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Secondary Return Air scheme For Air Conditioning System

The micro-electronic workshop with relatively small clean room area and limited radius of return air duct used to adopt the secondary return air scheme of air conditioning system. This scheme is also commonly used in clean rooms in other industries such as pharmaceuticals and medical care. Because the ventilation volume to meet the requirements of clean room temperature humidity is generally far less than the ventilation volume required to reach the cleanliness level, therefore, the temperature difference between the supply air and the return air is small. If the primary return air scheme is used, the temperature difference between the supply air state point and the dew point of the air conditioning unit is large, secondary heating is needed, resulting in the cold heat offset in the air treatment process and more energy consumption. If the secondary return air scheme is used, the secondary return air can be used to replace the secondary heating of the primary return air scheme. Although the adjustment of the primary and secondary return air ratio is slightly less sensitive than the adjustment of the secondary heat, the secondary return air scheme has been widely recognized as an air conditioning energy saving measure in small and medium-sized micro-electronic clean workshops.

Take an ISO class 6 microelectronics clean workshop as an example, the clean workshop area of 1 000 m2, the ceiling height of 3 m. Interior design parameters are temperature tn= (23±1) ℃, relative humidity φn=50%±5%; The design air supply volume is 171,000 m3/h, about 57 h-1 air exchange times, and the fresh air volume is 25 500 m3/h (of which the process exhaust air volume is 21 000 m3/h, and the rest is positive pressure leakage air volume). The sensible heat load in the clean workshop is 258 kW (258 W/m2), the heat/humidity ratio of the air conditioner is ε=35 000 kJ/kg, and the temperature difference of the room’s return air is 4.5 ℃. At this time, the primary return air volume of
This is currently the most commonly used form of purification air conditioning system in the microelectronics industry clean room, this type of system can be mainly divided into three types: AHU+FFU; MAU+AHU+FFU; MAU+DC (Dry coil) +FFU. Each has its advantages and disadvantages and suitable places, the energy-saving effect mainly depends on the performance of the filter and fan and other equipment.

1) AHU+FFU system.

This type of system mode is used in the microelectronics industry as “the way of separating air conditioning and purification phase”. There may be two situations: one is that the air conditioning system only deals with fresh air, and the treated fresh air bears all the heat and humidity load of the clean room and acts as a supplement air to balance the exhaust air and positive pressure leakage of the clean room, this system is also called MAU+FFU system; The other is that the fresh air volume alone is not enough to meet the cold and heat load needs of the clean room, or because the fresh air is processed from the outdoor state to the dew point specific enthalpy difference of the required machine is too large, and part of the indoor air (equivalent to a return air) is returned to the air conditioning treatment unit, mixed with the fresh air for heat and humidity treatment, and then sent to the air supply plenum. Mixed with the remaining clean room return air (equivalent to secondary return air), it enters the FFU unit and then sends it into the clean room. From 1992 to 1994, the second author of this paper cooperated with a Singaporean company and led more than 10 graduate students to participate in the design of the U.S.-Hong Kong joint venture SAE Electronics Factory, which adopted the latter kind of purification air conditioning and ventilation system. The project has an ISO Class 5 clean room of approximately 6,000 m2 (1,500 m2 of which was contracted by the Japan Atmospheric Agency). The air conditioning room is arranged parallel to the clean room side along the external wall, and only adjacentto the corridor. The fresh air, exhaust air and return air pipes are short and arranged smoothly.

2) MAU+AHU+FFU scheme.

This solution is commonly found in microelectronics plants with multiple temperature and humidity requirements and large differences in heat and humidity load, and the cleanliness level is also high. In summer, the fresh air is cooled and dehumidified to a fixed parameter point. It is usually appropriate to treat the fresh air to the intersection point of the isometric enthalpy line and the 95% relative humidity line of the clean room with representative temperature and humidity or the clean room with the largest fresh air volume. The air volume of MAU is determined according to the needs of each clean room to replenish the air, and is distributed to the AHU of each clean room with pipes according to the required fresh air volume, and is mixed with some indoor return air for heat and humidity treatment. This unit bears all the heat and humidity load and part of the new rheumatism load of the clean room it serves. The air treated by each AHU is sent to the supply air plenum in each clean room, and after secondary mixing with the indoor return air, it is sent into the room by the FFU unit.

The main advantage of the MAU+AHU+FFU solution is that in addition to ensuring cleanliness and positive pressure, it also ensures the different temperatures and relative humidity required for the production of each clean room process. However, often due to the number of AHU set up, occupy the room area is large, the clean room fresh air, return air, air supply pipelines crisscross, occupy a large space, the layout is more troublesome, maintenance and management is more difficult and complex, therefore, no special requirements as far as possible to avoid the use.


Post time: Mar-26-2024