Environmental testing technology is a useful basic technology and one of the important technological processes to stabilize and improve product quality. Its main task is to study the impact of environmental restrictions on product performance and to solve product reliability problems during storage, transportation or use. Among them, thermal cycle test is the most common type of climate test in environmental testing technology. During the test, due to changes in temperature and humidity of the environment in the test room, condensation may occur. This phenomenon usually occurs in low-temperature and high-temperature operating conditions where temperature and humidity fluctuate greatly. Many electronic products are exposed to water condensation, causing a rapid chemical reaction of metal oxidation or electrochemical corrosion on the surface of the temperature controller.
Electrical insulation performance is reduced or even malfunctions such as short circuits. Considering the damage caused by the product, some people first tried to prevent condensation by changing the test profile. However, since the test profile is often changed by the service sector as a whole and the change will directly affect the results of the product performance analysis, its power is reduced. Therefore, this article begins by identifying the causes and conditions of condensation. A simple way to reduce the humidity in the laboratory is discussed.
Why does the water boil during the test?
The low operating temperature means that when the temperature in the room is lower than the heating temperature and the air condition of the laboratory at that time, most of the water in the air humidity is n ‘the room is connected to the surface of the evaporator. , the lowest part of the indoor temperature, in the form of frost. When the room works at low temperature for a long time, there is a lot of frost on the surface of the evaporator. When the temperature rises, the cold layer above the evaporator absorbs heat from the air and slowly undergoes the process of liquefaction and degasification. The water then enters the air that circulates in the room in the form of vapor, which increases the humidity in the room.
At the same time, the walls of the building are heated by the circulating air and begin to heat up. The heat capacity of the house wall is much greater than the heat capacity of the gas, so the temperature rise rate of the house wall is behind the temperature of the air. As a result, the temperature gradient perpendicular to the building wall will increase with the rate of temperature rise. The temperature gradient causes the partial pressure of the air and water near the tank wall to be higher than that of the air passing through the tank, which means that the temperature of the dew point near the wall is higher, which means that it is much easier condense. Although the relative humidity of the humid air circulating in the room does not reach 100%, the air near the wall has reached 100%, causing condensation.
Solution:
When the temperature in the test room changes from low temperature to high temperature, the cold layer of the evaporator evaporates, increasing the water content in the room. Therefore, to completely reduce the water content in the room, try to remove the layer of frost on top of the evaporator. At present, the cooling methods in the refrigeration industry usually include: water evaporation, electric cooling, hot air cooling and other water cooling methods. The purpose of defrosting is to melt the frost layer by spraying liquid on the surface of the evaporator so that it can exchange latent heat with the solid ice. This method is only suitable for clearing when the machine is stopped.
During the test, water will be blown into the room due to direct contact with the circulating air, increasing the water content of the circulating air. Humid air with high water content quickly condenses into frost at low temperatures and sticks to the surface of the sample in the room, damaging the sample. Therefore, it is not suitable for thermal cycle testing. An electrical escape method is to run electrical wires around the evaporator to heat the frost layer. Melt the cold layer from the outside in. In this way, heat transfer takes place on the cold layer. The vaporized water enters the air and circulates directly to increase its content.
At the same time, the direct contact between the electric wire and the circulating air will cause a lot of heat, making the low stability difficult to control. Therefore, it is not suitable for thermal cycle tests. Refrigerant refers to the low level of refrigerant introduced into the evaporator during cooling. The advantage of this method is that the layer of frost melts inside. At the beginning of evaporation, the hot gas in the tube of the evaporator makes a heat exchange with the cold layer outside the tube. Heat is used to melt the layer of frost on the tube wall. During this time, the frost layer near the evaporator melts and forms an intermediate layer with a gas-water mixture between the frost layer and the air.
At this time, due to the outer layer of frost, most of the warm water is sealed in the middle layer, and the cold water gradually melts through from inside to outside until it completely melts and falls, then it is taken out of the tank Water collector. The whole process not only reduces the amount of water coming out of the evaporator, but also controls the water in the laboratory at a low level. In addition, the heat in the evaporator will appear outside only when the break is down, which is good to manage. The stability of the temperature of the air circulating in the test room during the frost period is a clear reason that it is better to use the warm air system in the cycling test. The hot gas exhaust system has two methods: the four-way bypass system and the compressor hot gas bypass system. The four-way reverse flow is the reverse flow of the refrigerant through the four-way flow valve that is arranged in the refrigeration system when it is required to turn the cold water into the refrigerant. At this time, the work of the evaporator at the heating end is changed to the condenser at the heat release end, and the high-temperature refrigerant flows into the low-temperature evaporator to melt the cold refrigerant.
The cooling method will make the refrigerator stop, which cannot ensure the temperature stability under low temperature conditions in the laboratory. At the same time, the melted water will re-enter the room, which will increase the humidity in the room and cause condensation to worsen. The high temperature refrigerant system uses a portion of the refrigerant that is extracted from the compressor and passes through the low level evaporator for evaporation. Some of them are used for post-throttling refrigeration, which will increase the humidity in the room and make condensation worse. In order to increase the stability of the temperature in the environmental laboratory and the water in the control room is low, the author plans to add an evaporator, several solenoid valves and two independent valves in the design of the traditional refrigeration system. The air duct consists of a laboratory system in a low dew point environment.
The system is different from the evaporator unit of the traditional refrigeration cycle. It uses a system with two evaporators. When two evaporators are working for refrigeration, the other evaporator is defrosting. The two evaporators are arranged individually with independent air outlets. The opening and closing times of air vents 1 and 2 are opposite. It is always noted that the refrigeration generator is connected to the air that circulates in the room, while the heater is isolated from the air in the room. This system can not only keep the refrigeration function stable under low temperature conditions, but also remove the water content from the humid air in the room.
