GB2416827A

GB2416827A - Condensing unit

Info

Publication number: GB2416827A
Application number: GB0417185A
Authority: GB
Inventors: Neil Arthur Sawers
Original assignee: BOULTER BUDERUS Ltd
Current assignee: BOULTER BUDERUS Ltd
Priority date: 2004-07-31
Filing date: 2004-07-31
Publication date: 2006-02-08
Also published as: GB0417185D0

Abstract

A condensing boiler includes a condensing unit 26 with heat exchange helical coils 28 fed from a common manifold 38 to a second common manifold 40. The coils are arranged with their axes parallel to one another and they allow parallel flow of a heat extraction medium within. An inlet 30 allows flue gas from the boiler to flow down, through, and around the coils before leaving via an outlet 32. The coils may have contact between adjacent turns or be spaced apart. A turbulent flow of gas may be provided by at least one baffle 42 in the casing 36 of the unit.

Description

Condensing Unit This invention relates to a condensing unit for a

condensing boiler, and to a condensing boiler fitted with such a unit.

Condensing boilers, for example for use in domestic central heating and/or hot water systems have a combustion chamber in which combustion of fuel takes place. A heat exchanger in the combustion chamber transfers the heat of combustion from gases in the combustion chamber to water in a water jacket, and the spent gases leave the chamber via a flue. The spent gases are still hot when they leave the chamber, and in a condensing boiler, these gases are passed through a secondary heat exchanger in which further heat is extracted and condensable gaseous fractions are condensed out so that they are not passed into the atmosphere.

According to the invention, there is provided a condensing unit for a condensing boiler, the unit comprising a chamber in which a plurality of heat exchange tubes in the form of helical coils are mounted, each tube being mounted to an inlet and to an outlet for the passage of heat extraction medium through the respective coil, the chamber also having a flue gas inlet and a flue gas outlet and the tube coils being arranged to define flue gas passages from the flue gas inlet to the flue gas outlet both through the centre of each coil and around the outside of each coil, the chamber also having a drain to allow condensate to be drained from the chamber.

The chamber preferably includes at least one baffle with at least one coil on either side of the baffle, the baffle being arranged so that flue gases flow in one direction on one side of the baffle and in an opposite direction on the other side of the baffle. The baffle arrangement is preferably such as to cause turbulent flow of the flue gases through the chamber. In one preferred arrangement there are four coils and a single baffle, with two coils on each side of the baffle.

The flue gas inlet and the flue gas outlet may both be on the same face of the chamber, and the heat exchange tubes can all be fed from and feed to common manifolds, such that the flow through the tubes is in parallel.

Adjacent turns of each coil may either be in contact with each other such that the gas path through the centre of each coil is separated from the gas path around the outside, or may be spaced from each other such that gas can flow from the centre of each coil to the outside and vice versa. A baffle plate can be fitted over the downstream end of each coil, or at intermediate positions along the coil axis to force gas to flow out from the centre of each coil, between the coil turns.

It is preferred for there to be no joints between or to the parts of the tubes which lie within the chamber. This makes the tubes less prone to corrosion damage from the - 3 corrosive flue gases.

The invention also provides a condensing boiler having a combustion chamber surrounded by a water jacket and a condensing unit, the condensing unit comprising an enclosure in which a plurality of heat exchange tubes in the form of helical coils are mounted, each tube being connected in a water circuit with the water jacket, the enclosure also having a flue gas inlet connected to a flue gas outlet from the combustion chamber and a flue gas waste outlet, and the tube coils being arranged to define flue gas passages from the flue gas inlet to the flue gas waste outlet both through the centre of each coil and around the outside of each coil, the enclosure also having a drain to allow condensate to be drained from the enclosure.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a condensing boiler, with a condensing unit shown separated from the boiler for ease of illustration; Figure 2 is a perspective view of a condensing unit in accordance with the invention; and Figure 3 is a section through the unit of Figure 2, the line III-III. - 4 -

The boiler 10 has a burner 12, which may burn any suitable liquid of gaseous fuel, burning in a combustion chamber 14. The combustion chamber is surrounded by a water jacket 16, and a set of baffles 18 in the chamber 14 direct the combustion gases to flow in a turbulent manner and to give up their heat to the water in the water jacket.

The heated water then flows in a closed circuit 20 to a central heating system and/or a hot water system indicated schematically at 22 and then back to the water jacket.

The combustion chamber has a flue gas outlet at 24.

In order to extract further heat from the exhausted flue gases, and to condense out undesirable constituents of the flue gas, the gas is passed through a condensing unit 26.

The unit 26 has four parallel (both in physical and in flow terms) heat exchange coils 28 which form part of the closed water circuit 20. The unit 26 also has a flue gas inlet 30, a waste gas outlet 32 and a condensate drain 34.

Figure 2 shows the condensing unit 26 in more detail. The unit has a closed casing 36, but in Figure 2 the front face is shown open to reveal the internal components. In practice, this unit will be housed immediately adjacent to the boiler, and within the same housing enclosure.

The four helical coils 28 are each separate from one another but are all fed from a common manifold 38 and all feed into a common manifold 40. Thus in terms of flow, they are in parallel. Connections between the coils and the manifolds 38, 40 are made outside the casing 36, and there are then no joints made within the casing to or between the tubes forming the helical coils. This is important because the flue gases flowing in the chamber are corrosive and tend to attack any joints which are in their flow path. Since water will be flowing in the tubes, it is necessary to take all precautions to prevent leakage.

Within the casing, a baffle plate 42 divides the internal space. Thus, gas entering the casing through the inlet 30 is forced to flow downwards through and around the coils 28a, 28b, under the bottom of the baffle plate 42 and then up through the coils 28c and 28d to the outlet 32, from where the gas escapes into the atmosphere. The heat exchange which takes place in the condensing unit transfers heat from the flue gases, which consequently cool down, to the water in the circuit 20, which thus reenters the water jacket 16 at a temperature higher than The coils may be tight coils, ie with adjacent turns in contact with each other, although it would not be intended that such contact would prevent all flow through between the turns. Alternatively, the coils may be wound with a small gap between the turn, and the centres 44 of the coils may be blanked off at the downstream end so that the gases flowing in the centre are forced to flow out between the turns.

It is desirable to make the flow of flue gases as turbulent as reasonable possible, and to this end, baffles can be placed randomly across the casing 36, either within the internal cross-section of the coils or across the casing externally of the coils, to force the gases to flow through the coils, making the best possible heat exchange contact with the coils.

Figure 3 shows how the incoming flue gases will flow from the inlet 30 both down the centre 44 of the coils 28 and around the outside, in the space 46 between the walls of the casing 36 and the coils. The coiled form of the tubes 28 provides a large heat exchange area in that the water flowing through the unit 26 has a long path to travel. It also provided an uneven surface past which the flue gases flow, and this helps to produce turbulence in the flow resulting in good mixing and good heat exchange.

As the flue gases cool, some constituents will condense out and will form on the surfaces of the coils. As these constituents are often acidic or corrosive, it is preferable to make the coils 28 from stainless steel.

Typically the flue gases will enter at about 180 - 200-C and leave at about 50 - 805 C., and this can give a temperature rise of 1 - 5 - C of the primary water in the circuit 20. This is a significant temperature rise in - 7 - terms of the overall efficiency of the boiler as a whole.

It will be clear that either more or less than four coils 28 can be provided, and different baffling arrangements can be used to direct the flow between the inlet to and the outlet from the condensing unit. In a typical domestic boiler, the length of tubing in the coils may be of the order of lOm, which (with a tube internal diameter of lOmm) would hold about 1 litre of water at any time.

The total flow cross section of the coils should be as close as possible to the flow cross section of the pipes 20, so as to avoid any slowing down of the flow in the circuit. For example, if the pipes 20 have an internal diameter of 2Omm (a typical value for a domestic installation), they will have a cross-sectional area of 314 mm2. If the coils each have a diameter of 10 mm, they will each have a cross-sectional area of 79 mm2. Four coils will therefore have a cross- sectional area of 79 x 4 = 316 mm2 which is substantially the same as the pipe cross-sectional area.

If the pipes 20 are instead 25 mm inside diameter and the coils are still 10 mm, the flow equalization may be met by adding a fifth coil. - 8

Claims

Claims 1. A condensing unit for a condensing boiler, the unit comprising a

chamber in which a plurality of heat exchange tubes in the form of helical coils are mounted, each tube having an inlet and an outlet for the passage of heat extraction medium through the respective coil, the chamber also having a flue gas inlet and a flue gas outlet and the tube coils being arranged to define flue gas passages from the flue gas inlet to the flue gas outlet both through the centre of each coil and around the outside of each coil, the chamber also having a drain to allow condensate to be drained from the chamber.
2. A condensing unit as claimed in Claim 1, wherein the chamber includes at least one baffle with at least one coil on either side of the baffle, the baffle being arranged so that flue gases flow in one direction on one side of the baffle and in an opposite direction on the other side of the baffle.
3. A condensing unit as claimed in Claim 2, wherein there are four coils and a single baffle, with two coils on each side of the baffle.
4. A condensing unit as claimed in any preceding claim, wherein the flue gas inlet and the flue gas outlet are both on the same face of the chamber.
5. A condensing unit as claimed in any preceding claim, 9 - wherein the heat exchange tubes are all fed from and feed to common manifolds, such that the flow through the tubes is in parallel.
6. A condensing unit as claimed in any preceding claim, wherein adjacent turns of each coil are in contact with each other such that the gas path through the centre of each coil is separated from the gas path around the outside.
7. A condensing unit as claimed in any preceding claim, wherein adjacent turns of each coil are spaced from each other such that gas can flow from the centre of each coil to the outside and vice versa, and wherein a plate is fitted over the downstream end of each coil to force gas to flow out from the centre of each coil, between the coil turns.
8. A condensing unit as claimed in any preceding claim, wherein, within the chamber, there are no joints between or to the tubes.
9. A condensing boiler having a combustion chamber surrounded by a water jacket and a condensing unit, the condensing unit comprising an enclosure in which a plurality of heat exchange tubes in the form of helical coils are mounted, each tube being connected in a water circuit with the water jacket, the enclosure also having a flue gas inlet connected to a flue gas outlet from the combustion chamber and a flue gas waste outlet, and the 10 tube coils being arranged to define flue gas passages from the flue gas inlet to the flue gas waste outlet both through the centre of each coil and around the outside of each coil, the enclosure also having a drain to allow condensate to be drained from the enclosure.
10. A boiler as claimed in Claim 9, wherein the condensing unit has the features of any one of Claims 2 to 8.
11. A condensing unit substantially as herein described with reference to the accompanying drawings.
12. A condensing boiler substantially as herein described with reference to the accompanying drawings.