A METHOD OF OPERATING A COMBUSTION APPARATUS HAVING A FIRED HEATER

Abstract: This invention relates to combustion apparatus incor­ porating a balanced draught system comprising a forced draught fan (6) for delivering combustion air to a burner section (7) and an induced draught fan (9) for extracting effluent gases from the burner section (7). A drive mechan­ ism (10) operates the fans. In order to improve the efficiency of the apparatus and simplify control of gas flow there­ through, according to the invention the fans (6, 9) are operatively coupled such that adjustment of both is simul­ taneous, maintaining a predeterminable relationship be­ tween their outputs. This enables system characteristics to be matched almost exactly. Coupling of the drive mechanism (10) to the respective fans can be by conventional means, but fan control is preferably accomplished by speed variation, electric motors using a frequency converter system.

Description: This invention relates to combustion apparatus, such as furnaces, boilers and fired heaters. Particularly it relates to such apparatus incorporating a balanced draught system, and is suited especially to the combination of liquid or gaseous hydrocarbon fuels. Primary uses of apparatus according to the invention are as a feed preheater in a reactor system and in the furnace section of steam generators. Energy conservation schemes are now a common feature of new installations and are increasingly being incorporated into old plant. In fired heaters, furnaces, boilers and the like, many of these schemes take the form of combustion air preheating by the outgoing hot flue gases. Such apparatus may comprise a flue gas air preheater of either recuperative or regenerative type, a forced air draught fan (FD fan) to blow the combustion air through the air side of the preheater thence through ducts to the burners, and an induced draught fan (ID fan) to. suck the flue gases fran the radiant section, over the convection banks and thence through the flue gas side of the air preheater to the ID fan which then discharges the flue gases to a stack or duct work of a common chimney systems. The FD fan and the ID fan have individual driving means, normally steam or electric, and each fan, which is generally a centrifugal type fan, has an output controlled by either radial inlet vanes or sane form of variable speed system which would alter the rotational speed of the fan itself. Control-of such apparatus is generally split in as much as the systems controlling the ID fan and the FD fan are independent of one another. In the case of a furnace the ID fan would be adjusted either manually or automatically to maintain a fixed pressure in the top furnace, typically 0.1 ins WG below atmospheric. The FD fan would be adjusted either manually or automatically to maintain sufficient air for satisfactory combustion of the fuel required to be burnt. Thus, separate criteria dictate the operation of the respective fans. It is important that the -0.1 ins WG arch pressure is attained consistently both during constant run conditions and whilst shifting from one load to another. Even under constant run conditions the ambient pressure reference point is subject to quite violent fluctuations due to wind pressure etc, and it has been found necessary to average the ambient pressure over a specific period; e.g., 15 seconds, otherwise during gusty weather conditions the ID fan system hunts badly. During change over from one load to another it is very easy to upset conditions, particularly under manual control, so that the furnace bgoes positive" and flame is emitted from any openings such as peepdoors, with its attendant danger. We have found that such a balanced draught system as is described above can be more effectively and simply controlled by coupling the FD fan and ID fan to each other and a common drive mechanism. According to the invention combustion apparatus has a burner section and a balanced draught system for the burner section, which draught system camprises a forced draught fan for delivering combustion gas to the burner section; an induced draught fan for extracting effluent gases from the section; and a drive mechanism for the fans, the fans being operatively coupled such that adjustment of both is simultaneous. By this means a predeterminable relationship is always maintained between the ID and FD fans. The fans are preferably variable speed fans and the adjustment thus a direct speed control, although inlet vane control may be used for fine tuning. Generally, in apparatus according to the invention a single drive motor is used, preferably of variable speed but a constant speed motor may be used with for example speed variable coupling to the fans. It is also possible to adopt a system in which a separate electric motor drives each fan, but with the motors powered through a common modulator. Wherever a variable speed electric motor is used, speed control is preferably accomplished using a frequency converter system. Combustion apparatus according to the invention may be used in many applications. A preferred use is as a direct or indirect feed preheater in a reactor system for heating a feedstock before it enters a main reactor It is though, equally useful in steam generation apparatus. The invention is based upon the recognition of the interdependence of certain of the parameters in a balanced draught system. Thus: 1) the system resistance in a furnace or the like for both the ID and FD sides varies as a square of the throughput; 2) the output from any centrifugal fan is as follows; ; throughput speed 2 head (speed)2 This means that by utilizing a common drive for the ID and FD fans, or a single modulated supply to otherwise independent electric motors for the fans, by adjusting the speed of the common drive or modulator alone, the output from both the fans can match the system characteristics almost exactly so that the balance point, usually the furnace arch, retains constant at varying throughput without the necessity for additional instrumentation and control The potential hazard of over or under pressurizing of a large radiant section in a furnace is thereby virtually eliminated without additional instrumentation contributing significantly to furnace safety Moreover, because a variable speed drive fan has a substantially constant efficiency over a wide speed range, both fans can be turned down yet still maintain high efficiency thus effecting a high energy economy compared with the known system of control by inlet vanes and constant speed drive where the efficiency falls off quite dramatically when a fan is turned down, Control of a system in accordance with the invention utilizing a variable speed common driving means is purely by linking the variable speed control to the fuel firing rate of the apparatus. The furnace pressure can be automatically maintained; e.g., 0.1 ins WG below atmospheric, although this furnace pressure can be trimmed manually by the stack damper in the normal way. In addition, it may be necessary to alter the relationship between the FD and ID fans (due to range of fuel etc.) and this can be effected by fitting the PT) fan with manual or automatic inlet vane control (IVC) this is a cheap device and may be arranged so that under nor- mal conditions the inlet vanes are said 100 closed (00 wide open; 900 fully shut) so that they may be opened fully to 0o to increase the air throughput ratio slightly or closed a 0 little to say 20 closed to slightly decrease the air through. put. These inlet vane movements will also slightly alter the furnace pressure thereby providing a further control In the event that the systems resistance alters due to lay down of soot in the flue gas side of the system over a prolonged period of operation it will be necessary to compensate. The first compensation is to adjust the stack damper to the fully open position. Should further adjustment be required it will be necessary to alter the ratio controller on the fuel/fan system so that the speed of the fans set is slightly increased It would then be necessary to close the IVC on the FD fan slightly to prevent too much combustion air being drawn in. The fans used in the system of the invention are normally of the centrifugal type and may be identical to those which have been used for FD and ID fans in prior balanced draught systems. The preferred use of variable speed drive means that the fan efficiency is virtually constant over the operating range (mechanical losses are the only variables) and thus efficiency of the order of 80% may be achieved for a good quality fan. This applies in general to both ID and FD fans although there will be a slight extra loss of efficiency on the FD fan when an inlet vane control is present The driving means used in the invention may be any of the conventional driving means, those in normal use being steam turbine and electric. Preferably the driving means has a variable speed control. The steam turbine does exhibit a significant loss of efficiency when the unit speed is turned down but this can be minimized to some extent by installing extra nozzles for high speed running to be brought in either automatically or manually. In the case of the electric drive a number of alternatives are available It is possible to use a constant speed motor and couple this to the fans by means of an hydrualic coupling. However, the hydraulic coupling does have significant losses about the middle range which is where the normal load is and the unit will spend most of its operating life in that range. The principal alternative to this, which is preferred, is to use a frequency converter system to alter the speed of the motor. Another alternative is of course the use of separate electric motors powered through a common modulator as described above. It is possible to couple the two fans together with a constant speed motor and to provide inlet vane control for each fan. However, this type of system requires sophisticated control of the inlet vanes to obtain a balanced system, and there is an appreciable loss of efficiency as the fan output is turned down since the power required for driving the fans with the constant speed motor will be the same for all outputs of the fans. The invention offers a number of advantages over known arrangements, including the following: 1. both fans may operate on a variable speed system of control hence minimum energy consumption; 2. if using variable speed electric motor control the motor efficiency remains high over speed range hence minimum energy consumption 3. only one speed control required instead of two hence considerable cost saving; 4. fan control much simpler and better because characteristics of fans match system resistance characteristic; ; 5. in the case of multiple fired heaters connected to a common single air preheater there is a potential problem of how to share out the air to the burners under each heater0 Up to now each take-off duct to an individual heater would have a dainper probably controlled off the burner instrumentation and generally. the FD fan would be equipped with inlet vane control. The tendency with such a system is for the IVC to remain wide open while all the dampers to the heaters are well shut in to effect good combustion on the heater The fan is using maximum power under such conditions and to arrange instrumentation to obviate this would be complex and expensive, However, in a system in accordance with the invention the speed of the fan set would be set by the sum of all the fuel flows to the various heaters and this would give just the right amount of air for combustion; hence the individual heater dampers would have to be set as far open as possible and merely ensure each heater gets its proper share of the air available The fan set would be running at minimum power for the load given. Other advantages also accrue from the invention, the following particularly from the use of a drive mechanism for the fans which incorporates a single drive mdtor: 6. one large drive means is cheaper than two smaller individual drive means which have conventionally been used; 7. common baseplate - minimises lining up problems; is smaller than for two individual fans, hence cheaper; occupies minimum plot area; 8. total weight less and unit more compact hence cheaper foundations. The invention will now be described by way of example and with reference to the accompanying diagrammatic drawings in which: - Figure 1 represents a schematic diagram of a fired heater and air heater plant incorporating an FD and ID fan and driving means in accordance with the invention; Figure 2 represents a side elevation of an arrangement of air heater and fans in accordance with the invention; Figure 3 represents a plan view of the arrangement of Figure 2; Figures 4a to 4e show various arrangements of FD and ID fans and common driving means; Figure 5 is a view similar to that of Figure 1, showing an alternative arrangement according to the invention; and Figure 6 represents a plot showing the draught level in various positions of a furnace having a balanced draught system. Figure 1 shows a fired heater 1, a convection bank 2, an air heater 3 and a stack 4. Combustion air is drawn into the system through the air inlet 5 and passes through a forced draught fan 6 through the air heater 3 in which it passes in heat exchange relationship with flue gas, and thence to the fired heater 1 where fuel is burnt by burners 7. The hot gases from the fired heater pass through the convection bank 2 of for example a feed preheater or a steam generator, past a damper 8 and through associated conduit into the air heater 3 and thence through an induced draught fan 9 to stack 4, The forced draught fan 6 and the induced draught fan 9 are connected to the same variable speed driver 10. In the arrangement of Figure 1 the speeds of the two fans 6,9 are always in the same relation to one another, and variation of the speed of the driver 10 simultaneously adjusts both fans 6,9, Control of the driver 10 may be linked to the fuel firing rate of the burners by means of a monitor (not shown). Additionally or alternatively, one or more pressure monitors may be mounted in the apparatus to monitor the pressures in the system, particularly the arch pressure over the burners. Signals generated by these monitors are used to control the driver 10 Figures 2 and 3 show an arrangement of an air heater and fans for use with a fired heater, furnace or the like. The apparatus comprises an air inlet stack 20 through which combustion air enters, and inlet vane control 22 and a forced draught fan 24. Air passes through the inlet vane control and forced draught fan into an air heater 25 in heat exchange relationship with flue gas from the fired heater. Upon exit from the air heater the heated combustion air passes through suitable conduits into the radiant-section of the heater. Flue gas from the fired heater passes through conduit 26 via a control damper 27 into the air heater 25, through the induced draught fan 28 and exits through conduit 29to a stack. The forced draught fan 24 is mounted on suitable bearings 30 and the induced draught fan 28 is mounted upon fan bearing 31. A variable speed driver 32 is coupled to the fans via couplings 33. The fans, driver and associated bearings and couplings are mounted upon a common base frame 34 thus providing a compact arrangement for controlling the balanced draught system. Various arrangements of driver and fans are illustrated in Figures 4a to 4e. Figure 4a illustrates a system in which the FD fan 40 and the ID fan 41 are coupled directly to a variable speed driver 42 mounted between the two fans. An inlet vane control 43 is provided on the inlet side of the FD fan, The ID and FD fans are driven at identical speeds which may be varied by varying the speed of the driver The inlet vane control provides an additional means of balancing the pressure within the system. Figure 4a illustrates a similar system to that shown in Figure 4a with the exception that a fixed ratio gearbox 44 is provided between the driver and the ID fan whereby the ID fan is driven at a lower speed to the FD fan. The fans are driven at a fixed ratio to each other which ratio remains the same as the speed of the driver is varied. Figure 4c shows an arrangement utilizing a constant speed driver 45 which is connected to the FD fan 40 and ID fan 41 via a variable speed hydraulic coupling 46. An oil cooler 47 for the variable speed hydraulic coupling may be conveniently provided in the FD fan ducting. The speed of the two fans may-be varied by controlling the variable speed hydraulic coupling, This arrangement is not as efficient as utilizing a variable speed driver in as much as the constant speed driver with hydraulic coupling utilizes more power at the middle portion of the turn-down range of output of the fans. Figure 4d shows a constant speed driver 50 positioned between the FD fan 40 and the ID fan 41, driving the fans at the same speed, This arrangement requires an inlet vane control 43 for the FD fan and an inlet vane control 51 for the ID fan. Control of the balanced draught system is obtained by suitable adjustment of the inlet vane controls for each fan, This arrangement suffers from loss of efficiency as the fan output is turned down and may require sophisticated controlling means linked to both the inlet vane controls in order to achieve a balanced draught, Figure 4e illustrates a further arrangement comprising a variable speed driver 42 linked directly to an FD fan 40 having an inlet vane control 52. The FD fan is linked to an ID fan 41 via a fixed ratio gearbox 53 arranged such that the ID fan runs at a lower speed than the FD fan. Figure 5 shows the same arrangement as is illustrated in Figure 1 with the difference that the two fans 6,9 have their own separate electric motor 54 and 55. The two motors are supplied with power through a common electric modulator 56. Figure 6 is a diagram of the draught levels in a typical furnace system incorporating an air heater and shows that each fan has its own area of influence in a balanced draught system. In the case of the FD fan it is fron the air intake (at atmospheric pressure) Point A to the nil (atmospheric) pressure point in the burner, Point B. The position of these points never varies and this system operates under the normal laws governing ducted air systems, The draught occasioned by the furnace at the floor of the furnace remains virtually the same over the load range because the furnace temperature does not vary much For the ID fan its system starts with the arch at -0.1 ins WG Point C and ends at some nil point where the stack flue gas temperature does not change greatly over a wide range of operating levels because of the influence of the air heater hence the nil point does not alter position greatly, Thus because the ID fan head is great compared with the arch pressure we can consider the arch pressure to be atmospheric and because the stack end nil point does not move much we can again consider this system to obey the normal laws, If the temperature levels remained constant over the whole operating range the pressure drop of both systems would obey the square law EMI14.1 where G is the mass flow rate. However, because the air temperature to the burners varies with load, part of the FD fan system has a modified pressure drop that depends on the specific volume of the air which in turn is influenced by the air temperature. Similarly the ID fan system is influenced by flue gas temperature. The net result is a slight imbalance between the two systems, If however this is plotted on a typical fan performance curve the net effect is negligible and the controlling factor is the air/flue gas throughput. Any discrepancy occurring can be easily compensated for by use of the inlet vane control on the FD fan or the stack damper. Thus a balanced draught can be obtained and maintained by a system in accordance with the invention by maintaining the speed of each fan in a fixed ratio utilizing a common driving means or a common electric speed modulator,

Claims: CLAIMS:- 1. Combustion apparatus having a burner section and a balanced draught system for the burner section, which draught system comprises a forced draught fan for delivering combustion gas to the burner section, an induced draught fan for extracting effluent gases from the section, and a drive mechanism for the fans, the fans being operatively coupled such that adjustment of both is simultaneous. 2. Apparatus according to Claim 1 wherein each fan is a variable speed fan having a substantially constant efficiency over its operating speed range. 3. Apparatus according to Claim 1 or Claim 2 wherein the drive mechanism comprises a steam turbine. 4. Apparatus according to Claim 1 or Claim 2 wherein the drive mechanism comprises an electric motor. 5. Apparatus according to Claim 4 including a frequency converter system for adjusting the motor speed. 6. Apparatus according to Claim 4 or Claim 5 including a separate electric motor for each fan, both motors being connected to a common modulator. 7. Apparatus according to any of Claims 1 to 5 wherein the fans have different operating speed ranges, and are coupled to a single drive motor, one through a gearbox. 8. 'Apparatus according to any preceding Claim including inlet vane control on at least one of the two fans. 9. Apparatus according to any preceding Claim including means linking adjustment of the fans to the fuel firing rate of the apparatus. 10. Apparatus according to any of Claims 1 to 8 including at least one pressure monitor in the burner section, and means linking adjustment of the fans to signals generated by said at least one pressure monitor to match the system characteristics at all times. 11. Combustion apparatus comprising a fired heater and means for feeding combustion air and fuel thereto, a convection bank in the path of effluent gases from the heater, an air preheater for preheating combustion air to be fed to the burner, an induced draught fan for drawing air from the heater through the convection bank and the air preheater, and for discharging same, and a forced draught fan for feeding air into the preheater and thence to the heater, the fans being operatively coupled such that adjustment of both is simultaneous to match the system characteristics. 12. Apparatus according to Claim 11 including inlet vane control on at least one of the fans for fin adjustment of the output thereof. 13. Apparatus according to Claim 11 or Claim 12 wherein the two fans are both driven by a single drive motor. 14. Apparatus according to Claim 11 or Claim 12 wherein each fan is driven by a separate electric motor, power being supplied to the motors from a common modulator. 15. Combustion apparatus comprising a fired heater and means for delivering combustion air and fuel thereto, a convection bank in the path of effluent gases from the heater, an air preheater for preheating combustion air to be fed to the burner, a variable speed induced draught fan for drawing air from the heater through the convection bank and the air preheater, and for discharging same, a variable speed forced draught fan for feeding air into the heater through the preheater, a common drive mechanism for the fans comprising a single variable speed electric drive motor coupled to both fans, and a frequency converter system for adjusting the motor speed and hence the fan outputs in fixed relation to one another to match the system characteristics of the apparatus, 16. Combustion apparatus comprising a fired heater and means for delivering combustion air and fuel thereto, a convection bank in the path of'effluent gases from the heater, an air preheater for preheating combustion air to be fed to the burner, a variable speed induced draught fan for drawing air from the heater through the convection bank and the air preheater, and for discharging same, a variable speed forced draught fan for feeding air into the heater through the preheater, a common drive mechanism for the fans comprising a single variable speed electric drive motor coupled to each fan, an electric modulator for controlling the power supply to each fan to maintain the fan outputs in a predetermined relation to one another, and means for monitoring the fuel firing rate of the heater, the modulator being linked to the monitoring means to match the flow of gases through the heater to the characteristics of the heater,

Inventors: GILBERT, KEITH WILLIAM
Applicants: FOSTER WHEELER ENERGY LIMITED
Application year: 1982
Main classification: F23L17
IPC codes: F23L17, F23N3, F04D27, F23L15, F23L5, F04D25