Research on Optimal Matching of Water Traps and Vacuum Pumps in Vacuum Precooling
Preface Vacuum pre-cooling technology is to reduce the pressure of the pre-cooled food pre-cooling space, thereby reducing the boiling point of water, so that food moisture quickly evaporates at a lower temperature, thus taking away food heat, in a short period of time to reduce food temperature. In this process, in order to ensure the normal operation of the vacuum pump, the water vapor needs to be drained by the vacuum pump after passing through the catcher, that is, the load of the vacuum pump is determined by the amount of air in the vacuum chamber and the amount of moisture evaporation. From the analysis of the existing research results, most of the research results focus on the introduction of vacuum pre-cooling, technology, etc. Only a few scholars have studied the optimization and matching of the vacuum pre-cooling system, which gives theoretical calculations on the relevant components of vacuum pre-cooling. formula. In this paper, the matching relationship between the required cooling capacity of the water trap and the power of the vacuum pump is studied to determine the selection of the two types and to achieve the economical operation status of the vacuum precooling equipment. 1 Vacuum precooling energy consumption analysis 1.1 The vacuum pump energy consumption analysis assumes that the volume of the vacuum tank is Km3), the initial pressure is WPa), the power of the vacuum pump is WW), and the exhaust gas volume is a certain pressure against 屮3) (13/ 5) Consider the power consumption per unit mass of gas excluded under such conditions. If the gas in the true tank is identified as the ideal gas, the mass of the exhaust gas after the stabilization is calculated according to formula (1) according to the given conditions. Infinity 461; 7; for the temperature (K) in the vacuum tank at this time, under normal circumstances, the temperature in the vacuum tank can be identified as a constant; m is the exclusion of air quality (kg/s). Therefore, the power required to exclude a unit mass of gas is calculated according to equation (2). For a vacuum pump, the relationship between power and air flow can be expressed in Equation (3), which is generally atmospheric pressure (105 Pa), without considering the efficiency of the vacuum pump. The energy consumption of the vacuum pump required for exhaust quality and unit exhaust quality at different vacuum pressures and temperatures is shown separately. It can be seen that the temperature in the vacuum tank has a small effect on the quality of the exhaust gas. When the pressure in the vacuum tank is 600 Pa and the exhaust gas volume is 5.555 L/S, the exhaust gas mass at a temperature of 288 K is 0.0251 g, and the exhaust gas mass at a temperature of 296 K is 0.0244 g. For the same temperature conditions, the pressure in the vacuum tank has a greater impact on the quality of the exhaust gas. When the temperature in the vacuum tank is 288 K and the exhaust gas volume is 5.555 L/S, the exhaust gas mass at a pressure of 100 OPa is 00418 g, which is much larger than 0.0251 g at 600 Pa. For the analysis, as the pressure in the vacuum tank rises, the energy consumption per unit mass of the vacuum pump is reduced. When the pressure is 600Pa, the energy consumption per unit mass discharge is 22KW/kg, and when the pressure is 100OPa, the energy consumption per unit mass discharge is 12.8KW/kg. This shows that when dealing with the same temperature drop, the pressure in the vacuum tank is better when the pressure is higher, but this is controlled by the temperature required to achieve the pre-cooling. The higher the vacuum tank pressure, the higher the minimum temperature the pre-cooling can reach, and the boiling temperature of the free water at this pressure cannot be exceeded. The curve of the boiling temperature of free water at different pressures is shown. As can be seen from the figure, the minimum temperature that the precooled food can reach increases as the pressure in the vacuum tank rises. When the pressure in the vacuum tank is 611Pa, the minimum temperature for food precooling can reach 0°C, and when the pressure is 100OPa, the minimum temperature for food precooling can reach 7°C. Therefore, for those foods with frostbite, vacuum preheating is performed. When cold, it is necessary to consider the ability of food moisture to evaporate, so as to select the appropriate vacuum pump to achieve the best pre-cooling effect. 1.2 Energy Consumption Analysis of Water Catcher The ability of the water trap to capture moisture has a decisive influence on the working performance of the vacuum pump. It is one of the two energy-consuming components in the system components. The energy consumption efficiency determines the energy consumption of the entire system. effectiveness. The energy consumption of the water trap is the same as the energy consumption of the refrigeration unit. Assume that the surface temperature of the water trap ST2 (K), the compressor condensation temperature t3 (k), the diameter of the water trap smooth tube is D (mm), the flow of the refrigerant is / (kg / s), the compression process is not Overheating requirements and isentropic compression. The energy consumption of the isentropic compression process can be calculated according to equation (4). The index, for polyatomic molecules, is 1.29; a and k are the corresponding saturation pressure at temperature and the specific volume under isentropic compression: points and K2 correspond to the saturated pressure and specific volume at temperature 72 respectively. These specific values ​​can be determined by the corresponding refrigerant state equations. The power consumption of the refrigeration unit compressor is shown. The calculation conditions are: condensing temperature 310K, refrigerant R22, vacuum pump exhaust capacity w precooled item mass (kg). For mi can be calculated according to the ideal gas state equation, substituting equation (1) into equation (5), and equation (6). The curve of the relationship between Afs and vacuum pump displacement R of different quality spinach under ideal conditions is shown. It is evident from this that at a certain pumping rate, the temperature drop per unit time decreases as the quality increases, while for a certain quality spinach, the unit time temperature increases with the increase of the pumping volume. However, it has also been found that the change in the temperature drop per unit time is not a linear change, which requires that the change in the temperature drop per unit time should be taken into consideration when determining the pumping amount, not just the change in the pumping amount. 2.2 Total energy consumption analysis of pre-cooling process As mentioned above, the total energy consumption of the pre-cooling process consists of two parts: the energy consumption of the compressor and the energy consumption of the vacuum pump. The actual power consumption of the compressor can be calculated using equation (7). The preheating system enclosure heat transfer, 0 is equal to the latent heat of moisture; f is the refrigeration coefficient of the refrigeration unit, for the air-cooled cryogenic unit, considering the irreversible degree of heat transfer temperature difference, the real refrigeration coefficient f is smaller than the theoretical calculation value Much more, from the analysis of experimental results, the true F value is between 0.951.2. Considering the irreversibility of the vacuum pump and the refrigeration system at the same time, the total power consumption of the precooling system can be expressed as Equation (8). From the figure, it can be seen that as the surface temperature of the water trap increases, the power of the compressor decreases. When the surface temperature of the water trap is 268K, the power consumption is about 14W; when the surface temperature is 273K, the power becomes about 12W. Therefore, increasing the surface temperature of the water trap is beneficial to improving the operation effect of the refrigeration unit. 2 Analysis of energy consumption indicators 2.1 Temperature drop per unit time From the previous analysis, it can be known that the temperature drop of food in a unit of vacuum precooling depends on the amount of water evaporation of the food. According to the energy balance of the first law of thermodynamics, the latent heat required for water evaporation From the sensible heat of the food itself, there is formula (5). The temperature drop (°C/s); c, the specific heat of the precooled article (/kg°C); shows the curve of the total precooling power consumption as a function of the vacuum pump displacement VI. It can be seen from the above that, when other conditions are fixed, the total power consumption required for vacuum precooling is directly proportional to the amount of air extracted, and it is not related to the type of fruits and vegetables. For precooled fruits and vegetables that require a certain temperature reduction, the energy consumed in the entire process is the time required to multiply the total power consumption by the entire pre-cooling process and can be calculated according to Equation (9). It has nothing to do with the pumping capacity of the vacuum pump. The specific heat of cucumber above freezing point is 4.06K/kg, while the specific heat of jujube is 3 Conclusion Through the above analysis, the following conclusions can be drawn: The energy consumption of water trap and vacuum pump in vacuum precooling mainly depends on the pumping of vacuum pump. Capacity, the choice of water traps should be determined according to the pumping capacity of the vacuum pump; the overall energy consumption of the system depends on the fruits and vegetables that need to be pre-cooled. When the vacuum pre-cooling device is certain, the ability to evaporate the moisture of fruits and vegetables should be considered. The quality of cold fruits and vegetables, otherwise there will be frost damage. Shows the drop in temperature of 23T for several different fruits and vegetables: the total energy (W) required. It can be seen from the figure that when different fruits and vegetables are reduced by vacuum pre-cooling to reduce the same temperature, the energy needed by cucumbers is the most, and jujube needs the least, depending on the specific heat of the different fruits and vegetables above the freezing point. Refined Hemp Oil is clear and colorless. It doesn't have much flavor and also lacks the natural vitamins and antioxidants that hemp is known for. All these oils are both extracted from the plant and processed in different ways. More importantly, all these oils offer different benefits. Understanding what properties each of these oils have will help you understand which hemp oil to shop for the next time you`re looking to try one. 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