GB2422892A

GB2422892A - Condensing boiler with diverter

Info

Publication number: GB2422892A
Application number: GB0502327A
Authority: GB
Inventors: John Frederick Michae Bosworth
Original assignee: Heating World Group Ltd
Current assignee: Heating World Group Ltd
Priority date: 2005-02-04
Filing date: 2005-02-04
Publication date: 2006-08-09
Anticipated expiration: 2025-02-04
Also published as: GB0502327D0; GB2422892B

Abstract

An oil- or gas-fired condensing boiler 10 has a condensing heat exchanger 12 through which the exhaust gases normally pass to an exhaust flue 20. The boiler 10 includes a control arrangement which performs an ignition sequence for initial firing of the boiler. A diverter 13,21 is disposed at an inlet 23 to the condensing heat exchanger and is operated at the commencement of an ignition sequence to direct exhaust gases from the boiler to the flue 20 so by-passing the condensing heat exchanger. Once combustion is established, the diverter 21 is returned to its normal setting to direct the exhaust gases through the condensing heat exchanger 12.

Description

BOILERS

This invention relates to boilers and, more particularly, to oil-fuelled or gas-fuelled condensing boilers. The invention further relates to a method of operating a condensing boiler and to a diverter for use in a condensing boiler.

In condensing boilers intended for use in commercial or domestic central heating systems, fuel oil or gas is burnt in a burner assembly, the heat produced thereby being used to heat water passing through a primary heat exchanger. The exhaust gases (sometimes also referred to as flue gases) produced at the burner by the combustion of the fuel are directed from the burner assembly through the primary heat exchanger and then through a secondary heat exchanger in which heat from the already cooled gases is taken up by water which is being returned to the boiler, such water having completed its passage through a heating system. This system will usually comprise a plurality of interconnected radiators through which the heated water from the primary heat exchanger flows.

The exchange of heat in the secondary heat exchanger has the effect of simultaneously heating the return water and further cooling the exhaust gases from the primary heat exchanger. This cooling causes water vapour in the exhaust gases to condense out of those gases within the heat exchanger, the condensate bringing down with it, perhaps, particles formed during the combustion of the fuel, which particles have, up to then, been entrained in the exhaust gas flow. Because of its condensing action, the secondary heat exchanger is often referred to as a condensing unit. The condensed water and any other material contained within the water condensed in the secondary heat exchanger are removed from that heat exchanger through a suitable drain arrangement.

On ignition, and in the moments after ignition of the fuel oil or gas, smoke containing carbon and other particles is formed, which has the potential to cause sooting both in the boiler (i.e. in the primary heat exchanger) and in the condensing unit (i.e. the secondary heat exchanger). These deposits from the smoke can build up to such an extent that total blocking of the condensing unit can occur. In the case of an overbaffled' primary heat exchanger, the addition of a secondary heat exchanger thereto can exacerbate the sooting potential in the boiler to such an extent that blockages can also be formed therein.

When a condensing boiler is ignited, there is no combustion at the start of the ignition sequence, and when combustion occurs, it will be at relatively low temperatures. As a consequence, and particularly in the case of fuel oil, the exhaust gases are extremely acid due to the relatively high sulphur content in fuel oil. This means that substantial damage can be caused to the metal of the secondary heat exchanger by the condensate, leading to metal corrosion within the exchanger.

This invention seeks to address the above-described problems of excess sooting and also possible corrosion with a condensing boiler, particularly when it is fired by fuel oil.

According to one aspect of this invention there is provided an oil- or gas- fired condensing boiler comprising a burner having an ignition device arranged to perform an ignition sequence at the commencement of a period of firing of the boiler, an arrangement for the passage of exhaust gases from the burner, a condensing heat exchanger having an inlet for exhaust gases from the arrangement, a diverter operable to direct exhaust gases away from said inlet, and control means arranged to cause operation of the diverter at the commencement of an ignition sequence of the ignition device whereby exhaust gases are diverted away from the heat exchanger for at least a part of an ignition sequence.

According to a second, but closely related, aspect of this invention there is provided a method of operating a method of operating a condensing boiler having a burner, an ignition device, an arrangement for the passage of exhaust gases from the burner and a condensing heat exchanger having an inlet for exhaust gases from the arrangement, the method involving the operation of a diverter at the commencement of an ignition sequence of the ignition device whereby exhaust gases are diverted away from the heat exchanger for at least part of the ignition sequence.

It will be appreciated that with this invention, the exhaust gases from the burner are directed away from the condensing heat exchanger at the commencement of an ignition cycle. As such, the likelihood of sooting in the condensing heat exchanger is greatly reduced, as is the production of highly acidic condensate, before the boiler has reached normal operating temperature. An appropriate period after the commencement of an ignition cycle, which period is determined as will be described below, the exhaust gas flow through the condensing heat exchanger is reinstated so that the boiler may operate in the normal condensing mode.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Figure 1 illustrates a housing forming part of a condensing boiler, the housing having connections inter a/ia to a secondary heat exchanger and to a main exhaust pipe; Figure 2 shows the housing of Figure 1 with a cover thereof removed, and also shows the arrangement of a diverter valve in the housing; Figure 3 is a view similar to Figure 2 but with the flap of the diverter valve blocking an inlet to the secondary heat exchanger; Figure 4 is similar to Figure 3, but with the flap of the diverter valve blocking an inlet to the exhaust pipe, for normal boiler operation; and Figure 5 is a further view of the housing specifically showing the arrangement of ducts connecting with the secondary heat exchanger.

In the drawings, only part of a condensing boiler 10 is shown, and purely diagrammatically since the internal arrangement of the boiler forms no part of this invention. Within the condensing boiler body there is a burner (not shown) to which fuel oil is fed. The burner has an ignition device (also not shown) including spark electrodes, a transformer or other spark generator connected to those electrodes and the usual safety systems to ensure that fuel will be cut off if the ignition device fails to ignite the fuel oil or if the flame at the burner should fail during normal use of the boiler. A control box associated with the ignition device is arranged to initiate an ignition sequence when triggered for example by an external timer or temperature control, as well as to control the timing of the ignition sequence and a lock-out procedure in the event that ignition does not properly take place.

The boiler further comprises a primary heat exchanger (not shown) through which water to be heated is passed, the heated water then being circulated, usually by a pump, around a heating system comprising a plurality of interconnected radiators. With a non-condensing boiler, the cooled return water is supplied to the inlet of the primary heat exchanger.

The exhaust gases produced by the burning of the fuel at the burner are conducted away from the boiler, normally with the assistance of a fan associated with or incorporated within the boiler. In the case of a condensing boiler, these exhaust gases are supplied to a secondary heat exchanger 12 through suitable ducts, as described below. The return water from the heating system is first passed through the secondary heat exchanger, to be heated by the exhaust gases from the primary heat exchanger, before the water is thereafter piped to the inlet of the primary heat exchanger.

The water flowing through the secondary heat exchanger takes heat from the exhaust gases so that the temperature of the return water is raised and, simultaneously, the temperature of the exhaust gases is reduced. The reduction in temperature of the exhaust gases causes water vapour to condense out of those gases, but the condensed water usually includes particulate matter carried by the exhaust gases from the primary heat exchanger. The condensate is removed from the secondary heat exchanger, utilising a suitable drain arrangement.

From the secondary heat exchanger, the remaining exhaust gases then pass to an outlet flue; in a normal arrangement this will be situated alongside or concentric with an air inlet conduit for at least part of the length of the outlet flue and further heat will pass from the exhaust gases to the air entering the boiler via the inlet conduit. Thereby the exhaust gases leaving the boiler are brought to a relatively low temperature, perhaps as low as 55 C compared to 190 C in a non-condensing boiler. This means that a substantial proportion of the heat produced by the boiler is utilised rather than being lost to atmosphere in the exhaust gases. Further, by achieving condensation in the secondary heat exchanger, the latent heat of condensation is also released and so it is possible to achieve a very high fuel efficiency with a condensing boiler.

As shown in the drawings, the boiler 10 includes a housing 13 which, as shown, is of bucket-like form having a main cylindrical wall 14 and a base 15.

The housing 13 is further provided with a cover 16 adapted to engage and effect a seal to the upper part of the cylindrical wall 14 of the housing 13. An opening 17 in the base 15 of the housing 13 communicates with a duct 18 from the boiler primary heat exchanger and two outlet ducts 19,20 pass through the cylindrical wall 14 of the housing, to communicate with the interior thereof.

Within the housing, there is provided a control flap 21 whereby the flow of exhaust gases from duct 18 may be directed to either one of the outlet ducts 19,20.

The duct 18 could simply be formed by an opening in the jacket of the primary heat exchanger, through which the exhaust gases pass. Depending upon the design of boiler, the duct 18 could comprise a flexible heat resistant pipe or other conduit through which the exhaust gases produced by the burner pass. In either case, fan assistance may be provided, in a manner known in the art. A suitable sealing arrangement (not shown) should be provided around opening 17 in the base 15 of the housing, where duct 18 connects thereto. The duct 18 thus feeds exhaust gases into the interior of the housing 13 from whence they are directed either to the secondary heat exchanger 12 through outlet duct 19 or to a main exhaust duct 20 connected to atmosphere (see below).

The outlet duct 19 passes through an aperture in the cylindrical waIl 14 of the housing 13, a sealing arrangement being provided between the duct 19 and the wall 14. The duct 19 thus has an inlet 23 within the interior of the housing 13, such inlet being formed in effect by the end of the duct 19. As such, the inlet 23 effectively constitutes an inlet to the heat exchanger 12.

Also connected to the housing 13 is the main exhaust duct 20 which connects with atmosphere and selectively allows the exhaust gases to bypass the secondary heat exchanger 12. The duct 20 passes through another aperture in the cylindrical wall 14 of the housing 13, an appropriate sealing arrangement being provided between the duct and the wall. The duct 20 has an inlet 24 within the interior of the housing 13, said inlet being, in effect, the end of duct 20.

As shown, the ducts 19,20 pass through the cylindrical waIl 14 of the housing 13 at approximately the same level and their main longitudinal axes are set at an angle of approximately 900 to one another.

The secondary heat exchanger 12 itself has an exhaust pipe 25 extending therefrom which pipe 25 is connected into the exhaust duct 20 at a point 27 spaced from the housing 13. Thus, exhaust gases which have passed through the secondary heat exchanger 12 and having transferred heat to water flowing therethrough, pass into duct 20 at point 27, and thence to atmosphere.

The control flap 21 provided within housing 13 allows gases flowing into the housing 13 from outlet duct 18 to be supplied to the heat exchanger 12 via outlet duct 19 or alternatively to the outlet duct 20, depending upon the setting of the control flap. Normally, when the boiler 10 is operating, the exhaust gases will be directed through the secondary heat exchanger 12, by virtue of the flap 21 covering the inlet 24 to the exhaust duct 20. It is only at commencement of an ignition sequence of the ignition device that the control flap 21 is operated to cause the exhaust gases from the burner to be diverted directly into the outlet duct 20 and so bypass the secondary heat exchanger 12.

The control flap 21 is fixedly mounted on a spindle 28, between two cylindrical collars 29,30 which are themselves secured on the spindle adjacent its respective ends, to locate the spindle. The ends of the spindle 28 are carried in bearings provided respectively in the base 15 and cover 16 so that the spindle is rotatable through approximately 90, to move the control flap 21 between the positions shown in Figures 3 and 4. In Figure 3 the flap 21 healingly engages inlet 23 to duct 19 thereby preventing flow through the secondary heat exchanger; in Figure 4, the flap 21 sealingly engages the inlet 24 to outlet duct 20 thereby forcing exhaust gas flow through the secondary heat exchanger.

In the illustrated embodiment of the invention, the spindle 28 is rotatable by the action of a solenoid 32 mounted on the cover 16 of the housing 13, which solenoid 32 when energised moves the control flap 21 from its normal first position shown in Figure 4 to its second position shown in Figure 3, where flow through the secondary heat exchanger is inhibited for a short period, for at least a part of the ignition sequence. During that period the outlet duct 20 to the flue is opened, and exhaust gases coming from the burner via duct 18 pass directly into duct 20 and bypass the secondary heat exchanger 12. This avoids the production in the secondary heat exchanger of very acidic and sooty gases, when the burner is initially ignited. After that short period has elapsed, the solenoid 32 is de-energised, so allowing the flap 21 to return to its normal position under the action of a spring, so that the inlet 23 to the secondary heat exchanger 12 is open and the inlet 24 to outlet duct 20 is closed. The exhaust gases passing through the secondary heat exchanger 12 then heat return water from the heating system, before that water flows through the primary heat exchanger within the boiler.

As illustrated, the solenoid 32 is connected by an electrical cable 33 to a control box for the burner. The control box is a sequential controller which times the initial ignition cycle, energising spark electrodes to effect ignition, and also controls other lock-out and switching-off mechanisms. However, though here described as being directly controlled by the control box of the burner, other timing devices may be employed, to cause the flap 21 to operate as described, during initiation of a combustion period. For example, the period may simply be pre-determined (at say twenty seconds) in line with the normal - 10- start-up and operating conditions to be expected for a particular model of boiler with which the invention is to be used: a conventional timer may then be utilised to operate the solenoid 32 to move the flap 21.

Alternatively, the period may be determined in response to one or a plurality of the actual instant operating conditions or parameters of the boiler.

In one arrangement the solenoid 32 responds to the actuation of ignition electrodes at the burner. As the electrodes are first operated the solenoid is energised to move the flap 21 to its second position and after a pre-set period of time the solenoid will be de-energised to allow the flap to move back to its first position.

In another arrangement there is a photocell mounted adjacent the region of the flame produced by the burner to detect the presence of a flame. When the flame is first detected the solenoid 32 is energised to move the flap 21 to its second position. The solenoid is de-energised a pre-set period of time thereafter or, in an alternative design, is de-energised in response to the photocell sensing a specific intensity of flame, indicative of the combustion gases no longer being too acidic.

In a further embodiment a heat detector is mounted adjacent the flame region of the burner and this will cause energisation and/or deenergisation of the solenoid 32 to move the flap 21 in response to specific temperatures being sensed and/or specific periods of time elapsing. For example, the solenoid may be de-energised a predetermined time after a pre-set temperature is sensed in that region or when a second higher temperature is sensed.

In a yet further embodiment there is provided a water temperature sensor which will cause energisation and/or de-energisation of the solenoid 32 to move the flap 21 in response inter a/ia to (a) specific temperature(s) being sensed in the water which is heated by the boiler e.g. in the water passing through the primary heat exchanger, or passing through the secondary heat exchanger or passing some other point in the heating system.

It is to be noted that the cover 16 can be removed to gain access to the interior of the housing 13, for example to allow servicing of the flap 21 and the removal of deposits in the housing.

Instead of a housing 13 as illustrated, an elbow structure could be provided between the ducts 18, 19 and 20, a diverter valve assembly being provided with the elbow structure. More particularly, such an elbow structure may be incorporated in a Y-shaped conduit, where the main limb of the Y is connected to or forms part of the exhaust gas duct 18 leading from the burner and the diverging limbs of the Y are respectively connected to or form part of the secondary heat exchanger duct 19 and the outlet duct 20. Such an arrangement may incorporate at or adjacent the intersection of the three limbs of the Y a diverter valve operable selectively to connect the exhaust gas duct 18 from the primary heat exchanger to the outlet duct 20 or to the secondary heat exchanger duct 19. Such a diverter arrangement may, for example, comprise a flap valve similar to the valve illustrated in the accompanying drawings. Alternatively, it may comprise a shuttle valve having a generally cylindrical element formed with one or more passages and/or one or more peripheral recesses and lands, the element being movable longitudinally or - 12- rotationally, selectively to connect and disconnect the appropriate ducts. In any event, the valve element will be caused to move by operation of a solenoid or some other actuator in response to a simple elapse of time and/or to one or more of the operational conditions/parameters detailed in respect of the illustrated embodiment, as described above. - 13-

Claims

1. An oil- or gas-fired condensing boiler comprising a burner having an ignition device arranged to perform an ignition sequence at the commencement of a period of firing of the boiler, an arrangement for the passage of exhaust gases from the burner, a condensing heat exchanger having an inlet for exhaust gases from the arrangement, a diverter operable to direct exhaust gases away from said inlet, and control means arranged to cause operation of the diverter at the commencement of an ignition sequence of the ignition device whereby exhaust gases are diverted away from the heat exchanger for at least a part of an ignition sequence.

2. A boiler as claimed in claim 1, wherein said arrangement for the passage of exhaust gases from the burner includes a primary heat exchanger through which the exhaust gases pass before passing through the condensing heat exchanger.

3. A boiler as claimed in claim 1 or claim 2, comprising a main exhaust conduit through which the exhaust gases are exhausted to atmosphere, said main exhaust conduit having an inlet adapted to receive said exhaust gases when the exhaust gases are diverted away from the condensing heat exchanger.

4. A boiler as claimed in claim 3, wherein said diverter comprises a valve assembly.

5. A boiler as claimed in claim 4, wherein the valve assembly includes an element selectively movable to engage with an inlet to the condensing heat exchanger or with an inlet to the main exhaust conduit.

- 14 -

6. A boiler as claimed in claim 5, wherein said control means comprises a solenoid operable to move the valve element.

7. A boiler as claimed in claim 5 or claim 6, wherein the element is mounted for pivoting movement.

8. A boiler as claimed in any one of claims 3 to 7, wherein the inlet to the heat exchanger, the main exhaust conduit and the arrangement for the passage of exhaust gases from the burner are all connected to the interior of a common housing.

9. A boiler as claimed in claim 8, wherein the inlet to the condensing heat exchanger is provided by the inlet to a heat exchanger conduit connected to the heat exchanger.

10. A boiler as claimed in claim 9, wherein the inlet to the heat exchanger conduit and the inlet to the main exhaust conduit are arranged at approximately the same level in the housing.

11. A boiler as claimed in claim 9 or claim 10, wherein the longitudinal axes of the main exhaust conduit and of the heat exchanger conduit are arranged at an angle of approximately 900 to one another.

12. A boiler as claimed in any one of claims 8 to 11, wherein the arrangement for the passage of exhaust gases from the burner is connected to the interior of the housing through the base thereof.

13. A boiler as claimed in any one of claims 8 to 12 when appendent to claim 6, wherein the solenoid is mounted on the housing.

14. A boiler as claimed in any one of claims 8 to 13, wherein the housing has a removable cover. - 15-

15. A boiler as claimed in any one of the preceding claims, wherein the control means is arranged to cause operation of the diverter for a preset period of time after commencement of the ignition sequence.

16. A boiler as claimed in claim 15, wherein said period of time is determined at least partly by one or more instant operating parameters of the boiler.

17. A boiler as claimed in claim 15 or claim 16, wherein an ignition electrode is operatively connected to said control means.

18. A boiler as claimed in claim 16, wherein a photocell is adapted to sense the presence of a flame at said burner, said photocell being operatively connected to said control means.

19. A boiler as claimed in claim 16, wherein a heat detector is arranged adjacent the burner, said heat detector being operatively connected to said control means.

20. A boiler as claimed in claim 18, wherein a sensor is adapted to sense the temperature of a liquid passing through the boiler, said sensor being operatively connected to said control means.

21. A boiler as claimed in any of the preceding claims, wherein there is a timer arranged to be started on initiation of an ignition sequence for the burner, the timer being operatively connected to said control means.

22. A method of operating a condensing boiler having a burner, an ignition device, an arrangement for the passage of exhaust gases from the burner and a condensing heat exchanger having an inlet for exhaust gases from the arrangement, the method involving the operation of a diverter at the commencement of an ignition sequence of the ignition device whereby exhaust - 16 - gases are diverted away from the heat exchanger for at least part of the ignition sequence.

23. A method of operating an oil-fired condensing boiler as claimed in claim 22, said boiler having the features of any one of claims 1 to 21.

24. A diverter for use in a condensing boiler which comprises an arrangement for the passage of exhaust gases from a burner, a condensing heat exchanger having an inlet for exhaust gases from the arrangement and a main exhaust conduit connected to atmosphere, said diverter being adapted selectively to supply exhaust gases from the arrangement to the inlet of the heat exchanger or to the main exhaust conduit, the diverter being operated at the commencement of an ignition sequence of an ignition device so as to supply exhaust gases from the arrangement to the main exhaust conduit for a period of time, and after elapse of said period to supply exhaust gases from the arrangement to the inlet of the heat exchanger.

25. A condensing boiler substantially as hereinbefore described and as illustrated in the accompanying drawings.

26. A method of operating a condensing boiler substantially as hereinbefore described with reference to the accompanying drawings.

27. A diverter valve for use with a condensing boiler, said diverter being substantially as hereinbefore described and as illustrated in the accompanying drawings.