EP2841869B1 - Method for providing a cooling medium in a secondary circuit - Google Patents

Description

The present invention relates to a method for providing a cooling medium with controlled flow temperature in a secondary circuit, wherein the cooling medium absorbs heat in the secondary circuit of one or more process coolers and then gives off heat to primary water in a primary circuit before it flows back to the process coolers. Furthermore, the invention relates to a device for carrying out the method according to the invention, wherein the device comprises one or more process coolers in the secondary circuit and at least one temperature sensor in the flow to the process coolers. WO 2011/149487 discloses a device having the features in the preamble of claim 7.

In many process engineering processes, it is necessary to dissipate heat from equipment or plant components. For this purpose, primary water is often used, which is available in particular for large-scale processes as recooling water, river water or seawater. From the point of view of process management, it is desirable that the cooling of the apparatuses or parts of the plant takes place at a temperature which is as constant as possible. From the point of view of safety and environmental protection, it should be prevented that, for example, potentially harmful substances enter the natural waters during leakages. These requirements can be met by the primary water is not brought directly into contact with the devices to be cooled, but a cooling medium in a closed intermediate circuit, also called secondary circuit, is used. The cooling medium is selected according to the respective needs. A common cooling medium is water, other examples are glycol water mixtures or methanol.
The cooling medium in the secondary circuit flows with a certain flow temperature into one or more process coolers and absorbs heat from the process, where it heats up. In order to bring the cooling medium back to the desired flow temperature, it is fed to one or more heat exchangers, in which it is cooled by primary water. The cycle of primary water, such as recooling water, river water or seawater, is referred to as the primary cycle. Accordingly, the heat exchangers in which the cooling medium is cooled by the primary water, called the primary heat exchanger.
The division of the cooling into a primary circuit through which a primary water flows and a secondary circuit through which a cooling medium flows is an established technique and offers several advantages. In the case of a leak on an apparatus or a process cooler, harmful substances may possibly enter the cooling medium, but the primary water in the primary circuit is spared from contamination. On the other hand, because the secondary circuit is closed, the process coolers in the system pollute less than, for example, direct cooling with river water.
This procedure also makes it possible, independent of daily and seasonal fluctuations in the primary water, the flow temperature to the process coolers to a desired Value to set. The primary heat exchangers are usually designed with regard to number and dimensioning in such a way that they can extract enough heat from the cooling medium in the event of high load. To define the high load case, a state is assumed in which the temperature difference between the entering into the heat exchanger cooling medium and the incoming primary water has a certain minimum allowable value. For a given value for the incoming cooling medium, this results in a maximum permissible value for the incoming primary water. In Central Europe, the high load case is therefore usually given in the summer months, in which river water can reach, for example, temperatures of 28 ° C or more.
On the other hand, it is referred to a reduced load case when the temperature difference between cooling medium entering the primary heat exchangers and entering primary water assumes large values. In Central Europe, this occurs usually in the winter months, when the temperature of river water can drop to levels of 4 ° C or below, for example. Furthermore, it is spoken of a reduced load case, when only little heat from the cooling medium in the secondary circuit must be transferred to the primary water, for example, when a system is not operated at maximum capacity or is completely turned off, so that less heat from the cooling medium to the primary water in the primary cooler is transmitted. In these cases, to keep the flow temperature of the cooling medium to the process coolers at the same value as in high load case, less primary water is required, so usually the flow rate of primary water is reduced to the primary heat exchangers.
This approach is disadvantageous in that in the case of low load due to the low flow rates, the primary heat exchanger on the primary water side tend to pollution, the so-called fouling. In addition, regulation of the flow temperature to the process coolers via the primary water feed to the primary heat exchangers is sluggish. It set itself the task of providing a generic method for cooling, in which contamination of the primary heat exchanger can be reduced and the flow temperature to the process coolers can be controlled quickly and robust.
This object is achieved by an inventive method according to claim 1 and by an inventive device according to claim 7. Advantageous embodiments of the invention are in the respective dependent claims 2 to 6. In the inventive method for providing a cooling medium with controlled flow temperature in a secondary circuit that takes Cooling medium in the secondary circuit of one or more process coolers heat and then gives off heat to primary water in a primary circuit before it flows back to the process coolers. There are at least two primary heat exchangers for cooling the cooling medium. After exiting the process coolers and before entering the primary heat exchanger, a bypass line for bypassing the primary heat exchanger branches off in the secondary circuit. The bypass line opens into the Conduction of the secondary circuit from the primary heat exchangers to the process coolers. According to the invention, the regulation of the flow temperature to the process coolers in the secondary circuit via the adjustment of the bypass current. The "flow temperature" is understood here and below to mean the temperature in the secondary circuit in the flow to the process coolers. The term "flow to the process coolers" refers to the section of the secondary circuit which is located between the entry of the bypass line into the secondary circuit and the first process heat exchanger. "Overrun" is the section of the secondary circuit which is located between the exit from the at least one process heat exchanger and the branch of the bypass line. In the case of multiple process heat exchangers, the term "after-run" refers to the section of the secondary circuit which is located between the confluence of the outlet lines from the process heat exchangers and the branch line of the bypass line.

In a preferred embodiment of the invention, a temperature sensor is in the flow to the process coolers, and the bypass line is provided with an actuator, by means of which the flow temperature to the process coolers in the secondary circuit can be controlled. It is also possible to provide a plurality of temperature sensors in the flow to the process coolers, for example to realize a redundant measurement. Suitable temperature sensors, for example thermocouples, detect the temperature of the flowing cooling medium and provide a value for forwarding to a control device.

In a further embodiment of the invention, there is a temperature sensor or are several temperature sensors in the wake of the secondary circuit, and the bypass line is provided with an actuator by means of which the flow temperature to the process coolers in the secondary circuit can be controlled.

Suitable actuators make it possible to set the flow through the bypass line between a minimum value and a maximum value. Preferably, the minimum value is zero, which means a completely closed bypass line. The maximum value preferably corresponds to a completely open bypass line. Any values for the flow can be set between the extreme values, preferably steplessly continuously. Suitable actuators are known to the person skilled in the art, e.g. Flaps, ball valves or three-way valves.

In a preferred embodiment, a control device is provided which has a setpoint for the flow temperature. From a comparison of the setpoint with the detected by the temperature sensor flow temperature, the control device generates an output signal for forwarding to the actuator. The temperature of the cooling medium flowing through the bypass line is higher than the temperature of the cooling medium flowing from the primary heat exchangers to the process coolers. The regulation therefore provides for the current through the bypass line to be reduced in the event of a flow temperature that is too high in comparison with the setpoint value and accordingly to increase the flow through the bypass line when the flow temperature is too low compared to the setpoint value.

In a further embodiment of the invention, a control device is provided which has a setpoint for the follow-up temperature. From a comparison of the setpoint with the detected by the temperature sensor follow-up temperature, the control device generates an output signal for forwarding to the actuator. In this embodiment, too, the control provides for the current through the bypass line to be reduced when the overrun temperature is too high compared to the setpoint value, and the current through the bypass line to be increased accordingly when the setpoint temperature is too low compared to the setpoint value.

The flow of the cooling medium through the bypass line can also be influenced by the fact that actuators are present in the cooling medium lines to the primary heat exchangers or from the primary heat exchangers and adjusted in their degrees of opening. However, a regulation of the bypass current by influencing an actuator in the bypass line is easier to implement and is therefore preferred.

In the control device may be an independent device, such as a compact controller, which is connected in terms of information technology with the temperature sensor and the actuator. The control device can also be realized in combination with the actuator, for example in the form of a control valve. Alternatively, the control device can also be integrated in a higher-level system for process control, for example in a process control system.

In a preferred embodiment of the method according to the invention, the primary heat exchangers are designed with regard to number and dimensioning to a high load case. In the case of reduced load, the cooling capacity of the primary circuit is adjusted by switching off one or more of the primary heat exchangers, with at least one primary heat exchanger remaining in operation. The terms "high load case" and "low load case" are understood to mean operating states as defined above.

In a further preferred variant, the primary heat exchangers are designed such that in a reduced load, a heat exchanger is sufficient to cool the cooling medium to the desired flow temperature by utilizing a maximum allowable temperature difference between primary water inlet and outlet. For the high load case, correspondingly more heat exchangers are provided, which are preferably also individually designed for the maximum permissible temperature difference between primary water inlet and outlet, and are in total suitable for the minimum temperature difference in the high load case. The maximum permissible temperature difference between primary water inlet and outlet is often prescribed by the authorities, for example to a value of 15 K. Through this measure, the primary water can be efficiently used over the entire seasonal load range and the required minimum amount of primary water can be significantly reduced.

The bypass line is preferably designed in terms of their capacity such that the control range of the bypass line is sufficient to control the flow temperature in the secondary circuit bumplessly when switching off and the connection of a primary heat exchanger.

In the design of the bypass line is further preferably considered that Tageszeitliche fluctuations in the primary water temperature can be compensated solely by regulating the flow through the bypass line. A connection or disconnection of primary heat exchangers is not provided for this case.

Particularly preferably, the flow of primary water through the primary heat exchanger or is kept substantially constant. The flow is not actively regulated, but results from the pressure difference between the primary water side inlet and the outlet of the primary heat exchanger. Variations in this pressure difference also result in fluctuations in the flow rate. If the temperature of the primary water drops or the amount of heat to be removed from the cooling medium decreases, e.g. by reducing the flow rate of cooling medium through a primary heat exchanger or by reducing the cooling medium temperature when exiting the process coolers, the cooling medium temperature drops at the outlet of this primary heat exchanger. Accordingly, the outlet temperature of the cooling medium increases with a temperature increase of the primary water and / or an increase in the amount of heat to be removed from the cooling medium.

In a further preferred embodiment of the invention, the capacity adjustment is made by switching on or off of primary heat exchangers such that the pressure drop of the primary water when flowing through the primary heat exchanger in operation is at least 300 mbar, more preferably at least 800 mbar. This significantly reduces the likelihood of deposits forming on the primary water side.

The at least two primary heat exchangers can be interconnected in different ways. The primary heat exchangers are preferably connected in parallel both on the side of the primary circuit and on the side of the secondary circuit.

All heat exchangers known to the person skilled in the art for this purpose can be used as the primary heat exchanger, preferably plate heat exchangers or tube bundle heat exchangers are used, particularly preferably sealed or welded plate heat exchangers. The most preferred plate heat exchangers are usually designed for a high pressure loss. This is advantageous if the bypass is to be realized without additional conveying devices such as pumps.

In a preferred embodiment of the method according to the invention, the primary water is recooling water, river water, sea water or brackish water. By "recooling water" is meant water, which has been cooled by a device such as a cooling tower or a recooling plant in process plants.

Compared with the method known from the prior art, the invention offers several advantages. The provision of at least two primary heat exchangers working together in the High load case provide the required cooling capacity, but in partial load can be partially shut down, makes it possible to operate the individual primary heat exchanger with almost constant flow on the primary water side, which prevents premature fouling. Furthermore, this offers the possibility, in the case of reduced load, the primary heat exchanger alternately on and off, which allows easy inspection and possibly maintenance or cleaning. Furthermore, the minimum amount of primary water required to provide the cooling capacity is significantly reduced. Another advantage is the fact that a control of the flow temperature by means of the bypass flow is much easier, faster and more robust to accomplish than a regulation on the flow rate of primary water, as practiced in the prior art.

Fig. 1:

Prinzipskizze eines Kühlsystems gemäß dem Stand der Technik

Fig. 2:

Prinzipskizze eines erfindungsgemäßen Kühlsystems

Fig. 3:

Prinzipskizze eines erfindungsgemäßen Kühlsystems mit sekundärseitiger Reihenschaltung der Primärwärmetauscher

Fig. 4:

Prinzipskizze eines erfindungsgemäßen Kühlsystems mit primärseitig flexibler Verschaltung der Primärwärmetauscher

Fig. 5:

Zeitlicher Verlauf der Primärwassertemperatur und der Anzahl an in Betrieb befindlichen Primärwärmetauschern

With reference to the drawings, the invention will be further explained in the following, the drawings being to be understood as schematic representations. They represent no limitation of the invention, for example, in terms of the number, type and interconnection of heat exchangers, dar. It shows:

Fig. 1:

Schematic diagram of a cooling system according to the prior art

Fig. 2:

Schematic diagram of a cooling system according to the invention

3:

Schematic diagram of a cooling system according to the invention with secondary side series connection of the primary heat exchanger

4:

Schematic diagram of a cooling system according to the invention with primary side flexible interconnection of the primary heat exchanger

Fig. 5:

Time history of the primary water temperature and the number of primary heat exchangers in operation

Fig. 1 shows a cooling system according to the prior art in which flows in a secondary circuit 20, a cooling medium to process coolers 22, receives heat there and in a primary heat exchanger 12 heat to primary water in a primary circuit 10 before it flows back to the process coolers 22. The process coolers can be of different types, for example plate, tube bundle, spiral heat exchangers or casings of pipes or containers for their cooling. The flow temperature of the cooling medium before the process coolers 22 is detected by means of a temperature sensor and regulated by a control device 24 to a specific setpoint. The amount of primary water in the primary circuit 10 acts as a control variable for the control.

In Fig. 2 a first preferred embodiment of the method according to the invention is shown. The cooling medium leaving the process cooler 22 is passed through two primary heat exchangers 12, 14, where it gives off heat to primary water in a primary circuit 10. In the illustrated, preferred case, the primary heat exchangers are connected in parallel both on the side of the primary circuit and on the side of the secondary circuit. Between the outlet of the cooling medium from the process coolers 22 and its entry into the primary heat exchanger 12, 14 branches off a bypass line 26, which opens after the exit of the cooling medium from the primary heat exchangers back into the secondary circuit 20. The control 24 of the flow temperature the cooling medium to the process coolers 22 via the adjustment of the current in the bypass line. In case of high load, both primary heat exchangers 12 and 14 are in operation, while in the low load case, the capacity of a primary heat exchanger is sufficient to sufficiently cool the cooling medium in the secondary circuit 20. In this case, one of the primary heat exchangers is switched off by closing the corresponding valves in the secondary circuit.

In Fig. 3 a further preferred embodiment of the method according to the invention is shown. In this example, the primary heat exchangers 12 and 14 are connected in parallel by the primary circuit and in series by the secondary circuit. In order to switch off the primary heat exchanger 12 or 14 in the case of reduced load, bypasses are provided in the secondary circuit, which can be switched on and off via valves.

Fig. 4 shows a further preferred embodiment of the method according to the invention, in which the primary heat exchanger 12, 14 are connected in parallel on the secondary side. On the side of the primary circuit, the interconnection is kept flexible. By appropriately opening and closing the valves shown as examples, a series connection or a parallel connection can be realized on the primary side. In addition, the primary heat exchangers 12, 14 can be switched off alternately by closing the corresponding valves in the secondary circuit.

The illustrations are for illustration only. Of the representations deviating configurations and interconnections are of course also fall under the invention fall, as long as the regulation of the flow temperature in the secondary circuit is done by adjusting the bypass current.

In particular, the number of heat exchangers shown in the figures is merely exemplary and not limited thereto. It is advantageous if more than two primary heat exchangers are present. The more primary heat exchangers are available, the more flexible it is possible to switch on and off individual primary heat exchangers in order to optimally match the currently available load case. On the other hand, this also increases the investment costs. Preferably, two to three primary heat exchangers are provided.

A selection criterion for the number of primary heat exchangers can be derived from the temperature gradients of the primary water. Preferably, the optimum number of primary heat exchangers is estimated by the quotient of the maximum permissible temperature difference and the typical temperature difference in the high load case. For example, for the location Ludwigshafen am Rhein, Germany, the maximum allowable temperature difference between primary water entry and exit is 15 K when using river water as primary water. In addition, the water returned to the river must not exceed 33 ° C. In the high load case in the summer months, in which the river water can reach temperatures of 28 ° C, temperature differences between primary water entry and exit of 5 K are therefore common. This results in a quotient of 15 K / 5 K = 3. It is therefore advantageous to provide three primary heat exchangers which are designed such that each of the three heat exchangers alone can extract the required amount of heat from the cooling medium in the secondary circuit when the maximum permissible temperature difference of 15 K is exhausted.

In Fig. 5 schematically shows the time course of the primary water temperature T (dashed curve) and the number of operating in primary heat exchangers N (solid lines, right scale) over a period of 12 months. In this exemplary representation, it is assumed that the amount of heat to be dissipated from the secondary circuit remains constant over the considered period of time. The primary water used is river water, which has the lowest temperature in the winter months of December and January, eg 4 ° C. The maximum permissible temperature difference can be fully exploited, so that a primary heat exchanger is sufficient to sufficiently cool the cooling medium in the secondary circuit. As soon as the river water rises above a value above which, taking into account a certain fluctuation range, the maximum permissible temperature difference can no longer be guaranteed, a further primary heat exchanger is put into operation. Assuming a fluctuation range of 3 K and a maximum value of 33 ° C for the water discharged into the flow, this results in a value of 15 ° C, from which the second primary heat exchanger is put into operation. In the example according to Fig. 5 this is the case in mid-April. At the beginning of June, the temperature of the river water rises to a level above which the third primary heat exchanger becomes necessary in order reliably to dissipate the required amount of heat and not to exceed the maximum value of 33 ° C. During the summer months of June, July and August, three primary heat exchangers are in operation until the river water temperature has fallen again enough that two primary heat exchangers are sufficient, in the example at the beginning of September. At the end of October, the river water temperature has dropped even further, eg below 15 ° C, so that once again a primary heat exchanger is sufficient.

The method according to the invention causes flexible cooling capacities to be made available adapted to the current needs. In the in Fig. 5 Example shown are a primary heat exchanger for a period of five and a half months, three primary heat exchangers for a period of three and a half months and three primary heat exchangers for a period of three months. Compared to a pure design for the high load case can be drastically reduced by the inventive method, the required amount of primary water.

On the primary water side, each primary heat exchanger flows through a substantially constant amount of water, which prevents soiling and fouling. Except in the summer months, the heat exchangers that are currently not in operation can be easily maintained and cleaned without affecting the operation of the systems in the secondary circuit. Assuming that in systems according to the state of the art, in which only a primary heat exchanger designed for high load cases is used, in which the amount of primary water is reduced in the case of reduced load, once a year a pollution-related shutdown of about 3 days is required, can be through the inventive Method increase the plant capacity by approx. 1%. For more frequent or longer shutdown times, the economic advantage increases accordingly.

Claims (7)

1. A method for providing a refrigerant medium having a controlled flow temperature in a secondary cycle (20), wherein the refrigerant medium in the secondary cycle (20) takes up heat from one or more process coolers (22) and then gives off heat to primary water in a primary cycle (10) before it flows back to the process coolers (22), wherein at least two primary heat exchangers (12, 14) are present for cooling the refrigerant medium, in addition a bypass line (26) in the secondary cycle (20) branches off after exit from the process coolers (22) for bypassing the primary heat exchangers (12, 14), and the temperature in the secondary cycle (20) in the flow to the process coolers (22) is controlled via the setting of the bypass stream.
2. The method according to claim 1, wherein number and dimensioning of the primary heat exchangers (12, 14) are designed for a high load case, and in a low load case the refrigeration capacity of the primary cycle (10) is adapted by shutting off one or more of the primary heat exchangers (12, 14), wherein at least one primary heat exchanger remains in operation.
3. The method according to claim 2, wherein the capacity is adapted in such a manner that the pressure drop of the primary water across the primary heat exchangers (12, 14) in operation is in each case at least 300 mbar, preferably at least 800 mbar.
4. The method according to any one of claims 1 to 3, wherein the primary heat exchangers (12, 14) are designed for a maximally permissible temperature difference between primary water intake and primary water outlet.
5. The method according to any one of claims 1 to 4, wherein the primary heat exchangers (12, 14) are connected in parallel both on the part of the primary cycle (10) and also on the part of the secondary cycle (20).
6. The method according to any one of claims 1 to 5, wherein the primary heat exchangers (12, 14) are plate heat exchangers or tube-bundle heat exchangers, in particular sealed or welded plate heat exchangers.
7. Apparatus for carrying out the method according to at least one of claims 1 to 6, comprising one or more process coolers (22) in the secondary cycle (20) and also at least one temperature sensor in the flow to the process coolers (22), wherein at least two primary heat exchangers (12, 14) are present in which the refrigerant medium of the secondary cycle (20) can release heat to the primary water of the primary cycle (10), and wherein, in addition, a bypass line (26) is present which, in the secondary cycle (20), branches off after exit from the process coolers (22) for bypassing the primary heat exchangers (12, 14) and is provided with an actuator, using which the temperature in the secondary cycle (20) is controllable in the flow to the process coolers (22), wherein the primary heat exchangers (12, 14) are connected in parallel not only on sides of the primary cycle (10) but also on sides of the secondary cycle (20).

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