EP0295967B1

EP0295967B1 - Indirect liquid heating system

Info

Publication number: EP0295967B1
Application number: EP88305611A
Authority: EP
Inventors: Geoffrey Lund Finney
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-06-19
Filing date: 1988-06-20
Publication date: 1991-10-30
Also published as: DE3865894D1; GB8814646D0; EP0295967A3; GB8714419D0; EP0295967A2; GB2207740A; GB2207740B

Description

This invention relates to an indirect liquid heating system and particularly, although not exclusively, to such a system used for the heating of raw water.
The indirect heating of raw water, for example by a gas burner, is effected by locating the heat source in a combustion chamber in or under the tank or reservoir into which cold water is introduced and from which heated water is withdrawn. To optimise heat utilisation the products of combustion of the heat source are preferably taken through the water in the tank or reservoir, through one or more vertical tubes which extend from a generally flat area of the bottom of the tank or reservoir to a smoke box above it, and from thence to a flue or chimney. Turbulators may be included in the tube or tubes to break up laminar flow of the combustion gases and enhance heat transfer to the ambient water.
One of the major problems affecting this type of equipment, especially in areas where the water supply is hard, is the formation of scale, particularly lime scale, on the wetted sides of the heat exchanger. On the surface of a tube itself scale formation is not a serious problem as expansion and contraction of the metal, especially on start-up and close-down, tends to break the scale off. However the broken scale falls to and collects on the bottom of the tank or reservoir, usually where it is directly subjected to the heat of the burner. This inhibits heat transfer to the water in the tank or reservoir and causes excessive heating of the bottom. The system becomes progressively less efficient and eventually the bottom of the tank or reservoir may fail through over heating.
Various proposals have been made to overcome this problem. In one of them a secondary, horizontal plate or "false bottom" is located in the tank or reservoir above its bottom to catch the falling scale particles. In another water is introduced through one or more approximately tangential jets immediately above the bottom of the tank or reservoir so as to agitate the precipitated scale particles and, as far as possible, prevent them from settling and solidifying.
A principal object of the present invention is to provide an improved solution to the problem of the deposition of scale, or like precipitate, on the part of a heat exchanger directly subjected to the heat e.g. of a fuel gas burner.
Patent Specification FR-A-1142539 discloses an indirect liquid heating system having the features of the pre-characterising part of claim 1. However this Specification is not concerned with the problem of scale formation and water entering the tank is slowed down rather than accelerated. The invention as claimed inhibits the formation of scale, by locally accelerating the incoming liquid, and ensures that scale or other precipitate which is formed will not accumulate directly above the heating means.
The said sloping region is preferably constituted by a central region of the bottom of the tank or reservoir, which central region is downwardly concave and reduces in cross-sectional area upwardly to a top aligned with the orifice of the liquid inlet, whereby inflowing liquid is distributed substantially over the whole surface of said central region presented inwardly of the tank or reservoir.
This central region may be dome-shaped, generally conical or pyramidal.
The mean angular relation to the horizontal of any side of said central region is preferably an acute angle in excess of 50 degrees. If the central region is pyramidal, the apex of the pyramid may divide the orifice into a plurality of separate, approximately D-shaped openings.
The heating means may be a gas burner and there may extend upwardly from said sloping region within the tank or reservoir at least one outlet for combustion gases which extends to a smoke box above the tank or reservoir. An annular array of circumferentially spaced combustion gas outlet tubes may extend upwardly from said central region from a horizontal plane intermediate its top and bottom.
The flow of combustion gases through each outlet tube therefor may be obstructed by formations on or in the tubes so as to promote heat transfer to the surrounding water.
A preferred embodiment of the invention will now be described by way of example with reference to the accompanying diagrammatic drawings, in which:

Figure 1 is a side sectional elevation of a liquid heating system in accordance with the invention taken generally on the line I-I of Figure 2, and
Figure 2 is a cross sectional view taken on the line II-II of Figure 1.

The water heating system illustrated comprises a tank or reservoir 20 having a peripheral wall 9 closed at its bottom end by a bottom plate 21 and at its top of a top plate 22. The tank has a hot water outlet 7 and cold water is introduced through a generally central inlet pipe 5 which extends downwardly into the tank 20 from an inlet connection 6 on one side of a smoke box 10 above the tank.
Beneath the bottom plate 21 of the tank is a combustion chamber 23 in which a burner 1 for a mixture of fuel gas and air is located. Gas burners are well known per se so that the burner 1 will not be particularly described except to say that it is preferably of the naturally-aspirated type, i.e. that air to be mixed with the burning fuel gas is not supplied under pressure. Flame from the burner 1 plays on the bottom of the tank 20 and the products of combustion are taken through an array of heat exchanger tubes 2 to the smoke box 10 above the tank, and thence vented through a flue 8. The tubes 2 transfer heat to the ambient water and to promote this the flow of combustion gases through each tube 2 is preferably interrupted or obstructed either by the provision in each tube 2 of a turbulator (not shown) or by giving each tube 2 a dimpled configuration through the provision of alternately inwardly and outwardly projecting areas 4.
As so far described the system is conventional. In accordance with the invention, however, a central region 3 of top plate 21 of the combustion chamber 23, also constituting the bottom plate of the tank 20, is not flat but generally conical. This downwardly concave formation 3 is immediately above the burner 1 and tapers to an apex or point 24 which extends into the orifice of the inlet pipe 5 with a small clearance. The coaxial alignment of the cone 3 and the pipe 5 ensures that the incoming water flow is distributed generally uniformly over the wet side of the cone, while the projection of the tip 24 into the orifice of the pipe 5 accelerates the incoming water flow which inhibits local scale deposition. As will be apparant, the bottom ends of the array of heat exchanger tubes 2 extend from the cone 3 in a horizontal plane intermediate its tip 24 and its bottom integral with the flat part of the plate 21.
The angle A formed by the side of the cone to the horizontal is an acute angle in excess of 50 degrees. If the cone is replaced by a pyramid, then each side of the pyramid forms such an angle with the horizontal.
With heat variation, especially on start-up and shut down, the cone 3 along with the tubes 2 is subject to differential expansion and contraction which has the effect of shedding scale which has adhered. The scale particles sink through water in the tank 20, and on striking the sloping surface of the cone 3 are directed by it to the annular area represented by the flat part of the plate 21 beneath the base of the cone 3. This area, however, is not immediately above the burner 1 but laterally offset from it. Consequently, during the continued use of the system there is no scale accumulation immediately above the burner 1 which will impair the efficiency of the system or cause overheating of the bottom of the tank 20.
In an alternative construction (not shown) the formation 3 is pyramidal rather than cone-shaped so that its apex, projecting into the orifice of the inlet pipe 5, forms an approximately D-shaped, restricted opening above each side of the pyramid.

Claims

1. An indirect liquid heating system comprising a tank or reservoir (20) having liquid inlet (5) and outlet means (7) and heating means (1) isolated by a combustion chamber (23) from the interior of the tank or reservoir (20), an upper region (3) of the combustion chamber (23) vertically above the heating means (1) sloping upwardly toward the liquid inlet (5) and said liquid inlet means (5) extending downwardly within the tank or reservoir (20) characterised in that said inlet (5) is of uniform diameter throughout its length and terminates at a bottom end orifice above and directed vertically downwardly in proximity with a highest part of said sloping region (3) locally to increase the velocity of liquid entering the tank or reservoir (20) whereby scale precipitated from the liquid and shed from said sloping region (3) will fall to a position laterally offset with respect to the heating means (1).

2. A system as claimed in claim 1, characterised in that said sloping region (3) forms part of the bottom (21) of the tank or reservoir (20).

3. A system as claimed in either preceding claim, characterised in that said sloping region (3) is constituted by a central region (3) of the bottom (21) of the tank or reservoir (20), which central region (3) is downwardly concave and reduces in cross-sectional area upwardly to an apex (24) which extends into the orifice of the liquid inlet (5), whereby said apex (24) restricts said orifice and inflowing liquid is distributed substantially over the whole surface of said central region (3) presented inwardly of the tank or reservoir (20).

4. A system as claimed in claim 3, characterised in that said central region (3) is generally conical.

5. A system as claimed in claim 3, characterised in that said central region (3) is pyramidal.

6. A system as claimed in any one of claims 3-5, characterised in that the mean angular relation to the horizontal of any side of said central region (3) is an acute angle in excess of 50 degrees.

7. A system as claimed in claim 5 or claim 6 characterised in that the apex (24) of the pyramid (3) divides the orifice (5) into a plurality of approximately D-shaped openings.

8. A system as claimed in any one of the preceding claims, characterised in that the heating means (1) is a gas bumer and there extends upwardly from said upper region (3) within the tank or reservoir (20) at least one outlet (2) for combustion gases which extends to a smoke box (10) above the tank or reservoir (20).

9. A system as claimed in claim 8 as appendant to any one of claims 3-7, characterised in that an annular array of circumferentially spaced combustion gas outlet tubes (2) extends upwardly from said central region (3) from a horizontal plane intermediate its top and bottom.