High-precision constant temperature and humidity air conditioning technology is relatively mature, but its use in high-level clean rooms is a subject lacking practice and experience. Due to the need to maintain a high level of cleanliness, the ventilation volume is much greater than that of conventional air-conditioned rooms, which makes the high-precision constant temperature and humidity control of clean rooms have its own particularity.
    In order to meet the dual technical indicators of high level and high precision, and maintain good system adjustment quality, it is also necessary to consider economic factors such as reducing initial investment and daily operating costs, which increases the difficulty of design. Taking actual projects as an example, this article introduces the corresponding measures and solutions taken in the comprehensive solution of the above problems from the whole process of design, installation and commissioning, as well as the final practical results.
2. Technical requirements and design scheme of high-precision constant temperature and humidity clean room
  The main technical indicators of the constant temperature and humidity clean room of a research institute of the Ministry of Electronics undertaken by the project (as shown in Figure 1) are as follows:
Cleanliness level: US Federal Standard FS2209EM315 (i.e. the commonly used British Class 100)
Clean room effective space: 215m (L) × 210m (W) × 215m (H)
Indoor temperature: 21±011℃
Indoor humidity: Φ55±5%
Suite parameters: Cleanliness is US Federal Standard FS2209EM615 (i.e. the commonly used British
Class 100000) with a temperature of 25±2℃.
    Due to the limitations of building conditions, the design considers that the clean room airflow organization scheme can only adopt the form of horizontal parallel flow. Since the size of the airflow direction is not large, the cross-sectional wind speed design value is 0128m/s, which is slightly lower than the conventional value, and the ventilation frequency has reached 400AC/h. The air supply wall is covered with high-efficiency air filters, and the entire wall opposite is the return air grille (as shown in Figure 1). Since the air supply ventilation times that meet the constant temperature and humidity accuracy requirements are much smaller than the ventilation volume required for the purification level, a secondary return air solution must be adopted. The air treatment process and air conditioning solution are shown in Figures 2 and 3 as follows:
    The air treatment process is: the new and return air are mixed once and enter the air conditioner, first cooled and dried to the machine dew point K, then fine-tuned to O', and then mixed with the return air twice to point O, and then sent into the room by the clean room air supply fan through the high-efficiency filter in state S. 
    Considering that after the secondary return air, a pressurized fan is set up, which can fully mix and stir the air treated by the air conditioning unit (K state point in Figure 3) and the secondary return air (air state N), there is no need to worry about the uneven supply air temperature and humidity caused by the different temperature and humidity of the air treated by the air conditioning unit and the secondary return air, which will cause the room temperature and humidity accuracy to fail to meet the requirements. Therefore, the capacity of the air conditioning unit can be selected completely according to the needs of the room's heat and humidity load, and there is no need to make any restrictions on its outlet temperature and humidity. 
    Since the heat and humidity load of the clean room equipment and lighting is stable, the staff is fixed, and the suite is maintained at around 25°C all year round, the heat transfer through the enclosure structure is quite small and the fluctuation is not large. Therefore, the indoor heat and humidity load is relatively stable in general, with little change throughout the year. From this perspective, it is a relatively favorable condition to meet the requirements of indoor temperature and humidity accuracy.
3. Control of constant temperature and humidity clean room
    The adjustment quality of automatic control is not only determined by the control device, but also closely related to the link characteristics of the controlled object. Due to the large number of air supply and ventilation in the clean room, the difficulty of control is increased. Based on the comprehensive analysis of the above two aspects, it is concluded that the temperature accuracy of this system is more difficult to guarantee. Therefore, this article mainly focuses on temperature analysis.
1 From the perspective of the controlled object, the clean room, although it is
a dynamic system, a qualitative analysis shows that the rate of change of its energy storage is: 
    Since the heat and humidity load in the clean room is relatively stable, and the air supply volume is much larger than that of the general air conditioning system, under the design conditions, the air supply temperature difference is relatively small, so the air supply temperature is not considered in the monitoring, and the return air temperature is used as the input for regulation.
    Although the air heat storage coefficient is very small, due to the large air supply flow rate in the clean room, its heat capacity is also quite large, which is not conducive to high-precision control. Fortunately, due to the horizontal parallel flow airflow adopted by the high-level clean room, the movement of the air is similar to the piston flow, which makes the air in the clean room in a relatively orderly state. The airflow in the clean room is small because the space of the clean room is small, and the relative distance between the air supply and return air walls is not large. The air stays in the clean room for a short time, less than 10 seconds, so the lag effect of the clean room is not large.
2 From the perspective of the control system, its determination must be based on the characteristics of the
    controlled object. The clean room uses the return air temperature as the input for regulation. From the previous analysis, it can be seen that the load and interference changes in the clean room are relatively flat. Considering the initial investment and operating costs, the control system adopts a single-loop control method.
    The clean room also has humidity requirements, so the air treatment process must include dehumidification. Since this system has excess heat and humidity all year round, the surface cooler is used to cool and dehumidify at the same time. In order to adjust the air parameters sensitively and accurately, an electric heater and an electrode humidifier are set after the surface cooler. Although the heat and humidity load will be offset and energy consumption will increase, the direct evaporative surface cooler, which is more difficult to adjust, can be uncontrolled, reducing the control links. At the same time, the electric heater and the electrode humidifier have small hysteresis, sensitive response, and are easy to control, which greatly improves the control effect.
    The selection of control law should focus on making the control system and the controlled object work well together. The PID parameter
adjustment method is technically mature and is widely used in single-loop air-conditioning control. It can solve problems such as large lag in the air-conditioning system, effectively eliminate static error, and improve the dynamic quality indicators of the system. The law is: 
    The PID regulator outputs a 0-10mA continuous signal. The thyristor element is used to adjust the voltage of the electric heater and the electric humidifier so that its power can be continuously changed. This is also an effective aspect to improve control accuracy. The specific control scheme is shown in Figure 4.
System operation and debugging
1 According to theoretical analysis and design calculations, I purchased the parts and assembled a refrigeration and air-conditioning unit. The rated air volume of the selected direct evaporative surface cooler is 2500m3/h. A wind valve is installed at the outlet of the refrigeration and air-conditioning unit. The air volume passing through the unit can be adjusted, and the cooling capacity can be adjusted accordingly. After the completion of the project site, when the system was adjusted, it was clearly seen from the measured data that when the air valve was opened, the air volume increased and the cooling capacity also increased accordingly. From the perspective of energy saving, the cooling capacity of the refrigeration machine should be reduced as much as possible to make it only slightly larger than the heat and humidity load of the room under the premise of meeting the temperature and humidity parameter requirements of the clean room. Through repeated adjustment tests, it was determined that when the air volume of the refrigeration unit was about 2200m3/h, the temperature and humidity of the clean room could be well maintained within the range of T=21±011℃ and Φ=55±5%.
2 The clean room adopts a fixed value control system. After the hardware is completed, the parameter setting of the PID regulator is the key. In the debugging process, the empirical trial and error method is adopted, and the accuracy and stability of the controlled object are taken as the basic goals of the adjustment. After repeated debugging, the appropriate PID parameters are obtained, among which the proportional band is between 013-014, and the most suitable integral time is in the range of 4-8 seconds. The differential effect is not obvious in this system, and the differential time generally varies between 0-3 seconds.
3 Compared with the 5000m3/h commercial air conditioning unit selected in the original design, the initial investment of the self-designed refrigeration and air conditioning unit is reduced by about 20%, and the operating cost is reduced by 30%, which proves that the design of the unit is successful in application.
V. Conclusions and Suggestions
1. The air supply volume of a small area, high precision, high-level constant temperature and humidity clean room is large, the airflow stays in the room for
a short time, and the heat and humidity load is relatively stable. It is feasible to use a single-loop PID adjustment method. However, reasonable design and repeated adjustment are required to ensure that the various technical parameters specified meet the requirements.
2 The PID regulator determines the control accuracy of the system to a large extent. Under the regulation of this system, the proportional and integral regulation play a key role and can fully meet the control requirements, while the differential regulation plays a lesser role. However, the selection of PID parameters must be analyzed according to the actual situation on site, adjusted repeatedly, and observed for a period of time in order to achieve the best effect.
3 The adjustment of the actuator must be carried out in a continuous manner. The duty cycle adjustment method will bring pulse disturbances to the system and the control accuracy of the system cannot be guaranteed.
4 The air conditioning system should be optimized and designed, and the ratio of primary and secondary return air should be reasonably determined. The method of only processing part of the return air will not affect the control accuracy of the system and reduce the initial investment.