thermostat working principle

The constant humidity machine is composed of refrigeration system, heating system, control system, humidity system, air supply circulation system, and sensor system. 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 humidity machine. Generally speaking, the refrigeration mode is mechanical refrigeration and auxiliary liquid nitrogen refrigeration, mechanical refrigeration adopts steam compression refrigeration, which mainly consists of compressor, condenser, throttling mechanism and evaporator. If the low temperature of our test is up to -55℃, the single-stage refrigeration cannot meet the requirements. Therefore, the refrigeration mode of the constant humidity machine generally adopts the cascade refrigeration. The refrigeration system of the constant humidity machine is composed of two parts, which are called high temperature part and low temperature part respectively. Each part is a relatively independent refrigeration system. Evaporation of the refrigerant in the high temperature part absorbs the heat from the refrigerant in the low temperature part and vaporizes. The evaporation of the cryogenic portion of the refrigerant absorbs heat from the cooled object (the air inside the machine) to obtain the cooling capacity. The high temperature part and the low temperature part are connected by an evaporative condenser, which is both the condenser of the high temperature part and the condenser of the low temperature part.

2. Heating system: heating system is relatively simple compared with refrigeration system. It is mainly composed of high-power resistance wire. Due to the large heating rate required by the test, the power of the heating system is relatively large. Moreover, there is also a heater on the base plate of the testing machine.

3. Control system: the control system is the core of the comprehensive test box, which determines the heating rate, accuracy and other important indicators of the testing machine. At present, most controllers of the testing machine adopt PID control, and a few adopt the combination of PID and fuzzy control. As the control system basically belongs to the category of software, and this part in the use of the process, generally will not appear problems.

4. Humidity system: the temperature system is divided into two subsystems: humidification and dehumidification.

Generally, the humidification method is steam humidification, in which the low-pressure steam is directly injected into the test space to humidify. This humidification method humidifies the ability, speed, humidification control sensitive, especially in the cooling easy to achieve forced humidification.

There are two ways of dehumidification: mechanical dehumidification and drying dehumidification. The dehumidification principle of mechanical cooling dehumidification is to cool the air below the dew point temperature, so that water vapor greater than the saturation moisture content condensation precipitation, thus reducing the humidity. Dryer dehumidification is the use of air pump will be out of the air in the test box, and the dry air into the air, at the same time the wet air into the recyclable dry dry, dry and then into the test box, so repeatedly cycle dehumidification. At present, most comprehensive test boxes adopt the former dehumidification method and the latter dehumidification method, which can make the dew-point temperature reach 0℃. Suitable for special requirements, but more expensive.

5. Sensor system: the sensors are mainly temperature and humidity sensors. Platinum electrogroup and thermocouple are widely used in temperature sensors. There are two methods for measuring humidity: wet and dry bulb thermometer and solid-state electronic sensor. Because the measuring precision of wet and dry ball method is not high, now the humidity machine is gradually replacing the wet and dry ball with a solid sensor to measure the humidity.

6. Air circulation system: the air circulation system is generally composed of centrifugal fans and motors to drive its operation. It provides a circulation of air inside the machine.

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