Environmental conditions are a key factor affecting device quality and reliability. For equipment used in environments where the ambient air temperature changes rapidly, the impact of the temperature shock environment must be considered. This environment brings a variety of typical environmental effects to equipment, such as deformation or fracture of parts, failure of insulation protection, pinching or loosening of moving parts, changes in electrical and electronic components, and electrical or mechanical failure caused by rapid condensation or frost. Whether the equipment can work normally in the temperature shock environment directly reflects the adaptability of the equipment to this environment.

According to Method 503.4, (Temperature Shock Test) of U.S. military standard ML-STD-810 F, equipment that may be deployed in environments with rapid changes in air temperature requires a temperature shock test. The environmental test chamber must have the ability to re-stabilize the test conditions within 5 minutes after changing the test piece. The test transition time is 1 minute, and the air used around the test piece shall not exceed 1.7m/s.

Temperature Shock Test Chambers-01

How to set up the temperature shock test equipment? What test mode does the device use? What cooling method is used for the equipment? How 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 the test plan

The structure of temperature shock test equipment usually has three types: single-chamber type, vertical lifting type, and horizontal double-chamber type. Compared with the above three types, the single-chamber type has poor feasibility and less practical application due to its large cooling and heating capacity; The vertical lifting type is converted by internal lifting, avoiding the influence of the external environment. However, since the lifting device itself is a heat load and consumes cooling or heat, this method is generally suitable for small laboratories.

For large and medium-sized test chambers, this method is not applicable because the lifting device is too heavy; by converting the two chambers mutually, the horizontal double-chamber type reduces the load on the chamber, thereby reducing cooling and heating capacity.

However, a horizontal translation device is required and can be affected by the external environment. Therefore, the choice of test method should be analyzed according to the specific situation. For small equipment, the vertical hoisting method can save one chamber and save costs; for medium and large test equipment, as long as the scheme is reasonable and feasible. The control box connects and controls the positive assembly.

2. Equipment composition and structure

The equipment composition and structure of the temperature shock test chamber include the following three points:

1) Equipment Composition

The temperature shock test equipment is composed of a low-temperature test chamber, a high-temperature test chamber, a refrigeration system, a heating system, a control system, a conversion device, and other equipment. The low-temperature test chamber provides a low-temperature platform for the temperature shock test, and can also conduct low-temperature tests independently.

The high-temperature chamber provides a high-temperature platform for the temperature shock test, and can also conduct high-temperature tests; 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 the equipment and test process; the conversion device converts the specimen during testing.

Temperature Test Chambers

2) Equipment structure

To meet the requirements of the temperature shock test, it is necessary to carefully design the structure of the test chamber and the way of airflow. The structure of the low-temperature test chamber should meet the requirements of rapid cooling of the equipment from normal temperature to the required low temperature and the process of temperature shock, and ensure the uniformity of airflow and temperature in the chamber; the structure of the high-temperature test chamber should meet the requirements of the equipment from normal temperature to the required high temperature.

The convenience and the requirement of rapid temperature rise during the temperature shock process, and ensure the uniformity of airflow and temperature in the box. The air distribution method is an important link in the equipment design.

Commonly used air supply methods include upper-side air supply and lower-side return air and full orifice top air supply and lower-side return air. Because the whole orifice air supply method has the advantages of fast air mixing, good mixing, uniform, and parallel airflow diffusion, rapid temperature difference and wind speed attenuation, etc., it makes the temperature and wind speed distribution in the working area more uniform.

Therefore, the air circulation in the low-temperature room and the high-temperature room is uniform Adopt the return air mode under the full hole plate air supply. The air circulation process is: the airflow in the cavity sucked by the fan is mixed with the cold air generated by the refrigeration system or the hot air generated by the heating system. Install full-size orifice plates at a certain height from the top wall. The full-size orifice forms a stable pressure layer with the top wall. At the front end of the chamber is a gate, and at the back end is a circulating air duct and circulating fan.

3) Converter

To realize the fast conversion function, the conversion device adopts the rail-type conversion method, which is composed of a rail car and a test-piece car. The specimen trolley is used as the support of the specimen, and is transferred and tested together with the model between the two test chambers; the transfer rail car is used to quickly transfer the specimen and the specimen trolley from one chamber to another. The lower wheel rolls on the ground track, while the upper track facilitates docking with the two-chamber track and movement of the test piece.

Temperature Shock Test Chambers-02


3. Determination of refrigeration and heating process

At present, the cooling method of the low-temperature box is usually steam compressor cooling or air cooling. Compared with vapor compressor refrigeration, air refrigeration has the following advantages: high low-temperature refrigeration coefficient, easy access to low temperature, wide temperature adjustment range; insensitivity to equipment leakage. The air leakage is small, which has little effect on the refrigeration performance, and the refrigeration performance is relatively stable; the refrigerant is air, which has no harm to the environment: reliable operation, simple operation, convenient maintenance, and low operating cost.

For large-scale temperature shock test equipment, it is required that the temperature change speed is fast, and air cooling is a better choice. The positive pressure supercharging refrigeration method uses a turbo expander for secondary compression. Increasing the expansion ratio of the turbine increases the temperature drop of the turbine and improves the cooling capacity. Since the positive boost refrigeration method has the advantages of a high refrigeration coefficient, good regulation performance, stable refrigeration performance, smooth start-stop, and regulation process, installed power, operating energy consumption, and less investment in equipment, the system adopts a positive boost refrigeration system.

The air refrigeration system is divided into two parts: air source and refrigeration. The air source part includes an air compressor unit, aftercooler, drying tower, water separator, etc.; the refrigeration part includes a turbine unit, sub-cooler, water cooler, filter, etc. The high-temperature box is heated by an electric heater; the electric heater is adjusted and controlled by a thyristor regulator to realize the step-less adjustment of the heating amount.

4. Conclusion

Through the air-cooling method and the track-type switching device, the index requirements of rapid recovery of temperature within 5 minutes and rapid switching of specimens between the two chambers within 1 minute are realized. The dual-chamber scheme is adopted to reduce the cooling and heating capacity of the equipment. The successful development of the two-chamber temperature shock test device has certain reference significance for the development of similarly large and medium-sized temperature shock test devices and temperature shock tests.