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In the first issue of "Beer Technology" in 2000, a topic entitled "Several Problems in the Temperature Control of Beer Conical Fermentors" was published. Some articles in the relevant magazines also discussed this issue. Think it necessary to discuss it again. This article is ready to discuss the following issues:
1. Definition of temperature and basic criteria for temperature measurement 2. Temperature and heat 3. The role of temperature and the essence of control in beer fermentation process 4. Temperature control in beer fermentation process 5. Evaluation of temperature control system A few 奌 view one. The definition of temperature and the basic criteria for temperature measurement Temperature is one of the physical quantities most closely related to human daily life and production practice. It is closely related to temperature in our surroundings and everywhere. But not everyone understands the definition of temperature and temperature measurement very clearly, even some technicians engaged in temperature measurement and control engineering are no exception. This is mainly due to the fact that people easily confuse the concept of temperature and heat, and sometimes it is related to human subjective intuition. Among all basic physical quantities, temperature is the most difficult to understand and least accurate physical quantity.
The most popular temperature concept describes temperature as the physical quantity that characterizes the degree of hot and cold of an object. However, such statements are not rigorous and are prone to illusion. For example, when we touch wooden blocks, foam plastics, and metal rods at the same temperature indoors, we feel a slight difference. Obviously, we cannot judge them to be different in temperature. The reason is not simple, it is not only related to the nature of the object, body temperature and room temperature, but also related to the way people feel hot. Therefore, to correctly understand the meaning of object temperature, we must make a scientific definition of the concept of temperature.
The scientific definition of temperature is derived from the zeroth law of thermodynamics, which is not stated one by one. It can be easily accepted that temperature is a state parameter that characterizes the thermal equilibrium of an object. When two objects in thermal equilibrium contact and no heat exchange occurs, this is the case. The two objects have the same temperature; when two objects in thermal equilibrium contact and exchange heat, the two objects have different temperatures, and the heat flows from the higher temperature object to the lower object. There are two things that must be explained here: First, the thermal equilibrium state of the object refers to the thermal equilibrium of the non-heat exchange inside the object; second, the heat balance between the two objects when there is no heat exchange between the objects. In contrast, the purpose of heat balance is to deepen the impression that the absolute ideal heat balance actually does not exist, and it can only be close to thermal equilibrium. This is why temperature, the physical quantity, is the most difficult to understand and the most difficult to measure.
We use a thermometer to measure the temperature of an object. It is not just that the temperature part of the thermometer touches the measured object. The display value of the thermometer is the true temperature of the measured object. The thermometer actually shows the temperature of the temperature sensing part of the thermometer. It is the temperature of the measured object only when there is no heat exchange or thermal equilibrium between the temperature sensing part of the thermometer and the measured object. Therefore, when using a thermometer to measure the temperature of an object, minimizing the heat flow between the temperature sensing part of the thermometer and the measured object and controlling it within the allowable range is a basic criterion for temperature measurement. The larger the heat flow through the temperature sensing part per unit time is, the larger the error of temperature measurement is; on the contrary, the smaller the error is. For this purpose, when using a thermometer to measure temperature, there should be a certain depth of insertion so that the heat flow from the outside through the thermometer rod is diverted before reaching the temperature sensing part, so that the heat flow through the temperature sensing part is reduced to the allowable range. To further reduce the error, the entire thermometer can also be wrapped with a heat-insulating material to block the influence of ambient temperature changes on the temperature measurement. jwZBWt)5
Another point must be emphasized: the temperature of the measured object is virtually impossible to be absolutely uniform, and the temperature of the temperature measurement point only represents the temperature of this part of the object.
Second, temperature and heat temperature are the state parameters that characterize the macroscopic thermal state of an object. From the microscopic point of view, temperature is the characterization of the intense thermal motion of molecules within an object. The input or output of heat can aggravate or reduce the intense heat of molecular motion within an object. The temperature of the object will also increase or decrease with it. Visible, the temperature is to describe the thermal state of the object, and the heat is a form of energy exchange with the outside world during the change of the state of the object. The temperature is related to the thermal state of the object, and the heat is related to the process of changing the state of the object. There is a connection between the two, but there are two concepts. Temperature is the basic physical quantity, and heat is a form of energy, which can be related by the first law of thermodynamics.
Specific to the object's temperature control issues, not to discuss the thermal machinery, does not consider the exchange of objects and external mechanical energy. For a given mass of object, the increase or decrease of the temperature of the object depends entirely on the heat flow direction and size of heat exchange with the outside world. To increase the temperature, the heat is input. When the temperature is lowered, the heat is output. To maintain the temperature, it is necessary to completely insulate or balance the input and output heat. Controlling the temperature is actually choosing a heat transfer channel that can be controlled so that the input or output heat of the controlled object is balanced with the output or input heat of the controllable heat transfer channel. Therefore, controlling the temperature is essentially controlling the amount of heat exchange in the controllable heat transfer channels. The so-called controllable heat transfer channels are what are commonly referred to as heat exchangers, heaters, refrigerators, etc. The cooling jacket on the outer wall of the fermentation tank is a kind of heat exchanger. The thermometer installed on the outer wall of the fermentation tank is used to control the heat taken away by the cooling medium in the cooling jacket. It shows the temperature of the fermentation liquid at the measurement point. The temperature distribution in the entire tank is related to many factors and the whole fermentation process. The different stages in the process are also different. We will discuss this issue specifically below.
Third, in the beer fermentation process, the role of temperature and control of the temperature in the fermentation plant of the brewery, whether brewer or operator, the temperature of the fermentation liquid in the fermentation tank is the most concern. Especially for operators, the temperature and time are basically written on the process sheet, so it is easy to have an illusion: It seems that the beer fermentation process is based on temperature and time. In fact, during the entire process, the fermentation broth must be periodically taken out of the sampling port and sent to the laboratory for testing. According to the results of the laboratory tests, the temperature and time may change. Therefore, from the operation of the entire process, it can be seen that: in the beer fermentation process, the temperature is the process parameter that completes the process, not the characteristic parameters that characterize the fermentation state; it is the biochemical reaction environment that completes the beer fermentation process, not Identification. The physicochemical indexes such as the sugar content, ethanol content, live yeast number and diacetyl content of the fermentation broth tested and tested in the laboratory are the characteristic parameters characterizing the fermentation state, and are the marks of the fermentation process.
The beer fermentation process is a biochemical reaction process referred to as a biochemical reaction, not a chemical reaction process. There are reaction temperatures in the chemical reaction. The chemical reaction cannot proceed below the reaction temperature. There is no reaction temperature in the biochemical reaction. It is said that the main reaction body is microorganisms instead of elements, and the reaction can take place as long as the microorganisms have biological activity. The main difference is the reaction rate and the metabolites produced in the reaction. Of course, it is not only the temperature that determines the speed of the biochemical reaction, but also the type and quality of the substances participating in the reaction. Therefore, the maximum temperature is often limited in biochemical reactions. For example, the two existing fermentation processes, ie, high-temperature fermentation and low-temperature fermentation, stipulate the highest temperatures. The purpose of limiting the maximum temperature is to prevent the production of unwanted metabolites. It can be seen that as long as the temperature is lower than the maximum temperature limit, biochemical reactions can proceed normally. However, an excessively low temperature will affect the production efficiency. Excessive temperature fluctuations will affect the quality of metabolites. Therefore, depending on the quality and oxygenation of the wort, the type of yeast and the number of live yeast, the type of beer, and the production efficiency, the appropriate temperature is selected and controlled within a certain range, and then based on the sugar content measured during the fermentation process. Physical and chemical indicators such as ethanol content, number of live yeasts, and diacetyl content were used to determine whether the temperature should be raised or lowered or kept constant, and with the time to control the entire fermentation process. In short, the temperature provides a biochemical reaction environment and is a process parameter; the adjustment is the reaction rate. This is the role of temperature in the beer fermentation process.
It can be seen that the temperature measurement in the beer fermentation process does not require high accuracy, but rather satisfies a higher degree of accuracy under certain accuracy. This is also the case in the chemical industry sector—generally using a platinum resistance thermometer of class A. The only reason to use B grade in beer brewing.
In the previous section of temperature and heat, it has been pointed out that the essence of temperature control is to control the amount of heat exchange through the controllable heat transfer channels. In the beer fermentation process, heat is not mainly transferred from the outside to the fermentation broth, but is the internal heat of biochemical reaction. This heat is produced by the concomitant conversion of sugars in the fermentation broth into ethanol, carbon dioxide and other metabolites under the action of yeast. These heats must be passed through the cooling jackets or cold strips of the controllable heat transfer channels in a timely manner so that the temperature of the fermentation broth can be maintained at a value below the maximum temperature limit, providing a more stable reaction environment for biochemical reactions. If you want to warm up, you reduce the amount of transmission; if you want to cool, you increase the amount of transmission. Therefore, in the beer fermentation process, the biochemical heat produced is passed through the cooling jacket in a timely manner, which is the essence of temperature control in the beer fermentation process.
At this point, one may ask, since the thermometer installed on the outer wall of the fermentation tank measures the temperature near this point and is used to control the heat flow through the cold zone, how is the temperature in other parts of the fermentation tank distributed? What? Because of the great role of temperature in beer fermentation and the fact that the temperature distribution inside the tank is unknown, many misunderstandings arise. If the fermentation broth is discharged from the sampling port, the temperature is measured; the thermometer is inserted deeper and grows to one meter or even one meter five, and it is desirable to insert it into the center of the tank. These are all due to the fact that the basic concepts described above are not very clear. This is not only unnecessary but harmful. This is because the temperature of heat exchange with the surrounding environment during the release of the fermentation liquid from the sampling port has changed, and the thermometer used for measurement is not the same, and will send wrong information to people. The thermometer is too long and the rigidity is reduced. It is easy to generate vibration when the fermentation liquid is highly fluid, and it is not easy to clean and sterilize.
IV. Temperature Control in Beer Fermentation Process At present, most of the breweries in China use cylindrical conical-bottom type fermentation tanks referred to as conical tanks. Generally, two to three cooling jackets are welded in the cylindrical part. There is also a cooling jacket. The entire tank except the tank top device and the bottom of the tank, the import and export, all wrapped with thermal insulation material, with it to block the heat exchange with the outside world. In this way, the amount of heat exchange between the fermentation broth in the tank and the outside and the biochemical heat generated by the fermentation broth can be neglected. The heat taken away from the cooling medium through the cooling jacket in the temperature control is mainly biochemical heat. Qu:nV"~_
Throughout the entire beer fermentation process, as long as there is biological activity of yeast, there will be biochemical reactions, there is also the production of biochemical heat and metabolites, which is different from other liquids without autothermal reaction is an important feature. Another characteristic is that the main ingredient in beer is water, and the density of water varies with temperature over a temperature that is higher than the freezing point temperature, pure water is 4°C, and beer is about 3°C. In other words, the beer density increases when the beer temperature drops above 3°C, and the beer density decreases when the beer temperature drops below 3°C.
The features described above and some of the basic concepts discussed in the previous section, together with the basic laws of fluid motion and the basic knowledge of heat transfer, are the basis for our scientific analysis of problems.
First of all, let's look at the next simplest method for controlling fluid temperature. Almost all of the breweries use plate heat exchangers to warm or cool the fluid, such as lowering the filling temperature before the filling machine. In order to control the temperature of the beer, a thermometer must be used to detect it. Then the regulator controls the flow of refrigerant through the heat exchanger to control the temperature. Obviously, it is impossible to install the thermometer at the inlet of the heat exchanger, but at the outlet, and the pipeline from the outlet to the filling machine needs to be insulated. This is a typical fluid heat transfer control method. This is also the way to control the sprayer's temperature. This is similar in beer fermentation tanks, but there is one difference: in beer heat exchangers, beer flows under pressure and in the fermentation tank, the flow of fermentation liquid is in gravity or buoyancy and CO2 bubble rising force. The role of flow. The former is called forced convection, while the latter is called natural convection.
During the main fermentation period of beer fermentation, a large amount of heat and carbon dioxide will be produced. In order to remove the heat from the cooling medium immediately by the cooling jacket, the heat exchange method must be convection heat transfer, and the structure of the conical tank and the characteristics of the fermentation liquid itself can be used without mechanical agitation during fermentation. Strong natural convection. We can take a fluid micelle in the fermentation broth for research.
At the bottom of the fluid micelles, the heat generated during the biochemical reaction causes the volumetric expansion density to decrease; along with the generated carbon dioxide—which partially rises in the form of bubbles, and the other dissolves in the liquid, the more fluid micelles at the bottom. Due to the influence of static pressure, the amount of dissolution also increases. All of the above factors make the fluid micro-cluster on the bottom have an upward force so that it moves upward in anti-gravity direction. In the upper part, the fluid micelles near the cooling jacket are moved downward by gravity due to the increased density of the cooled volume. When the original liquid micro-cluster on the bottom reaches the upper part, due to the continuity of the fluid, the liquid micelles in the upper part of the inner wall of the tank are cooled down and then moved downward, they flow toward the tank wall, and then flow down along with the fluid moving down the front. Exercise and transfer the increased heat to the cooling medium in the cooling jacket. At this time, the fluid micelle moves under the action of gravity and has a certain speed. If you want to keep the speed constant, you need to take away the biochemical heat generated during the movement in the next cooling jacket. Therefore, it is not scientifically reasonable to think that in the main fermentation period, the setting value of temperature is gradually raised from the top down to strengthen convection. On the contrary, doing so will weaken the convection. During this period, the liquid in the tank formed a middle upward and downward strong convection. The upward driving force is the buoyancy force, and the downward force is the gravity. Under the action of these two forces, the fluid is allowed to circulate up and down. Between the middle and outer layers, vortices are formed due to the viscous force of the fluid, so that mass exchange occurs between the fluid micelles between the inner and outer thermal-cooled fluid layers, and heat is also transferred along with the transfer, which is called mass transfer heat transfer. . The resulting large circulation flow can not only greatly increase the heat exchange efficiency, but also make the temperature in the tank more uniform, which is equivalent to the effect of mechanical stirring. This is one of the biggest advantages of the conical tank fermentation.
According to our test data, the non-uniformity of temperature within one meter of the tank wall is less than 0.5°C during the main fermentation period. Therefore, there is no need to insert the thermometer deeply. There is no scientific basis for making a fuss on the insertion depth of the thermometer. The insertion depth of the thermometer does not affect the measurement as long as the external temperature changes. The kind of thick and long thermometer used here is unreasonable, not to mention it is not easy to clean and sterilize. It is generally possible to use a platinum resistance of 30 cm. According to my personal opinion, the thermometer does not need to extend into the tank at all, and it is attached to the outer wall of the tank with a surface thermometer. It can also be used to control the heat taken away by the cooling medium, but the entire thermometer must be tightly insulated. Measure, measures the wire also needs to prevent the heat from passing to the temperature sensing part measures. Since the inner wall of the tank has no other substance at this time, cleaning and sterilizing are very convenient and the effect is good, which is of great benefit to the improvement of beer quality. Of course, it is impossible to achieve complete thermal insulation or that outside temperature changes have no effect on temperature measurement, but it is entirely feasible if we take into account this effect in advance and correct it. This kind of control method has been adopted not only in foreign countries but also in China, such as the surface thermometer used in Beijing Yanjing Beer's partial fermenter.
The temperature control during the main fermentation period is generally not a problem and is better controlled. There is also no major problem in the cooling phase, as long as the cooling rate is well controlled and the convective intensity is gradually reduced to facilitate yeast settling. After a large number of yeasts have been discharged and continue to cool down, the problem arises. Since a large number of yeasts have been eliminated at this time, the biological activity of the remaining yeasts is reduced due to the decrease in temperature, the biochemical heat generated is also reduced, and the convective velocity is greatly reduced, but at a temperature higher than the maximum density of the wine liquid by 3°C. Under the circumstances, the flow of liquid in the tank is still in the middle, upwards and downwards. When the temperature continues to decrease to the maximum density of 奌3°C, the volume of the fluid micelles on the inside of the tank wall is reduced to the minimum. If the temperature of the fluid micelles increases, the density of the fluid micelles increases and the density decreases, resulting in an upward buoyancy. Suspension of downward flow, and then change the direction of upward movement, when the fluid micro-cluster leaves the cold zone, due to self-heat and external heat transfer, the temperature rise density becomes larger, the motion is stagnated and changes direction downwards, Local circulation, so that the formation of the overall circulation in the tank, resulting in a larger temperature gradient. The reason for this phenomenon is caused by the characteristics of water, but it is also related to the temperature control scheme.
When the direction of fluid flow changes, the position of the thermometer used to control the temperature should also be switched so that it is still at the exit of the heat exchanger. In other words, when the temperature is higher than the maximum density, the fluid flows downwards through the cold band and the thermometer should be placed below the cold band; when below the maximum density point, the fluid flows upwards through the cold band and the thermometer should be placed on the cold band. Above. In this way, the heat taken away by the cooling medium can be controlled. Otherwise, when the flow direction changes and the thermometer is still in position, it is equivalent to inserting a thermometer at the inlet of the heat exchanger. Obviously, it is impossible to control the temperature at all and not only cannot form a large cycle, but also The top floor is also very easy to freeze. 6D&{+;
In order to solve this problem, some people proposed a number of methods: early braking, fuzzy control, expert systems, etc., can not say no, can only say that it failed to grasp its essence. If the thermometer is still under the cold belt, it is equivalent to the temperature of the wine liquid when the wine liquid flows out of the heat exchanger outlet and then turns around and then returns to the heat exchanger. This not only delays the time, In addition, the heat transfer in the outer circle will be influenced by many factors. Therefore, this is not a very scientific method. The scientific method is based on the basic concept of controlling the temperature and controlling the exchange of heat in controllable heat transfer channels. When the direction changes, the position of the temperature detector should be changed in a timely manner. The thermometer originally located under the cold belt should be discarded, and the thermometer located above the cold belt should be used as the temperature-control detection thermometer. At this time, the main part of the control is the cold zone at the bottom of the cone, and it is sometimes necessary to control the cold zone below the cylindrical body. The topmost thermometer, which is located near the liquid surface, can only be used as a monitor.
Of course, if the diameter of the tank is too large, or the insulation effect of the tank is not good, a large temperature gradient will be formed. At this time, the internal heat exchange can be enhanced by the carbon dioxide blowing at the bottom of the tank, so that the temperature in the tank tends to be uniform.
V. Several points of view of the evaluation temperature control system The temperature control system generally adopts closed-loop control, that is, the sensing part acquires the controlled parameter signal from the control object, passes it to the regulator and compares the set value, and the output of the regulator is controlled according to the comparison result. Signal to the actuator to adjust the controlled parameter of the control object. The control object is a very important part of the closed loop. Most of the problems discussed above are related to this. Do not assume that we have thermometers, regulators, and actuators to make the temperature control stable and uniform. First of all, there must be a scientific analysis of the thermal structure of the control object, use the thermometer correctly and determine the installation location, otherwise it may produce temperature fluctuations and larger temperature gradients. Therefore, when evaluating a temperature control system, it is necessary to first look at the thermal structure analysis in the control scheme. This is the premise and basis.
Secondly, in industrial production, reliability is always the top priority. Whether it is mechanical or electronic, the simpler the function is, the lower the reliability of the intermediate links. As far as the entire temperature control system is concerned, the most important part is the sensing part and the implementing agency. The failure rate of these two parts is based on the statistics of the famous Honeywell company in the United States: nearly 80% of the failures in traditional control systems are caused by sensing. The faults were caused by the failure of some parts and actuators, and the sensing part of them accounted for 45%. Therefore, the focus of improving reliability is sensing and executing two parts, not others. This is often overlooked, especially for those who work in computers. Their attention is often placed on the hardware and software of the computer, while ignoring other control links. In our country, the use of computer temperature control has just been broken down. Some people think that using a computer to control the temperature is not a problem in the computer, but rather a problem in other areas.
Let us talk about the "advancedness" issue. Under the market economy, there is only "advanced" economic returns or valuable returns. The so-called "advancedness" of non-valued returns is meaningless. A system is not because you have adopted a new technology or new components, the entire control system is advanced. The advanced nature of the entire system should be fully evaluated from the entire control link, and it is not static and should be changed according to the actual situation. What's more, in the end, it is up to the user to speak with actual economic indicators.
1. Definition of temperature and basic criteria for temperature measurement 2. Temperature and heat 3. The role of temperature and the essence of control in beer fermentation process 4. Temperature control in beer fermentation process 5. Evaluation of temperature control system A few 奌 view one. The definition of temperature and the basic criteria for temperature measurement Temperature is one of the physical quantities most closely related to human daily life and production practice. It is closely related to temperature in our surroundings and everywhere. But not everyone understands the definition of temperature and temperature measurement very clearly, even some technicians engaged in temperature measurement and control engineering are no exception. This is mainly due to the fact that people easily confuse the concept of temperature and heat, and sometimes it is related to human subjective intuition. Among all basic physical quantities, temperature is the most difficult to understand and least accurate physical quantity.
The most popular temperature concept describes temperature as the physical quantity that characterizes the degree of hot and cold of an object. However, such statements are not rigorous and are prone to illusion. For example, when we touch wooden blocks, foam plastics, and metal rods at the same temperature indoors, we feel a slight difference. Obviously, we cannot judge them to be different in temperature. The reason is not simple, it is not only related to the nature of the object, body temperature and room temperature, but also related to the way people feel hot. Therefore, to correctly understand the meaning of object temperature, we must make a scientific definition of the concept of temperature.
The scientific definition of temperature is derived from the zeroth law of thermodynamics, which is not stated one by one. It can be easily accepted that temperature is a state parameter that characterizes the thermal equilibrium of an object. When two objects in thermal equilibrium contact and no heat exchange occurs, this is the case. The two objects have the same temperature; when two objects in thermal equilibrium contact and exchange heat, the two objects have different temperatures, and the heat flows from the higher temperature object to the lower object. There are two things that must be explained here: First, the thermal equilibrium state of the object refers to the thermal equilibrium of the non-heat exchange inside the object; second, the heat balance between the two objects when there is no heat exchange between the objects. In contrast, the purpose of heat balance is to deepen the impression that the absolute ideal heat balance actually does not exist, and it can only be close to thermal equilibrium. This is why temperature, the physical quantity, is the most difficult to understand and the most difficult to measure.
We use a thermometer to measure the temperature of an object. It is not just that the temperature part of the thermometer touches the measured object. The display value of the thermometer is the true temperature of the measured object. The thermometer actually shows the temperature of the temperature sensing part of the thermometer. It is the temperature of the measured object only when there is no heat exchange or thermal equilibrium between the temperature sensing part of the thermometer and the measured object. Therefore, when using a thermometer to measure the temperature of an object, minimizing the heat flow between the temperature sensing part of the thermometer and the measured object and controlling it within the allowable range is a basic criterion for temperature measurement. The larger the heat flow through the temperature sensing part per unit time is, the larger the error of temperature measurement is; on the contrary, the smaller the error is. For this purpose, when using a thermometer to measure temperature, there should be a certain depth of insertion so that the heat flow from the outside through the thermometer rod is diverted before reaching the temperature sensing part, so that the heat flow through the temperature sensing part is reduced to the allowable range. To further reduce the error, the entire thermometer can also be wrapped with a heat-insulating material to block the influence of ambient temperature changes on the temperature measurement. jwZBWt)5
Another point must be emphasized: the temperature of the measured object is virtually impossible to be absolutely uniform, and the temperature of the temperature measurement point only represents the temperature of this part of the object.
Second, temperature and heat temperature are the state parameters that characterize the macroscopic thermal state of an object. From the microscopic point of view, temperature is the characterization of the intense thermal motion of molecules within an object. The input or output of heat can aggravate or reduce the intense heat of molecular motion within an object. The temperature of the object will also increase or decrease with it. Visible, the temperature is to describe the thermal state of the object, and the heat is a form of energy exchange with the outside world during the change of the state of the object. The temperature is related to the thermal state of the object, and the heat is related to the process of changing the state of the object. There is a connection between the two, but there are two concepts. Temperature is the basic physical quantity, and heat is a form of energy, which can be related by the first law of thermodynamics.
Specific to the object's temperature control issues, not to discuss the thermal machinery, does not consider the exchange of objects and external mechanical energy. For a given mass of object, the increase or decrease of the temperature of the object depends entirely on the heat flow direction and size of heat exchange with the outside world. To increase the temperature, the heat is input. When the temperature is lowered, the heat is output. To maintain the temperature, it is necessary to completely insulate or balance the input and output heat. Controlling the temperature is actually choosing a heat transfer channel that can be controlled so that the input or output heat of the controlled object is balanced with the output or input heat of the controllable heat transfer channel. Therefore, controlling the temperature is essentially controlling the amount of heat exchange in the controllable heat transfer channels. The so-called controllable heat transfer channels are what are commonly referred to as heat exchangers, heaters, refrigerators, etc. The cooling jacket on the outer wall of the fermentation tank is a kind of heat exchanger. The thermometer installed on the outer wall of the fermentation tank is used to control the heat taken away by the cooling medium in the cooling jacket. It shows the temperature of the fermentation liquid at the measurement point. The temperature distribution in the entire tank is related to many factors and the whole fermentation process. The different stages in the process are also different. We will discuss this issue specifically below.
Third, in the beer fermentation process, the role of temperature and control of the temperature in the fermentation plant of the brewery, whether brewer or operator, the temperature of the fermentation liquid in the fermentation tank is the most concern. Especially for operators, the temperature and time are basically written on the process sheet, so it is easy to have an illusion: It seems that the beer fermentation process is based on temperature and time. In fact, during the entire process, the fermentation broth must be periodically taken out of the sampling port and sent to the laboratory for testing. According to the results of the laboratory tests, the temperature and time may change. Therefore, from the operation of the entire process, it can be seen that: in the beer fermentation process, the temperature is the process parameter that completes the process, not the characteristic parameters that characterize the fermentation state; it is the biochemical reaction environment that completes the beer fermentation process, not Identification. The physicochemical indexes such as the sugar content, ethanol content, live yeast number and diacetyl content of the fermentation broth tested and tested in the laboratory are the characteristic parameters characterizing the fermentation state, and are the marks of the fermentation process.
The beer fermentation process is a biochemical reaction process referred to as a biochemical reaction, not a chemical reaction process. There are reaction temperatures in the chemical reaction. The chemical reaction cannot proceed below the reaction temperature. There is no reaction temperature in the biochemical reaction. It is said that the main reaction body is microorganisms instead of elements, and the reaction can take place as long as the microorganisms have biological activity. The main difference is the reaction rate and the metabolites produced in the reaction. Of course, it is not only the temperature that determines the speed of the biochemical reaction, but also the type and quality of the substances participating in the reaction. Therefore, the maximum temperature is often limited in biochemical reactions. For example, the two existing fermentation processes, ie, high-temperature fermentation and low-temperature fermentation, stipulate the highest temperatures. The purpose of limiting the maximum temperature is to prevent the production of unwanted metabolites. It can be seen that as long as the temperature is lower than the maximum temperature limit, biochemical reactions can proceed normally. However, an excessively low temperature will affect the production efficiency. Excessive temperature fluctuations will affect the quality of metabolites. Therefore, depending on the quality and oxygenation of the wort, the type of yeast and the number of live yeast, the type of beer, and the production efficiency, the appropriate temperature is selected and controlled within a certain range, and then based on the sugar content measured during the fermentation process. Physical and chemical indicators such as ethanol content, number of live yeasts, and diacetyl content were used to determine whether the temperature should be raised or lowered or kept constant, and with the time to control the entire fermentation process. In short, the temperature provides a biochemical reaction environment and is a process parameter; the adjustment is the reaction rate. This is the role of temperature in the beer fermentation process.
It can be seen that the temperature measurement in the beer fermentation process does not require high accuracy, but rather satisfies a higher degree of accuracy under certain accuracy. This is also the case in the chemical industry sector—generally using a platinum resistance thermometer of class A. The only reason to use B grade in beer brewing.
In the previous section of temperature and heat, it has been pointed out that the essence of temperature control is to control the amount of heat exchange through the controllable heat transfer channels. In the beer fermentation process, heat is not mainly transferred from the outside to the fermentation broth, but is the internal heat of biochemical reaction. This heat is produced by the concomitant conversion of sugars in the fermentation broth into ethanol, carbon dioxide and other metabolites under the action of yeast. These heats must be passed through the cooling jackets or cold strips of the controllable heat transfer channels in a timely manner so that the temperature of the fermentation broth can be maintained at a value below the maximum temperature limit, providing a more stable reaction environment for biochemical reactions. If you want to warm up, you reduce the amount of transmission; if you want to cool, you increase the amount of transmission. Therefore, in the beer fermentation process, the biochemical heat produced is passed through the cooling jacket in a timely manner, which is the essence of temperature control in the beer fermentation process.
At this point, one may ask, since the thermometer installed on the outer wall of the fermentation tank measures the temperature near this point and is used to control the heat flow through the cold zone, how is the temperature in other parts of the fermentation tank distributed? What? Because of the great role of temperature in beer fermentation and the fact that the temperature distribution inside the tank is unknown, many misunderstandings arise. If the fermentation broth is discharged from the sampling port, the temperature is measured; the thermometer is inserted deeper and grows to one meter or even one meter five, and it is desirable to insert it into the center of the tank. These are all due to the fact that the basic concepts described above are not very clear. This is not only unnecessary but harmful. This is because the temperature of heat exchange with the surrounding environment during the release of the fermentation liquid from the sampling port has changed, and the thermometer used for measurement is not the same, and will send wrong information to people. The thermometer is too long and the rigidity is reduced. It is easy to generate vibration when the fermentation liquid is highly fluid, and it is not easy to clean and sterilize.
IV. Temperature Control in Beer Fermentation Process At present, most of the breweries in China use cylindrical conical-bottom type fermentation tanks referred to as conical tanks. Generally, two to three cooling jackets are welded in the cylindrical part. There is also a cooling jacket. The entire tank except the tank top device and the bottom of the tank, the import and export, all wrapped with thermal insulation material, with it to block the heat exchange with the outside world. In this way, the amount of heat exchange between the fermentation broth in the tank and the outside and the biochemical heat generated by the fermentation broth can be neglected. The heat taken away from the cooling medium through the cooling jacket in the temperature control is mainly biochemical heat. Qu:nV"~_
Throughout the entire beer fermentation process, as long as there is biological activity of yeast, there will be biochemical reactions, there is also the production of biochemical heat and metabolites, which is different from other liquids without autothermal reaction is an important feature. Another characteristic is that the main ingredient in beer is water, and the density of water varies with temperature over a temperature that is higher than the freezing point temperature, pure water is 4°C, and beer is about 3°C. In other words, the beer density increases when the beer temperature drops above 3°C, and the beer density decreases when the beer temperature drops below 3°C.
The features described above and some of the basic concepts discussed in the previous section, together with the basic laws of fluid motion and the basic knowledge of heat transfer, are the basis for our scientific analysis of problems.
First of all, let's look at the next simplest method for controlling fluid temperature. Almost all of the breweries use plate heat exchangers to warm or cool the fluid, such as lowering the filling temperature before the filling machine. In order to control the temperature of the beer, a thermometer must be used to detect it. Then the regulator controls the flow of refrigerant through the heat exchanger to control the temperature. Obviously, it is impossible to install the thermometer at the inlet of the heat exchanger, but at the outlet, and the pipeline from the outlet to the filling machine needs to be insulated. This is a typical fluid heat transfer control method. This is also the way to control the sprayer's temperature. This is similar in beer fermentation tanks, but there is one difference: in beer heat exchangers, beer flows under pressure and in the fermentation tank, the flow of fermentation liquid is in gravity or buoyancy and CO2 bubble rising force. The role of flow. The former is called forced convection, while the latter is called natural convection.
During the main fermentation period of beer fermentation, a large amount of heat and carbon dioxide will be produced. In order to remove the heat from the cooling medium immediately by the cooling jacket, the heat exchange method must be convection heat transfer, and the structure of the conical tank and the characteristics of the fermentation liquid itself can be used without mechanical agitation during fermentation. Strong natural convection. We can take a fluid micelle in the fermentation broth for research.
At the bottom of the fluid micelles, the heat generated during the biochemical reaction causes the volumetric expansion density to decrease; along with the generated carbon dioxide—which partially rises in the form of bubbles, and the other dissolves in the liquid, the more fluid micelles at the bottom. Due to the influence of static pressure, the amount of dissolution also increases. All of the above factors make the fluid micro-cluster on the bottom have an upward force so that it moves upward in anti-gravity direction. In the upper part, the fluid micelles near the cooling jacket are moved downward by gravity due to the increased density of the cooled volume. When the original liquid micro-cluster on the bottom reaches the upper part, due to the continuity of the fluid, the liquid micelles in the upper part of the inner wall of the tank are cooled down and then moved downward, they flow toward the tank wall, and then flow down along with the fluid moving down the front. Exercise and transfer the increased heat to the cooling medium in the cooling jacket. At this time, the fluid micelle moves under the action of gravity and has a certain speed. If you want to keep the speed constant, you need to take away the biochemical heat generated during the movement in the next cooling jacket. Therefore, it is not scientifically reasonable to think that in the main fermentation period, the setting value of temperature is gradually raised from the top down to strengthen convection. On the contrary, doing so will weaken the convection. During this period, the liquid in the tank formed a middle upward and downward strong convection. The upward driving force is the buoyancy force, and the downward force is the gravity. Under the action of these two forces, the fluid is allowed to circulate up and down. Between the middle and outer layers, vortices are formed due to the viscous force of the fluid, so that mass exchange occurs between the fluid micelles between the inner and outer thermal-cooled fluid layers, and heat is also transferred along with the transfer, which is called mass transfer heat transfer. . The resulting large circulation flow can not only greatly increase the heat exchange efficiency, but also make the temperature in the tank more uniform, which is equivalent to the effect of mechanical stirring. This is one of the biggest advantages of the conical tank fermentation.
According to our test data, the non-uniformity of temperature within one meter of the tank wall is less than 0.5°C during the main fermentation period. Therefore, there is no need to insert the thermometer deeply. There is no scientific basis for making a fuss on the insertion depth of the thermometer. The insertion depth of the thermometer does not affect the measurement as long as the external temperature changes. The kind of thick and long thermometer used here is unreasonable, not to mention it is not easy to clean and sterilize. It is generally possible to use a platinum resistance of 30 cm. According to my personal opinion, the thermometer does not need to extend into the tank at all, and it is attached to the outer wall of the tank with a surface thermometer. It can also be used to control the heat taken away by the cooling medium, but the entire thermometer must be tightly insulated. Measure, measures the wire also needs to prevent the heat from passing to the temperature sensing part measures. Since the inner wall of the tank has no other substance at this time, cleaning and sterilizing are very convenient and the effect is good, which is of great benefit to the improvement of beer quality. Of course, it is impossible to achieve complete thermal insulation or that outside temperature changes have no effect on temperature measurement, but it is entirely feasible if we take into account this effect in advance and correct it. This kind of control method has been adopted not only in foreign countries but also in China, such as the surface thermometer used in Beijing Yanjing Beer's partial fermenter.
The temperature control during the main fermentation period is generally not a problem and is better controlled. There is also no major problem in the cooling phase, as long as the cooling rate is well controlled and the convective intensity is gradually reduced to facilitate yeast settling. After a large number of yeasts have been discharged and continue to cool down, the problem arises. Since a large number of yeasts have been eliminated at this time, the biological activity of the remaining yeasts is reduced due to the decrease in temperature, the biochemical heat generated is also reduced, and the convective velocity is greatly reduced, but at a temperature higher than the maximum density of the wine liquid by 3°C. Under the circumstances, the flow of liquid in the tank is still in the middle, upwards and downwards. When the temperature continues to decrease to the maximum density of 奌3°C, the volume of the fluid micelles on the inside of the tank wall is reduced to the minimum. If the temperature of the fluid micelles increases, the density of the fluid micelles increases and the density decreases, resulting in an upward buoyancy. Suspension of downward flow, and then change the direction of upward movement, when the fluid micro-cluster leaves the cold zone, due to self-heat and external heat transfer, the temperature rise density becomes larger, the motion is stagnated and changes direction downwards, Local circulation, so that the formation of the overall circulation in the tank, resulting in a larger temperature gradient. The reason for this phenomenon is caused by the characteristics of water, but it is also related to the temperature control scheme.
When the direction of fluid flow changes, the position of the thermometer used to control the temperature should also be switched so that it is still at the exit of the heat exchanger. In other words, when the temperature is higher than the maximum density, the fluid flows downwards through the cold band and the thermometer should be placed below the cold band; when below the maximum density point, the fluid flows upwards through the cold band and the thermometer should be placed on the cold band. Above. In this way, the heat taken away by the cooling medium can be controlled. Otherwise, when the flow direction changes and the thermometer is still in position, it is equivalent to inserting a thermometer at the inlet of the heat exchanger. Obviously, it is impossible to control the temperature at all and not only cannot form a large cycle, but also The top floor is also very easy to freeze. 6D&{+;
In order to solve this problem, some people proposed a number of methods: early braking, fuzzy control, expert systems, etc., can not say no, can only say that it failed to grasp its essence. If the thermometer is still under the cold belt, it is equivalent to the temperature of the wine liquid when the wine liquid flows out of the heat exchanger outlet and then turns around and then returns to the heat exchanger. This not only delays the time, In addition, the heat transfer in the outer circle will be influenced by many factors. Therefore, this is not a very scientific method. The scientific method is based on the basic concept of controlling the temperature and controlling the exchange of heat in controllable heat transfer channels. When the direction changes, the position of the temperature detector should be changed in a timely manner. The thermometer originally located under the cold belt should be discarded, and the thermometer located above the cold belt should be used as the temperature-control detection thermometer. At this time, the main part of the control is the cold zone at the bottom of the cone, and it is sometimes necessary to control the cold zone below the cylindrical body. The topmost thermometer, which is located near the liquid surface, can only be used as a monitor.
Of course, if the diameter of the tank is too large, or the insulation effect of the tank is not good, a large temperature gradient will be formed. At this time, the internal heat exchange can be enhanced by the carbon dioxide blowing at the bottom of the tank, so that the temperature in the tank tends to be uniform.
V. Several points of view of the evaluation temperature control system The temperature control system generally adopts closed-loop control, that is, the sensing part acquires the controlled parameter signal from the control object, passes it to the regulator and compares the set value, and the output of the regulator is controlled according to the comparison result. Signal to the actuator to adjust the controlled parameter of the control object. The control object is a very important part of the closed loop. Most of the problems discussed above are related to this. Do not assume that we have thermometers, regulators, and actuators to make the temperature control stable and uniform. First of all, there must be a scientific analysis of the thermal structure of the control object, use the thermometer correctly and determine the installation location, otherwise it may produce temperature fluctuations and larger temperature gradients. Therefore, when evaluating a temperature control system, it is necessary to first look at the thermal structure analysis in the control scheme. This is the premise and basis.
Secondly, in industrial production, reliability is always the top priority. Whether it is mechanical or electronic, the simpler the function is, the lower the reliability of the intermediate links. As far as the entire temperature control system is concerned, the most important part is the sensing part and the implementing agency. The failure rate of these two parts is based on the statistics of the famous Honeywell company in the United States: nearly 80% of the failures in traditional control systems are caused by sensing. The faults were caused by the failure of some parts and actuators, and the sensing part of them accounted for 45%. Therefore, the focus of improving reliability is sensing and executing two parts, not others. This is often overlooked, especially for those who work in computers. Their attention is often placed on the hardware and software of the computer, while ignoring other control links. In our country, the use of computer temperature control has just been broken down. Some people think that using a computer to control the temperature is not a problem in the computer, but rather a problem in other areas.
Let us talk about the "advancedness" issue. Under the market economy, there is only "advanced" economic returns or valuable returns. The so-called "advancedness" of non-valued returns is meaningless. A system is not because you have adopted a new technology or new components, the entire control system is advanced. The advanced nature of the entire system should be fully evaluated from the entire control link, and it is not static and should be changed according to the actual situation. What's more, in the end, it is up to the user to speak with actual economic indicators.
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