When the tested product (components, components, parts or complete machine) is placed in the climate environment chamber for testing, in order to ensure that the tested product
The ambient atmosphere can meet the environmental test conditions specified in the test specifications. The following provisions shall be followed between the working dimension of the climate chamber and the external dimension of the tested product:
a) The volume of the tested product (Wide x Deep x High) shall not exceed (20 ~ 35)% of the effective working space of the test chamber (recommended)
20%). It is recommended that no more than 10% of the products that generate heat in the test be selected.
b) The ratio of the windward sectional area of the tested product to the total area of the test chamber on the section shall not be greater than (35 ~ 50)% (push)
c) The distance between the outer surface of the test product and the wall of the test chamber shall be at least 100 ~ 150 mm (150 mm is recommended).
The above three points are interdependent and unified. Taking a cubic meter cube chamber as an example, the area ratio is 1: (0.35 ~ 0.5) is equivalent to a volume ratio of 1: (0.207 ~ 0354). 100 ~ 150 mm from the tank wall corresponds to a volume ratio of 1: (0.343 ~ 0.512).
To sum up the above three points, the working chamber volume of the climate environment test chamber shall be at least 3-5 times of the external volume of the tested product. The reasons are as follows:
1) After the test piece is placed in the chamber, the smooth channel is occupied, and the narrowing of the channel will lead to the increase of the airflow velocity. Accelerated airflow and quilt
Heat exchange between test pieces. This is inconsistent with the reproduction of environmental conditions, because it is specified in relevant standards that the air flow rate around the test sample in the test chamber shall not exceed 1.7 m/s for the temperature environment test, so as to prevent the test sample and the surrounding atmosphere from generating unrealistic heat conduction. When no-load, the average wind speed in the test chamber is 0.6 ~ 0.8m/s, not more than 1m / s. when the space and area ratio specified in a) and b) are met, the wind speed of the flow field may increase (50 ~ 100)% and the average maximum wind speed is (1 ~ 1.7) m / s. Meet the requirements specified in the standard. If the volume or windward cross-sectional area of the test piece is increased without limitation during the test, the airflow speed will increase to exceed the maximum wind speed specified in the test standard during the actual test, and the validity of the test results will be doubted.
2) The accuracy indexes of environmental parameters (such as temperature, humidity, salt fog sedimentation rate, etc.) in the working chamber of the climate chamber are all in the no-load state
Once the test piece is placed in the test chamber, the test results will have an impact on the uniformity of environmental parameters in the working chamber of the test chamber. The larger the space occupied by the test piece, the more serious the impact will be. The measured test data show that the temperature difference between the windward and leeward sides in the flow field can reach
3 ~ 8 ℃, and more than 10 ℃ in severe cases. Therefore, the requirements of a) and b) must be met as much as possible to ensure the uniformity of environmental parameters around the tested product.
3) According to the principle of heat conduction, the temperature of the air flow near the tank wall is usually 2 ~ 3 ℃ different from the temperature of the flow field center
At the upper and lower limits, it may reach 5 ℃. The difference between the temperature of the tank wall and the temperature of the flow field near the tank wall is 2 ~ 3 ℃ (depending on the structure and material of the tank wall). The larger the difference between the test temperature and the external atmospheric environment, the larger the above temperature difference. Therefore, the space within the distance of 100 ~ 150 mm from the tank wall is unusable.
2.Selection of Temperature Range
At present, the range of temperature test chamber abroad is generally – 73 ~ + 177 ℃ or – 70 ~ + 180 ℃. Most domestic manufacturers (1)
Generally, it is – 80 ~ + 130 ℃, – 60 ~ + 130 ℃, – 40 ~ + 130 ℃. There are also high temperatures up to 150 ℃. These temperature ranges can generally meet the needs of the temperature test of most domestic military and civil products. Unless there are special needs, such as the products installed near the engine and other heat sources, the upper temperature limit cannot be blindly increased. Because the higher the upper limit temperature, the greater the temperature difference inside and outside the chamber, and the worse the uniformity of the flow field inside the chamber. The smaller the available workroom space. On the other hand, the higher the upper limit temperature, the higher the heat resistance requirements of the insulation materials (such as glass wool) in the interlayer of the chamber wall. The higher the requirements of the sealing property of the chamber, the higher the manufacturing cost of the chamber.
3.Selection of humidity range
The humidity indexes given by the domestic and foreign environmental test chambers are mostly 20 ~ 98% RH or 30 ~ 98% RH. If the humid heat test chamber does not
For dehumidification system, the humidity range is 60-98%. This type of test chamber can only do high humidity test, but its price is much lower. It should be noted that the corresponding temperature range or the lowest dew point temperature should be indicated after the humidity index. Because the relative humidity is directly related to the temperature, for the same absolute moisture content, the higher the temperature, the smaller the relative humidity. For example, the absolute moisture content is 5 g / kg (refers to 5 g of water vapor in 1 kg of dry air). When the temperature is 29 ℃, the relative humidity is 20% RH; when the temperature is 6 ℃, the relative humidity is 90% RH. When the temperature drops below 4 ℃, the relative humidity exceeds 100%, and condensation will occur in the chamber.
To achieve high temperature and high humidity, only spray steam or atomized water droplets into the air of the chamber for humidification. Low temperature and low humidity are relatively difficult to control because the absolute moisture content at this time is very low, sometimes much lower than the absolute moisture content in the atmosphere. It is necessary to dehumidify the air flowing in the chamber to make the air dry. At present, most temperature and humidity chamber at home and abroad adopt the principle of refrigeration and dehumidification, which is to add a group of refrigeration light pipes in the air conditioning room of the chamber. When the wet air passes through the cold pipe, its relative humidity will reach 100% RH. Because the air is saturated and condenses on the light pipe, the extended air becomes drier. Theoretically, this dehumidification method can reach the dew point temperature below zero. However, when the surface temperature of the cold point reaches 0 ℃, the water droplets on the surface of the light pipe will freeze, which will affect the heat exchange on the surface of the light pipe and reduce the dehumidification capacity. In addition, because the chamber cannot be completely sealed, the humid air in the atmosphere will permeate into the chamber and make the dew point temperature rise. On the other hand, the wet air flowing between the light pipes only reaches the saturated state at the moment of contact with the light pipe (cold point) and the water vapor is separated out. Therefore, this dehumidification method is difficult to keep the haze point temperature in the chamber below 0 ℃. The actual minimum cloud point temperature is 5 ~ 7 ℃. The dew point temperature of 5 ℃ is equivalent to the absolute moisture content of 0.0055 g/kg, and the temperature corresponding to the relative humidity of 20% RH is 30 ℃. If the temperature is required to be 20 ℃ and the relative humidity reaches 20% RH, the dew point temperature at this time is – 3 ℃. It is very difficult to dehumidify by refrigeration, and the air drying system must be selected.