With the rapid development of modern industry, the technological content of its products has become increasingly complex. To meet the increasing reliance of cleanrooms for product production and assembly, and to ensure the quality and longevity of equipment, modern cleanrooms are increasingly being used across a wide range of industries.
A cleanroom is first designed by an architectural design institute based on the requirements of the equipment design unit and the environmental characteristics of the products in the respective industry, matching the cleanliness level to the appropriate level. This is a prerequisite and fundamental step for cleanroom design and construction.
1. Establishing a Cleanroom Pressure Differential
Cleanroom testing and inspection is a series of tests and inspections conducted by the equipment design unit after all ventilation and air conditioning systems are installed, based on the design unit’s construction drawings. These tests primarily include air volume and pressure differential adjustments, cleanliness testing, and measurement of cleanroom temperature and humidity.
1. Air Volume Testing: Design and calculate the supply and return air ducts based on the design and construction drawings; determine the design air volume, determine the opening status of the supply and return air valves in each duct, and determine the air volume of each branch.
2. Cleanliness Testing: This is divided into testing under empty conditions, testing under static conditions, and testing under dynamic conditions, each measuring the dust content in the cleanroom.
Testing under empty conditions refers to testing when the system (cleanroom) is in normal operation but before process equipment and production personnel enter.
Testing under static conditions refers to testing when the system (cleanroom) is in normal operation, process equipment is installed but not in operation, and there are no production personnel in the room.
Testing under dynamic conditions refers to testing when the system (cleanroom) is in normal operation, process equipment is installed but not in operation, and there are no production personnel in the room.
Based on the above test results, the equipment and installation units will each test dust concentrations under various production environment conditions to achieve the cleanliness level specified by the equipment and installation designer and the equipment and installation unit. This ensures that the dust concentration remains below this value and that the equipment and installation unit is qualified for use.
3. Pressure Differential Adjustment: To prevent external contaminants from entering the cleanroom and increasing cleanliness, the indoor pressure must be maintained higher than the external pressure. This means that a certain pressure gradient must be maintained within the cleanroom between rooms with different cleanliness levels. This maintains a certain pressure differential and effectively prevents contamination of the cleanroom from adjacent rooms or contamination of adjacent rooms. The pressure differential adjustment process aims to ensure the cleanroom’s air change rate and the effectiveness of the equipment’s exhaust and dust removal, minimizing changes in the supply air volume and the airflow of the equipment’s exhaust and dust removal fans. Pressure differential adjustment is primarily achieved by adjusting the return air volume and system exhaust volume. The “Cleanroom Design Specification” clearly states that a cleanroom with an indoor pressure higher than the external pressure is considered a positive pressure cleanroom, while a negative pressure cleanroom is considered a negative pressure cleanroom. Positive and negative pressure are relative terms. A cleanroom may have positive pressure relative to the atmosphere, but may have negative pressure relative to another room. The pressure differential between cleanrooms of different grades and between clean and non-clean areas must be no less than 5 Pa, and the pressure differential between a clean area and the outside must be no less than 10 Pa.
The functional rooms within the cleanroom should be arranged along the cleanroom route: non-clean corridor, shoe changing room, primary changing room, secondary changing room, de-confliction room, and cleanroom corridor.
A positive pressure differential of at least 30 Pa should be maintained between the cleanroom corridor and the non-cleanroom area.
The basic principle for establishing this pressure differential is that the supply air volume exceeds the return air volume, exhaust air volume, and leakage air volume to maintain positive pressure. The pressure differential is established by achieving a balance between the cleanroom’s supply air volume and the exhaust air volume plus the pressure differential air volume (excess air volume).
For fresh air systems: fresh air volume = exhaust air volume + pressure differential air volume.
For a circulating air system: Fresh air volume + return air volume = return air volume + exhaust air volume + pressure differential air volume.
Ultimately, the final cleanroom pressure differential is defined as fresh air volume = exhaust air volume + pressure differential air volume, and a balanced relationship is established between them.
In summary, design, construction, and commissioning are three key steps in ensuring the smooth operation of a purification system. Through commissioning, we can gain an understanding of any issues that may arise during system operation, thereby improving the design plan, standardizing construction operations, and avoiding these problems. Commissioning is an important and necessary step in ensuring that the cleanroom’s indoor pressure differential and cleanliness meet standards.
II. Factors that Fluctuate and Interfere with Cleanroom Pressure Differentials and Control Methods
Changes in outdoor wind pressure and speed can cause changes in the cleanroom’s maintained pressure differential relative to the outside world.
Coastal cities and cities with high wind speeds require a recalculation of the headwind pressure to adjust the pressure differential.
As the air conditioning system runs over time, the system resistance will change. This is mainly due to the change in filter resistance, which causes the change in air supply volume and affects the establishment of the pressure difference in the clean room. When the air conditioning system runs for a period of time and the system resistance changes to a certain value, the system resistance increases, which affects the total air supply volume. The actual air supply volume is less than the set air supply volume, and the establishment of the original indoor pressure difference cannot be guaranteed. At the same time, due to the flow of personnel in the clean room, the windows and doors in the clean room are frequently opened and closed, the original sealing function of the clean room is reduced, and serious air leakage eventually affects the establishment of the original pressure difference in the clean room. In order to maintain a certain pressure difference in the clean room, certain measures must be taken to maintain the positive pressure in the room.
(1) Install an air damping layer at the return air inlet.
(2) Install an electric sealing valve with a medium-efficiency sprayer on the exhaust duct.
(3) The residual pressure valve is generally installed on the downwind wall of the clean room. When this method is adopted, it is required that there is sufficient residual air volume in the room.
(4) Adjust the return air valve or exhaust valve.
(5) Adjust the fresh air valve.
(6) Interlock control of the fan and air valve.
(7) Replace some necessary filters regularly.
