When there is a high incidence of foreign objects in the workshop products, engineers investigate the reasons according to the human machine material method and environmental testing. When the environment is identified, the pressure difference in the dust-free workshop must be investigated.
Principle of pressure difference control
1. A certain pressure difference must be maintained between the clean room and the surrounding space, and a positive pressure difference should be determined according to process requirements. Generally, areas with high cleanliness flow towards areas with low cleanliness, so that the cleanliness of the clean room is not disturbed by polluted air.
2. The pressure difference between adjacent rooms of different levels should be at least 10Pa. Special attention should be paid to the protection of critical areas. EU GMP/FDA: The pressure difference used for different levels is 10-15 Pa, and it is recommended to use 12.5 Pa.
China GMP2010 version: It is required that the pressure difference between clean and non clean areas, as well as between different levels of clean areas, should not be less than 10Pa.
3. The air volume required to maintain different pressure differentials in a clean room is generally determined using the method of air exchange rate.
Establishment of pressure difference in clean area
The pressure in a room is determined by the difference in air volume between the incoming and outgoing rooms. If the air volume entering the room is greater than the air volume exiting the room, positive pressure will be formed, and vice versa, negative pressure will be formed.
In order to maintain the pressure difference in the room at a specific value, the following formula must be satisfied:
Fresh air system, supply air volume=fresh air volume=exhaust air volume+maintenance pressure difference air volume;
Circulating air system, supply air volume=fresh air volume+return air volume=return air volume+exhaust air volume+maintenance pressure difference air volume.
For the entire system, as long as the following conditions are met: fresh air volume=exhaust air volume+maintenance pressure difference air volume; It can maintain the pressure balance of the system.
According to Article 48 of the new version of GMP (revised in 2010), the pressure difference between clean and non clean areas, as well as between different levels of clean areas, should not be less than 10Pa. If necessary, appropriate pressure gradients should also be maintained between different functional areas (operating rooms) of the same cleanliness level.
Maintain the pressure gradient in the clean area
1. In general, there are many ways to adopt a fixed air volume system, which first ensures a relatively constant supply air volume to the clean room, adjusts the return or exhaust air volume of the clean room, thereby controlling the pressure difference air volume of the clean room and maintaining the pressure difference of the clean room;
2. Install an electric actuator system on the regulating valve shaft of the clean room return (exhaust) branch pipe, which is matched with the corresponding valve to form an electric regulating valve. Based on the feedback of the cleanroom pressure difference, fine tune the valve opening and automatically adjust the cleanroom pressure difference to return to the set value. This method is reliable and accurate for controlling the pressure difference in clean rooms, and the cost of the control system is not high. It is widely used in engineering practice. The system can be installed on the regulating valve of the return (exhaust) branch pipe in clean rooms or typical clean rooms that require pressure difference display.
Differential pressure testing process
1. Before testing, all doors should be kept closed and the sealed structure of the room should be intact.
2. If there are multiple clean rooms, first measure the pressure difference between the innermost room and the adjacent room, and then measure the outer rooms in sequence until the pressure difference between the outermost sealed area and the auxiliary area, as well as between the outdoor environment, is measured.
3. Measure and record the pressure difference between the clean room and the surrounding environment.
4. It is recommended to set up long-term measuring points, which should be located far away from the air supply and return ports that may affect the local pressure of the measuring points.
5. During each test, the pressure gauge should be zeroed, and the test hose should be placed in the middle of the room or in a relatively low airflow area. If possible, the hose testing height can be raised appropriately.