlet, (not shown) and the f lue gas helical path in the tubular convection element, with a direction that varies from being substantially perpendicular to the longittudinal axis of the convection element to a direction substantially parallel to the same.

Arrows 19, 20 indicate two such flow directions within the convection element. It should be observed, that the arrow 20 8 in reality is located f arther to the right in the tubular convect-ion element (which is only partly shown).

Heating boilers with turbulence creating means according to the invention, compared with prior art boilers,offer great advantages. If the exit f lue gas temperature is kept at 240' for instance, the heating effect can be increased by about 50%.

on the other hand, if the exit flue gas temperature is reduced to 2000C, the effect can still increase by 25-40% with increased efficiency.

Ine ei-,ibodinieni:s snown of heating boilers according to the inven.zlon are only examples of course, other embodiments being possiGle within the scope of the invention.

C 1 n 7 = 9 c / n

Claims

CLAIMS.

1. A heating boiler for the combustion of gaseous, liquid or solid fuels, comprising a combustion chamber, the flue gas flow from which is arranged to contact at least one convection element for the transfer of heat from the flue gas to a heat absorbing medium, such as a liquid, through a heat conducting wall over which, in use, flue gas flows, characterized in that at least one turbulence creating means (3, 15) is provided on the flue gas side of said wall, with its height extending in a direction away from the wall, substantially within the boundary laver of the flue gas flow.

2. A heating boiler according to Claim 1, charact-erised in that the turbulence creating means (3, 15) has a subsuantially larger elongation than height.

A heating boiler according to Claim 2, characterized in creating means (3, 15) seen in its has a crosssection limited by substan- 3.

that said turbulence longitudinal direction, tially straight lines.

4. A heating boiler according to Claim 3, characterised in that said crosssection is in the form of a parallel trapezium.

5. A heating boiler according to Claim 4, characterised in that the cross section is in the form of a rectangle.

!er according to Claim 4, characterized in 6. A heating bo-L that said cross section has the size from 2 x 2 to 7 x 7 mm.

7. A Heating boiler according to any of the Claims 1 to 6, and in which several turbulence creating means are provided, -hat the turbulence creating means (3) are characterised in arranged in such a way on the flue gas side of said wall, that n.

their longitudinal extension form an angle of from 151 to 45' wit'n the flow direction of the flue gas.

8. A heatina boiler according to Claim 7, and in which said wall is preferably plane, characterised in that the turbulence creating means (3) are substantially evenly distributed over 'd wall, arranged wih spaces in between, in zigzag fashion.

s a L - - 9. A heating boiler according to any one of Claims 1 to 6, ana in which the convection element has the form of a tube, preferably with a circular cross section, through which flue gas is arranged to flow, characterised in that the turbulence creaiing means ( 15) with its longitudinal axis, or their loncitudinal axes, extend(s) in one first helical path along the inner wall of the tube, this helical path forming an ancle of from 5 to 4- 0 with planes perpendicul-ar to ne long-L-:udinal di--ec--ion of the tube.

10. A heating boiler according to Claim 9, characterised in tha-- said ancle is abou: 10'.

1 -,. A heatinc boiler according to Claim 9 or 10, and in which a flue gas inlet (10) is provided rangeniially to said tube, so that flue cas will flow in a second helical path through the t ub e, characterised in that the second helical path runs - helical path (15).

c--un.ier to said fi--st 1 -,.

A heatinc boile-- according to any one of Claiins 1 to 11, - is manufact and in which said convecz-ion elemenIL ured from cast iron, characterised in that the turbulence creating means (3, 15, is (are) manufacujred inz:ec---allv with the flue gas side of said wall.

1 A m 13. A heating boiler according to any of Claims 1 to 11, and in which said convection element is manufactured from plating, characterised in that the turbulence creating means (3, 15) is(are) embossed in the plating.

14. A heating boiler constructed, arranged and adapted for use substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

j = g Published 1985 P.', The Patent Office, State House, 66'71 High Holborn, London WCIR 4TP. Further copies Tney be obtained from The Patent, Office, Sales Branch. St Mary Cray. Orpington. Kent BRS 3RD. Printed 1:y Multiplex techniques)td, St Mary Cray, Kent. Con. 1/87.