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Evaporator design

Views:1     Author:Site Editor     Publish Time: 2019-07-19      Origin:Site


Evaporator design For the excavation of domestic air conditioners, only a few new products need to rediscover new molds. The evaporator is designed with a new shape structure. Most of the product excavations are only designed to redesign the heat exchanger in the original size. When designing, the structural size of the heat exchanger is basically impossible to adjust. Of course, if the selected steamer can not meet the specifications under the given structural size, the most commonly used method is to increase the size of the original. Block fins to increase the heat exchange area. If it still fails to meet the specifications, we have only tried to use indoor units with large heat exchange areas. Now let's talk about the method of circuit design for a certain geometry of the evaporator. First we need to determine the number of flow paths in the evaporator, and then consider the flow of refrigerant in each flow path based on the number of flow paths. The number of flow paths is determined. The change of the refrigerant in the evaporator is that the saturated liquid (which actually contains a small amount of flashing gas after throttling) starts to absorb heat. After the liquid is vaporized, it turns into a gas. As the refrigerant flows, the gas in the copper tube As the volume increases, the flow rate of the refrigerant increases with the volume, and the flow resistance at this time also increases. When all the refrigerant has turned into a gas, if the heat exchange continues, what the refrigerant does is Sensible heat and heat, its heat transfer coefficient is very low, so in order to ensure the high utilization of the evaporator, we should try to make the refrigerant just fully evaporated in the evaporator when debugging the system, of course, this problem and the determination of the number of flow paths It is not relevant and will not be discussed here. According to the basic knowledge of heat transfer, we know that a higher refrigerant flow rate can obtain the heat transfer coefficient. The design of the evaporator improves the cooling capacity of the refrigeration system, but from the knowledge of fluid mechanics, we can know that the flow resistance of the refrigerant varies with The increase of the flow rate increases the pressure drop of the refrigerant in the evaporator, which reduces the suction pressure of the compressor. The suction pressure of the compressor has a significant effect on the output of the compressor. The influence of these two aspects should be considered in a compromise when determining the number of flow paths, so as to maximize the utilization of the evaporator. According to general experience, the gas flow rate in the evaporator is more suitable at 6 ~ 8m / s. In this way, we can calculate the liquid flow rate based on the ratio of the specific volume of the refrigerant gas and liquid. For R221 and R4071C, the liquid flow rate is 0.11 ~ 0.151m / s. In this way, we can roughly estimate the heat exchange capacity of each flow path is approximately: ф9.53mm copper tube heat exchange amount per flow path is 1600 ~ 2101Wф7.94mm copper tube heat exchange amount per flow path is 1001 ~ 1400Wф7. The heat transfer capacity of each flow channel of 0mm copper tube is 801 ~ 1001W. For R4110A, the liquid flow rate is 0.151 ~ 0.21m / s, so we can roughly estimate the heat transfer capacity of each flow channel is approximately: ф9.53mm copper tube per The heat transfer capacity of each flow path is 2000 ~ 2500W ф7.94mm copper tube. The heat transfer capacity of each flow path is 1300 ~ 1700W ф7.0mm copper tube. The heat transfer capacity of each flow path is 901 ~ 1310W. Based on the above data, we can determine the heat transfer first. The number of flow paths before designing the flow path. Flow path design. After we have determined the number of flow paths based on the cooling capacity, we have to consider how to allocate these copper tubes to ensure the best heat exchange effect. Before designing the flow path, we must first determine a large direction, namely the evaporator Whether to adopt the co-current or counter-current design. Under normal circumstances, the use of counter-current will help increase the heat transfer temperature difference and achieve the purpose of increasing the heat exchange capacity. However, if the heat pump type air conditioner is used, the evaporator will change during heating when the counter-current design is used. It has formed a co-current heat exchange form, which will cause very little heat exchange temperature difference between the refrigerants in the back, which will seriously affect the utilization rate of the heat exchanger. For comprehensive consideration, for heat pump air conditioners, we usually adopt co-current design in the evaporator. Another point of attention is to ensure that the liquid is placed downward and the gas is placed upward when designing the flow path of the heat exchanger. After determining the direction of the refrigerant, we next need to consider how to allocate the number of tube passes for each flow path. A main principle of tube pass distribution is to distribute less tubes with good heat exchange and poor heat exchange. Allot some copper pipes. For domestic air conditioners, because of structural limitations, but in order to pursue a higher energy efficiency ratio or achieve higher capacity, we often adopt a multi-fold design for the heat exchanger, but usually our heat exchangers are bent The shape does not meet the distribution of the fan's flow field well, that is to say, the air flow velocity of each folded block of the heat exchanger will be much different, so we have some copper pipe distribution when the wind speed is lower You can distribute more appropriately and try to ensure that the refrigerant in each flow path can completely evaporate. Of course, we can also adjust the flow rate of refrigerant in each flow path by adjusting the length of the manifold on the distributor, so that each path The refrigerant can completely evaporate, but in the design of the flow path, we should try to assume that the flow rate of each path is the same, so that we can only adjust the liquid separation when a certain degree of imbalance occurs in each flow path. The length of the capillary is used to solve the problem. If we do not consider the differences between the various parts of the evaporator in the design of the flow path, when the balance of the flow paths is too large during actual measurement, we can adjust the flow path. Liquid separation capillary length may not solve the problem. Another problem should also be noted in the circuit design, that is, we try not to concentrate the refrigerant outlets together. This will cause a large difference in air temperature after the various parts of the evaporator are processed. The evaporator design is mixed in the air duct like this. After that, condensed water will be generated, and the air outlet will blow water in severe cases. Such problems are usually caused by condensation tests.

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