FURNACE CONTROL

REGULATEUR DE LA TEMPERATURE D'UN FOUR

English Abstract

K 5373
A B S T R A C T
A process for the control of a furnace having at least
two parallel coils over which the flow of matter to be
heated and/or evaporated is distributed, characterized
in that
a) the temperature Ti of the matter is measured at the
end of each coil and the average value <IMG> is
computed,
b) each difference T - Ti is used to produce a signal by
which the flow of matter through the bundle concerned is
adjusted so as to cause the said difference T - Ti
to decrease,
c) of the temperatures Ti the highest Timax is chosen and
the difference Timax - Ts, (where Ts is a set value related
to the temperature To of the combined flow of matter
leaving the furnace) is used to produce a signal by which
the heat generation in the furnace is adjusted so as
to cause the said difference Timax - Ts to decrease.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for the control of a furnace having at least two parallel
coils over which the flow of matter to be heated and/or evaporated is
distributed, comprising
a) measuring the temperature Ti of the matter at the end of each coil
and deriving the average value <IMG>,
b) deriving each difference ? - Ti, said difference producing a signal
by which the flow of matter through the coil concerned is adjusted so as to
cause the said difference ? - Ti to decrease,
c) taking the highest Timax of the temperatures Ti and deriving the
difference Timax - Ts (where Ts is a set value related to the temperature
To of the combined flow of matter leaving the furnace) said difference pro-
ducing a signal by which the heat generation in the furnace is adjusted so
as to cause the said difference Timax - Ts to decrease.
2. The process as claimed in claim 1, wherein each signal produced
according to l-b is made to adjust the set value for the control of the flow
of matter through the coil concerned in proportion to the difference ? - Ti.
3. The process as claimed in claim 1, comprising deriving for each
coil the ratio <IMG>, where C is a constant and Fs the set value
for the total flow of matter to the furnace, this ratio Ri being used to
control the flow of matter through the coil concerned.
4. The process as claimed in claim 3, wherein for a liquid flow of
matter Fs is derived from the level of the liquid flow to be supplied from
a vessel.
5. The process as claimed in claim 1, wherein the difference To - Ts
is used for non-linearizing the control of the heat output in the furnace,
namely by increasing the proportional control factor and the integral action
time of the heat output control as the said difference becomes greater.
12

6. The process as claimed in claim 2, wherein the difference To - Ts
is used for non-linearizing the control of the heat output in the furnace,
namely by increasing the proportional control factor and the integral action
time of the heat output control as the said difference becomes greater.
7. The process as claimed in claim 3, wherein the difference To - Ts
is used for non-linearizing the control of the heat output in the furnace,
namely by increasing the proportional control factor and the integral action
time of the heat output control as the said difference becomes greater.
8. The process as claimed in claim 4, wherein the difference To - Ts
is used for non-linearizing the control of the heat output in the furnace,
namely by increasing the proportional control factor and the integral action
time of the heat output control as the said difference becomes greater.
9. The process as claimed in claim 5, 6 or 7, wherein the said
increase amounts to at most a factor of 6.
10. The process as claimed in claim 8, wherein the said increase
amounts to at most a factor of 6.
11. An apparatus for controlling a furnace having at least two parallel
coils over which the flow of matter to be heated and/or evaporated is dis-
tributed, comprising
d) a meter for the temperature of the matter at the end of each coil,
e) a meter for the flow of the matter through each coil,
f) a control device for the flow of the matter through each coil,
g) a computing element for deriving <IMG> and .DELTA.Ti = ? - Ti for
each coil, for which purpose the computing element is connected to the said
meters for the temperature of the matter at the end of each coil,
h) a controller for each coil of which controller the input for the
measured value is connected to the said corresponding meter for the temper-
ature of the matter at the end of each coil and of which the input for the
set value is connected to the output of the said computing element for the
13

corresponding value of .DELTA.Ti and of which the output is connected to the said
corresponding control device for the flow of the matter through each coil,
i) a computing element for deriving the difference Timax - Ts for which
purpose the computing element is connected to the said meters for the temper-
ature of the matter at the end of each coil,
j) a meter for the temperature of the combined outgoing stream,
k) a control device for the fuel stream to the burner of the furnace,
l) a controller of which the input for the measured value is connected
to the said meter for the temperature of the combined outgoing stream and
of which the input for the set value is connected to the output of the
said computing element for deriving the difference Timax - Ts and of which
the output is connected to the said control device for the fuel stream to
the burner of the furnace.
12. The apparatus as claimed in claim 11, comprising a computing
element for deriving a multiplication factor, for which purpose the comput-
ing element is connected to the output of the said computing element for
deriving the difference Timax - Ts and to the said meter for the temperature
of the combined outgoing stream and of which the output is connected to an
input of the said controller of which the input for the measured value is
connected to the said meter for the temperature of the combined outgoing
stream, for setting the proportional control factor and the integral action
time of that controller.
14

Description

10"7766
-- 2 --
The invention relates to a process and to an apparatus for
the control of a furnace with at least two parallel coils of
tubes over which the flow of matter to be heated and/or
evaporated i~ distributed.
In principle, induqtrial furnaces consist of a system of
tubeq grouped around one or more flames. Through the system of
tubes flows the matter to be heated and/or evaporated. This is
often a liquid, quch as an oil or an oil product. Besides its
use for heating or evaporation, a furnace iq also used for
chemical conversions in the matter to be passed through. An
example is cracking naphtha to form ethylene. The heat is trans--
ferred by radiation and by convection.
As a rule the ~ystem of tubes consists of two or more
parallel coils. The incoming matter i~ distributed over the coil~;
at the ends of the coils the flows are combined again. The
various flowq can be controlled individually by valves. In doing
thi-q it is important to choose such a distribution of the
matter that local overheating of a tube and/or of the matter
flowing through it is prevented. It is also desirable to
distribute the heat load evenly over the coil~. This promotes an
economic use of the furnace and prolong~ its life. Induqtrial
furnaces are usually kept in continuous operation for very long
periods. With such furnaces it is not sufficient merely to
maintair, a pre-selected distribution of the flow of matter over
tne coils, becau3e there are influences which call for adju3tment~
in this distribution. Examples of such influences are changes
~k

1097766
in the distribution of fuel and air supply to the burners, changes in the
position(s) of the flame(s), clogging of part of the tubes, weather
conditions.
The invention enables measures to be taken by which the above-
mentioned wishes can be satisfied.
The invention relates to a process for the control of a furnace
having at least two parallel coils over which the flow of matter to be
heat~d and/or evaporated is distributed, comprising
a) measuring the temperature Ti of the matter at the end of each coil
0 and deriving the average value T = n ~ Ti'
b) deriving each difference T - Ti, said difference producing a signal
by which the flow of matter through the coil concerned is adjusted so as to
cause the said difference T - Ti to decrease,
c) taking the highest Ti of the temperatures Ti and deriving the
difference Ti ~ Ts~ (where Ts is a set value related to the temperature
TQ of the combined flow of matter leaving the furnace) said difference pro-
ducing a signal by which the heat generation in the furnace is adjusted so
as to cause the said difference Ti ~ Ts to decrease.
max
The invention also relates to apparatus for carrying out the
method of the preceding paragraph.
This control system continuously aims at eliminating differences
in temperature at the end of the coils. For this purpose the average of
temperatures Ti is used, not the temperature of the combined flows leaving
the furnace. The latter temperature is a weighted average and it may deviate
from the arithmetic average T. A deviation may also occur through evapor-
ation of matteT after the point where Ti is measured and before the
r. ^,

10~7766
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point where To is measured. Therefore the control ~ystem accor-
ding to the invention always enables temperatures Ti to be
equalized, irre~pective of differences in magnitude of the
flows of matter through the coils.
The control of the heat ~eneration in the furnace is
based on the highest temperature occurring in a coil. The impor-
tant re~ult i~ that this highest temperature will never exceed
the permis~ible temperature. Moreover, the furnace can now
actually be operated at its maximum load, because all tempe-
rature3 Ti are continuously made to approach the maximum
permis3ible temperature.
As a consequence of the control according to the invention
as described hereinbefore the total flow of matter through the
furnace doe~ not remain constant. In a number of cases, however,
it iq important to keep that flow constant or to be able to
control it. According to another characteristic feature of the
invention this requirement can be met when each signal produced as
de~cribed hereinb~fore under b) is made to ad~ust the set value
for the control of the flow of matter through the coil concerned
in proportion to the difference T - Ti. Now the total flow of
matter does remain constant, ~ecause the sum Or the deviation~
from the (arithmetic) mean ~(T - Ti) is always zero and hence
the sum of changes which are each proportional to the deviation
concerned is also zero.
There are also situations in which a variable ~upply of
matter to the furnace has to be reckoned with. That may be the
result of a non-con~tant supply of matter, e.g. from another

10~7766
plant, or of a variable demand for the end product of the
furnace. The invention meets this situation by computing for
each coil the ratio QRi=CT Ti, where C is a constant and
F~ the set value for the total flow of matter through the
furnace, thiq ratio ARi being used to control the flow of
matter through the coil concerned. In this way each coil
gets its share Fi of the flow matter supplied, while the
advantages of the control system according to the invention
are retained. The ratio Ri=Fi. A frequently encountered
situation involving a variab~e incoming flow of matter
is one in which an incoming liquid flow comes from a space in
which a specified liquid level must be maintained, for instance
at the bottom of a distilling column. The set value Fs can then
be derived from that liquid level. When the liquid level changes
F~ i5 adjusted such that the liquid level goes back to the
desired value.
An interesting improvement of the process according to the
invention is achieved when the difference To - T~ iq used for
non-linearizing the control of the heat output in the furnace,
namely by increasing the proportional control factor and the
integral action time of the heat output control as the said
difference becomes greater. The said increase can amount to
at most a factor of 6. The result of this control i~ that
upward deviations of Ti are reduced.
max
The control according to the invention may be implemented
using conventional controllers and dedicated computing ele-
ments, the latter, if de~ired, of the electronic type.

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It is also possible to set up the whole control system
according to the principle of direct digital control or super-
visory control. Thi~ alternative will be preferred in the case
Or large manufacturing plants where a digital computer or a
number of microcomputers is already in use, e.g. to control
other units of the plant and for the processing and presentation
Or data. The invention will now be further elucidated with
reference to some figures and examples.
Figures 1, 2 and 3 are diagrammatic representations of
a furnace with four parallel coils of tubes with variants of
the control ~y~tem according to the invention. Figures 4/8
show results of experiments with control systems according to
the invention.
In Fig. 1 numeral 1 represents the flame that heats coils
2, 3, 4 and 5. Fuel for flame 1 is supplied at 6 and the rate
of flow is controlled by means Or valve 7. The matter to be
heated and/or evaporated enters the sy~tem at 8 and is distri-
buted by mean~ of valves 9, 10, 11 and 12 over coils 2, 3, 4
and 5, respectively. The collected matter leaves the system at
50.
Each coil is equipped with a temperature meter (13, 14,
15, 16) a~d a flow meter (17, 18, 19, 20). The flow meters
measure the quantity of matter passing through per unit
time. A flow meter 21 measures fuel stream 6, a temperature
meter 22 measures the temperature of the stream Or matter 50
leaving the furnace. A temperature controller 23 generates the
set value for ~uel flow controller 24.
Flow controllers 25, 26,
27 and 28 control valves 9, 10, 11 and 12, re~pectively.

~0~77~6
-- 7 --
The numerals used so far have the same meaning in Figs. 2
and 3.
In the scheme according to Fig. 1 the signals of tempera-
ture meters 13, 14, 15 and 16 go to a computing element 29 and
to a selector 30. ~omputing element 29 computes the average
value T of the temperatures measured, so, in this case,
1 4
Ti. The average value T is the set value for control-
lers 31, 32, 33 and 34. In controller 31 T is compared with the
temperature measured by meter 16 in coil 5. The output signal
of controller 31 is the set value for flow controller 28. This
control aims at equalizing the temperature measured by meter 16
with average temperature T. Similar elucidations are applicable
to controllers 32, 33 and 34.
From the measured temperatures Or coi 19 2, 3 9 4 and 5
selector 30 selects the highest value Ti . In controller 35
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Ti is compared with a set value Ts re~ulting in an output
max
signal which i3 the set value for controller 23. From the
comparison with the measured temperature of stream 50 follows
the ~et value for flow controller 24 for fuel stream 6. The
temperature of stream 50 follows the set value for flow con-
troller 24 for fuel stream 6. The temperature of stream 50
leaving the furnace is thus determined by the highest value
Ti occurring in a coil, which temperature Ti cannot
max max
exceed set value Ts.
In the scheme according to Fig. 2 the task of computing
element 36 is to control the streams through coils 2, 3, 4 and

lQ"77S6
-- 8 --
5 such that total stream 8 (or 50) remains constant, while
obviously the aforementioned task of controllers 25, 26, 27 and
28, which was to equalize each temperature Ti with average
temperature T, remains the same. To this end the signals of
temperature meters 13, 14, 15 and 16 are transmitted to compu-
ting element 36 and average temperature T is computed. Sub-
sequently, T - Ti i3 determined for each coil. Then for each
flow controller 25, 26, 27 and 28 a set value is now computed,
based on the difference T - Ti, which set value is now incre-
mented in proportion to the difference ~ - Ti. The desired
value Fs for the total flow of matter 8 is taken into account
here, which is indicated by arrow 37. Computing element 38
fulfils the tasks of selector 30 and controller 35 discussed
with Fig. 1. Set value Ts is indicated here by arrow 39. Thus,
computing element 36 controls the entire flow through the coils
and the balance between them, while computing element 38 con-
trols the balance between the desired and the maximum permis-
sible temperature.
In Fig. 3 the incoming stream 8 is withdrawn from the
bottom of a column 41 by a pump 40.
The liquid level in column 41 must remain within certain li-
mits. For this purpose a level gauge 42 and a controller 43 are
present. The output signal of level controller 43 goes to
computing elements 44, 45, 46 and 47. Each computing element
computes the ratio between the output ~ignal of level control-
ler 43 and the difference T - Ti computed for each coil by

10~7766
computing element 4~. The amount of liquid allowed to leave the
column, determined by level controller 43, is thus distributed
over the various coils. Furthermore, on the basis of set value
Ts (arrow 49), computing element 48 computes the set value for
controller 23 as described in relation to Fig. 2 for computing
element 38.
Fig. 3 also shows a computing element 51 by which the
proportional control factor and the integral action time of
controller 23 can be ad~usted to the difference Ti - Ts
computed by computing element 48.
The aforementioned control schemes may be realized with
conventional controllers and dedicated computing elements, such
as computing a ratio, computing an average, etc. However, it is
also possible to use a digital computer for all computations.
Finally, the functions of the controllers, too, can be perfor-
med entirely by a digital computer.
Example I
Through a furnace with four coils of tubes oil was passed
at a mass velocity of about 3000 t/d. The maximum permissible
temperature TmaX was 390C. Fig. 4A shows, for an arbitrary
time interval of 120 minutes, the changes in temperature at the
end of each of the coils 1, 2, 3 and 4 for the case where only
manual control waq used. There was a spread of about 11C. The
changes in temperature To of the combined outgoing stream is
shown in Fig. 4B on the same temperature and time scale as used
in Fig. 4A. The set value Ts is also shown. Owing to the wide

l~q7766
- lo -
qpread a temperature of about 380C had to be chosen for Ts, to
avoid the risk of any of the coil temperatures exceeding TmaX.
Figs. 4C and 4D show the results upon application of the
control system according to the invention for a constant oil
supply. The temperature curves for coils 1, 2, 3 and 4 now
coincide and the curve for To is identical with each of the
curves 1, 2, 3 and 4. The set value Ts can now be chosen much
closer to TmaX~ viz. by a distance a as compared with the case
of manual control. That distance was here 6.3C.
Example II
The effect of changing the total oil throughput is shown
for a furnace similar to that used in Example I with a control
system according to the invention. Fig. 5A shows the changes in
temperatures Ti and in the rate of flow Fi for the four coils
of tube~ in case of manual control. Temperature To of the
combined oùtgoing stream is also given. This temperature To is
lower than any of the temperatures Ti which is a result of
evaporation between measuring points Ti and To .Line Ts shows
the set value for temperature To~
Arrow b in Fig. 5B indicates the time at which the total
oil supply was reduced by 292 tJd, arrow c indicates the time
at which the supply was increased by 292 t/d. Figs. 6A and 6B
show the re~ults of these step-wise changes when the control
system according to the invention is used. ~learly, the initial
disturbing effect on temperatures Ti and To is rapidly neutra-
lized. The relative spreads in Ti are very small.

lQ9776~
1,
Example III
For a furnace similar to that used in the previous exam-
ples the effect of non-linearity of the fuel supply controller
is measured as a function of the difference To ~ Ts (controller
23 in Fig. 3).
There were four cases: I refers to the normal setting of
the controller, II refers to multiplication of the proportional
control factor and the integral action time by a factor of 2,
III by a factor of 4 and IV by a factor of 6.
Fig. 7 shows the changes in temperatures Ti ~ which coin-
cide each time for all four coils - for the cases I/IV when the
difference To ~ Ts exceeded 1C. Fig. 8 Chows setting VP of
valve 7 in the fuel line (Fig. 3).
The fluctuation in the temperatures Ti is found to decrea-
se as the multiplication factor increases. The valve qettingfor cases III and IV is shown. Case III is more favourable than
IV as regards minimizing the fluctuations in the fuel supply.