Environmental testing technology is a practical basic technology and one of the important technical measures to stabilize and improve product quality. Its main task is to study the impact of environmental stress on product performance and solve the reliability problems of products in storage, transportation or use. Among them, thermal cycle test is a common type of climate test in environmental test technology. During the test, due to the change of temperature and humidity environment in the test chamber, condensation may occur. This phenomenon often occurs in the low temperature to high temperature working condition stage where the temperature and humidity change dramatically. Many electronic products are affected by condensation water drops, resulting in accelerated chemical reaction of metal oxidation or electrochemical corrosion on the surface of thermal control coating, The electrical insulation performance is reduced, or even functional damage such as short circuit. In view of the damage caused by condensation to the product, some predecessors have tried to prevent condensation from changing the test profile. However, because the test profile is generally changed by the overall project unit, and the change will directly affect the product performance assessment results, its feasibility is greatly reduced. Therefore, this paper starts from exploring the causes and conditions of condensation. The feasible means to reduce the relative humidity in the test chamber are discussed.
Why will the water condense during test ?
Low temperature working condition means that when the temperature in the chamber is lower than the freezing point temperature under the atmospheric conditions of the laboratory at that time, most of the moisture in the wet air in the chamber is attached to the evaporator surface, the lowest temperature part in the chamber, in the form of frost. When the chamber operates at low temperature for a long time, there is a lot of frost on the evaporator surface. When the temperature rises, the frost layer on the evaporator surface absorbs the heat in the air and gradually undergoes the process of liquefaction and regasification, The water again enters the circulating air in the chamber in the form of steam, which increases the moisture content in the chamber.
At the same time, the wall of the chamber heated by the circulating air also starts to heat up. The heat capacity of the chamber wall is far greater than the gas heat capacity, causing the temperature rise speed of the chamber wall to lag far behind the air temperature rise speed. As a result, the temperature gradient perpendicular to the wall of the chamber will increase with the speed of temperature rise. The temperature gradient makes the partial pressure of air and water vapor near the tank wall greater than that of circulating air in the tank, which means that the dew point temperature of air near the wall is higher, which means that it is easier to condense. Even if the relative humidity of circulating wet air in the chamber does not reach 100%, the air near the wall has reached 100%, causing condensation.
As the temperature in the test chamber changes from low temperature to high temperature, the evaporator frost layer will vaporize, increasing the moisture content in the chamber. Therefore, in order to completely reduce the moisture content in the chamber, try to remove the frost layer on the evaporator surface. At present, the defrosting methods in the refrigeration industry mainly include: water spraying defrosting, electric defrosting, hot air defrosting and other water spraying defrosting methods. The purpose of defrosting is to melt the frost layer by spraying liquid water on the surface of the evaporator so that it can conduct latent heat exchange with the solid ice. This method is only applicable to defrosting when the machine is stopped.
During the test, the water will be blown into the chamber due to direct contact with the circulating air, increasing the moisture content of the circulating air, The wet air with increased moisture content quickly condenses into frost under low temperature and adheres to the surface of the test object in the chamber, causing damage to the test object. Therefore, it is not applicable to thermal cycle test. The electric defrosting method refers to arranging an electric heating wire around the evaporator to heat the frost layer. Melt the frost layer from the outside to the inside. In this way, heat transfer is carried out on the frost layer surface. The vaporized water directly enters the circulating air to increase its moisture content. At the same time, the direct contact between the electric heating wire and the circulating air will bring a lot of heat, making the low temperature stability difficult to control. Therefore, it is not suitable for the thermal cycle test. Hot gas defrosting refers to the low temperature refrigerant introduced into the evaporator during the dewing stage. The advantage of this method is that the frost layer melts from the inside out. In the early stage of defrosting, the hot gas in the evaporator tube conducts latent heat exchange with the frost layer outside the tube. The heat is used to melt the frost layer on the tube wall. At this time, the frost layer close to the evaporator melts and forms a interlayer containing gas-liquid mixture between the outer frost layer and the evaporator.
At this time, due to the external frost coating, most of the water vapor is sealed in the interlayer, and the frost layer gradually melts from the inside to the outside until it completely melts and falls off, and then it is discharged out of the tank through the water collector. The whole process not only reduces the amount of water vapor vaporized from the evaporator, but also controls the moisture content in the test chamber at a low level. In addition, the heat on the evaporator will radiate outward only after the defrosting falls off, which is conducive to controllingThe temperature stability of the circulating air in the test chamber during the frost stage is obviously better to use the hot air defrosting method in the thermal cycle test.
The traditional hot gas defrosting system has two ways: four-way reversing method and compressor hot gas bypass method. The four-way reversing method is to switch the refrigerant flow path direction through the four-way reversing valve arranged on the refrigeration system when defrosting is required to reverse the refrigerant flow. At this time, the functions of the evaporator at the heat absorbing end and the condenser at the heat releasing end are exchanged, and the high-temperature refrigerant flows into the low-temperature evaporator to melt the frost layer
The defrosting method will lead to the interruption of refrigeration, which cannot ensure the stability of the temperature under low temperature conditions in the test chamber. At the same time, the melted water vapor will re-enter the chamber, which will increase the moisture content in the chamber and aggravate the condensation. The compressor hot gas bypass method uses a part of the high-temperature refrigerant discharged from the compressor to pass through the low-temperature evaporator for defrosting. Some of them are used for refrigeration after throttling, which will also increase the moisture content in the chamber and aggravate the condensation.
In order to maintain the temperature stability of the temperature and humidity environment test chamber and the moisture content in the control chamber at a low level, the author considers adding an evaporator, several solenoid valves and two independent valves to the structure of the traditional refrigeration systemThe air duct constitutes the low dew point environmental test chamber system.
The system is different from the single evaporator structure of the traditional steam compression refrigeration cycle system. It uses one system with two evaporators.When two evaporators are running for refrigeration, the other evaporator defrosts. The two evaporators are respectively equipped with independent air ducts. The opening and closing periods of air ducts 1 and 2 are opposite. It is always ensured that the refrigeration generator is connected with the circulating air flow in the chamber, while the defrosting evaporator is isolated from the air flow in the chamber. This system can not only continuously maintain the stable refrigeration work under low temperature conditions, but also continuously remove the moisture content of the wet air in the chamber.