The Application of Fresh Air Systems in Clean Factories
Introduction
With the development of the industrial society, clean air conditioning is applied in a wide range of social fields, including the electronics industry, aviation industry, pharmaceutical industry, medical care, textile industry, etc. And fresh air plays a crucial role in clean air conditioning. In electronic clean factories, the cleanliness levels basically range from Class 300,000 to Class 100, or even higher. As a result, the air supply and ventilation rates of air conditioners are required to vary from dozens to hundreds of times, making it an industry with high energy consumption. Through specific designs in recent years, it has been found that centralized pre - treatment of fresh air has many advantages in terms of energy conservation, control, and management. This article will briefly elaborate on the calculation of fresh air volume in electronic clean factories, the advantages of centralized pre - treatment of fresh air, and its air heat and humidity treatment and control solutions.
1.1 In clean air conditioning, fresh air is of great significance for production safety and product quality. Due to the need for metabolism, the human body needs to continuously inhale oxygen and exhale carbon dioxide. Working in an environment without fresh air can lead to oxygen deficiency phenomena such as dizziness, nausea, irritability, and low work efficiency. Therefore, in an air - conditioning purification system, the fresh air volume to meet the hygienic requirements of personnel is generally not less than 40m³/(h·person).
1.2 Clean spaces generally have exhaust air required by the process. To balance the amount of air discharged outdoors, the purification system needs to compensate with an appropriate amount of fresh air.
1.3 To prevent the air from outdoors or adjacent rooms for other purposes from infiltrating into the air - conditioned and purified workshop, interfering with its cleanliness, temperature, or humidity, the amount of air required to maintain positive pressure will be supplemented by fresh air. The calculation of this part of the fresh air volume mainly includes the crack method and the air change rate method, which will not be introduced in detail here.
1.4 The fresh air volume should meet a certain proportion of the total air volume. According to experience, considering that the positive pressure in the air - conditioning units and air - supply ducts in the clean air - conditioning system is relatively high, there may be significant leakage along the way. For safety reasons, especially in industries such as pharmaceuticals, the proportion of fresh air volume should meet 10% - 30% of the total air volume of non - unidirectional flow clean rooms and 2% - 4% of the total air volume of unidirectional flow clean rooms. At this time, if there are few people in the clean room, little process exhaust air, and sufficient positive pressure, and the fresh air volume calculated by proportion is greater than the system's demand, measures should be taken to increase the exhaust air volume of the clean air - conditioning system.
2.1 For clean rooms, fresh air is a major source of pollution in the air - conditioning system. Fresh air without filtration can block the surface cooler, reduce the heat transfer coefficient of the surface cooler, and harmful chemical particles are corrosive to the surface cooler. A large number of bacteria also multiply near it, reducing the service life of the surface cooler and the high - efficiency air filter. To ensure the cleanliness requirements of the clean room, outdoor fresh air must go through multi - stage filtration before being sent to the clean room.
2.2 In a clean air - conditioning system, because the enthalpy value of outdoor fresh air often differs greatly from that of indoor air, the heat and humidity load of fresh air accounts for a large proportion in the purification air - conditioning system. If fresh air is directly sent into the clean room without heat and humidity treatment, it is impossible to meet the temperature and humidity requirements of the clean room.
2.3 Generally, most clean air - conditioning systems are designed as classic once - through return air systems. Outdoor fresh air is directly mixed with indoor return air and then filtered and subjected to heat and humidity treatment. This air - conditioning method is the most common one. In electronic clean factories, the indoor moisture generation in most rooms is very small, while the heat generation of equipment is very large, and the indoor air heat - humidity ratio is infinitely large, which is a vertical line in the psychrometric chart. Therefore, considering aspects such as investment, management, and energy conservation, it is recommended to adopt the combination of centralized pre - treatment of fresh air and circulating air - conditioning units (MAU + RCAU).
In the "MAU + RCAU" system, the air flow process of centralized pre - treatment of fresh air is: fresh air inlet → coarse - effect filter → medium - effect filter → heater → humidifier → surface cooler → supply fan → air - supply duct → each circulating air - conditioning unit (RCAU);
The air flow process of the circulating air - handling unit is: fresh air and return air mixing (fresh air from MAU) → surface cooler → heater → supply fan → medium - effect filter → air - supply duct → high - efficiency filter air outlet → clean area → louvered return air inlet → return air duct → fresh air and return air mixing;
3.1 In some clean workshops with high precision requirements for temperature and humidity control, each workshop needs to be equipped with an independent air - conditioning unit, and these air - conditioning units need to be equipped with a complete functional section such as a fresh air treatment function section. This means that all air - conditioning boxes are fully equipped. If there are many clean workshops in the factory building, the number of air - conditioning boxes will be large, resulting in a large air - conditioning machine room, increased investment, and increased maintenance workload. If centralized fresh air treatment is adopted and then distributed to each air - conditioning purification system, although an additional fresh air unit (MAU) is added, many functional sections of other circulating air - conditioning boxes (RCAU) are reduced, saving equipment investment, the area of the air - conditioning machine room, and the number of fresh air louver openings on the outer wall of the machine room.
3.2 In northern regions where the outdoor temperature is relatively low in winter, fresh air often needs to be pre - heated. Otherwise, when directly mixed with once - through return air, there is a high possibility of generating condensed water. Clean workshops generally have humidity requirements, and the treatment process in winter often requires humidifying the air, thus increasing the humidification load in vain. In summer, the fresh air has a high moisture content and a high temperature. After being directly mixed with return air, it may require multi - stage and multi - row surface coolers for treatment, greatly increasing the burden on the surface coolers of the air - conditioning unit. However, after centralized treatment, the fresh air is sent into the circulating air - conditioning box and mixed with return air, reducing the functional sections of the circulating air - conditioning box and the replacement frequency of the filter, reducing the fan pressure head of the circulating air - conditioning box, and avoiding energy waste.
3.3 As an important source of pollution controlled in clean rooms, centralized pre - treatment can strengthen the filtration of fresh air and facilitate management, providing a more secure guarantee for the operation of clean rooms.
3.4 In the "MAU + RCAU" method, fresh air is processed to the indoor air - supply dew point to meet the humidity requirements of the clean room. The processed fresh air is sent to each circulating air - conditioning box through the air duct, and after being mixed with return air, it is processed by surface cooling or heating to meet the temperature requirements of the clean room. This makes the humidity and temperature of the clean air - conditioning system be respectively responsible for by the fresh air treatment unit and the air - conditioning circulation unit, making the temperature and humidity control independent and operable. Especially when the fresh air volume in the clean room changes, its superiority is more obvious.
Suzhou Pharma Machinery Co.,Ltd.
2025/12/09
Gino
