Solution of automatic control system in clean room

With the development of economy and the improvement of living standard, the requirements of clean room air conditioning system in electronic, pharmaceutical, food, bioengineering, medical and other fields are higher and higher, and the clean technology is also developed It integrates technology, architecture, decoration, water supply and drainage, air purification, HVAC and other aspects of technology According to the standard of the people's Republic of China gbj73-84 code for design of clean workshop, the main technical indexes related to air conditioning system are as follows: 1 Air cleanliness: grade ≥ 0.5 μ m dust particles in air per m3 grade ≥ 0.5 μ m dust particles in air per m3 Grade 100 ≤ 35 × 100 grade 1000 ≤ 35 × 1000 ≤ 250 grade 10000 ≤ 35 × 10000 ≤ 2500 grade 100000 ≤ 35 × 100000 ≤ 25000 2 Temperature and humidity: (1) meet the production requirements; (2) when the production process has no temperature and humidity requirements, the temperature of the clean room is 20-26 ℃, and the humidity is less than 70%; 3) the temperature of the personnel purification room and the living room is 16-28 ℃ 3 Positive pressure of clean room: the clean room must maintain a certain positive pressure The static pressure difference between clean rooms of different grades and between clean areas and non clean areas shall not be less than 4.9pa, and the static pressure difference between clean areas and outdoor areas shall not be less than 9.8pa In addition, there are technical requirements for air volume, wind speed, etc In short, the clean room indicators are very strict, so it is necessary to control it 1.2 the significance of air conditioning automatic control in clean room in modern commercial and industrial buildings, air conditioning system equipment is more, automatic management is an important guarantee for its safe work and good operation At the same time, the energy consumption of air conditioning generally accounts for more than 40% of the total energy consumption, so air conditioning energy saving is an important means of energy saving This is especially true for clean rooms The use of air-conditioning automatic control products will produce a series of advantages as follows: first, because the air-conditioning system realizes automatic monitoring, it can make the system run more safely, and Zui greatly improves the comfort level For the clean room, it becomes the necessary condition to ensure the production In addition, due to the realization of automatic monitoring, it can meet the requirements of safe operation of the system and ensure various technical indicators of the system, while Zui can achieve energy-saving control to a large extent, in line with the increasingly prominent energy-saving and environmental protection needs Relevant data show that 10% of the annual operation cost of air conditioning system equipment can be saved by using the air conditioning automatic control system A more optimistic estimate is 15% - 30% However, the investment of air-conditioning automatic control products accounts for less than 0.5% of the total investment of the whole building or workshop, and the investment recovery time is short At the same time, the realization of automatic control and management of equipment can reduce personnel maintenance, save personnel expenses, improve the comprehensive management level, reduce the occurrence of accidents and equipment damage, thus bringing potential benefits 1.3 the control system of clean room air conditioning function simple Excel 20 Chinese version controller is a member of advanced excel 5000 controller family of Honeywell company in the United States It is especially suitable for the air conditioning control of clean rooms such as operating rooms and clean workshops According to the national standards such as code for construction acceptance of clean rooms, code for design of clean workshops, code for design of ventilation and air conditioning, and taking into account the inherent of the above systems, we use excel 20 as the core, a relatively complete automatic control system of clean room air conditioner is built, which has the following characteristics: 1 Constant temperature and humidity proportional integral control 2 Indoor remote start and stop of air conditioner 3 Indoor temperature setting 4 Key fault (fire) alarm and interlock 5 Non key fault (filter plugging / air supply overheating) alarm and interlock 6 Preventing air supply condensation in summer / antifreezing in winter 7 Startup sequence and interlocking, etc 8 Custom start stop time program, etc Next, its composition, working principle and so on are introduced in detail 2 The automatic control system of air conditioner in clean room is composed of analog controller (also called analog control instrument), sensor, actuator and controlled object See Figure 2-1 for the automatic control system of analog instrument This is the single loop control system diagram Because of its mature theory, simple structure, less investment, easy to adjust and other factors, it has been widely used in air conditioning, cold and heat sources, water supply and drainage systems in the past Generally, the analog controller is electrical or electronic, only hardware part, without software support Therefore, the process of adjustment and operation is relatively simple Its composition is generally a single loop control system, which can only be applied to small-scale air conditioning system From the perspective of development trend, it has been adopted less, so no further explanation will be given here 2.2 with the development of computer technology, control technology, communication technology and image technology, the application of microcomputer control technology in automatic control of refrigeration and air conditioning is becoming more and more common The traditional control system can make full use of the powerful arithmetic operation, logic operation, memory and other functions of the computer after the introduction of the microcomputer, and use the microcomputer instruction system to compile the software that conforms to the control law When the computer executes these programs, it can realize the control and management of the controlled parameters, such as data acquisition and data processing The control process of computer can be divided into three steps: real-time data acquisition, real-time decision-making and real-time control These three steps will make the whole system control and adjust according to the given law At the same time, monitoring, over limit alarm and protection are also carried out for the controlled variables, equipment operation status and faults, and historical data is recorded It should be said that the computer control in the control functions such as accuracy, real-time, reliability and so on is incomparable to analog control What's more, the enhancement of management functions (such as alarm management, history record, etc.) brought by the introduction of computer is impossible for analog controller to realize at all Therefore, in recent years, in the application of automatic control of refrigeration and air conditioning, especially in the automatic control of large and medium-sized air conditioning systems, computer control has taken the leading position 2.2.1 the so-called direct digital control refers to the control mode of directly driving the actuator by inputting the output signal of the sensor or transmitter or the output signal of the system directly into the microcomputer without the aid of the analog instrument on the basis of the microprocessor, which is called DDC (direct digital) for short This kind of computer is called direct digital controller (DDC controller for short) The CPU in DDC controller runs very fast, and it has many input and output ports (I / 0) configured Therefore, it can control multiple loops at the same time, which is equivalent to the characteristics of high analog price ratio 2.2.2 total distributed system is abbreviated as TDS Compared with the traditional computer control method in the past, its control function is as decentralized as possible, and its management function is as centralized as possible The basic structure is shown in Figure 2-3 It is connected with communication channel by central station, substation, field sensor The substation is the DDC controller with microprocessor as the core It is distributed in the field of the whole system controlled equipment, and directly connected with the field sensors and actuators, so as to realize the detection and control of the field equipment The central station realizes centralized monitoring and management functions, such as centralized monitoring, centralized start and stop control, centralized parameter modification, alarm and record processing, etc It can be seen that the centralized management function of the distributed control system is completed by the central station, while the control and regulation function is completed by the sub station, namely DDC controller 2 Realization of air conditioning automatic control system in clean room 3.1 air filter is used for air purification in general clean room space system In general, after the installation of the primary filter and the intermediate filter, the air cleanliness can reach 10000 levels For the ultra clean room, filters should also be installed In this way, the air cleanliness can reach a higher level (such as 100 or even higher) When the filter is used for a long time, the dust on the filter material will slowly increase, which will increase the air flow resistance and affect the operation of the whole air conditioning system Therefore, the air flow resistance of the filter should be automatically detected and alarmed in engineering The differential pressure method is usually used to measure the differential pressure PD before and after the filter, and display the differential pressure signal and alarm according to the set differential pressure limit, so as to clean or replace it in time 4.2 temperature control 3.2.1 primary heating control air primary heating, also known as pre heating, is used to heat the fresh air or the mixture of the heating fresh air and the primary return air The primary heating is generally only used in cold winter areas to prevent fresh air and primary return air from reaching saturation after mixing, resulting in water mist or ice The primary heating is also used in the super clean air conditioning system where the primary mixing is not allowed to change When steam or hot water is used for heating, the temperature control is generally realized by controlling the opening of the regulating valve of steam or hot water; when electric heating is adopted, the temperature control is realized by controlling its heating electric power through the thyristor electric controller The principle is shown in Figure 3-2 3.2.2 the secondary heating of control air for secondary heating and tertiary heating is usually set behind the surface cooler or the mixing section of secondary return air The purpose of secondary heating is to ensure the temperature of air supply or air conditioning room under the condition of relative humidity Its control mode is basically the same as that of primary heating Three times of heating is refined heating, which is usually a heating section set for temperature fine-tuning in high-precision temperature control The control shall be carried out according to the above principles 3.3 humidity control 3.3.1 in the humidification treatment and control of clean room air conditioning project, the humidification operation is generally carried out in winter or in the transitional season when the air is dry There are many ways to humidify the air Generally, the switch control or power regulation of steam humidifier and electric humidifier are used When the steam is humidified, according to the humidity control requirements, it can be realized by the position control of the solenoid valve or the continuous adjustment of the two-way regulating valve 3.3.2 dehumidification (drying) treatment and control air cooling and drying treatment are usually completed by surface cooler There are two processes of surface cooling, i.e equal humidity cooling When the surface cooler is used for humidity control, it is realized by regulating the flow of refrigerant (such as chilled water) of the surface cooler When the humidity is higher than the required value, the flow can be increased by increasing the opening of the cold water valve to achieve dehumidification (i.e drying); otherwise, the flow can be reduced to achieve humidification It should be noted that due to the physical properties of air, the humidity control is relatively complex and there are many methods Moreover, the two parameters of air temperature and humidity affect each other in the process of regulation For example, if the indoor temperature increases due to some reasons, the change of saturated partial pressure of water vapor in the air will reduce the relative humidity when the moisture content is constant Therefore, when one of the parameters is adjusted, the other will change For example, in summer, the surface cooler is used for dehumidification regulation When the cooling water valve is opened, the humidity is restored to normal and the temperature is reduced Therefore, this feature should be fully considered in process design and automatic control scheme design 3.4 according to the national standard of positive pressure control, clean rooms of different levels should