In the complex flow of people in large shopping malls and hospitals, in order to ensure the good quality of indoor air, it is very important to supply fresh air indoors. With the popularization of the application of central fresh air system, in order to strengthen the practicability of fresh air system, it is very important to manage the fresh air system through automatic control. This paper expounds the basic control theory of fresh air system and the principle of automatic control of central fresh air system

Xia, Cui

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Urban Transportation & Construction (E-Journal)

Urban Transportation & Construction- 36 -Research on Automatic Control of Central Fresh Air SystemCui XiaTaishan University, Taian 271000, Shandong, China.Abstract: In the complex flow of people in large shopping malls and hospitals, in order to ensure the good quality of indoorair, it is very important to supply fresh air indoors. With the popularization of the application of central fresh air system, inorder to strengthen the practicability of fresh air system, it is very important to manage the fresh air system through automaticcontrol. This paper expounds the basic control theory of fresh air system and the principle of automatic control of centralfresh air system.Keywords: Central Air Conditioning; Fresh Air; Auto-ControlIntroductionWith the continuous growth of China’s economy this year and the continuous improvement of the quality of life ofChinese residents, people also put forward higher requirements for buildings and pay more and more attention to the airquality in buildings. In large public buildings such as hospitals and large-scale commerce and trade, the interior of thebuilding has the characteristics of people flow and personnel density. The air quality has a great impact on the internalenvironment and the use feeling of the building. In the current fresh air system of central air conditioning, most of them adoptautomatic control to ensure the effect of internal ventilation and internal air quality. With the acceleration of urbanization inChina in recent years, large buildings are emerging, and the automatic control of fresh air system has also developed rapidly[1].1. Indoor air quality and fresh air volume1.1 Indoor air qualityIn the closed environment of a building, the indoor air will be affected by all aspects of the building. Especially in theair-conditioned room, because the air is an internal circulation mode, it will cause a large number of low concentrationpollutants in the indoor air. Affected by these pollutants, most people living in closed buildings will feel stimulated oruncomfortable. In relation to air quality, Danish scientists first put forward the definition of air quality in 1989. However, atthis time, the research on air quality is subjective, and the definition of air quality is mostly people's subjective perception ofwhether the air quality is good. This method has no reference value. After that, in the study of air quality, people analyze theconcentration of indoor pollutants and the content of different components in the air, so as to gradually realize the goal ofquantifying air quality.1.2 Standard for fresh air volume of building ventilationWith the development of architecture, in the 1970s, people gradually realized the impact of fresh air on the internalcontrol quality of buildings, which is not only limited to the impact of people’s feelings, but also has a negative impact onpeople's health. With the development of social construction, people are more and more resistant to the building mode at theexpense of health and comfort. Under this background, indoor air quality standards have been formulated. Based on this, theEuropean Union puts forward the calculation method of fresh air volume. China has also formulated clear requirements forVolume 7 Issue 4 - 37 -this. In the current design of central air conditioning, it is required to meet three basic conditions: first, central airconditioning can dilute the pollutants brought by people's activities. Secondly, the indoor exhaust air volume shall besupplemented to ensure that the indoor is in a positive pressure state. Finally, the indoor fresh air volume shall be greater than10% of the air supply volume of the system. The specific marking requirements are shown in the table below:Table 1-1. Indoor air quality standardSerialnumberReference category Parameter Unit Standard value Note1Physical propertyTemperature ℃ 22–28 Summer airconditioning2 16–24 Winter heating3 Relative humidity % 40–80 Summer airconditioning4 30–60 Winter heating5 Air velocity m/s 0.3 Summer airconditioning6 0.2 Winter heating7Chemical propertyFresh air volume m3/(h • p) 30a8 Sulfur dioxide mg/m3 0.5 1h mean value9 Nitric oxide mg/m3 0.24 1h mean value10 Carbon monoxide mg/m3 10 1h mean value11 Carbon dioxide % 0.1 Daily meanvalue12 Ammonia mg/m3 0.2 1h mean value13 Ozone mg/m3 0.16 1h mean value14 Formaldehyde mg/m3 0.1 1h mean value15 Benzene mg/m3 0.11 1h mean value16 Toluene mg/m3 0.2 1h mean value17 Xylene mg/m3 0.2 1h mean value18 Benzopyrene ng/m3 1 Daily meanvalue19 Inhalableparticulate mattermg/m3 0.15 Daily meanvalue20 Volatile organicmattermg/m3 0.6 8h mean value21 Biological property Total colony cfu/m3 2500 According tothe instrument2. Principle introduction of central fresh air system2.1 Composition principle of fresh air systemIn the whole fresh air unit, its components mainly include fresh air duct, air supply valve, air supply fan, etc. In theexhaust system, the main components are return air pipe, return air valve, exhaust fan, etc. There are two filter sections in thewhole fresh air unit, namely, primary effect filter section and medium effect filter section. Besides the first time, there are hotspots at the fresh air and exhaust air. In the central air-conditioning system, the transmission mode of fresh air generallyUrban Transportation & Construction- 38 -adopts the form of replacement. After the external fresh air passes through the initial effect and medium effect, the filtered airis transmitted to the new air outlet, and the fresh air is transmitted to each room through the new air outlet. At the same time,there will also be sealing in each room, which corresponds to the new air outlet, so that the air in the whole room forms acycle, takes away the indoor waste gas, and allows the indoor air to be replaced regularly to ensure the quality of indoor air.In the spatial layout, the fresh air outlet of the fresh air system is generally arranged at the ceiling, and the exhaust outlet isinstalled on the floor or lower position of the room to ensure that the indoor air can form a relatively stable flow state afterthe fresh air enters, so that the dirty air in a room can be discharged through the return air outlet. When cooling or heating theroom in summer, the air will be cooled or heated in the fan coil at the exhaust outlet to adjust the indoor temperature. At thesame time, in order to achieve the effect of energy saving, the general fresh air and exhaust system will be controlled byfrequency conversion to ensure the ventilation effect in each room and reduce the power consumption of the whole system[2].Figure 2-1. Structure diagram of fresh air system.2.2Automatic control theory of fresh air systemWith the development of computer technology, automatic control system has played a great value in many fields. Underthe background that the current building is developing towards intelligence, the control of central air conditioning is alsodeveloping towards intelligence. The operation mode of traditional buildings is replaced by intelligent systems, which greatlyimproves the regulation of office and living environment in buildings and the convenience of people's life. In the automaticcontrol of central air conditioning system, there are many control modes, such as single chip microcomputer (PIC) control,PLC control, analog instrument control and direct digital control (DDC). In the current specific application, analoginstrument control mode is less used. At the same time, using DDC and DCS for control also has the problem of excessivefraud. Most of the control through single chip microcomputer will integrate circuits, so there will be the problem ofinsufficient control function and difficult expansion. In the current air conditioning automatic control, PLC is the mostcommonly used control mode, and the stability of operation can also be guaranteed.PLC has the advantage that the program can be written freely, and among the above control modes, this mode has thestrongest anti-interference ability. But the problem with this is that the operation speed of PLC is also the slowest. Thecommunication function of PLC is good, and it is very convenient to carry out or expand in the process of use. Therefore, thecurrent automatic control of central air conditioning is the main control method. The specific control composition is shown inthe figure below.Figure 2-2. Basic composition of PLC.Volume 7 Issue 4 - 39 -However, nowadays, the building scale of Suihe building is increasing, and the complexity of the building is alsoincreasing. In some special areas, higher requirements are put forward for air quality. For example, electronic workshop,operating room, etc. Therefore, it puts forward higher requirements for the technical level of automatic control. In order tomeet the higher computational requirements of the current central air conditioning system, on the basis of the traditional PLCcontroller, it will also be combined with computer technology, communication technology and microelectronics technology.3. Correlation control between indoor pollutants and fresh air volumeIn the traditional ventilation control, CO2 concentration is generally used as the control reference standard. That is, inthe calculation of ventilation volume, it will be adjusted and calculated according to CO2 concentration. When the sensordetects that the CO2 concentration in the air exceeds the standard, the system will control the fresh air valve and adjust thefresh air fan at the same time, so as to change the input of indoor fresh air. In the initial design, the main reason for takingthis as an index is that in people’s activities, the most significant index is the CO2 concentration in the air, so it is regarded asthe only standard for air quality identification in the central air conditioning system. However, the concentration of CO2 inthe air cannot be used as the only standard. In the whole building, not only the changes of air quality brought by humanactivities, but also furniture and decorative materials will bring various air pollutants, which will affect the air quality. Theconcentration of these organics exceeds the standard, even greater than that of CO2, and greater harm to human body.Therefore, it is unscientific to take CO2 concentration as the only standard in control. However, there are many reasons forindoor air pollution, and the concentration of some pollutants is very low. The complex problem of air components makes itimpossible to detect all components. After all, which one do you choose? Therefore, in the quality control measurement, airpollutants will be measured according to different scenarios. By combining the grey clustering analysis of different buildings,taking the representative pollutants in different types of buildings as the key reference standard, the correlation controlbetween air pollutants and fresh air volume is established. There are mainly the following different site types [3].3.1 Underground shopping mallThe underground supermarket has the characteristics of high personnel density and strong mobility. When people passthrough underground shopping malls, it will not only increase the concentration of carbon dioxide, but also aggravate theconcentration of dust in the air. Underground supermarket shops need to be decorated, so there will be a large number oforganic pollutants such as formaldehyde brought by decoration materials. However, with the improvement of the level ofurban architecture, it has brought more and more serious space problems, resulting in the continuous improvement of thenumber and scale of underground shopping malls. Underground shopping malls have the characteristics of high closeness.They can’t open windows for ventilation like aboveground buildings. If they don’t change air in time, it will inevitably leadto a sharp decline in air quality. However, if the mechanical air supply and exhaust system is adopted, it will bring moreenergy consumption problems. In order to calculate the grey correlation degree of pollutants affecting air quality in the air,classifying and detecting the pollutants, and correlating the fresh air system are very key. In the relevant gray correlationstatistical research, carbon monoxide, carbon dioxide and radon can be classified into one category in the air pollutants ofunderground shopping malls. Inhalable particulate matter and TVOC can be classified into one category, and organic mattersuch as formaldehyde can be classified into one category.3.2 Teaching buildingsIn the attention to air quality, teaching buildings and office buildings are the key objects. In order to ensure thepractical effect of air conditioning, the early classroom design will adopt a closed design method. Therefore, it will lead tobad smell and poor air quality in the classroom in summer or winter. Classroom is the main activity place for students. Airquality has a great impact on students’ study, life and health. According to relevant statistics, students spend more than halfof their time in the classroom. Improving classroom air quality is of great practical significance to students’ study and life. Inthe study of gray correlation statistics related to teaching buildings, carbon dioxide, formaldehyde and radon can be classifiedas one of the air pollutants in classrooms. There is no necessary correlation between carbon monoxide, inhalable particulateUrban Transportation & Construction- 40 -matter and TVOC.3.3 Office buildingThe study of indoor pollutants in office buildings is the earliest. The office building has the characteristics of stablepersonnel and high density. At the same time, there are a large number of electronic equipment and office furniture in theoffice building, which will have a certain impact on air quality. Combined with the carbon dioxide brought by personnelbreathing, the air composition in the office is complex. In the gray correlation statistical study of office buildings, carbonmonoxide, formaldehyde and inhalable particulate matter can be classified into one category. Carbon dioxide,microorganisms and sulfur dioxide can be classified into one category. Nitrogen dioxide is classified separately.4. ConclusionIn the automatic control of the central fresh air system, the detection of the inhalable pollutants in each room, and thenthe adjustment of fresh air volume after computer calculation are the key to the automatic control of the fresh air system. Thekey point is to monitor the concentration of pollutants in the air. However, air has the characteristics of complex components.This paper expounds the correlation degree of air pollutants based on different public scenes, and analyzes and studies thewhole central fresh air system in combination with the controllable principle of fresh air system, in order to provide referencevalue for relevant technicians.References[1] Xu Y. Connection and difference between central air conditioning and fresh air system. Popular Utilization of Electricity2017; 32(8): 46.[2] Han G. Quality control method of central air conditioning fresh air system. Construction Materials & Decoration 2016;(53): 186.[3] Li L. Energy saving problem of new wind system installation of center air-condition in large marketplace. ShanxiArchitecture 2009; 35(25): 207–208.

Research on Automatic Control of Central Fresh Air System

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Research on Automatic Control of Central Fresh Air System

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