1. Introduction
To ensure the high precision, high purity, and high yield of manufactured products, higher requirements have been put forward for air cleanliness. Cleanrooms are widely used in various industries such as electronics, photovoltaics, aerospace, and pharmaceuticals.
2. Characteristics of Cleanrooms in the Electronic Industry
Cleanrooms in the electronic industry have distinct features in terms of air distribution, cleanliness level, and temperature-humidity control:(1) Uniform air distribution requirement: To maintain high indoor cleanliness, cleanrooms must remove micro-particle contaminants generated indoors, thus imposing strict demands on air distribution.(2) High-precision and stable temperature-humidity control: Since the manufacturing processes of electronic products are extremely sensitive to changes in temperature and humidity, air parameters in the process areas of cleanrooms must be strictly controlled within a narrow range.(3) Large fresh air volume and high air change rate: Cleanrooms in the electronic industry are equipped with process equipment such as ovens, which generate significant heat. This heat is mainly removed by process cooling water and process exhaust air. The large-volume exhaust air in the room requires supplementary fresh air (after heat and humidity treatment) to maintain positive indoor pressure. Meanwhile, a high air change rate is necessary to filter dust particles and maintain high indoor cleanliness.(4) Large air-conditioning cooling load and minimal moisture load, with a heat-moisture ratio close to +∞: Electronic cleanrooms are generally located in the middle of buildings; although the heat gain through the envelope structure is small, the heat generated by process equipment in cleanrooms is considerable, meaning excess indoor heat needs to be removed throughout the year.
3. MAU+FFU+DCC Clean Air-Conditioning System
The MAU+FFU+DCC system refers to the combination of a Make-up Air Unit (MAU), Fan Filter Unit (FFU), and Dry Cooling Coil (DCC). In this air-conditioning mode, outdoor fresh air is treated by the MAU and then supplied into the return air plenum of the cleanroom. The DCC adjusts the air to the required parameters, and the FFU circulates the air to achieve the air change rate needed for cleanliness. Typically, the MAU treats fresh air to the indoor dew-point temperature, handling the fresh air load and indoor moisture load; the DCC undertakes the indoor sensible heat load; and the FFU is responsible for air circulation and filtration. In other words, this is an air treatment system where the MAU controls humidity, the DCC controls temperature, and the FFU ensures cleanliness. Compared with other systems, the MAU+FFU+DCC system has the following advantages:(1) High flexibility and high coverage rate: Flexibility is crucial for clean air-conditioning systems in electronic factories. If the indoor cleanliness needs to be adjusted regionally due to process changes, the cleanliness of local areas can be modified by adjusting FFU air volume or replacing blind plates. When densely arranged, the coverage rate of FFUs can reach 85.5%.(2) Negative pressure in the ceiling above FFUs, enabling a sealing effect: The ceiling cavity is under negative pressure, and the air pressure in the cleanroom is higher than that in the ceiling space. If there are gaps between the FFU and its support frame, the negative pressure in the ceiling prevents unfiltered air from entering the room, which is beneficial for maintaining cleanroom cleanliness.(3) Low resistance of the air flow system and low outlet air velocity: The DCC has a large air-facing area, resulting in an air flow resistance of approximately 50 Pa. The return air plenum also has a large cross-sectional area, with a resistance of less than 15 Pa. The resistance of porous floors and grilles is also low, and the total resistance of the entire return air system is within 250 Pa.(4) Low energy consumption per unit air volume: With FFUs, there is no need to install additional pressure fans. Fans are decentralized (instead of centralized), eliminating the need for a dedicated equipment room and reducing the resistance of supply air ducts. When FFUs adopt DC/EC (electronic commutation) motors, the outlet air velocity can be adjusted between 0.25–0.45 m/s, with a motor efficiency of 75%–80%. Additionally, speed control can be individually adjusted based on filter pressure drop to save energy. Therefore, the energy consumption per unit air volume is generally lower than that of centralized systems with large centrifugal fans.
4. Engineering Design of the MAU+FFU+DCC Clean Air-Conditioning System
The core of the engineering design for the MAU+FFU+DCC clean air-conditioning system lies in determining the air change rate. The total air volume calculated based on the indoor air-conditioning cooling load is generally different from that calculated based on the allowable indoor dust concentration, resulting in two different values for the air change rate. When designing the FFU+DCC clean air-conditioning system, these two air change rates should be calculated separately, and the larger value should be adopted. If the finally determined air change rate has a significant surplus relative to the air-conditioning cooling load, the DDC (Direct Digital Control) system can automatically adjust the inlet water temperature and outlet air parameters of the DCC to maintain stable indoor temperature and humidity. Since FFU motors generate significant heat (equivalent to increasing the indoor air-conditioning cooling load), additional air volume must be allocated to remove the heat produced by FFUs. Therefore, after initially calculating the ventilation volume and the number of FFUs, the total air volume and the number of FFUs need to be revised based on the heat generated by FFUs.
The MAU is selected based on the fresh air volume, which must meet the requirements of hygiene, positive pressure maintenance, and exhaust air compensation. The indoor positive pressure is adjusted by a pressure relief valve installed on the wall panel. For electronic cleanrooms, the number of indoor staff is usually small, so the maximum fresh air volume can generally be determined based on the requirements of positive pressure maintenance and exhaust air compensation. The MAU is selected according to the fresh air volume and load, and its filtration efficiency is determined. The load of the DCC is calculated based on the return air volume (equal to the supply air volume minus the fresh air volume), from which the return air ratio is derived. The model and quantity of DCCs are selected based on the DCC load and the cleanroom return air volume. The inlet water temperature of the DCC should generally be higher than the dew-point temperature of the indoor air.
Calculation of indoor dust generation: Indoor dust sources mainly include humans, building surfaces, equipment surfaces, and process operations. After determining the filtration efficiency of FFUs and MAUs, the dust concentration in the supply air from FFUs can be calculated. Based on the required indoor dust concentration, the air change rate is derived, and the number of FFUs is reconfirmed accordingly.
5. Engineering Design Example
A Class 1000 cleanroom of a liquid crystal panel enterprise in Beijing covers an area of 40,000 m² and has a volume of 578,000 m³. The MAU+FFU+DCC air-conditioning system is adopted, with air returned through the return air plenum. Fresh air is supplied into the ceiling after purification treatment. The key parameters are as follows:
(1) Air state parameters at each point:
Outdoor air: Dry-bulb temperature 35.8℃, wet-bulb temperature 27.6℃, enthalpy 87.57 kJ/kg;
Indoor air: Dry-bulb temperature 23℃, relative humidity 55%, enthalpy 47.74 kJ/kg;
MAU dew-point temperature: 13.5℃, relative humidity 95%, enthalpy 38.81 kJ/kg.
(2) Total air volume and air change rate calculated based on air-conditioning cooling load:The indoor air-conditioning cooling load is 28,698 kW. Initially selected FFUs have dimensions of 1200×1200 mm, an air velocity of 0.38 m/s, and an air volume of 31.21 m³/min, with a filtration efficiency of 99.99%. The total circulating air volume is 17.7877 million m³/h, and the calculated number of FFUs is 9,500. The air change rate required for heat removal is 30.77 times/h.
(3) MAU parameters and DCC load:The fresh air volume is 900,000 m³/h. The filtration efficiency of the MAU's primary, medium, and high-efficiency filters is 35%, 85%, and 95% respectively. The return air volume is 16.8877 million m³/h, with a return air ratio (s) of 0.949. The calculated dew-point temperature of the air passing through the DCC is 13.5℃, with an enthalpy of 41.72 kJ/kg. The total load of the DCCs is 36,421 kW.
(4) Air change rate calculated based on cleanroom dust concentration:The indoor dust generation from staff is G = 5064 particles/(m³·min), the average indoor dust concentration is 11,766 particles/m³, and the dust concentration in the supply air from FFUs is 8.786 particles/m³. The air change rate calculated based on the uniform distribution theory is 25.84 times/h; taking ψ = 1.19, the air change rate calculated based on the non-uniform distribution theory is 30.75 times/h. After comparison, the air change rate for this cleanroom is determined to be 30.77 times/h.
In practice, 9,538 FFUs, 470 DCCs of 3000×1600 mm, and 54 DCCs of 350×1600 mm were installed in the cleanroom. Third-party testing results show that all indicators of the cleanroom meet the design standards and satisfy process requirements.
6. Conclusion
The MAU+FFU+DCC clean air-conditioning system is a cleanroom air-conditioning technology widely used in the electronic industry, especially for vertical unidirectional flow cleanrooms with high cleanliness levels. With the advancement of theoretical analysis, accumulation of practical application experience, and development of manufacturing technology for the MAU+FFU+DCC system, the system cost will be well-controlled, and its application will have broader prospects.
Suzhou Pharma Machinery Co.,Ltd.
2025/11/20
Gino
