Working principle of constant temperature and humidity machine:
The constant temperature and humidity machine is composed of a refrigeration system, a heating system, a control system, a humidity system, an air circulation system, and a sensor system. The above-mentioned systems are divided into two aspects: electrical and mechanical refrigeration. The working principle and working process of several main systems are described below.
1. Refrigeration system: The refrigeration system is one of the key parts of the constant temperature and humidity machine. Generally speaking, the refrigeration methods are mechanical refrigeration and auxiliary liquid nitrogen refrigeration. Mechanical refrigeration adopts vapor compression refrigeration, which are mainly composed of compressor, condenser, throttling mechanism and evaporator. If the temperature and low temperature of our test are to reach -55℃, single-stage refrigeration is difficult to meet the requirements, so the refrigeration method of constant temperature and humidity machine generally adopts cascade refrigeration. The refrigeration system of the constant temperature and humidity machine consists of two parts, called the high temperature part and the low temperature part, each of which is a relatively independent refrigeration system. The evaporation of the refrigerant in the high temperature part absorbs heat from the refrigerant in the low temperature part and vaporizes; the evaporation of the refrigerant in the low temperature part absorbs heat from the object to be cooled (the air in the testing machine) to obtain cold energy. The high temperature part and the low temperature part are connected by an evaporative condenser, which is not only the condenser of the high temperature part, but also the condenser of the low temperature part.
2. Heating system: Compared with the refrigeration system, the heating system is relatively simple. It is mainly composed of high-power resistance wire. Because the heating rate required by the test is relatively large, the heating system power is relatively large, and a heater is also provided on the bottom plate of the testing machine.
3. Control system: The control system is the core of the comprehensive test box, which determines the temperature rise rate and accuracy of the test machine and other important indicators. Most of the controllers of the testing machine use PID control, and a small number of them use a combination of PID and fuzzy control. Because the control system basically belongs to the category of software, and this part is generally not problematic during use.
4. Humidity system: The temperature system is divided into two subsystems: humidification and dehumidification.
The humidification method generally adopts steam humidification, that is, low-pressure steam is directly injected into the test space for humidification. This humidification method has high humidification capacity, high speed, and sensitive humidification control, especially when the temperature is lowered, it is easy to realize forced humidification. There are two dehumidification methods: mechanical refrigeration dehumidification and dry dehumidification. The dehumidification principle of mechanical refrigeration dehumidification is to cool the air below the dew point temperature to condense and precipitate the water vapor that is greater than the saturated moisture content, thus reducing the humidity. Dryer dehumidification is to use an air pump to extract the air in the test box and inject dry air. At the same time, the humid air is sent to the recyclable dryer for drying, and then sent to the test box after drying. Repeated cycles for dehumidification . At present, most comprehensive test chambers use the former dehumidification method, and the latter dehumidification method, which can make the dew point temperature below 0℃. Suitable for occasions with special requirements, but the cost is more expensive.
5. Sensor system: The sensors are mainly temperature and humidity sensors. The most commonly used temperature sensors are platinum group and thermocouple. There are two methods for measuring humidity: dry and wet bulb thermometer method and solid-state electronic sensor direct measurement method. Due to the low measurement accuracy of the wet and dry bulb method, the current constant temperature and humidity chamber is gradually replacing the wet and dry bulb with solid-state sensors for humidity measurement.
6. Air circulation system: The air circulation system generally consists of a centrifugal fan and a motor that drives its operation. It provides circulation of air in the testing machine.