GB2256701A

GB2256701A - Gas fired boiler

Info

Publication number: GB2256701A
Application number: GB9212372A
Authority: GB
Inventors: Mark Longrigg; Craig Barnes
Original assignee: BURCO DEAN APPLIANCES Ltd
Current assignee: BURCO DEAN APPLIANCES Ltd
Priority date: 1991-06-11
Filing date: 1992-06-11
Publication date: 1992-12-16
Anticipated expiration: 2012-06-11
Also published as: GB9212372D0; GB2256701B

Abstract

A gas fired boiler comprises, in vertical array, an upper down-firing gas burner (18), an intermediate heat exchanger (15) and a lower suction fan (16). The arrangement of the boiler provides for a large reduction in the size of the unit. Located on the burner (18) is a premixer (17) having an air inlet tube (25) which passes into and across a mixer changer and ends adjacent a hexagonal lateral gas injector (28). The provision of a good quality air/gas pre-mixer (17) obviates the need for a conventional upstream blower fan. The downstream location of the fan (16) ensures that any leaks in the burner or heat exchanger cannot result in dangerous gas leakage and also permits the connection of the boiler to relatively long air inlet and exhaust flue ducts. <IMAGE>

Description

A GAS FIRED BOILER This invention relates to gas fired boilers, especially but not exclusively domestic gas fired central heating boilers.

US Patent No. 4366778 discloses a boiler enclosed in a sealed casing. The boiler has a down firing tubular burner the upper part of which has a mixer in the form of an annular jacket. Combustion air is injected under pressure from a fan in an inlet duct into the casing around the boiler and into the top of the burner where it is mixed with fuel gas.

UK Patent application No. 2213243 discloses a gas fired hot water boiler which comprises a down firing gas burner above a heat exchanger. Above the burner is a fan which draws air in from an adjacent inlet duct and receives fuel gas from a feed pipe. The air and gas are mixed in the fan to provide a premixed forced draught gas and air mixture to the burner.

UK Patent No. 1422503 describes a forced draught gas heater having an upward firing gas burner above which is located a heat exchanger. An extractor fan is located above the heat exchanger adjacent an exhaust duct. An inlet duct is located coaxially around the outlet duct. The assembly is enclosed in a sealed unit. Fresh air is admitted through the inlet duct and is drawn downwards to the burner so as to establish a circulation of combustion products from the bottom (burner) to the top (exhaust) of the unit.

It is well known in the field of boilers or heaters that downfiring burners are preferred as they allow for a more compact arrangement by virtue of a smaller flame size. However, the downfiring burner of UK 2213243 requires an upstream fan to be mounted above the burner, the fan mixing air and gas before the mixture is forced under pressure into the burner. This arrangement requires that a casing of the fan and the burner must be reliably sealed to prevent leaks of mixed air and gas or combustion products. This requirement increases the size and expense of the boiler/heater assembly. The arrangement described in US4366778 has a tubular burner which has flame emitting holes along its length which is disposed in the heat exchanger.This arrangement requires an upstream fan to blow air into the air/gas premixer, making reliable sealing very important, and occupies a large space longitudinally.

A suction fan disposed in a heater exhaust duct is known from UK Patent No. 1422503. This arrangement however is used with an upwardly firing atmospheric burner which does not have a premixer.

A 1986 International Gas Research Conference report entitled "Development and Evaluation of A High Efficiency Condensing Boiler", Dr. M. Searle and C.G. Allen, describes the design of a "fully premixed" down firing boiler. The document describes a forced draught type premixer in which gas is fed into the inlet of and mixed with air within the fan. Again this requires the combustion chamber to be very carefully sealed.

It is an object of the present invention to provide a gas fired boiler which for a given capacity or rating is substantially reduced in bulk and dimensions compared with an existing gas fired boiler of the same capacity or rating.

According to the present invention there is provided a gas fired boiler comprising, in a vertical array, a static premixer to which air at atmospheric pressure and fuel gas is admitted, a burner located beneath the premixer, a heat exchanger located beneath the burner, and an exhaust fan located beneath the heat exchanger to draw fuel gas and air mixture from the premixer into the burner and combustion products from the burner through the heat exchanger to an exhaust.

The term "static" premixer is used herein to indicate a premixer which has no moving parts, in contrast to an arrangement in accordance with the prior art discussed above in which a fan is used to achieve premixing.

The exhaust fan may be a motor driven centrifugal fan, and a diffuser baffle may be located between the heat exchanger and the fan. The premixer preferably comprises an air inlet tube arranged to direct a flow of air onto a gas injector. The injector may be arranged to produce a plurality of gas jets directed transversely to the flow of air, the gas jets being directed radially relative to a point on a main axis of the air inlet tube.

Preferably the injector is supported adjacent a wall of a chamber into which the air inlet tube opens, the air flow being directed perpendicular to the wall and the gas jets being directed parallel to the wall. The air inlet tube may be arranged to terminate close to the said wall, for example at a distance of 15 mm from the wall, but preferably less than 10 mm from the wall, for example 8 mm from the wall. The air inlet tube may have a diameter of for example 40 mm. The injector may be hexagonal with a gas jet being produced from each of a series of apertures each provided in a respective face of the hexagon.

Preferably the chamber comprises a lower wall beneath the air inlet tube, an aperture being formed in the lower wall to enable gas to flow from the mixer to the burner. The aperture may be formed centrally within the lower wall such that the air inlet tube projects horizontally beyond the aperture. The aperture may have a diameter of substantially 35 mm and the inlet tube may project horizontally at least 40 mm beyond the vertical plane through the centre of the aperture.

The premixer may be connected to a rearwardly extending air inlet and the fan exhaust may be connected to a rearwardly extending outlet. The vertical array may be detachably mounted on a tray-like plinth which may be secured to a supporting wall. One or more of the walls of the plinth may be provided with one or more duct openings for passage therethrough of an air or exhaust gas duct. The or each duct opening may normally be closed by a knock-out plate. An air inlet duct and the exhaust of the fan may be connected by elbow pipe connections.

Preferably the vertical array comprises a rear frame to which the premixer, burner, heat exchanger and fan are secured, the boiler being adapted to be secured to a wall by securing the rear frame to that wall. The frame may removably house below the fan a removable control box carrying gas boiler controls.

It is a further object of the present invention to provide a static premixer the structure of which is such that it can reliably premix fuel gas and air to enable its use in for example a down-firing boiler.

To achieve this further object, the present invention further provides a premixer for mixing fuel gas with air drawn through the premixer by a fan downstream of the premixer, comprising a gas injector located adjacent a substantially planar wall of a mixing chamber and arranged to direct jets of gas away from the injector in a direction substantially parallel to the planar wall, an air inlet in the form of a tube extending across the chamber towards the injector, the inlet tube being open to the atmosphere at its end remote from the injector, and an outlet from the chamber located in a bottom wall of the chamber, wherein the injector is substantially smaller in directions perpendicular to the inlet tube than the internal diameter of the end of the inlet tube adjacent the injector, and the tube extends across the chamber to a position beyond the edge of the aperture which is nearest to the injector.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a front view of a domestic gas fired boiler in accordance with the present invention with an external casing removed; Fig. 2 is a corresponding side view of the boiler of Fig. 1, the external casing being indicated in dotted outline; Fig. 3 is a part sectioned side view of a premixer and burner assembly of the boiler of Fig. 1; Fig. 4 is a perspective view of a diffuser baffle of the boiler of Fig. 1; and Figs. 5, 6 and 7 illustrate the effect on burner efficiency of varying the dimensions of components of the premixer of Fig. 3.

The illustrated gas fired boiler comprises a metal rectangular frame generally indicated at 10 to which is detachably mounted by screws or bolts (not shown) a rear tray-like plinth 11, there being a removable external casing 12 of aesthetic appearance detachably secured again by screws or bolts (not shown) to either the frame 10 or the plinth 11.

The tray-like plinth 11 has four walls 13 each of which is formed with two spaced duct openings normally closed by knock-out plates or discs (not shown). As a result an air inlet pipe or duct and a flue products outlet pipe or duct can be connected to the gas fired boiler respectively at any one of eight locations to suit installation requirements. Reference will be made to this again later.

The gas fired boiler per se is a sealed module consisting of, in vertical spaced array, an upper down-firing gas burner assembly 14, an intermediate heat exchanger 15, and a lower motor-driven centrifugal fan assembly 16.

The gas burner assembly comprises a gas/air premixer 17 and a rectangular gas burner 18 having a double walled convex perforated lower surface 19. A horizontal perforated baffle plate divides the burner 18 into upper and lower components, the baffle plate being secured between flanges 20 extending around upper and lower burner body components 21 and 22 as shown in Fig. 3. The gas boiler module 14 to 16 is, in effect, of rectangular box configuration with a front plate 23 removable for access purposes. Removal of this front plate 23 allows access to both the top and bottom of the gas boiler module 14 to 16 for maintenance and repair purposes.

The gas boiler module 14 to 16 is sealed in terms of air, gas, flue products and water inflows and outflows save through specific inlets and outlets. If however any small leaks develop upstream of the fan 16, leakage will be into the assembly from the atmosphere and therefore safe conditions will be maintained.

As shown in Fig. 3, the pre mixer 17 is in the form of a rectangular mixing chamber 24 supporting an air inlet tube 25 which passes into and across most of the width of the chamber towards a side wall 26. The spacing between the end of the tube 25 and the side wall 26 is 8 mm. A gas supply tube is connected to the chamber 24 via an elbow connector 27. On the other side of the chamber wall 26 to the elbow connector 27 is a hexagonal lateral gas injector 28. The injector 28 has a hole 29 on each flat of the hexagon through which gas is injected (see inset to Fig. 3 which indicates the six radial gas jets issuing from the injector by arrows 30). The injector 28 has a maximum outside diameter of 14 mm measured to the line joining the edges of any two adjacent flats.The mixer 17 is located directly above the burner 18 and is fixed and sealed thereto, the lower wall of the chamber 24 being defined by an upper wall 31 of the burner.

The gas/air mix passes through a circular aperture 32 in the upper wall of the burner 18, the aperture 32 having a diameter of 35 mm.

The burner 18 is structured such that only a relatively small pressure drop occurs across its depth, which makes available more pressure to pump out the exhaust gas through the suction fan 16.

A combustion chamber 33 is disposed beneath the gas burner 18 adjacent the heat exchanger 15. A water inlet to the heat exchanger 15 is indicated at 34, a water outlet from the heat exchanger at 35, and water flow connection pipes between heat exchanger sections (three for example) at 36.

The motor-driven fan assembly 16 comprises a centrifugal suction fan 37 which is driven by an external electric motor 38. A diffuser plate 39 as shown in Fig. 4 is located between the heat exchanger 15 and the fan 37. The diffuser 39 is a substantially flat perforated plate having down-turned side edges. The diffuser plate 39 acts to distribute evenly the suction force across the heat exchanger 15.

The gas boiler module 14 to 16 is detachably secured by screws or bolts (not shown) onto the rear frame 10. The centrifugal fan 37 has a rear combustion products outlet 40, and this outlet and the air inlet 25 both traverse the rear frame 10. An elbow connection 41, 42 respectively connects the air inlet 25 to an air inlet pipe or duct 43 and the flue products outlet 40 to an exhaust flue pipe or duct 44 which extends through selected duct openings in one of the sidewalls 13 of the plinth 11. The ducts 43 and 44 may alternatively be directly connected to inlet 25 and outlet 40 to provide rectilinear flow paths into and out of the gas boiler 14 to 16. In such an arrangement the tray-like plinth 11 may be dispensed with if desired, or if necessary as a result of dimensional installation restrictions for example.

The gas supply to the mixer/injector device 17 is via a gas inlet pipe 45 from the gas supply mains, a gas control valve 46 mounted on the rear frame 10, and a supply pipe 47 between the latter and the mixer/injector device 17.

The controls for the gas fired boiler are electronic in nature and are supported in a removable tray 48 below the gas control valve 46 and the motor-driven suction fan assembly 16. The front of the tray 48 mounts a fault indicator LED 49, an on/off switch 50 and a thermostat control knob 51. A gas pressure switch 52 is mounted on the rear frame 10 and is operatively associated with the gas control valve 46. Within the combustion chamber 33, a spark ignition device 53 and a flame sensing probe 54 are provided. Electronic controls of the gas-fired boiler include, inter alia, an overheat thermostat and pump over-run device (both not shown). The removable nature of the controls tray assists in production, installation and maintenance.

The efficiency of the described boiler is dependent upon the premixer ensuring good mixing of the fuel gas with air. The performance of the premixer is dependent upon careful selection of the dimensions of components of the premixer. The effects on premixer performance of adjusting certain critical dimensions are described below with reference to Figs. 5, 6 and 7.

With reference to Fig. 5, this illustrates the carbon monoxide and carbon dioxide content in percent in the flame issuing from the burner which has been described above, measurements having been taken at points spaced apart along the 100 mm length of the burner, that is along a line perpendicular to Fig. 1 and located centrally of the curved burner surface 19. The results are plotted for six different positions of the end of the inlet tube 25 relative to the aperture 32. The line labelled 0 mm corresponds to the plane defined by the end of the tube 25 being positioned so as to extend through the centre of the aperture 32. It can be seen that combustion is improved by ensuring that the end of the tube overhangs the aperture 32. The larger the overhang, the better the performance.

Further tests have indicated that the width of the gap between the end of the air inlet tube 25 and the facing wall 26 of the premixer chamber affects mixing efficiency. In the illustrated arrangement, the optimum gap width was found to be 8 mm. A gap width of less than 8 mm resulted in excessive restriction, giving poor combustion at high outputs. Increasing the gap width substantially above 8 mm resulted in poor combustion, particularly at the edges of the burner. Thus it is preferred that the gap width is less than 15 mm, and preferably less than 10 mm.

Further tests have indicated that injector size relative to inlet pipe size significantly affects premixer efficiency. Fig. 6 illustrates the effect of varying injector diameter, all other parameters of the premixer being as described with reference to Fig. 3. It can be seen that performance falls off rapidly as the injector diameter is increased. Thus it is preferred that the injector diameter is less than half the inlet tube diameter, advantageously approximately one third of the inlet tube diameter.

Further test have been conducted to compare the performance of a simple inlet pipe geometry as shown in Fig. 3 with an inlet pipe of larger diameter but supporting a radially inwards extending flange adjacent the outlet end. No significant advantages have been detected with such a relatively complex geometry. Further tests have been conducted to assess the effect on combustion of adjusting the mixing chamber dimensions. These tests indicated that adjusting the length of the chamber (in the direction of the air inlet tube) had negligible effect, although combustion was improved with reducing width and height, each dimension being reduced with no reduction in the other dimensions. These effects are illustrated by lines 55 (length), 56 (width) and 57 (height) in Fig. 7. The initial conditions for each test were a mixing chamber 55 mm wide, 88 mm long and 63 mm high.This gave an overall volume of approximately 300,000 cu mm. Adjusting all of these dimensions equally revealed that combustion was improved as indicated by curve 58 until the total volume reached approximately 150,000 cu mm. Further reductions in volume rapidly degraded the combustion performance.

A gas fired boiler as described above having a 40,000 B.T.U.

rating may be, for example, 300 mm wide (side-to-side), 275 mm deep (back to front) with the plinth 11 fitted and 200 mm deep with the plinth 11 removed, and 400 mm in height. This size of boiler represents around a 60% reduction in size compared with previous boilers of a comparable capacity. A gas fired boiler according to this invention may, however, have a rating which is higher or lower than 40,000 B.T.Us.

Due to the provision of the powerful suction fan it is envisaged that an exhaust flue pipe or duct length of up to 10 metres (straight) or 6 m (with bends) may be provided. Also the flue pipe or duct external of the gas fired boiler may incorporate up to, say, three changes in direction to suit installation requirements.

Claims

CLAIMS:

1. A gas fired boiler comprising, in a vertical array, a static premixer to which air at atmospheric pressure and fuel gas is admitted, a burner located beneath the premixer, a heat exchanger located beneath the burner, and an exhaust fan located beneath the heat exchanger to draw fuel gas and air mixture from the premixer into the burner and combustion products from the burner through the heat exchanger to an exhaust.

2. A gas fired boiler as claimed in claim 1, in which the exhaust fan is a motor-driven centrifugal fan.

3. A gas fired boiler as claimed in claim 1 or 2, wherein a diffuser baffle is located between the heat exchanger and the fan.

4. A gas fired boiler as claimed in any preceding claim, wherein the premixer comprises an air inlet tube arranged to direct a flow of air onto a gas injector.

5. A gas fired boiler as claimed in claim 4, wherein the injector produces a plurality of gas jets directed transversely to the flow of air, the gas jets being directed radially relative to a point on a main axis of the air inlet tube.

6. A gas fired boiler as claimed in claim 5, wherein the injector is supported adjacent a wall of a chamber into which the air inlet tube opens, the air flow being directed perpendicular to the wall and the gas jets being directed parallel to the wall.

7. A gas fired boiler according to claim 6, wherein the air inlet tube ends substantially 8 mm from the wall.

8. A gas fired boiler according to claim 6 or 7, wherein the air inlet tube has a diameter of substantially 40 mm.

9. A gas fired boiler according to claim 6, 7 or 8, wherein the injector is hexagonal, a gas jet being produced from each of a series of apertures each provided in a respective face of the hexagon.

10. A gas fired boiler according to any one of claims 6 to 9, wherein the chamber comprises a lower wall beneath the air inlet tube, an aperture being formed in the lower wall to enable gas to flow from the mixer to the burner.

11. A gas fired boiler according to claim 10, wherein the aperture is formed centrally within the lower wall such that the air inlet tube projects horizontally beyond the aperture.

12. A gas fired boiler according to claim 11, wherein the aperture has a diameter of substantially 35 mm and the inlet tube projects horizontally at least 40 mm beyond a vertical plane through the centre of the aperture.

13. A gas fired boiler according to any preceding claim, wherein the premixer is connected to a rearwardly extending air inlet and the fan exhaust is connected to a rearwardly extending outlet.

14. A gas fired boiler as claimed in any preceding claim, in which the vertical array is detachably mounted on a tray-like plinth which may be secured to a supporting wall.

15. A gas fired boiler as claimed in claim 14, in which one or more of the walls of the plinth is provided with one or more duct openings for passage therethrough of an air or exhaust gas duct.

16. A gas fired boiler as claimed in claim 15, in which the or each duct opening is normally closed by a knock-out plate.

17. A gas fired boiler as claimed in claim 15 or 16, in which an air inlet duct and the exhaust of the fan are connected to air and exhaust gas ducts by elbow pipe connections.

18. A gas fired boiler as claimed in any preceding claim, in which the vertical array comprises a rear frame to which the premixer, burner, heat exchanger and fan are secured.

19. A gas fired boiler as claimed in claim 18, which is adapted to be wall mounted by securing the rear frame to a wall.

20. A gas fired boiler as claimed in claim 18 or 19, in which the frame removably houses below the fan a removable control box carrying gas boiler controls.

21. A premixer for mixing fuel gas with air drawn through the premixer by a fan downstream of the premixer, comprising a gas injector located adjacent a substantially planar wall of a mixing chamber and arranged to direct jets of gas away from the injector in a direction substantially parallel to the planar wall, an air inlet in the form of a tube extending across the chamber towards the injector, the inlet tube being open to the atmosphere at its end remote from the injector, and an outlet from the chamber located in a bottom wall of the chamber, wherein the injector is substantially smaller in directions perpendicular to the inlet tube than the internal diameter of the end of the inlet tube adjacent the injector, and the tube extends across the chamber to a position beyond the edge of the aperture which is nearest to the injector.

22. A gas fired boiler substantially as herein before described with reference to Figs. 1 to 4 of the accompanying drawings.