Analysis of temperature control problems in the co

2022-07-31
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Analysis of problems in temperature control of beer conical fermentation tank (Part 2)

III. in the beer fermentation process, the role of temperature and the essence of temperature control

in the fermentation workshop of the brewery, whether the brewer or the operator, pay the most attention to the temperature of the fermentation liquid in the fermentation tank. Especially for the operators, the process sheet basically refers to the temperature and time, which is easy to create an illusion: it seems that the beer fermentation process is completed according to the temperature and time. In fact, during the whole process, the starter yeast should be regularly taken from the sampling port and sent to the laboratory for test. According to the test results, the temperature and time may change. Therefore, from the operation of the whole technological process, it can be seen that temperature in the beer fermentation technological process is the technological parameter to complete the technological process, rather than the characteristic parameter to characterize the fermentation state; It is the biochemical reaction environment to complete the beer fermentation process, not the identification. The physical and chemical indicators such as sugar content, ethanol content, number of live yeast and diacetyl content of fermentation broth tested and detected in the laboratory are the characteristic parameters to characterize the fermentation state and the identification of the fermentation process

beer fermentation process belongs to biochemical reaction process, which is called biochemical reaction for short, not chemical reaction process. There is a reaction temperature in the chemical reaction, and the chemical reaction cannot be carried out without the reaction temperature; There is no reaction temperature in the biochemical reaction. That is to say, the main body of the reaction is the microorganism rather than the element. As long as the microorganism has biological activity, it can react. The temperature mainly affects the reaction speed and the metabolites produced in the reaction. Of course, the speed of biochemical reaction is not only determined by temperature, but also by the type and quality of substances participating in the reaction. Therefore, the maximum temperature is often limited in biochemical reactions. For example, the two existing fermentation processes, namely high-temperature fermentation and low-temperature fermentation, specify the maximum temperature. 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, the biochemical reaction can be carried out normally. However, too low temperature will affect the production efficiency, and too large temperature fluctuation will affect the quality of metabolites. Therefore, according to the quality and oxygenation of wort, yeast type and number of live yeast, beer variety, etc., as well as production efficiency, select the appropriate temperature and control it within a certain range, and then determine that the temperature should increase or decrease or remain unchanged according to the physical and chemical indicators such as sugar content, ethanol content, number of live yeast and diacetyl content measured in the fermentation process, Controlling the whole fermentation process with time can avoid the heat bridge effect to the greatest extent. In a word, temperature provides biochemical reaction environment and process parameters; It regulates the reaction speed. 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 need high accuracy, but meets the high accuracy measurement under a certain accuracy. This is also the reason why class a platinum resistance thermometer is generally used in the chemical industry sector, but only class B thermometer is used 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 heat exchange through controllable heat transfer channels. In the beer fermentation process, the heat of fermentation broth is not mainly from the outside, but from the internal biochemical reaction heat. The heat is produced when the sugar in the fermentation broth is converted into ethanol, carbon dioxide and other metabolites under the action of yeast. These heat must be timely transmitted through the controllable heat transfer channel cooling jacket, i.e. the cold belt, so that the temperature of the fermentation broth can be maintained at a certain value lower than the maximum temperature limit, providing a more stable reaction environment for the biochemical reaction. If you want to heat up, reduce the output; If you want to cool down, increase the outgoing volume. Therefore, the essence of temperature control in beer fermentation process is to transmit the generated biochemical heat through the cooling jacket in time

at this point, people 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 from the cold belt, how is the temperature distribution in other parts of the fermentation tank? Because the temperature plays a very important role in beer fermentation, and the temperature distribution in the tank is unknown, many misunderstandings arise. For example, measure the temperature of the yeast from the sampling port; Lengthen the insertion depth of the thermometer to one meter or even one meter five. If it can be inserted into the center of the tank, we should vigorously develop green packaging. These are caused by not being very clear about the basic concepts mentioned above. Forcing enterprises to invest in controlling product quality is not necessary, but harmful. This is because the heat exchange temperature with the surrounding environment has changed during the release of yeast from the sampling port, and the thermometer used for measurement is not the same branch, which will send wrong information to people. However, if the thermometer is too long, its rigidity will be reduced, and it is easy to vibrate when the fermentation broth flows fiercely, and it is not easy to clean and sterilize

IV. temperature control in the beer fermentation process

at present, the vast majority of breweries in China adopt circular column conical bottom fermentation tanks, which are called conical tanks for short. Generally, two or three cooling jackets are welded on the cylindrical part, and there is a cooling jacket at the conical bottom. Except for the inlet and outlet of the tank top device and the tank bottom, the whole tank body is wrapped with thermal insulation materials to block the heat exchange with the outside world. In this way, the heat exchange capacity between the fermentation liquid in the tank and the outside world and the biochemical heat generated by the fermentation liquid can be ignored. The heat taken away by the cooling medium through the cooling jacket in the temperature control is mainly biochemical heat

in the whole beer fermentation process, as long as the yeast has biological activity, there will be biochemical reaction, and biochemical heat and metabolites will be produced, which is an important feature different from other liquids without autothermal reaction. Another feature is that the main component of beer is water, and the density of water has a maximum value near the freezing point temperature with the change of temperature. The temperature of pure water is about 4 ℃ and that of beer is about 3 ℃. In other words, when the beer temperature is above 3 ℃, the beer density increases; When the temperature was lower than 3 ℃, the beer density decreased

the above characteristics and some basic concepts mentioned in the previous part, together with the basic laws of fluid motion and the basic knowledge of heat transfer, are the basis for our scientific analysis

first, let's take a look at the simplest method to control fluid temperature. Plate heat exchangers are almost used in breweries to heat or cool the fluid, such as lowering the filling temperature before the filling machine. In order to control the beer temperature, Chinese chemical enterprises must first use a thermometer to detect when deciding whether to carry out M & A, and 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. Moreover, the pipeline from the outlet to the filling machine also needs to be insulated. This is a typical fluid heat exchange control mode, and the temperature control of the spray water of the sterilizer is also this mode. The same is true in the beer fermentation tank, but there is still a difference: in the plate heat exchanger, the beer flows in the pipe under pressure, while in the fermentation tank, the fermentation liquid flows under the action of gravity or buoyancy and the lift of carbon dioxide bubbles. The former is called forced convection, while the latter is called natural convection. (to be continued)

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