RU2006762C1 - Method of automatic control of evaporator liquid cooling process - Google Patents

Abstract

FIELD: refrigerating engineering. SUBSTANCE: method consists in measuring the temperature and flow rate of liquid being cooled, pressure and temperature of refrigerant vapor after evaporator, level of refrigerant in evaporator, stabilizing the level of refrigerant in evaporator through changing the rate of flow of liquid refrigerant to evaporator, changing the temperature of the coolant at evaporator inlet, stabilizing the temperature of liquid being cooled at evaporator outlet through change of control action on pressure regulator of refrigerant vapor depending on temperature of liquid being cooled at evaporator outlet, its flow rate, level of refrigerant in evaporator, limiting the control action on pressure regulator of refrigerant vapor depending on temperature of liquid being cooled at evaporator inlet, determining the overheating of refrigerant vapor by its temperature and pressure and forming (depending on its magnitude) the control action on refrigerant level regulator in evaporator and limiting the control action on refrigerant level regulator, determining the thermal head by pressure of refrigerant vapor and temperature of coolant at evaporator inlet and outlet, determining the low limit on control action to pressure regulator of refrigerant vapor depending on magnitude of thermal head and level of refrigerant in evaporator and limiting the control action on pressure regulator of refrigerant vapor after evaporator. EFFECT: enhanced efficiency. 2 dwg

Description

 The invention relates to the automation of heat transfer processes and can be used, in particular, for automatic control of the processes of cooling liquids in evaporators in the chemical, petrochemical, metallurgical, food industries.

 There is a method of automatically controlling the process of cooling liquid in an evaporator, which consists in measuring the temperature and pressure of the refrigerant vapor after the evaporator, determining the amount of superheat of the refrigerant vapor, adjusting the amount of superheat by changing the flow rate of the liquid refrigerant to the evaporator (V. Uzhansky. Automation of refrigeration machines and installations. 2 -th ed., revised and additional M.: Light and food industry, 1982, S. 79-102, 231-290).

 The disadvantage of this method is the relatively low accuracy of automatic control of the temperature of the liquid being cooled and the high cost of cooling it. This is due to the fact that the known control method does not provide for the measurement and use of the level in the evaporator when forming the control action.

Using to control the process of cooling the liquid in the evaporator only the value of the superheat, without taking into account information about the level in the evaporator, does not provide the same effect as when they are used simultaneously. This is due to the fact that the superheat should be minimal, for example 1.5 ° ^C. In operation, overheating can be more (2 or 3 ° ^C), the superheat value can only decrease to zero. Thus, an undesirable non-linearity arises in the automatic control system (in the direction of reducing overheating - the maximum deviation is 1.5 ^° C, and in the direction of increasing - the maximum deviation is not limited). If this does not use information about the level of the refrigerant in the evaporator as an intermediate controlled variable, then the indicated non-linearity in some cases leads to unsatisfactory quality of temperature control of the cooled liquid.

 In addition, it is known that excessive filling of the evaporator causes instability of the outlet temperature. In this case, changes occur in the value of overheating (fluctuations).

 A disadvantage of the known method is that this control method does not provide for determining the lower limit on the control action of the refrigerant vapor pressure regulator and the restriction of this control action.

 There is also a method of automatically controlling the process of cooling liquid in the evaporator during the polymerization of conjugated dienes, which consists in regulating the temperature of the liquid being cooled by changing the vapor pressure of the refrigerant in the evaporator, measuring the temperature and pressure of the liquid refrigerant supplied to the evaporator, comparing the measured pressure with its acceptable value, moreover, if it is greater than or equal to this value, then the set value of the temperature of the cooled liquid is left unchanged, and if it is less, then its adjust proportionally to the deviation of the refrigerant vapor pressure from its permissible value, and the latter is determined by summing the saturated vapor pressure of the refrigerant at the measured temperature of the liquid refrigerant with a threshold value of the pressure of the liquid refrigerant, which is selected empirically (A.S. N 1139736, class C 08 F 36/04; G 05 D 27/00).

 The disadvantage of this method is the relatively low accuracy of temperature control of the liquid being cooled and the high cost of the cooling process.

 Measurement of the pressure and temperature of the refrigerant supplied to the evaporator makes it possible to only approximately estimate the boiling rate.

 The known method does not provide for the determination and use to control the magnitude of the superheat of the refrigerant vapor.

 Closest to the technical essence of the present invention is a method for automatically controlling the process of cooling the liquid in the evaporator, used in the polymerization of isoprene rubber, which consists in measuring the temperature and flow rate of the cooled liquid, vapor pressure of the refrigerant after the evaporator, the level of refrigerant in the evaporator, stabilization of the level of refrigerant in the evaporator by liquid refrigerant flow rate to the evaporator, stabilization of the temperature of the cooled liquid by changing the control action I regulator of vapor pressure of the refrigerant, depending on the temperature of the liquid to be cooled, its flow rate, level of refrigerant in the evaporator, limiting the control action to the regulator of vapor pressure of the refrigerant (A.S. N 1255626, CL 08 F 136/08, G 05 D 27/00 )

 The disadvantage of this method is the relatively low accuracy of temperature control of the liquid being cooled and the high cost of cooling it. This is due to the lack of control and regulation of the magnitude of the superheat of the refrigerant vapor, as well as the fact that in the known method, the change in the magnitude of the superheat of the refrigerant vapor is not taken into account when generating control actions. Lack of control of the amount of overheating adversely affects the accuracy of regulating the degree of filling of the evaporator with refrigerant. The efficiency and safety of the operation of the evaporator depends on the degree of filling of the evaporator with boiling refrigerant. Insufficient filling of the evaporator leads to the necrosis of part of the heat transfer surface, and therefore, to an increase in cooling costs. The overfilling of the evaporator negatively affects the operation of the compressor, which sucks the resulting refrigerant vapor.

 A disadvantage of the known method is that the known method does not provide for determining the lower limit on the control effect of the pressure regulator of the refrigerant vapor after the evaporator, depending on the temperature head. This impairs the accuracy of the automatic control of the temperature of the liquid being cooled and leads to an increase in the cost of the cooling process due to a decrease in the heat transfer coefficient. This happens due to the fact that when a certain temperature head is reached, the process of boiling the refrigerant passes from the bubble boiling mode to the film boiling mode. In this case, the heat transfer coefficient decreases sharply. This can be avoided by lowering the control action of the refrigerant vapor pressure regulator from below. Moreover, the lower limit of the refrigerant vapor pressure regulator should be determined depending on the temperature head.

 By limiting the control action of the refrigerant vapor pressure regulator, a limitation of the boiling point of the refrigerant is achieved, which means that the temperature head is limited.

 The essence of the invention lies in the fact that according to the method for automatically controlling the process of cooling the liquid in the evaporator, which consists in measuring the temperature and flow rate of the cooled liquid, the vapor pressure of the refrigerant after the evaporator, the level of refrigerant in the evaporator, stabilization of the level of refrigerant in the evaporator by changing the flow rate of liquid refrigerant to the evaporator, stabilization of the temperature of the cooled liquid by changing the control action of the refrigerant vapor pressure regulator depending on the temperature tours of the cooled liquid, its flow rate, the level of the refrigerant in the evaporator and the limitation of the control action of the refrigerant vapor pressure regulator, additionally measure the temperature of the refrigerant vapor after the evaporator, the temperature of the refrigerated liquid at the evaporator inlet, adjust the control effect of the refrigerant vapor pressure regulator depending on the temperature of the refrigerant inlet evaporator, adjust the control action to the regulator of the refrigerant level in the evaporator, depending on the amount of overheating and limit the control action to the refrigerant level controller if its value is less than the lower or greater than the upper limit with the help of a non-linear block of the “stop” type. The temperature of the head is determined from the vapor pressure of the refrigerant and the temperature of the liquid to be cooled at the inlet and outlet of the evaporator.

 The lower restriction on the control action of the refrigerant vapor pressure regulator is determined depending on the temperature head and the refrigerant level in the evaporator and the control effect on the refrigerant vapor pressure regulator after the evaporator is limited if its value is less than the lower or more specified upper limit using a non-linear block of type “stop” .

 The use of information on overheating of refrigerant vapor makes it possible to correct the set level value, to control and maintain the degree of filling of the evaporator with greater accuracy, on which, ultimately, cooling costs depend. The use of information on the level of refrigerant in the evaporator facilitates the task of automatically controlling the degree of filling, leading to an increase in the quality of automatic control of the temperature of the liquid being cooled and lowering the cost of cooling.

Using for control only the value of superheat without taking into account information about the level in the evaporator does not provide the same effect as with their simultaneous use. This is due to the fact that the superheat should be minimal, for example 1.5 ° ^C. In operation, overheating can be more (2 or 3 ° ^C), the superheat value may decrease only to zero. Thus, an undesirable non-linearity arises in the automatic control system. If this does not use the level of refrigerant in the evaporator as an intermediate controlled variable, then the specified non-linearity in some cases leads to unsatisfactory quality of temperature control of the cooled liquid. However, the use of both the level in the evaporator and the value of overheating precludes this possibility and leads to an increase in the accuracy of automatic control of the temperature of the liquid being cooled.

 In addition, it is known that excessive filling of the evaporator causes instability of the outlet temperature. In this case, changes (fluctuations) in the superheat occur. With an increase in the degree of filling of the evaporator, the fluctuations of overheating increase.

 With a decrease in the degree of filling of the evaporator, a "necrosis" of part of the heat exchange surface of the evaporator occurs.

 The intensification of heat transfer with an insufficient degree of filling of the evaporator by lowering the boiling point of the refrigerant is not practical, since with a decrease in the boiling point of the refrigerant the heat transfer coefficient decreases (Heat exchange intensification in evaporators of refrigeration machines. A. A. Gogolin, G.N. Danilova, V.M. Azaranov, N. M. Medninova, M.: Light and Food Industry, 1982, p. 4).

The stabilization of the value of the superheat of the refrigerant vapor at a level of 1-1.5 ^about With also provides the removal of oil droplets with vapors leaving the evaporator and their accumulation in the liquid separator.

 Thus, it is advisable to stabilize the amount of heating, and if it deviates from the set value, it is necessary to adjust the set value of the refrigerant level in the evaporator.

 Depending on the temperature head, various modes of boiling of the refrigerant are possible in the evaporator: free convection, bubble feeding, film boiling. At a certain temperature head, a transition from bubble boiling to film boiling occurs. In this case, the heat transfer coefficient decreases sharply, and the evaporator control system becomes unstable. The temperature head can be limited by limiting the control action of the refrigerant vapor pressure regulator, i.e., by limiting the boiling point of the refrigerant. Moreover, the lower limit of the refrigerant vapor pressure regulator should be determined depending on the temperature head.

 The essence of the invention is illustrated by an example of its implementation and the drawing (Fig. 1), which shows a system that implements the considered method of automatic control of the process of cooling the liquid in the evaporator.

 In FIG. 2 presents a block diagram of an automatic control system that implements the proposed method.

 Liquid refrigerant is fed into the annulus 1 of the evaporator 1 where it boils. Ammonia, propane or refrigerant is usually used as the refrigerant.

 In this case, the refrigerant is propane. The resulting refrigerant vapor from the upper part of the evaporator 1 enters the liquid separator 2, in which the unevaporated refrigerant is separated from its vapor. From the liquid separator 2, the refrigerant vapor is sucked off by the compressor.

 A control system that implements the considered method for automatically controlling the process of cooling the liquid in the evaporator includes a sensor 4 for the flow of the cooled liquid, a sensor 5 for the temperature of the cooled liquid at the outlet of the evaporator, a sensor 6 for the refrigerant level in the evaporator, a pressure sensor 7 and a sensor 8 for the temperature of the refrigerant vapor after the evaporator , a temperature sensor 9 of the cooled liquid at the inlet of the evaporator, a regulator 10 of the refrigerant level in the evaporator and a regulator 11 of the vapor pressure of the refrigerant after the evaporator.

 The proposed method for automatically controlling the process of cooling the liquid in the evaporator is as follows. The inputs of the control computer complex 3 receive signals from the sensor 4 of the flow rate of the cooled liquid, the sensor 5 of the temperature of the cooled liquid at the outlet of the evaporator, the sensor 6 of the refrigerant level in the evaporator, the pressure sensor 7 and the sensor 8 of the temperature of the refrigerant vapor after the evaporator, sensor 9 of the cooled liquid inlet evaporator. The refrigerant level in evaporator 1 and the vapor pressure of the refrigerant after evaporator 1 are stabilized at the set values, respectively, with the help of regulators 10 and 11. The set values for the regulators 10 and 11 are received from the control computer complex 3.

K₄(T_{ж вых i}-T_{ж вых з})Δt+K₅((T_{ж вых i}-T_{ж вых i-1})/Δt) где Р_хвых.о - начальное значение давления паров хладагента после испарителя 1, кг/см²;
F_жi, F_жо - текущее и начальное значения расхода охлаждаемой жидкости, кг/ч; H_i, Н_о - текущее и начальное значения уровня хладагента в испарителе, % ; Т_жвыхi; Т_жвыхз - текущее и заданное значения температуры охлаждаемой жидкости на выходе испарителя, ^оС;
Δ t - дискретность формирования управляющего воздействия, ч;
К₁, К₂, К₃, К₄, К₅ - настроечные коэффициенты.The control action of the regulator 11 of the vapor pressure of the refrigerant is formed depending on the temperature of the cooled liquid at the outlet of the evaporator 1, its flow rate, the level of refrigerant in the evaporator:
_{O zi} P _x = P _{x vyh.o} + K ₁ (F -F _{zhi jo)} + K ₂ (H _i -H _{a) s} + K (T _i -T _{w O w O z)} +
+

K ₄ (T _{w output i} -T _{w output h} ) Δt + K ₅ ((T _{w output i} -T _{w output i-1} ) / Δt) where Р _hvy.o is the initial value of the vapor pressure of the refrigerant after the evaporator 1, kg / cm ² ;
F _Жi , F _жо - current and initial values of the flow rate of the cooled liquid, kg / h; H _i , N _o - current and initial values of the level of refrigerant in the evaporator,%; T _live ; T _zhvyz - current and set temperature values of the cooled liquid at the outlet of the evaporator, ^о С;
Δ t is the discreteness of the formation of the control action, h;
To ₁ , To ₂ , To ₃ , To ₄ , To ₅ - tuning factors.

Correct the control action of the regulator 11 of the vapor pressure of the refrigerant depending on the temperature of the cooled liquid at the inlet of the evaporator 1:
Р ^к _ххыхзi = Р _хвыхзi + К ₆ (Т _жвхi - Т _жвх.о ), (2) where Р ^к _хвыхзi is the adjusted value of the control action to the pressure regulator of 11 refrigerant vapor, kg / cm ² ;
T _zhvi ; T _LHW - current and initial temperature values of the cooled liquid at the inlet of the evaporator, ^о С; To ₆ - tuning factor.

The superheat of the refrigerant vapor is determined by the temperature and pressure of these vapor:
V = Т _хвыхi - Т _кi , (3) where V is the value of overheating, ^о С;
T _hvyh - temperature of the refrigerant vapor after the evaporator, ^о С;
T _Ki - the boiling point of the refrigerant, determined by the vapor pressure of the refrigerant after the evaporator, ^about C.

Determination of the boiling point of the refrigerant is carried out by piecewise linear approximation of the dependence T _to = f (P _xout ). For this, in the working range of this dependence, control points m are set at which the values of P _xoutm and the corresponding values of T _{km are known} . The determination of T _{Ki is} carried out according to the following formula:
T _ki = T _km + (T _{k m + 1} -T _km ) / (P _{x out m + 1} -P _{x out m} ) (P _{x out i} -P _{x out m} ) where m is the number of the control point of the dependence T _to = f (P _xout );
T _to - the value of the boiling point of the refrigerant, ^about ; P _hvyi - the value of the vapor pressure of the refrigerant, kg / cm ² .

, (5) H_зi=

где Е_нi - отклонение между текущим и заданным значениями уровня хладагента в испарителе, % ; Н_о - начальное значение значения уровня, % ; Е_нд - допустимое отклонение уровня от заданного значения, % ;
К_н, К_v - настроечные коэффициенты.Next, a control action is formed for the regulator 10 of the refrigerant level in the evaporator, depending on the amount of overheating. Carry out this, for example, as follows. The value of the refrigerant level is compared with its predetermined value and, if the level value does not differ from the set value by more than the permissible deviation, then the control action of the regulator 10 of the refrigerant level in the evaporator is determined depending on the amount of superheat of the refrigerant vapor, otherwise the control effect to the regulator 10 levels of refrigerant in the evaporator are left unchanged at the achieved level:
ε _нi = Н _i -

, (5) H _zi =

where Е _нi is the deviation between the current and set values of the refrigerant level in the evaporator,%; N _about - the initial value of the level value,%; E _nd - the permissible deviation of the level from the set value,%;
To _n , To _v - tuning factors.

< ε_нд дает возможность повысить устойчивость системы автоматического регулирования и качество автоматического регулирования температуры охлажденной жидкости.Changing the set value of the level N _s only under the condition

<ε _nd makes it possible to increase the stability of the automatic control system and the quality of automatic control of the temperature of the chilled liquid.

где

- ограниченное управляющее воздействие регулятору 10 уровня хладагента в испарителе, % ;

- ограниченное управляющее воздействие регулятору 10 уровня хладагента в испарителе не предыдущем шаге формирования управляющего воздействия, т. е. в момент времени i-1, % ; Δ Н_зi - изменение (приращение) управляющего воздействия на последнем шаге формирования управляющего воздействия, т. е. за время t_i-t_i-1, % ; Н_зmin,Н_зmax - нижнее и верхнее ограничения на управляющее воздействие регулятору 10 уровня хладагента, % .The control action of the regulator 10 of the refrigerant level is limited by means of a non-linear block of the “stop” type:

Where

- limited control action to the regulator 10 of the refrigerant level in the evaporator,%;

- limited control action to the regulator 10 of the refrigerant level in the evaporator not at the previous step of forming the control action, i.e., at time i-1,%; Δ N _zi - change (increment) of the control action at the last step of the formation of the control action, that is, for the time t _i -t _i-1 ,%; N _zmin, N _zmax - lower and upper limits on the control action of the regulator 10 levels of refrigerant,%.

 Limiting the set value of the refrigerant level in the evaporator makes it possible to increase the reliability of the system that implements the proposed method, to reduce the likelihood of level changes in unacceptably wide limits.

 Using for this purpose a non-linear block of the “stop” type makes it possible to increase the accuracy of automatic control of the value of overheating and temperature of the cooled liquid (Poplavsky V.F., Arbuzov S.P., Dorofeev V.I. -3. M.: NIITEKHIM, 1983, issue 3, pp. 8-14).

Next, determine the temperature head from the vapor pressure of the refrigerant and the temperature of the cooled liquid at the inlet and outlet of the evaporator. To do this, the boiling point of the refrigerant is preliminarily determined by the vapor pressure of the refrigerant. Then determine the temperature head: ΔT = [(T _zhvii -T _ki ) - (T _zhvii -T _ki )] / ln [(T _zhvii -T _ki ) / (T _zhvi -T _ki )], (11) where ΔТ - temperature head, ^about C.

где P_xвыхi ^min - нижнее ограничение на управляющее воздействие регулятору 11 давления паров хладагента после испарителя, кг/см²;
Δ Т_д - допустимое значение температурного напора, ^оС; Δ Н_д - допустимое значение отклонения уровня хладагента в испарителе от заданного значения, % ; θ - постоянная времени усредняющего фильтра, ч;
P_xвых ^min.o - минимальное начальное нижнее ограничение на управляющее воздействие регулятору 11 давления паров хладагента после испарителя 1, кг/см².Depending on the temperature head and the level of refrigerant in the evaporator, the lower restriction on the control effect of the regulator 11 of the vapor pressure of the refrigerant after the evaporator is determined:

where P _xouti ^min is the lower limit on the control action of the regulator 11 of the vapor pressure of the refrigerant after the evaporator, kg / cm ² ;
Δ T _d - the permissible value of the temperature head, ^о С; Δ N _d - the permissible value of the deviation of the refrigerant level in the evaporator from the set value,%; θ is the time constant of the averaging filter, h;
P _xout ^min.o is the minimum initial lower limit on the control action of the regulator 11 of the vapor pressure of the refrigerant after evaporator 1, kg / cm ² .

The value of P _xout ^min.o is determined empirically and is set when debugging a system that implements this control method.

Thus, if Δ T _i > Δ T _d or H _zi - H _i > Δ N _d and at the same time P> P _xout ^min.o , then the lower restriction on the control action of the pressure regulator of 11 refrigerant vapors is equal to the current pressure value P _xouti .

If Δ Ti ≅ΔT _zi and H _d - H _i ≅ΔN _h, the lower limit is averaged over time by the averaging filter and thus return smoothly to its original value P _xvyh ^min.o.

где

- ограниченное управляющее воздействие регулятору 11 давления паров хладагента после испарителя, кг/см²;
Р^к _хвыхзi - изменение скорректированного управляющего воздействия на последнем шаге формирования управляющего воздействия, кг/см²;
P_xвых ^max - верхнее ограничение на управляющее воздействие регулятору 11 давления паров хладагента, кг/см²;

- ограниченное управляющее воздействие регулятору 11 давления паров хладагента после испарителя 1 на предыдущем шаге, кг/см². (56) Авторское свидетельство СССР N 1255626, кл. С 08 F 136/08, 1986. The control effect of the regulator 11 of the vapor pressure of the refrigerant after the evaporator 1 is limited using a non-linear block of the type "stop":

Where

- limited control action to the regulator 11 of the vapor pressure of the refrigerant after the evaporator, kg / cm ² ;
Р ^к _хххзi - change in the adjusted control action at the last step of the formation of the control action, kg / cm ² ;
P _xout ^max - the upper limit on the control action of the regulator 11 of the vapor pressure of the refrigerant, kg / cm ² ;

- limited control action of the regulator 11 of the vapor pressure of the refrigerant after the evaporator 1 in the previous step, kg / cm ² . (56) Copyright certificate of the USSR N 1255626, cl. C 08 F 136/08, 1986.

Claims (1)

 METHOD FOR AUTOMATIC CONTROL OF THE LIQUID COOLING PROCESS IN THE EVAPORATOR, which consists in measuring the temperature and flow rate of the cooled liquid, the vapor pressure of the refrigerant after the evaporator, the refrigerant level in the evaporator, the stabilization of the refrigerant level in the evaporator by changing the flow rate of the liquid refrigerant in the evaporator, the pressure regulator, the stabilization refrigerant vapor depending on the temperature of the liquid to be cooled, its flow rate, the level of refrigerant in the evaporator and in limiting the control action of the refrigerant vapor pressure regulator, characterized in that the temperature of the refrigerant vapor after the evaporator is additionally measured, the temperature of the coolant at the inlet of the evaporator, the control action of the refrigerant vapor pressure regulator is adjusted, depending on the temperature of the coolant at the inlet of the evaporator, the superheat of the refrigerant vapor is determined by temperature and vapor pressure of the refrigerant and form a control action to the regulator of the level of refrigerant in the evaporator in depending on the amount of overheating, limit the control action to the refrigerant level regulator, if its value is less than the lower or more upper limit, determine the temperature head from the pressure of the refrigerant vapor and the temperature of the cooled liquid at the inlet and outlet of the evaporator, determine the lower restriction on the control effect of the regulator of vapor pressure in depending on the temperature head and the level of refrigerant in the evaporator and limit the control action of the regulator of vapor pressure agent downstream of the evaporator, if its value is less than the lower or greater than a predetermined upper limit.

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