[0001] 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.
[0002] 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cold water is introduced and from which heated water is withdrawn.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.
[0003] 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.
[0004] 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 theprecipitated scale particles and, as far as possible, prevent
them from settling and solidifying.
[0005] 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.
[0006] In accordance with the invention there is provided an indirect liquid heating system
comprising a tank or reservoir having liquid inlet and outlet means and heating means
isolated by a combustion chamber from the interior of the tank or reservoir, wherein
an upper region of the combustion chamber vertically above the heating means slopes
upwardly toward the liquid inlet, the arrangement being such that solids such as scale
precipitated from the liquid and shed from said sloping region will fall to a position
laterally offset with respect to the heating means, and wherein said inlet means extends
downwardly within the tank or reservoir to open in proximity with said sloping region
such that the inlet is restricted locally to increase the speed and pressure of liquid
entering the tank or reservoir.
[0007] A system in accordance with the invention thus combines two features which respectively
inhibit the formation of scale, by locally accelerating the incoming liquid and increasing
its pressure, and ensure that scale or other precipitate which is formed will not
accumulate directly above the heating means.
[0008] In a preferred construction the said sloping region forms part of the bottom of the
tank or reservoir.
[0009] 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.
[0010] This central region may be dome-shaped, generally conical or pyramidal.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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 as
well as increasing its pressure 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.
[0018] 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 pryramid, then each side of the
pyramid forms such an angle with the horizontal.
[0019] 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.
[0020] 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.
1. An indirect liquid heating system comprising a tank or reservoir (20) having liquid
inlet (5) and outlet (7) means 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), the arrangement being such that solids such as 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), characterised in that said liquid inlet means (5) extends downwardly within the tank or reservoir (20)
to open in proximity with said sloping region (3) such that the inlet (5) is restricted
locally to increase the speed and pressure of liquid entering the tank or reservoir
(20).
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 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 a top aligned with the orifice of the liquid inlet
(5), whereby 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 dome-shaped.
5. A system as claimed in claim 3, characterised in that said central region (3) is generally conical.
6. A system as claimed in claim 3, characterised in that said central region (3) is pyramidal.
7. A system as claimed in any one of claims 3 - 6, 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.
8. A system as claimed in claim 6, or claim 7 as appendant to claim 6, characterised in that the apex (24) of the pyramid (3) divides the orifice (5) into a plurality of approximately
D-shaped openings.
9. A system as claimed in any one of the preceding claims, characterised in that the heating means (1) is a gas burner and there extends upwardly from said sloping
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).
10. A system as claimed in claim 9 as appendant to any one of claims 3 - 8, characterised in that an annular array of circumferentially spaced combustion gas outlet tubes (2) extends
upwardly from said central region (3) form a horizontal plane intermediate its top
and bottom.