US3672812A

US3672812A - Fuel burner unit for mounting in a common air box

Info

Publication number: US3672812A
Application number: US93625A
Authority: US
Inventors: Knud Erik Bendixen
Original assignee: Burmeister and Wains Motorog Maskinfabrik AS
Current assignee: Burmeister and Wain AS; Burmeister and Wains Motorog Maskinfabrik AS
Priority date: 1969-12-09
Filing date: 1970-11-30
Publication date: 1972-06-27
Anticipated expiration: 1989-06-27
Also published as: SE356119B; GB1283629A; FI52000C; NO128345B; DE2059020A1; JPS5018618B1; DE2059020B2; DK120810B; DE2059020C3; FI52000B

Abstract

A FUEL BURNER UNIT DESIGNED TO BE CONNECTED TO A COMMON AIR SUPPLY BOX IN PARALLEL WITH ONE OR MORE LIKE BURNER UNITS. THE BURNER COMPRISES A DUCT SYSTEM FOR CONVEYING COMBUSTION AIR FROM THE AIR BOX TO THE BURNER HEAD AND INCLUDES A SINGLE ANNULAR OUTER DUCT CONNECTED AT ITS FRONT END TO THE AIR BOX AND AT ITS REAR END TO INDIVIDUAL INNER DUCTS FOR PRIMARY AND SECONDARY COMBUSTION AIR, RESPECTIVELY. THE VOLUME RATIO BETWEEN PRIMARY AND SECONDARY AIR IS DETERMINED BY AN ADJUSTABLE, BUT NORMALLY FIXED BAFFLE DEVICE AT THE REAR OUTLET END OF THE OUTER DUCT, THE TOTAL AMOUNT OF COMBUSTION AIR SUPPLIED TO EACH BURNER IS CONTROLLED BY A VALVE DEVICE LOCATED AT THE INLET END OF THE ANNULAR OUTER DUCT.

Description

June 27, 1972 E. BENDIXEN FUEL BURNER UNIT FOR MOUNTING IN A COMMON AIR BOX Filed NOV. 30. 1970 2 Sheets-Sheet 1 June 1972 K. E. BENDIXEN 3,

FUEL BURNER UNIT FOR MOUNTING IN A COMMON AIR BOX Filed Nov. 30. 1970 2 Sheets-Sheet 2 United States Patent Office 3,672,812 Patented June 27, 1972 Int. (:1. F23n US. Cl. 431-89 7 Claims ABSTRACT OF THE DISCLOSURE A fuel burner unit designed to be connected to a common air supply box in parallel with one or more like burner units. The burner comprises a duct system for conveying combustion air from the air box to the burner head and includes a single annular outer duct connected at its front end to the air box and at its rear end to individual inner ducts for primary and secondary combustion air, respectively. The volume ratio between primary and secondary air is determined by an adjustable, but normally fixed bafile device at the rear outlet end of the outer duct. The total amount of combustion air supplied to each burner is controlled by a valve device located at the inlet end of the annular outer duct.

BACKGROUND OF THE INVENTION The present invention relates to a fuel burner unit for boilers, furnaces and other heat exchanging apparatus, and of the type which is intended to be mounted in an air box common to several units and which has at least two parallel flow ducts for the supply of primary and secondary combustion air. It is well-known that an almost completely or essentially stoichiometric combustion is advantageous by yielding flue or exhaust gases with a low sulphur trioxide content and, thereby, permits low smoke or flue temperatures without risk of corrosion damages. However,

stoichiometric combustion requires a precise matching of the air volume supplied to each burner unit in dependence upon the amount of fuel supplied, which, to date, has proved difficult to effect in actual practice. Boilers have been constructed having a completely separate and relatively long air supply duct for each burner, i.e. without a common air box, but this represents a complicated and expensive solution from a constructional point of view and, moreover, presents difficulties with respect to the desired automatic control of the firing plant. In plants having a common air box it is known to effect a separate control of the volumes of primary and secondary air supplied to the individual burners, but this complicates the control and, consequently, in other known designs, the entire volume of combustion air has been supplied collectively to each burner without any division into primary and secondary air being effected, while various designs of the burner and the air duct have been employed in an attempt to obtain an acceptable course of the combustion process. However, experience gained from actual practice shows that the chances of optimizing the combustion process by means of this solution are limited.

BRIEF SUMMARY OF THE INVENTION According to the invention there is provided a fuel burner unit comprising an open front end and a closed rear end, duct means for supplying fuel to said burner front end, at least two parallel duct means for supplying primary and secondary combustion air, respectively, to said burner front end, an annular duct surrounding said two duct means and connected thereto at its rear end,

fixed air distributor means for said primary and secondary air duct means located at the rear end of said annular duct, means at the front end of said annular duct for connecting the duct to an air box common to a plurality of burner units, adjustable air valve means at said front end of said annular duct, means for measuring the air flow rate through said annular duct, and means for adjusting said air valve means in dependence upon the measured air flow rate.

It has surprisingly been found that the invention provides a satisfactory solution of the problems pointed out above. The supply of the total air volume to each burner via a common duct and the control of the air volume with the aid of a valving means located at the inlet of the duct where it is connected to the common air box provides a structurally simple design of the burner and the associated control system; it has also been found possible to ensurewithin a wide load rangea satisfactory distribution of the total air volume into the primary and secondary air system, respectively, without any change in the setting of the distributor provided at the rear end of the burner. Consequently, an adjustment of this setting is required only when the firing plant is taken into service or when there are material changes in the operating conditions, for instance, when changing over to a different fuel. When, according to the invention, primary and secondary air is supplied separately to the burner head or front end, while the volumetric control is effected on the total air volume supplied to the unit, it is possible to optimize the combustion process and generally to obtain those advantages which stem from dividing the air into primary and secondary air streams and from using a common air supply, respectively, without the respective concomitant drawbacks to which the known designs were subject.

The flow rate measuring means may be adapted to measure the difference between the static pressure at the valving means and at a location intermediate the ends of the duct, respectively, as this pressure difference depends on the air velocity in the duct and, hence, on the air flow rate.

According to a feature of the invention, there may be provided a measuring chamber connected to the measuring means, which chambercommunicates with the annular duct but is otherwise closed, adjacent and downstream of the valving means at the front end of the annular duct. In this way, a particularly accurate value of the static pressure at the inlet of the duct is obtained independently of the pressure conditions prevailing in the air box and the possible pressure drop across the valving means.

The annular duct may be designed as a Venturi nozzle, and the static pressure intermediate the ends of the duct may be measured at the narrowest cross section of the nozzle, whereby a certain recovery of the pressure drop in the constriction of the nozzle is ensured which favourably affects the economy of operation of the burner.

If the available space does not permit the relatively great projecting length of the burner unit which the Venturi shape requires, an orifice restrictor or diaphragm may alternatively be mounted in the annular duct and the measuring means may be connected on the upstream and the downstream side of the orifice restrictor, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a longitudinal section through an embodiment of the burner unit according to the invention mounted in a common air box,

FIGS. 2 and 3 show two corresponding longitudinal sections through modified embodiments of the burner unit,

FIG. 4 shows a cross section along the line 44 in FIG. 1, and

FIG. 5, shows a schematic damper control.

3 DETAILED DESCRIPTION All the burner units shown in the drawings are designed for being mounted in an air box 7 common to several burner units, which box extends along a combustion chamber 15 (not shown in detail) and is provided with means (not shown) for supplying combustion air in the requisite amount thereto.

Each burner unit comprises a centrally located supply duct or line 16 for the fuel which may be of any type, liquid, gaseous or pulverized solid fuel. In the front end of the fuel line 16, that is to say at the left-hand side of FIGS. 1, 2 and 3, there may be provided means (not shown) for distributing and/or atomizing the fuel into the combustion chamber 15. Such means may be of any known type.

In all the embodiments shown, the air necessary for the combustion of the fuel is supplied to the burner head or front end' via two separate ducts, viz a central duct 1 surrounding the supply line 16 and through which primary air is supplied, and a secondary air duct 2 surrounding the duct 1 coaxially. At the front end of each of the ducts 1 and 2, so-called

turbulators

3 and 4, respectively, are schematically shown, which turbulators may consist of inclined or curved plates which impart a suitable rotation to the two air streams before they enter the combustion chamber through a primary air nozzle 5 and a secondary air nozzle 6, respectively. The

tubulators

3 and 4 may be fixed or they may be adjustable with the aid of moving means (not shown).

In the embodiment of FIGS. 1 and 4, an annular flow duct 9 is defined between the external wall of the burner unit and the secondary air duct 2, and the front end of the flow duct 9 communicates with the air box 7 via a conically constricted inlet 8. At the rear end of the duct 9, an air distribution means in the form of an annular baifie device 10 is mounted comprising a central sleeve 17, which is guided on the wall of duct 1, and an annular baffie plate 18, the outermost edge of which is curved so as to extend into the discharge or rear end of duct 9. The bafile device 10 divides the air stream coming from the duct 9 into a primary air stream which continues through the internal duct 1, and a secondary air stream flowing through duct 2, respectively. The sleeve 17 may be displaceable in its longitudinal direction along the wall of the duct 1, so that it is possible to vary the ratio between the cross-sectional areas through which the air flows from duct 9 into each of the ducts 1 and 2, and hence, the mutual ratio between the volumes of primary and secondary air. However, during the operation of the burner, the battle device 10 may remain stationary since an adjustment of the said ratio normally only is required when there are significant changes in the operational conditions.

Duct 9 including the inlet 8 is shaped as a Venturi nozzle, and in the constricted section thereof there is an aperture 19 in the outer wall of the duct for connecting a schematically indicated differential pressure gauge 20. The other measuring point of gauge 20 is constituted by a similar aperture 21 in the peripheral wall of a measuring chamber 13 which wall is closed except for the aperture 21 and is coaxial with the

ducts

1, 2 and 9. The chamber 13 is located closely adjacent and downstream of an annular damper or valve 14 which is mounted in the air box 7 coaxially with the burner unit and which may be displaced axially to vary the inflow cross section area of the duct inlet 8. The measuring chamber 13 communicates, via one or more apertures 22 in its end wall, with the inlet 8.

As shown in FIG. 1, the pressure gauge 20 senses the difference between the static pressure prevailing in the measuring chamber 13 and, hence, immediately downstream of the damper 14, and the pressure at the narrowest section of the duct 9. Consequently, the differential pressure measured will indicate the total air volume passing through duct 9 per unit of time since the cross sectional area at the measuring point 19 is known. The said differential pressure which, in FIG. 1, is indicated schematically in the form of a liquid column Ap, may in a manner known per se be utilized as input signal in a control system (see FIG. 5) for continuously adjusting the damper 14 in such a way that the total volume of air supplied to each burner is kept at a predetermined value which, in turn, depends on the amount of fuel supplied via the line 16. With a single measuring device for each burner unit and a correspondingly simple control system, it is thus possible, independently of any pressure variations in the air box, to control the air volumes supplied to the individual burners of a firing plant in a desired manner and in such a way that optimal conditions for combustion in the individual burner, units are obtained. During the running-in period of the firing plant, it is possible, for each individual burner unit, to set or adjust experimentally the transfer function between the measuring signal derived from the differential pressure gauge 20 and the driving means (not shown) which displace the damper 14 to vary the inflow area of duct 9 from the air box 7. In an analogous manner, it is possible to experimentally effect the adjustment of baffie device 10 which adjustment may normally be maintained in the course of the subsequent operation of the unit.

The embodiment illustrated in FIG. 2 corresponds, so far as important elements are concerned, to that shown in FIG. 1, and for these elements, the same reference numerals are employed as in FIG. 1.

The difference resides in the constructional design of the common air supply duct surrounding the primary and secondary air ducts 1 and 2. While the duct in FIG. 1

was constructed in the form of a Venturi nozzle and,

consequently, was relatively long, so that the air could expand downstream of the narrowest cross section of the nozzle, the corresponding duct 11 in FIG. 2 is considerably shorter. The reduction in the space requirement of the burner unit achieved hereby is accompanied by a minor decrease in the economy of operation due to the increased energy loss on account of the higher air velocity of the duct 11 and in the transition area to the ducts 1 and 2 including the bafile device 10. The mode of operation is, however, generally the same as for the embodiment in FIG. 1. a

FIG. 3 shows an embodiment which, like that shown in FIG. 2, is distinguished by a short structural length of the complete burner unit, but in which the total air volume supplied is measured by measuring the dilferental pressure across an orifice restrictor 12 or apertured diaphragm located in the annular duct between the inlet 8 and the bafile device 10. In FIG. 3, the measuring points of the differential pressure gauge 20 are located on the downstream side of the orifice restrictor 12 and in the measuring chamber 13 described above, respectively, but the latter measuring point might also be located directly on the upstream side of the orifice restrictor in the outer wall of the annular air duct.

I claim:

1. A fuel burner unit comprising an open front end and a closed rear end, duct means for supplying fuel to said burner front end, at least two parallel duct means for supplying primary and secondary combustion air, respectively, to said burner front end, an annular enclosure surrounding said two duct means, air distributor means located at the rear end of said annular enclosure to divide an incoming stream for said primary and secondary air duct means, means at the front end of said annular enclosure for connecting it to an air box, adjustable air valve means at said front end of said annular enclosure, means for measuring the air flow rate through said annular enclosure, and means for adjusting said air valve means in dependence upon the measured air fiow rate.

2. A fuel burner unit as claimed in claim 1, wherein said flow rate measuring means comprises means for sensing the difference between the static air pressure in the region of said air valve means and at a location intermediate the front and rear ends of said annular enclosure.

3. A fuel burner unit as claimed in claim 2, comprising means defining a measuring chamber located adjacent and downstream said air valve means and communicating with said annular enclosure and with said flow rate measuring means.

4. A fuel burner as claimed in claim 3, wherein said measuring chamber is annular and comprises a closed peripheral wall coaxially surrounding said duct means and having an apertured end wall facing said rear end.

5. A fuel burner unit as claimed in claim 2, wherein said annular enclosure is formed as a Venturi nozzle, and said intermediate location in which the static pressure is measured, is at the narrowest cross-section of said nozzle.

6. A fuel burner unit as claimed in claim 2, comprising an apertured diaphragm located intermediate the ends of said annular enclosure, said intermediate location being situated at the downstream side of said diaphragm.

7. A fuel burner unit as claimed in claim 1, wherein said air valve means is in the form of an open-ended drum arranged coaxially with said annular enclosure and adjustable in the longitudinal direction thereof.

References Cited UNITED STATES PATENTS CARROLL B. DORITY, 111., Primary Examiner US. Cl. X.R.