Environmental conditions are the key factors that affect the quality and reliability of equipment. For equipment used in the environment where the ambient air temperature changes rapidly, the impact of temperature shock environment is a factor that must be considered. This environment brings a variety of typical environmental effects to equipment, such as deformation or fracture of parts, insulation protection failure, clamping or relaxation of moving parts, changes of electrical and electronic components electronic or mechanical failure caused by rapid condensation or frost. Whether the equipment can work normally under the temperature shock environment directly reflects the adaptability of the equipment to this environment.
According to method 503.4 (temperature shock test) of American military standard ML-STD-810 F, equipment that may be deployed in the environment with rapid air temperature change shall undergo temperature shock test. The test chamber must have the ability to re stabilize the test conditions within 5 min after the test piece is changed. The test conversion time is 1 min, and the air used around the test piece shall not exceed 1.7 m/s.
How to build temperature shock test equipment? What test mode is used for the equipment? What kind of cooling method is used for the equipment? How to determine the cooling capacity and heating capacity of the equipment is the main problem to be solved before the construction of the equipment.
1.Determination of test scheme
The structure of temperature shock test equipment usually has three types: single chamber type, vertical lifting type and horizontal two chamber type. Compared with the above three forms, the single chamber type has poor feasibility and few practical applications due to its large cooling and heating capacity; The vertical lifting type avoids the influence of the external environment through the conversion of internal lifting. However, as the lifting device itself is a thermal load, it consumes cold or heat, so this method is generally applicable to small-scale test chambers. For medium and large-scale test chambers, this method is not applicable because the lifting device is too heavy; The horizontal two chamber type reduces the load of the chamber through the mutual conversion between the two chambers, thus reducing the cooling capacity and heating capacity of the equipment. However, the horizontal conversion device is required and will be affected by the external environment. Therefore, the selection of test method should be analyzed according to the specific situation. For small equipment, the vertical lifting method can save one chamber, which can save costs; For medium and large-scale test equipment, as long as the scheme is reasonable and feasible and can meet the requirements of national and military standards, the horizontal two chamber test scheme is a better choice.
2.Equipment composition and structure
2.1 Equipment composition
The temperature shock test equipment is composed of low-temperature chamber, high-temperature chamber, refrigeration system, heating system, control system, conversion device and other equipment. The low-temperature chamber provides a low-temperature platform for the temperature shock test, and can also conduct low-temperature test independently; The high-temperature chamber provides a high-temperature platform for the temperature shock test, and can also conduct high-temperature test; The refrigeration system provides a low-temperature environment for the low-temperature chamber; The heating system provides a high temperature environment for the high temperature chamber; The control system completes the control and measurement of equipment and test process; The conversion device is used to convert the test piece during the test.
2.2 Equipment structure
In order to meet the requirements of temperature shock test, the chamber structure and air flow mode need to be carefully designed. The structure of the low-temperature chamber shall meet the requirements of rapid cooling during the process of equipment from normal temperature to required low temperature and temperature shock, and ensure the uniformity of air flow and temperature in the chamber; The structure of the high-temperature chamber shall meet the requirements for convenient heating of equipment from normal temperature to required high temperature and rapid heating in the process of temperature shock, and ensure the uniformity of air flow and temperature in the chamber.
Air distribution mode is an important link in equipment design. The commonly used air supply modes include upper side air supply and lower side air return and full orifice top air supply and lower side air return. Because the overall orifice air supply mode has the advantages of fast and good mixing of air flow, uniform and parallel diffusion of air flow, and rapid attenuation of temperature difference and wind speed, the temperature and air velocity distribution in the working area are more uniform. Therefore, the air circulation mode of the low-temperature chamber and the high-temperature chamber adopts the return air mode under the full orifice air supply. The air circulation process is: the air flow in the chamber sucked by the fan is mixed with the cold air generated by the refrigeration system or the hot air generated by the heating system, and then enters the pressure stabilization layer along the circulation air duct to make the air flow even and pressure, and then is sent into the chamber.
The low-temperature chamber and the high-temperature chamber adopt steel frame enclosure structure and are equipped with insulation layer. A full-scale orifice plate is installed at a certain height from the top wall. The full-scale orifice plate and the top wall form a stable pressure layer. The front end of the chamber is the gate, and the rear end of the chamber is provided with a circulating air duct and a circulating fan.
2.3 Conversion device
In order to realize the function of rapid conversion, the conversion device adopts the rail type conversion mode, which is composed of rail cars and specimen cars. As the support of the test piece, the test piece truck is transferred and tested between two chambers together with the test piece; The transfer rail car is used to quickly transfer the test piece and the test piece car from one chamber to another. The lower wheel is set to roll on the ground track, and the upper track is set to facilitate the docking with the tracks in the two chambers and the movement of the test piece car.
3.Determination of refrigeration and heating process
At present, the refrigeration mode of low-temperature chamber is usually steam compressor refrigeration or air refrigeration. Air refrigeration has the following advantages compared with steam compressor refrigeration: high coefficient of refrigeration at lower temperature, easy to obtain lower temperature and wide temperature regulation range; It is not sensitive to equipment leakage. Small air leakage has little impact on refrigeration performance, and the refrigeration performance is relatively stable; The refrigerant is air, without any harm to the environment: reliable operation, simple operation, convenient maintenance and low operation cost. For large-scale temperature shock test equipment, the temperature change speed is required to be fast, and air cooling is a better choice.
The positive pressure boosting refrigeration mode uses the turbo expander for secondary compression. Increase the expansion ratio of the turbine Increase the temperature drop of the turbine and improve the cooling capacity. As the positive pressure rising refrigeration mode has the advantages of high coefficient of refrigeration, good regulation performance, stable refrigeration performance, stable startup and shutdown and regulation process, less installed power, operating energy consumption and equipment investment, the system adopts the positive pressure rising refrigeration system. The air refrigeration system is divided into two parts: air source and refrigeration. The air source part includes air compressor unit, aftercooler, drying tower, water separator, etc; The refrigeration part includes turbine unit, recooler, water cooler, filter, etc.
The high temperature chamber is heated by electric heater; Silicon controlled regulator is used to regulate and control the electric heater to realize stepless adjustment of heating amount.
4.Conclusion
Through the air cooling mode and the track type conversion device, the index requirements of rapid recovery of temperature within 5 min and rapid conversion of test pieces within 1 min between two chambers are realized. The cooling capacity and heating capacity of the equipment are reduced by adopting the two chamber type scheme. The successful development of the two chamber type temperature shock test equipment has certain reference significance for the development of similar large and medium-sized temperature shock test equipment and the temperature shock test.