Method and apparatus for the temperature balancing control of a plurality of heat exchangers

A method for the temperature balancing control of a plurality of heat exchangers wherein, in case of operating the heat exchangers connected in parallel, the temperatures of a medium to-be-heated on the outlet sides of the respective heat exchangers are balanced. According to the method, the temperatures of the same positions of the heat exchangers except for the inlets thereof for the medium to-be-heated are sensed, the respective sensed values are compared with a temperature setting value so as to calculate control signals, all the control signals are subsequently revised so that the maximum value among the control signals may agree with a preset control reference value, and temperature regulation means disposed for the respective heat exchangers are controlled on the basis of the revised control signals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and apparatus for the temperature 
balancing control of a plurality of heat exchangers. 
2. Description of the Prior Art 
As various plants become larger in size, various heat exchangers for use in 
them ought to be enlarged correspondingly. It is the actual situation, 
however, that the enlargement of the heat exchangers is limited in 
relation to manufacturing equipment and fabricating techniques. 
For this reason, in a large-sized plant, the case of using a plurality of 
heat exchangers connected in parallel by piping is increasing. On that 
occasion, the control of the distribution of fluid flow rates to the 
respective heat exchangers becomes a problem. 
More specifically, even when the respective heat exchangers are fabricated 
in accordance with the same specifications, the dispersion of fluid 
resistances is inevitable, and dispersions arise also in the fluid 
resistances of pipes connecting the heat exchangers, the fluid resistances 
of valves disposed midway of pipes, etc. Therefore, the flow rate 
distribution to the individual heat exchangers becomes unbalanced, with 
the result that unbalanced temperatures develop in various parts of the 
heat exchangers. 
It is necessary to correct the unbalance and to operate all the parallel 
heat exchangers while their temperatures are being balanced. 
The temperature balancing control is performed by equipping the respective 
heat exchangers with control valves for regulating the fluid flow rates 
and regulating the control valves individually. When only the temperature 
balance is considered, the temperatures may be balanced with all the 
control valves kept close to their fully closed states. In order to 
realize the stable operation and efficient operation of the plant, 
however, the temperatures should preferably be balanced with the control 
valves kept close to their fully open states. 
A known prior-art method for the temperature control of a plurality of heat 
exchangers is disclosed in the official gazette of Japanese Patent 
Application Publication No. 51-30304. 
In the aforementioned known temperature control method for a multiple heat 
exchanger in which a plurality of heat exchangers are arranged in 
parallel, temperatures are sensed at the same positions of the respective 
heat exchangers except for the inlets thereof for a fluid subject to heat 
exchange, the mean temperature of the sensed temperatures is evaluated, 
and the sensed temperatures are compared with the mean temperature so as 
to regulate the flow rates of a heat exchanging fluid, whereby the 
temperatures of the fluid subject to the heat exchange are averaged. 
With this known method, the flow rates of the heat exchanging fluid in the 
respective heat exchangers are controlled using the mean temperature as a 
reference value. It is theoretically possible, however, that the balanced 
relationship of the temperatures holds in the state in which the openings 
of all control valves for controlling the flow rates are close to the full 
opening or the full closure. Therefore, the method left intact is 
problematic in practical use. 
In addition, a prior-art control method according to which the temperatures 
do not become balanced in the full closure direction is disclosed in the 
official gazette of Japanese Patent Application Publication No. 58-9920. 
In a multiple heat exchanger wherein a plurality of heat exchangers are 
used in parallel, this method consists in sensing the temperatures of the 
same positions of the respective heat exchangers except for the inlets 
thereof for a fluid subject to heat exchange and the inlets thereof for a 
heating fluid, selecting the temperature of any desired one of the 
positions as a control reference value, and adjusting the fluid flow rates 
of the respective heat exchangers so that the sensed temperatures may 
agree with the control reference value. 
In such method, using the desired position for the control reference value, 
the fluid flow rates of the respective heat exchangers are adjusted so 
that the sensed temperatures may agree. However, when control valves have 
become fully open, they cannot be opened more, and the method becomes 
uncontrollable. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method and apparatus for 
the temperature balancing control of a plurality of heat exchangers, which 
are free from the drawbacks mentioned above and which are high in 
reliability. 
In one aspect of performance of the present invention, a method for the 
temperature balancing control of a plurality of heat exchangers wherein 
temperatures of the same positions of the plurality of heat exchangers 
used in parallel, the positions being except for inlets of the heat 
exchangers for a medium to-be-heated, are respectively sensed, the sensed 
temperature values are respectively compared with a temperature setting 
value so as to calculate control signals for balancing temperatures of the 
medium to-be-heated which flows out of the respective heat exchangers, and 
regulation means for the respective heat exchangers are controlled on the 
basis of the control signals; is characterized by revising all the control 
signals so that a maximum value among said control signals may agree with 
a preset control reference value, and controlling said regulation means on 
the basis of the revised control signals. 
In another aspect of performance of the present invention, an apparatus for 
the temperature balancing control of a plurality of heat exchangers 
connected in parallel, comprising thermometers or temperature sensors 
which sense temperatures of the same positions of the heat exchangers 
respectively, the positions being except for inlets of the heat exchangers 
for a medium to-be-heated, regulation means to control temperatures of the 
medium to-be-heated in the heat exchangers respectively, and arithmetic 
control means to receive the sensed temperature values of the temperature 
sensors, to compare the respective sensed temperature values with a 
temperature setting value so as to calculate control signals for balancing 
temperatures of the medium to-be-heated which flows out of the respective 
heat exchangers, and to supply the control signals to the regulation 
means; is characterized in that said arithmetic control means has a 
function of revising all the control signals so that a maximum value among 
said control signals may agree with a preset control reference value, the 
revised control signals being supplied to said regulation means. 
Other objects and features of the present invention will become apparent 
from the following description taken with reference to the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Now, the present invention will be described in detail with reference to 
the drawings. 
FIG. 1 is a block diagram showing an embodiment of the present invention, 
while FIG. 2 is an operating flow diagram thereof. 
In FIG. 1, symbols H1, H2, H3, . . . and Hn denote heat exchangers, which 
have the function of affording the amount of heat of a heating medium A to 
a medium to-be-heated B. Here, the heating medium A signifies a heating 
fluid or a cooling fluid. In addition, the medium to-be-heated B signifies 
a fluid to-be-heated or a fluid to-be-cooled. Hereinbelow, the heating 
medium shall be explained as the heating fluid, and the medium 
to-be-heated as the fluid to-be-heated. Symbols 11, 12, 13, . . . and 1n 
indicate temperature sensors, which deliver electric signals corresponding 
to temperatures. The temperature sensors 11-1n are disposed in the same 
positions of the corresponding heat exchangers, the positions being except 
for the inlets of the fluid to-be-heated B in the heat exchangers. In the 
arrangement of FIG. 1, the temperature sensors 11-1n are installed at the 
outlets of the fluid to-be-heated B in the respective heat exchangers. 
Symbol 2A denotes a temperature sensor which is disposed at the entrance 
of the heating fluid A, and symbol 2B a temperature sensor which is 
disposed at the exit of the fluid to-be-heated B posterior to a 
confluence. Symbols CV1, CV2, CV3, . . . and CVn denote control means 
which are disposed on the outgoing sides of the fluid to-be-heated B in 
the corresponding heat exchangers H1, H2, H3, . . . and Hn so as to 
control the temperatures of the fluid to-be-heated B. In the arrangement 
of FIG. 1, control valves for controlling flow rates are employed as the 
control means. Shown at numeral 10 is arithmetic control means, which is a 
computer in FIG. 1. Each of the control valves CV1, CV2, CV3, . . . and 
CVn is actuated in accordance with a control signal (valve opening 
command) which is delivered from the computer 10. 
The apparatus shown in FIG. 1 operates as follows. The heat exchangers are 
supplied with the heating fluid A and the fluid to-be-heated B and supply 
the heat of the fluid A to the fluid B, so that the fluid B is heated. The 
temperatures of the heat exchanger outlets of the fluid to-be-heated B are 
sensed by the temperature sensors 11-1n, the sensed values T.sub.1 
-T.sub.n of which are applied to the computer 10. The computer 10 
calculates the optimum control signals on the basis of the sensed value 
inputs, and supplies them to the corrresponding control valves CV1-CVn so 
as to control the flow rates of the fluid to-be-heated B. The internal 
operations of the computer 10 are as illustrated in FIG. 2. The output of 
a timer (not shown), which delivers a start signal every fixed time, 
starts a control program so as to perform a series of operations. First, 
the sensed values T.sub.1 -T.sub.n of the respective temperature sensors 
11-1n are received as inputs (step S10). Next, a temperature setting value 
T.sub.s which serves as the reference of a temperature balancing control 
is calculated on the basis of the input values (step S20). Subsequently, 
each of the sensed temperature values T.sub.1 -T.sub.n is compared with 
the temperature setting value T.sub.s, whereupon the valve opening 
variation .DELTA.V.sub.i of each control valve is calculated on the basis 
of a deviation .DELTA.T.sub.i (i=1, 2, . . . , n) obtained by the 
comparison. The control signal (valve opening) V.sub.i of each control 
valve is evaluated from the variation .DELTA.V.sub.i. That is, the 
following is calculated: 
EQU .DELTA.T.sub.i =T.sub.i -T.sub.s (1) 
EQU .DELTA.V.sub.i =.alpha..multidot..DELTA.T.sub.i (2) 
EQU V.sub.i =V.sub.i.sup.(-1) +.DELTA.V.sub.i (3) 
where 
.alpha.: the coefficient of conversion, 
V.sub.i.sup.(-1) : the control signal of the i-th control valve in the last 
control. 
These are operations indicated in steps S30-S80. After all the control 
signals V.sub.i for the control valves have been calculated, the operating 
flow proceeds to the next step. At step S90, the maximum value V.sub.max 
is selected from among all the control signals V.sub.i. Next, the maximum 
value V.sub.max is compared with a preset control reference value V.sub.o 
at step S95. Subject to V.sub.max &gt;V.sub.o, the processing flow proceeds 
to step S100. The reference value V.sub.o is selected at a magnitude 
corresponding to a valve opening of 50%-100%, in consideration of the 
overall efficiency. However, V.sub.o is not restricted thereto, but any 
desired magnitude other than 0% can be selected therefor. Moreover, if 
necessary, V.sub.o can be altered during the operation of the apparatus. 
Step S100 executes the calculation of revising the control signal V.sub.i. 
This calculation is as follows: 
EQU .DELTA.k=.vertline.V.sub.max -V.sub.o .vertline. (4) 
EQU V.sub.i '=V.sub.i -.DELTA.k (5) 
where 
i=1, 2, . . . , n 
V.sub.i '; revised control signal. 
As the result of the calculation, V.sub.max is revised to V.sub.o, and also 
the other control signals V.sub.i are equally revised by .DELTA.k. 
When V.sub.max =V.sub.o or V.sub.max &lt;V.sub.o holds, the control flow 
proceeds to step S110. When V.sub.max &lt;V.sub.o holds at step S110, the 
processing flow proceeds to step S120, which revises the control signal 
V.sub.i as follows: 
EQU V.sub.i '=V.sub.i +.DELTA.k (6) 
As the result of the calculation, V.sub.max is revised to V.sub.o, and also 
the other control signals V.sub.i are equally revised by .DELTA.k. In case 
of V.sub.max =V.sub.o, the processing flow proceeds to step S130, and the 
control signal is not revised in this case. That is, V.sub.i '=V.sub.i is 
held. At step S140, the revised control signals V.sub.i ' are fed to the 
respective control valves. On the basis of the control signals V.sub.i ', 
the control valves regulate the valve openings so as to control the flow 
rates of the fluid to-be-heated B. 
The operations of FIG. 2 are intelligibly illustrated in FIG. 3. Let's 
consider the state in which, at a point of time t.sub.1, the valve opening 
of the control valve CV1 is 80%, that of the control valve CV2 is 60%, 
that of the control valve CV3 is 100%, and that of the control valve CVn 
is 90%. It is assumed that the calculations up to step S80 in FIG. 2 have 
given the control signals V.sub.i with which the valve openings of the 
control valves fall into a state b (CV1: 80%, CV2: 70%, CV3: 105%, CVn: 
85%). On this occasion, the control valve CV3 comes to have the valve 
opening of 105% and becomes uncontrollable in actuality. Accordingly, the 
actual control signals V.sub.i ' at a point of time t.sub.2 are revised so 
as to bring the valve openings of the control valves into an illustrated 
state c (CV1: 75%, CV2: 65%, CV3: 100%, CVn: 80%). The control reference 
value V.sub.o in the case of FIG. 3 corresponds to the valve opening of 
100%. 
Although, in the above example, the revision of the control signals V.sub.i 
has been made on the basis of the difference .DELTA.k between the maximum 
value V.sub.max and the reference value V.sub.o, this is not restrictive. 
For example, it is also allowed to take the ratio of the values V.sub.max 
and V.sub.o and to revise all the control signals on the basis of the 
ratio. The revision of the control signal in the case of employing the 
ratio can be realized with the following equations by way of example: 
EQU M=V.sub.o /V.sub.max (7) 
EQU V.sub.i '=V.sub.i .multidot.M (8) 
where M; proportion coefficient. 
Although the temperature control means in FIG. 1 has been the valves for 
controlling the flow rates of the medium to-be-heated B, the present 
invention is not restricted thereto. For example, it is also allowed to 
employ an appliance which changes the temperature or flow rate of the 
heating medium A. A heater may well be employed. Anyway, means capable of 
controlling the temperature of the medium to-be-heated B suffices. 
In the foregoing embodiment, the temperature setting value T.sub.s may 
concretely be any of the sensed temperature values T.sub.1 -T.sub.n 
mentioned before or the mean value of the values T.sub.1 -T.sub.n. It may 
well be the sensed value of the temperature sensor 2B which is located at 
the exit of the fluid to-be-heated B in FIG. 1. 
The sensed value of the temperature sensor 2A in FIG. 1 is utilized for a 
predictive control which predicts the temperature fluctuations of the 
fluid to-be-heated B attributed to a temperature fluctuation on the 
incoming side of the heating fluid A and which serves to mitigate the 
temperature fluctuations of the fluid B. The sensed value of the 
temperature sensor 2B is utilized, not only as the temperature setting 
value stated above, but also for a feedback control which maintains the 
temperature of the fluid to-be-heated B at a desired value. 
As described above, according to the present invention, the drawbacks of 
uncontrollability etc. can be eliminated, and the temperature balancing 
control of high reliability can be realized.