EP2128551A1 - Monitoring of heat exchangers in process control systems - Google Patents

Abstract

The method involves flowing heat from a product medium into a service medium e.g. cool water. Current heat flow is measured and compared with a reference heat flow that corresponds to a predetermined pollution degree of a heat exchanger (2). Calculation of the reference heat flow is performed by a simulation program, where the heat exchanger is dimensioned by the simulation program. The current heat flow is compared with a reference heat flow that corresponds with a pollution degree of zero, and with a reference heat flow that corresponds with a maximum pollution degree. Independent claims are also included for the following: (1) a device for controlling a system with a heat exchanger (2) a computer program product comprising a program-code sequence for executing a method for monitoring effectiveness of a heat exchanger.

Description

The invention relates to a method according to the preamble of claim 1, for monitoring the effectiveness of a heat exchanger in which heat flows from a first medium into a second medium. Furthermore, the invention relates to a device for controlling a system with at least one heat exchanger.

Heat exchangers are technical apparatus in which, for example, fluids of a first temperature deliver part of their heat to, for example, fluids of a second temperature below the first temperature. For example, a first medium (product medium) can be cooled or heated by means of a second medium (service medium). The service medium can be, for example, cooling water or heating steam. The service medium typically either flows through a piping arrangement which is disposed within the product medium or flows around the piping arrangement through which the product medium flows.

Depending on the nature of the product medium or service medium, deposits can form inside or outside the pipe arrangement (so-called fouling). The deposits reduce the effectiveness of the heat exchanger. If the thickness of the pads has exceeded a certain level, it is therefore necessary to perform a cleaning of the piping arrangement. For this purpose, the heat exchanger in question must be taken out of service as a rule. This is on the one hand very expensive and causes on the other hand considerable costs.

A particular disadvantage is that the pads are often not visible from the outside. Therefore, it is not clear when a cleaning is required. A cleaning is often only then made when problems caused by the poor performance of the heat exchanger occur. To avoid this, the heat exchanger must be cleaned preventively at regular intervals. This is also disadvantageous because the heat exchanger is cleaned in such a case, even if the deposits are not very strong.

Although simulation programs are known, which are used for the process engineering design and dimensioning of heat exchangers in the planning phase of a plant, which are based on a physico-thermodynamic modeling of the heat exchanger, which is mathematically divided into numerous segments, but it is not known, these simulation programs to use for on-line monitoring of heat exchangers during operation. Therefore, there is not yet a satisfactory solution to realize the monitoring of heat exchangers within a process control system, especially when the heat exchangers are driven in the operating phase at different operating points, for example, because flow or temperature of the product are not constant.

It is an object of the invention to form an initially mentioned method or an aforementioned control such that a statement about the effectiveness of a heat exchanger can be made.

The solution of this problem arises from the features of the characterizing part of claim 1. Advantageous developments of the invention will become apparent from the dependent claims.

According to the invention, a method for monitoring the effectiveness of a heat exchanger in which heat flows from a first medium to a second medium, characterized in that an actual heat flow is detected and with at least one each predetermined degree of contamination the heat exchanger corresponding reference heat flow is compared.

Furthermore, according to the invention, a device for controlling a system with at least one heat exchanger is characterized in that a memory is present in which at least one reference heat flow of the heat exchanger is stored.

Characterized in that a current heat flow is detected and compared with at least one respective predetermined degree of contamination of the heat exchanger corresponding reference heat flow, can be on the effectiveness of the heat exchanger make a very reliable statement; because by the inventive idea, to use the heat flow itself as a measure of the performance of the heat exchanger, a size is used as a measure of the effectiveness of the heat exchanger, which represents the most essential function of the heat exchanger. This eliminates problems associated with an indirect determination of the performance of the heat exchanger, d. H. when using another size characterizing the heat exchanger for determining the performance of the heat exchanger, may occur.

$${\dot{Q}}_{\mathrm{S}}={\mathrm{C}}_{\mathrm{P}\mathrm{,}\mathrm{S}}\mathrm{\cdot }{\mathrm{\rho }}_{\mathrm{S}}\mathrm{\cdot }{\mathrm{F}}_{\mathrm{S}}\mathrm{\cdot }\left({\mathrm{T}}_{\mathrm{S}\mathrm{,}\mathrm{Aus}}-{\mathrm{T}}_{\mathrm{S}\mathrm{,}\mathrm{Ein}}\right)$$

The determination of the current heat flow ( Q _act ) can be done by the flow (F _P ) of the product medium through the heat exchanger, the flow (F _S ) of the service medium through the heat exchanger, the temperature (T _{P, A} ) of the product medium at the inlet of the product medium in the heat exchanger, the Temperature (T _{P, Out} ) of the product medium at the outlet of the product medium from the heat exchanger, the temperature (T _{S, Ein} ) of the service medium at the entrance of the service medium into the heat exchanger and the temperature (T _{S, Out} ) of the service medium at the output of the service medium be detected the heat exchanger. On the basis of the measured values of the flows and the temperatures as well as the substance data c _{P, P} , c _{P, S} , ρ _P and ρ _S can be calculated from the stationary energy balances for product and service medium within the heat exchanger calculate the current heat flow for a liquid-liquid heat exchanger reliably and with little effort according to the following formulas: $${\dot{Q}}_{\mathrm{P}}={\mathrm{C}}_{\mathrm{P}\mathrm{.}\mathrm{P}}\mathrm{\cdot }{\mathrm{\rho }}_{\mathrm{P}}\mathrm{\cdot }{\mathrm{F}}_{\mathrm{P}}\mathrm{\cdot }\left({\mathrm{T}}_{\mathrm{P}\mathrm{.}\mathrm{Out}}-{\mathrm{T}}_{\mathrm{P}\mathrm{.}\mathrm{On}}\right)$$

$${\dot{Q}}_{\mathrm{S}}={\mathrm{C}}_{\mathrm{P}\mathrm{.}\mathrm{S}}\mathrm{\cdot }{\mathrm{\rho }}_{\mathrm{S}}\mathrm{\cdot }{\mathrm{F}}_{\mathrm{S}}\mathrm{\cdot }\left({\mathrm{T}}_{\mathrm{S}\mathrm{.}\mathrm{Out}}-{\mathrm{T}}_{\mathrm{S}\mathrm{.}\mathrm{On}}\right)$$

Due to the law of conservation of energy, the following theoretically holds: $${\dot{Q}}_{\mathrm{p}}=-{\dot{Q}}_{\mathrm{s}}$$

wobei

• Q̇ _P der Wärmestrom des Produktmediums,
• Q̇ _S der Wärmestrom des Servicemediums,
• Q̇ _act der aktuelle Wärmestrom,
• c_P,P die Wärmekapazität des Produktmediums,
• c_P,S die Wärmekapazität des Servicemediums,
• ρ_P die Dichte des Produktmediums und
• ρ_S die Dichte des Servicemediums ist.

Because of measurement inaccuracies, an average value of the absolute values is formed for the current heat flow: $${\dot{Q}}_{\mathrm{act}}=\frac{1}{2}\cdot \left(\left|{\dot{Q}}_{\mathrm{P}}\right|+\left|{\dot{Q}}_{\mathrm{S}}\right|\right)$$

in which

• Q _{P is} the heat flow of the product medium,
• Q _S the heat flow of the service medium,
• Q _act the current heat flow,
• c _{P, P is} the heat capacity of the product medium,
• c _{P, S is} the heat capacity of the service medium,
• ρ _{P is} the density of the product medium and
• ρ _{S is} the density of the service medium.

In cases of evaporation or condensation of product or service medium in the heat exchanger, these formulas must be adapted accordingly.

By means of the procedural simulation program, with which the heat exchanger has been designed or can be designed or dimensioned, can be calculated for different degrees of contamination of the heat exchanger in each case a theoretical heat flow, which can be used as a reference heat flow.

Advantageously, the calculation of the reference heat flow is carried out by means of the simulation program. This gives a simple way reference heat flows, which come very close to the actual heat flows of the heat exchanger in question under the same conditions. To increase the accuracy measurements are made to fine-tune parameters of the simulation program at a few operating points in the clean state of the heat exchanger.

By comparing the current heat flow with the reference heat flow determined with the simulation program, for example in the non-polluted state of the heat exchanger, a reliable statement can be made about the current efficiency of the heat exchanger. If the current heat flow corresponds to the reference heat flow, the efficiency of the heat exchanger is not affected by deposits. As the difference between the current heat flow and the reference heat flow increases, the performance of the heat exchanger decreases, i.e., decreases. H. the deposits have increased. The difference between the current heat flow and the reference heat flow thus provides a measure of the deposits, i. H. the pollution of the heat exchanger. The larger the difference, the larger the deposits.

Instead of comparing the current heat flow with the reference heat flow of the non-polluted heat exchanger, the current heat flow can be compared with the reference heat flow of the dirty heat exchanger. The difference between the current heat flow and the reference heat flow then forms a reciprocal measure of the deposits, ie, the smaller the difference, the larger the deposits.

The current heat flow is advantageously compared with a reference heat flow corresponding to a contamination level of zero and with a reference heat flow corresponding to a maximum permissible degree of contamination. This allows a characteristic value to be determined which corresponds to the degree of soiling of the heat exchanger from 0 to 100%.

bestimmt, wobei

HeatPerf

der Kennwert (Abnutzungsvorrat),

Q̇ _act

der aktuelle Wärmestrom,

Q̇ _dirty

der Referenz-Wärmestrom im verschmutzten Zustand des Wärmetauschers und

Q̇ _clean

der Wärmestrom im sauberen Zustand des Wärmetauschers ist,

Advantageously, the characteristic value is determined by the quotient of the difference between the current heat flow and the reference heat flow corresponding to the maximum permissible degree of contamination being added to the difference between the reference heat flow corresponding to the contamination level zero and the reference temperature corresponding to the maximum permissible contamination level. Heat flow is formed. The characteristic value, which can be referred to as wear stock, according to the formula $$\mathit{HeatPerf}=\frac{{\dot{Q}}_{\mathit{act}}-{\dot{Q}}_{\mathit{dirty}}}{{\dot{Q}}_{\mathit{clean}}-{\dot{Q}}_{\mathit{dirty}}}\cdot 100\%$$

determined, where

HeatPerf

the characteristic value (wear stock),

Q _act

the current heat flow,

Q _dirty

the reference heat flow in the dirty state of the heat exchanger and

Q _clean

the heat flow is in the clean state of the heat exchanger,

the characteristic value is 100% in the clean state of the heat exchanger and 0% in the maximum soiled state of the heat exchanger. The characteristic value can be continuously calculated and displayed in the process control system in which the heat exchanger is integrated as a trend over longer periods of time. As soon as the characteristic value falls below a specified limit, a maintenance message can be generated.

Advantageously, the reference heat flow is exactly the same operating point, the z. B. as a combination of the two flows product medium F _P and service medium F _S and the two input temperatures product medium T _{P, A} and service medium T _{S, Ein is} defined, as the basis of the current heat flow. This has a very advantageous effect on the accuracy of the method according to the invention. Other sizes can be used for the definition of the operating point, if z. B. phase transitions (evaporation or condensation) occur within the heat exchanger.

It is particularly advantageous if a multiplicity of reference heat flows are determined at different operating points and the operating point of the reference heat flow corresponding to the operating point of the current heat flow is determined by means of interpolation.

In this case, the theoretically transferable amount of heat for a large number of possible operating points is first calculated with the aid of the process engineering simulation program with which the heat exchanger was designed or could be designed. Such simulation calculations are carried out both for the reference state "freshly cleaned" and for a reference state "maximally contaminated" in which a cleaning of the heat exchanger is absolutely necessary. The calculated simulation values are used as interpolation points for two multidimensional maps each having a plurality of input variables (eg four input variables in each case) and one output variable.

After a multiplicity of interpolation points have been calculated, the reference heat flow for the current operating point can be taken from the relevant characteristic field. If the operating point lies between several interpolation points, the reference heat flow for the current operating point can optionally be determined by a map interpolation.

The time-consuming simulation calculation can advantageously be carried out offline in advance of the operation of the process plant or of the heat exchanger. During operation of the process plant or the heat exchanger, only the map interpolation may be necessary.

For interpolation, an approach known from mathematics is used: It is first checked in which hyperbolic cube in the high-dimensional lattice network of the input variables is the current operating point. This hyperbolic cube with the simulation values of all vertices is transformed into the coordinate origin and normalized. The searched starting point is then calculated by evaluating a multilinear polynomial. Such a method can be easily implemented in a controller.

In a transient transient between different work points, the computation is preferably temporarily frozen because the underlying model describes only the steady state thermal balance. To detect whether a stationary state is present, is preferably in the patent application PCT / EP2007 / 004745 described method used.

By means of the method according to the invention, it is advantageously possible to carry out a monitoring of heat exchangers with a variable operating point in process control systems. By direct consideration of the heat flow can be hard to interpret auxiliary quantities to determine the effectiveness are dispensed with the heat exchanger, whereby the associated problems are avoided. By using the process engineering simulation program, the operating point dependence of the transmittable heat quantity can be predicted, for example, at several hundred interpolation points without having to carry out correspondingly time-consuming measurements on the real system. Ideally, the model of the heat exchanger is used several times: first in the planning phase for dimensioning the heat exchanger and then at the beginning of the operating phase for parameterizing the monitoring.

By storing the simulated values in a map, it is possible to completely dispense with the computation time-consuming simulation of the procedural model in the process control system. The online monitoring function is based on a linear map interpolation and can be implemented seamlessly within a process control system.

By calculating characteristic values for the freshly cleaned and the maximum soiled heat exchanger, the actual wear stock of the heat exchanger can be calculated. If it is observed during operation that the wear reserve is slowly approaching the value zero, appropriate maintenance measures can be reasonably planned, for example between two batches of a batch plant or as part of another planned plant shutdown for a continuously operating plant.

Freezing the calculation during transient transients avoids false alarms.

Further details, features and advantages of the present invention will become apparent from the following description of a particular embodiment with reference to the drawings.

Fig. 1

eine schematische Darstellung einer einen Wärmetauscher aufweisenden Prozessanlage mit einem die Überwachung des Wärmetauschers betreffenden Teil einer Steuerung und

Fig. 2

eine schematische Darstellung eines dreidimensionalen Schnitts durch ein mit einem verfahrenstechnischen Simulationsprogramm erzeugtes fünfdimensionales Kennfeld der Größen F_S, Fp und Q̇ _Rer bei vorbestimmten Temperaturen T_S,Ein und T_P,Ein.

It shows:

Fig. 1

a schematic representation of a heat exchanger having a process plant with a monitoring of the heat exchanger part of a control and

Fig. 2

a schematic representation of a three-dimensional section through a generated with a procedural simulation program five-dimensional map of the sizes F _S , Fp and Q _Rer at predetermined temperatures T _{S, A} and T _{P, A.}

As Fig. 1 can be removed, a process plant 1, a heat exchanger 2. The heat exchanger 2 has a container 2a, in which a pipe assembly 2b is arranged. The container 2a has a first input 2 _EP and a first output 2 _AP . Via the first input 2 _EP , a product medium flows into the container 2 a, which leaves the container 2 a again at the first outlet 2 _AP .

The pipeline assembly 2 _{b is led} out of the container 2 a of the heat exchanger 2 via a second input 2 _ES and via a second output 2 _AS . Via the second input 2 _ES , a service _medium can be conducted into the pipeline arrangement 2 _b, which leaves the pipeline arrangement 2 b again at the second outlet 2 _AS .

By means of a first flow meter 3, the amount of the product medium supplied to the container 2a can be detected. By means of a second flow meter 4, the amount of the service medium supplied to the piping arrangement 2b can be detected. By means of a first temperature sensor 5, the temperature of the product medium supplied to the container 2a can be detected at the first inlet 2 _{EP of} the container 2a. By means of a second temperature sensor 6, the temperature of the pipe assembly 2b supplied service medium be detected at the second input 2 _{ES of} the pipe assembly 2b. By means of a third temperature sensor 7, the temperature of the product medium at the first output 2 _{AP of} the container 2a can be detected. By means of a fourth temperature sensor 8, the temperature of the service medium at the second output 2 _{AS of} the pipe assembly 2b can be detected.

The output signals 3a, 4a of the flow meter 3, 4 and the output signals 5a, 6a of the temperature sensors 5, 6 are fed to a first map module 9 and a second map module 10. In the map modules 9, 10 each have a high-dimensional map is stored, which was calculated by means of a procedural simulation program, with which the heat exchanger 2 has been designed or can be designed. A three-dimensional section through a five-dimensional characteristic field 16 stored in the map module 9 is shown in FIG Fig. 2 shown. The map 16 relates to a predetermined temperature of the product medium at the first input 2 _{EP of} the heat exchanger 2 and a predetermined temperature of the service medium at the second input 2 _{ES of} the pipe assembly 2b.

In the first map module 9 operating point-dependent maps 16 are deposited, which relate to the heat exchanger 2 in the clean state. In the second map module 10 maps are stored, which relate to the heat exchanger 2 in the maximum polluted state. The maps of the first map module 9 give in response to the output signals 3a, 4a of the flow meter 3, 4 and the output signals 5a, 6a of the temperature sensors 5, 6 a heat flow again, which can be used as a reference heat flow of the non-polluted heat exchanger 2. The maps of the second map module 10 are in response to the output signals 3a, 4a of the flow meter 3, 4 and the output signals 5a, 6a of the temperature sensors 5, 6 a heat flow again, which can be used as a reference heat flow of the maximum contaminated heat exchanger 2. The reproduced heat flows are each as an output signal 9a, 10a of the relevant map module 9, 10 a monitoring module 11 is supplied. In special cases, such as, for example, during phase transitions within the heat exchanger (evaporation, condensation), variables other than those given above may also be used as input variables in the characteristic diagrams.

The map building blocks 9, 10 have a computer, by means of the intermediate values, for which no interpolation point is stored, are calculated by interpolation. In addition to the heat flows taken directly from the maps, the monitoring module 11 is also supplied with the heat flows 9a, 10a determined by interpolation. Furthermore, the monitoring module 11 is supplied with the output signals 3a, 4a of the flowmeters 3, 4 as well as the output signals 5a, 6a of the temperature sensors 5, 6, which indicate the current operating point of the heat exchanger 2. In addition, the monitoring module 11 nor the output signals 7a, 8a of the third temperature sensor 7 and the fourth temperature sensor 8 are supplied. In special cases, such as, for example, during phase transitions within the heat exchanger (evaporation, condensation), variables other than those indicated above may also be supplied to the monitoring module.

In the monitoring module 11, a current heat flow can thus be calculated. The current heat flow is then linked to the characteristic block building blocks 9, 10 extracted operating point-dependent reference heat flows. As output signal 11a can then be given a value between 0 and 100%, indicating the degree of contamination of the heat exchanger 2.

In order to avoid that transient conditions are taken into account in the monitoring module 11, signals 12 _P , 13 _P , 14 _{P of} the process plant 1 to control modules 12, 13, 14 which are dependent on corresponding process parameters are output, which transmit the signals 12 _P , 13 _P , 14 _P evaluate whether the process plant 1 is in a stationary state. If the process installation 1 is in a stationary state, a signal 12a, 13a, 14a is applied to the outputs of the control modules 12, 13, 14, which signals are logically linked together in an AND gate 15. The output signal 15a of the AND gate 15 is applied to the monitoring module 11 as an enable signal.

Claims (9)

A method for monitoring the effectiveness of a heat exchanger in which heat flows from a first medium into a second medium, characterized in that an actual heat flow is detected and compared with at least one respective predetermined degree of contamination of the heat exchanger corresponding reference heat flow. A method according to claim 1, characterized in that the calculation of the at least one reference heat flow by means of a simulation program, by means of which the heat exchanger is dimensioned, is made. A method according to claim 1 or 2, characterized in that the current heat flow is compared with a pollution degree zero corresponding reference heat flow and with a maximum permissible degree of contamination corresponding reference heat flow. A method according to claim 3, characterized in that a quality value is determined, the quotient of the difference between the current heat flow and the maximum allowable degree of contamination corresponding reference heat flow to the difference between the pollution degree zero corresponding reference heat flow and the maximum permissible degree of contamination corresponds to the reference heat flow. Method according to one of claims 1 to 4, characterized in that the at least one reference heat flow is based on the same operating point as the current heat flow. A method according to claim 5, characterized in that a plurality of reference heat flows is determined at different operating points and the operating point of the current heat flow corresponding operating point of the reference heat flow is determined by means of interpolation. Device for controlling a system (1) with at least one heat exchanger (2), characterized in that a memory (9, 10) is present, in which at least one characteristic diagram (16) for a reference heat flow of the heat exchanger (2) is stored , Device according to claim 7, characterized in that in the memory (9, 10) more than ten different operating points corresponding and / or at least two different degrees of contamination of the heat storage corresponding maps (16) are stored for reference heat flows. Computer program product, characterized in that it comprises program code sequences, in the execution of which a method according to one of claims 1 to 6 is performed.

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