US3527445A - Furnace reversal system - Google Patents

Description

p 19 70 J. STEWART, JR, ETAL 3,527,445

FURNACE REVERSAL SYSTEM Filed July 8, 1968 3 Sheets-Sheet 1 FIG. I

FUEL RATE RECORDER RELAY SYSTEM TEMPERATUREPH REVERSAL REVERSAL REVERSAL FIG. in

I I l SET POINT I i I I TIME FUEL I FUEL FUEL ON OFF 0N OFF ON i OFF COMPUTE COMPUTE Sept. 8, 1970 J. STEWART, JR, ETAL 3,527,445

FURNACE REVERSAL SYSTEM Filed July 8, 1968 I 3 Sheets-Sheet 2 TIME F 2 I FUEL-0N TIME TOTAL TIME PIT DRAWN Sept. 8,1970

J. STEWART, JR, ET AL FURNACE REVERSAL SYSTEM 3 Sheets-Sheet 5 TIME FIG. 3 FUEL-0N TIME TOTAL TIME 100 90 l 70 so so 30 2o 10 0 J COVER OFF 3 Tmg FIG. 4

FUEL-0N TIME TOTAL TIME COVER OFF United States Patent O 3,527,445 FURNACE REVERSAL SYSTEM James Stewart, Jr., Philadelphia, Pa., and Vincent J. Cucuzzella, Silver Spring, Md., assignors to Leeds & Northrup Company, Philadelphia, Pa., a corporation of Pennsylvania Filed July 8, 1968, Ser. No. 743,176 Int. Cl. F27d 17/00 U.S. Cl. 263- 7 Claims ABSTRACT OF THE DISCLOSURE In a regenerative furnace the reversal is actuated each time that the burners are turned off. This criteria for reversal is used when the signal representing the ratio of fuel on time to total cycle time is below a preset, adjustable point. When the signal is above this point, the furnace is reversed when the burners have been on for a given time interval. Uniform heating of the steel to the desired temperature is indicated when an electrical signal representing the rate of change of fuel flow to the furnace is substantially constant.

BACKGROUND OF THE INVENTION Regenerative furnaces generally include chambers through which fuel, combustion air, and the products of combustion pass on the way to be ignited within the furnace. In order that maximum efficiency be obtained in the operation of the furnace, it is desirable to elevate the temperature of the combustion air before combustion Within the furnace. In order to do this, regenerative chambers at opposite ends of the furnace alternately receive heat from the products of combustion of the furnace and alternately give up the heat to the combustion air.

The operation of the furnace must be reversed periodically in order to prevent excessive cooling or overheating of the material of which the regenerative chambers are constructed. In the prior art, many techniques have been suggested for controlling the time at which the furnace is reversed. For example, U.S. Pat. 2,287,186 describes known techniques for determining the reversal time in regenerative furnaces. Of these techniques, perhaps the most straightforward is to reverse the furnace after definite time intervals.

Other, more complex techniques for determining the time of reversal are described in U.S. Pats. 2,851,221, 2,177,805, 2,531,200 Davis, and 2,804,268 Davis.

In a system in which the pit is reversed periodically, nothing prevents one side of the pit from carrying a dominant portion of the total firing time. This can result in a large checker-brick temperature differential between pit sides. Other of the prior art systems of the type de scribed above provide a system which is sensitive to a temperature differential in the chambers. Many of the systems of this type reverse at periodic time intervals unless there is an override caused by too great a temperature differential 'between the two sides. While systems using temperature differential provide a good safety factor, they do not basically equalize the B.t.u. input to each side of the furnace. That is, any temperature measurement lags behind B.t.u. input so there is never good equalization of B.t.u. input to both sides when 3,527,445 Patented Sept. 8, 1970 this input is solely temperature control. The placement of the thermocouple also may not provide the average checker temperature but only the temperature of a hot or cold area adjacent to the thermocouple. All of the techniques described in the foregoing patents have disadvantages which are obviated by the present invention and which will become apparent from the following detailed description.

SUMMARY OF THE INVENTION This invention relates to an improved regenerative furnace control system and to means for providing an indication of when the charge of steel is uniformly heated to the desired temperature and ready to be removed from the furnace Accordingly, it is an important object of the present invention to provide improved temperature performance in regenerative furnaces by uniformly heating the charge and balancing regenerative checker-brick temperature by controlling to an equal value the B.t.u.s fired from each side.

It is another object of the present invention to reduce the total heating cycle by reversing the pit while the fuel is off, rather than on a straight time basis which can reverse the furnace while the fuel is on.

It is another object of the present invention to provide an analog signal of the rate of change of fuel flow to batch type heating furnaces and to provede, in response to this signal approaching a constant value, an indication that the charge is uniformly heated to a desired temperature and ready to be discharged from the furnace.

It is a further object of the present invention to produce a signal representing the ratio of fuel on time to total cycle time, and to reverse the firing direction in response to the signal being above a preset, adjustable point, whenever the burner has been on for a predetermined time, and further to reverse the furnace, in response to the signal being below the preset adjustable point whenever the fuel to the furnace is cutoff.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the furnace reversal system of the present invention;

FIG. la shows furnace temperature as a function of time;

FIG. 2 shows the ratio of fuel on time to total time in a prior art system; and

FIGS. 3 and 4 show similar curves for a furnace of the present invention.

DESCRIPTION OF A PARTICULAR EMBODIMENT The invention in its preferred form has been shown applied to the control of a regenerative soaking pit furnace 1 which has regenerative chambers or checkers, 2 and '3 at its ends. In the operation of the furnace, air is forced through an air inlet 4 into one of the checkers which preheats the air to supply heated air for combustion. The furnace exhaust gases are forced out through the other checker to preheat that checker, and are exhausted through a stack 5. A damper 6, located in the junction of the supply and exhaust pipes, directs air to the other end of the furnace and is rotated by to reverse the furnace operation. A suitable fuel is supplied to the furnace through a pipe 7 which has branches that lead to the burners 8 and 9, one of which is located in each end of the furnace in such a position that the incomming air can be mixed with the fuel thereby insuring combustion. A reversing valve 10 operates to supply fuel to one or the other of the burners. The damper 6 and the valve 10 are actuated in a conventional manner to reverse the operation of the furnace.

What has been described thus far is a conventional regenerative type furnace which further includes a set of relays 11 for effecting the reversal of the furnace. That is, when the relay system 11 is actuated, a sequence is initiated which reverses the position of the damper 6 and reverses the position of the valve 10.

In order to effect this reversal at the proper time in accordance with the principles of this invention, a computing network is provided which includes integrating amplifiers 12 and 13 and the dividing, or ratio, network 14.

The computing network produces at the output of ratio network 14 a signal proportional to the ratio of the fuel on time to total time betweeen reversals. This is referred to as the fuel rate signal.

The integrating amplifier 12 produces an output signal which is indicative of the time that either one of the burners 8 or 9 is on. The burners 8 and 9 are temperature controlled. That is, a thermocouple 15 measures the temperature in the furnace and a contact is actuated as the temperature fluctuates about a set point. As shown in this particular embodiment, the contact 16 is included in the temperature recorder 17. The output of the thermocouple is applied to temperature recorder 17 which has a balancing network of the conventional type. Through the action of the balancing network, the pen on the recorder is moved up and down scale in accordance with the temperature. At a predetermined point in its travel, the contact 16 is opened and closed. This contact may be used to turn burners 8 and 9 on and off. Alternatively, actuation of contact 16 may merely be indicative of the time at which the burners 8 and 9 are turned on and off.

When contact 16 is closed, indicating that one of the burners is firing, potential is applied to the integrating amplifier 12 which produces as an output a signal indicative of the time that the burner is on. Coincidental with the closure of the contact 16, the time 18 is started. That is, timer 18 is started each time that one of the burners is turned on.

Assuming that the ratio of fuel on time to total time between reversals is above a preset point, assume a 50% value, then the timer 18 will time out and actuate the reversal relay system 11 after a given interval, say 4 minutes.

The reversal relay system 11 requires about 20 seconds to complete a reversal, that is, to go through the sequence of relay actuations which will effect reversal of the damper 6 and the valve 10. At the end of this reversal time, a relay contact is closed which applies a voltage to the integrating amplifier 13.

Integrating amplifier 13 produces a ramp voltage output. At the beginning of the next reversal period, the relay contact in system 11 is opened thereby disconnecting the input from integrating amplifier 13. At this time, integrating amplifier 13 has an output indicative of the total time between reversals. During the reversal time period (20 seconds in the example being discussed), the computation of fuel rate is performed. That is, the signal proportional to fuel on time from integrating amplifier 12 is divided by the signal proportional to time between reversals from integrating amplifier 13. This division is performed in the dividing network 14 which produces the fuel rate signal. During the reversal time period, the output of network 14 is sampled and is applied to recorder 19 for recordation of a fuel rate value. Also, during the time interval in which a reversal is effected, the integrators 12 and 13 are reset so that they are ready for the next computation. While the sample and reset functions for the computing network have not been indicated in FIG. 1, the provision of these functions at the times indicated is within the skill of the art.

When the fuel rate signal goes below a predetermined adjustable point (50% in the case under discussion), a different technique is used for effecting reversals. The recorder 19 responds through the usual self-balancing network to position the pointer 20 to the proper position. At a predetermined point in its travel, in this case the point indicating a 50% fuel rate, the mechanism associated with the pointer 20 closes the normally open contact 22. That is, as the fuel rate falls below the 50% point, the criteria for effecting reversals is changed. The reversal relay system 11 will no longer normally be actuated by timer 18. Instead, the reversal relay system 11 is actuated each time that the contact 16 is opened indicating that the burners are cut off. That is, each time that the furnace goes from fuel on to fuel off the pit will reverse.

The actuation of the relay reversal system under this condition may be under what is referred to as automatic control or manual control. In automatic control, energizaton is through the circuit including contact 16, closed contact 22 and contact 27a which is closed in automatic control only. In manual control, energiiation is through the contact 16, through a contact 16a which is closed when the temperature is approximately 20 below the set point, and through the contact 27 which is closed only for manual control.

The operation of the furnace under these conditions is depicted in FIG. 1a which shows furnace temperature as a function of time. FIG. la also shows the times at which the fuel is turned on and off and the time at which there is a reversal in the furnace. If, however, the fuel fires for 4 minutes without turning off, this timer will initiate reversal as a safety override.

Included in the circuit between the reversal system and the integrating amplifier 13 is a cover interlock switch 28 which is opened when the pit cover is removed. The function of this interlock is to interrupt the computation for the period that the pit cover is removed. When the pit cover is again closed, the generation of the total signal time continues. This interlock is necessary because furnaces of this type have another interlock which prevents the pit from firing when the cover is removed. If total time was allowed to accumulate while the firing circuit was disabled, the recorded data during this period would be in error.

Summarizing the operation of the system of FIG. 1, furnace reversal is regulated in response to two different criteria. When the signal representing the ratio of fuel on time to total time is greater than 50%, the reversal is controlled by the timer 18. When the timer 18 has accumulated a preset total of time that a burner has been under fire, then a reversal is effected. During this period, the furnace is on what is termed high fire. When the necessary firing time has been accumulated, the furnace reverses, a computation is made, and the fuel time to total time ratio is recorded. The principal advantage of this technique of operation is that it insures that both sides of the pit will receive an equal amount of fuel on time.

The second criterion of the pit reversal is applied during the period of time when the ratio of fuel on time to total time is below 50%. The change from the first criterion to the second is accomplished by actuation of contact 22 in the recorder 19. When the second criterion for pit reversal is employed, reversal of the furnace occurs every time that the burners are turned off. That is, every time that the switch 16 is actuated, the reversal relay system 11 is actuated.

As a further aspect of the present invention, there is provided an indicator 23 for indicating when the steel in the furnace is soaked out, that is, when the steel has uniformly reached the desired temperature. The basis for actuating this indicator is that as the soak out time nears, the ratio of fuel on time to total time approaches a constant value. As this time approaches, the burners are on merely for enough time to reesupply heat to the input which has been lost during the time that the burners have been off. As the signal representing fuel rate approaches a substantiallyconstant value, there is an indication that soak out has occurred. In order to determine the existence of such a condition, the signal representing fuel rate for one computation period is compared to the signal representing fuel rate for the last computation period. Note that the signal representing fuel rate for the last computation period is available from the balancing slidewire 24 in the recorder 19. This signal is applied to comparator amplifier 25. Also, the present value of the fuel rate signal, from dividing network 14, is applied to the comparator amplifier 25. If the comparator amplifier 25 indicates that the two signals are within a preset tolerance, a counter 26 is advanced by one count. When the counter 26 counts a selected number of cycles in which the previous fuel rate signal was within the tolerance of the present fuel rate signal, the counter 26 actuates the indicator light 23 to signal that soak out has occurred.

The advantages of the present invention over the prior art can be shown from FIGS. 24. FIG. 2 depicts a prior art type of operation in which the pit is reversed periodically. In this example, the pit was reversed every 4 minutes. FIG. 2 is a plot of the ratio of fuel on time to total time as a function of time. The chart of FIG. 2 was made on a furnace which was reversed every 4 minutes. Computation of the fuel on time to total time between reversals was made each time that the furnace was reversed.

At the bottom of FIG. 2, in the portion of the record marked a, both burners are firing 100% of the time. Since the pit is being reversed every 4 minutes, there is no problem. Both burners are on equal amounts of time and both regenerative chambers will be equally heated. However, at the point 12, the firing ratio drops to approximately 60% of the time. The left hand burner, which is being fired during this time, will be on only approximately 60% of the time. However, at the point marked 0, the burner on the right hand side is being fired approximately 96% of the time. The portion of the graph d shows the fuel on time after another reversal, that is, the fuel on time for the burner in the left hand chamber. As the reversals are followed along the graph, it will be seen that the right hand burner consistently is being fired for a greater period of time than the left hand burner. This is a very undesirable condition.

FIG. 3 shows the ratio of fuel on time to total time where the reversal is made each time the fuel is shut off. It can be seen that there are no wide variations in the record which means there was not a wide variation in the firing time from one burner to the other burner.

FIG. 4 is a chart similar to FIG. 3, except that the reversals have been averaged out. That is, computations have been made in which the computed values of heat rate have been averaged out for both combustion chambers rather than being computed separately for the left hand and the right hand combustion chamber. This is a desirable modification in order to get a better indication of when the steel is soaked out. Note that the relatively rapid variations in the chart of FIG. 3 make it difiicult to obtain consecutive readings within a narrow tolerance. However, when the variations have been averaged out between the two burners as was done in obtaining the record of FIG. 4, it now becomes possible to obtain a number of consecutive readings in which the fuel rate signal is constant.

While a particular embodiment of the invention has been shown and described, it will, of course, be understood that various modfiications may be made without departing from the principles of the invention. The appended claims are, therefore, intended to cover any such modification within the true spirit and scope of the invention.

What is claimed is:

1. An apparatus for controlling the reversal of the flow of combustion air, and the products of combustion through a regenerative furnace having regenerative chambers at two ends and means to direct alternatively the combustion air through said chambers and burners in each of said chambers comprising in combination:

temperature responsive means for controlling the on and off condition of the firing of said burners in firing cycles,

timer means to determine the fuel on time during each firing cycle, means for producing a signal representing the ratio of fuel on time to total cycle time, and

means operable when said signal is above a preset, ad-

justable, point for reversing the flow of combustion air and the products of combustion whenever said timer means exceeds a preset time interval, and operable when said signal is below a preset, adjustable, point to reverse the flow of combustion air and the products of combustion in response to a change in the firing condition of said burners.

2. The apparatus recited in claim 1 wherein said means responsive to a change in the firing condition of said burners includes a relay contact actuated when said burners are cut off to reverse said flow of combustion air and the products of combustion.

3. An apparatus for controlling the reversal of the flow of combustion air, and the products of combustion through a regenerative furnace having regenerative chambers at two ends and means to direct alternatively the combustion air through said chambers comprising in combination:

timer means to determine the fuel on time during each firing cycle, means for producing a signal representing the ratio of fuel on time to total cycle time,

means responsive to said signal being above a preset,

adjustable, point for reversing the flow of combustion air and the products of combustion whenever said timer means exceeds a preset time interval, and

means responsive to said signal being below a preset,

adjustable, point for reversing said flow of combustion air and the products of combustion whenever the fuel to said furnace is cutoff.

4. The method of controlling the reversal of the flow of combustion air and the products of combustion through a regenerative furnace comprising:

determining the ratio of fuel on time to total cycle time,

reversing the flow of combustion air and the products of combustion after preset time intervals of burner firing whenever said ratio is above a preset, adjustable, point, and

reversing the flow of combustion air and the products of combustion in response to said burner being turned off whenever said ratio is below said preset, adjustable, point.

5. The method of indicating when the steel in a batch type heating furnace has been uniformly heated to the desired temperature comprising:

generating an electrical signal representing the rate of change of fuel flow to said furnace, and

actuating an indicating device when said signal approaches a constant value to indicate that the steel is uniformly heated to the desired temperature.

6. The method recited in claim 5 wherein said actuating step includes:

comparing said signal for a previous cycle with said signal for the present cycle,

actuating a counter each time the signal for the past and present cycles are substantially equal, and energizing said indicating device when said counter has accumulated a predetermined number of counts.

7 7. The method of operating a regenerative furnace in which the flow of combustion air is alternatively directed through regenerative chambers comprising: a I

generating an electrical signal representing the rate of change of fuel flow to said furnace, t I actuating an indicating device when said signal approaches a constant value to indicate that the steel in said furnace is uniformly heated to the desired temperature, v reversing the flow of combustion air and the products of combustion after preset time intervals of burner firing whenever said signal is above a preset, adjustable, point, and i 8 t .reversingthe flow of combustion air and the products I of combustion in response to said burner being i .turned off whenever said signal is below said preset,

adjustable, point;

References Cited UNITED STATES PATENTS 3,393,868 7/1968 'Griem 23615 v U.S. Cl. X.R.

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