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Differential pressure control method and control requirements for clean rooms

Nov 08, 2019
Introduction
A clean room is a room in which the concentration of suspended particles is controlled. For cleanrooms, maintaining their cleanliness levels is essential and is a requirement for cleanroom-related production activities. In general, the design, construction and operation of the clean room should minimize the interference and influence of the surrounding environment on the clean room space, and the differential pressure control is the most important to maintain the clean room cleanliness level, reduce external pollution and prevent cross-contamination. The most effective means. The static pressure difference in the clean room has the following effects:
(1) When the doors and windows of the clean room are closed, the pollution of the surrounding environment is prevented from infiltrating into the clean room by the gap between the doors and windows.
(2) When the clean room doors and windows are opened, ensure sufficient airflow speed, minimize the airflow of doors and windows and personnel entering the clean room, and ensure the airflow direction to minimize the ingress pollution.
When clean indoor process production or activities make indoor air contain high-risk substances, such as high-sensitivity drugs such as penicillin, highly infectious high-risk viruses, bacteria, etc., the pressure difference of the clean room should be relatively negative. Domestic and foreign standards, norms [3], [4], [5], [6], [7], [8], [9] have specified the external pressure difference of the clean room, or give the differential pressure control Requirements and guidelines.

This paper will discuss the requirements of clean room differential pressure control and discuss the method of clean room differential pressure control.

a) Establishment of pressure difference in clean room

The test and inspection of the clean room is the design and construction drawings of the construction unit according to the design unit. After all the ventilation and air-conditioning design is installed and in place, the sub-items are tested and measured separately for some projects, mainly air volume debugging and differential pressure debugging, cleanliness Test, measure the temperature and humidity of the clean room.
(1) Air volume commissioning: According to the design and construction drawing design, the sending and returning pipelines are designed; the size of the design air volume is determined, the opening state of each pipeline returning air valve is determined, and the air volume of each branch is tested. This article is not covered here.
(2) Cleanliness test: It is divided into test under air condition, test under static condition, and test under dynamic condition, and the content of dust in clean room is tested separately.
Test under empty conditions means that the system (clean room) is in normal operation, but the process equipment and production personnel have not been tested.
Test under static conditions means that the system (clean room) is in normal operation, the process equipment has been installed but not running, and the test is carried out without production personnel in the room.
Test under dynamic condition means that the system (clean room) has been in normal operation, and the process equipment and production personnel have been working.
According to the above test results, the construction unit shall test the dust content concentration in different production environment conditions, so as to achieve the cleanliness level required by the construction designer and the construction unit, and ensure that the dust concentration is always lower than this value, and qualified delivery to the construction unit for use.
(3) the differential debugging: in order to prevent the external pollution into the clean room and make the room cleanliness higher, asked for indoor pressure to remain higher than that of external pressure that is in the clean room requirements of different cleanliness room must maintain a certain pressure gradient, which can maintain a certain indoors effectively avoid clean room under the pressure difference of adjacent room pollution or contamination adjacent rooms.In the process of differential pressure debugging, in order to ensure the number of air changes in the clean room and the effect of equipment exhaust air and dust removal, try not to change the air supply volume and the air volume of equipment exhaust fan and dust removal fan.The pressure difference is mainly adjusted by adjusting the return air volume and the system exhaust air volume.The clean room design specification clearly states that the pressure inside the clean room is higher than the external pressure, which is called positive pressure clean room and negative pressure clean room.Positive versus negative pressure, one room is a positive pressure room to the atmosphere, but another room may be a negative pressure room.It is also stipulated that the pressure difference between the clean room and the clean area and the non-clean area shall be no less than 5Pa, and the pressure difference between the clean area and the outdoor area shall be no less than 10Pa.
operation of the purification system. Through debugging, we can understand the problems that arise in the operation of the system, so that the design scheme can be improved, the construction operation can be standardized, and the above problems can be avoided. Commissioning work is an important and necessary step to ensure that the pressure difference and cleanliness of the clean room are up to standard. The normal operation of the system is a normal use of production.

2.Selection and discussion of pressure difference control method in clean room

Currently, there are basically two types of differential pressure control methods commonly used in clean rooms.One is called static control, constant air volume control, and the other is called dynamic control.

2.1 Air volume control principle

Air conditioning system through the system of the air supply, return and exhaust air volume reasonable design and regulation to achieve different clean levels and indoor and outdoor pressure difference requirements.

The pressure difference is established when the balance is reached between the inlet air volume and the exhaust air volume + the differential pressure air volume (residual air volume) in the clean room. Here, the discharge air volume includes: the exhaust air volume to be discharged to the outside and the return air volume.

For dc system, fresh air = exhaust air + PDC

For the circulatory system, the fresh air + the return air = the return air + the exhaust air + the differential pressure air

So in the final analysis, the essence of the pressure difference is: new air volume = exhaust air volume + pressure difference air volume

Therefore, the adjustment of the establishment of PDC in the clean room should reflect the balance between fresh air volume and exhaust air volume + PDC:

(1) feed a certain amount of air into the dc system and adjust the discharge air volume to the establishment of differential pressure;

(2) for the circulatory system, a certain amount of air is fed into it. Generally, the exhaust air volume can be adjusted as required, and then the return air volume can be adjusted to establish the pressure difference.

(3) when there is no separate exhaust air volume in some clean rooms, adjust the return air volume to establish the pressure difference.The inlet air volume of clean room is required to be certain, so it cannot be used as a control variable.Through reasonable design and debugging, the pressure difference can be achieved.


2.2 Control mode

There are two systems for constant air volume control:

(1) Manual control

After manually adjusting the differential pressure, lock the feed, return and exhaust air supply valves, and readjust them when the system resistance changes.The system is simple and easy to operate and maintain.Often used to restrict the factory - related clean room.

The feed and return fans (single or double fans) of its HUAC system are not equipped with automatic air volume regulation device.Of course, if automatic air volume regulation is set (such as, the static pressure on the air supply main pipe must control the single fan or the outlet control valve of the feed and return fan), the pressure difference in the clean room may be more stable, but the automatic air volume regulation is generally not set.

(2) Constant air volume valve control

Basic is in clean room send air tube, return air tube and exhaust air tube on each setting air volume valve, also have to be in only send air tube, exhaust air tube on setting, and return air tube does not set, with the former had better of course, but investment is some bigger.

Constant air volume valve as in spring and valve body composed of mechanical, can the change of static pressure inside the duct by spring driven body displacement compensation, make the air volume, maintain its resistance changes on HUAC system can automatically maintain into air volume must be (constant air volume valve is the role of resistance, some is 105 ~ 750 pa, some for 20 ~ 1000 pa), and air volume control is accurate.Therefore, there is no need to set up automatic air volume adjustment measures for the fans, which are widely used in isolation wards, operating rooms and laboratories.


2.3 Energy-saving measures for the control mode of constant air volume

(1) for energy saving constant air volume valve can be bistable, that is, the clean room in the work with rated air volume operation, when not working, using low air volume operation to save energy, the fan can be used double speed or variable frequency variable speed fan.The bistable state can be set manually (on duty) or controlled by light, etc.At present, it is often used in the operating room and some laboratories.

(2) in order to save energy, some pharmaceutical factories have adopted the connecting pin of return air and partial exhaust air (one opening and one closing) to maintain the indoor pressure difference.If the partial exhaust air of the indoor fume hood or equipment is not working, the exhaust valve shall be closed and the linkage return air valve shall be opened to carry out interlock switching with the same air volume to maintain the indoor pressure difference.At this time, send, return fan should use double fan, return fan and is variable speed, exhaust fan is also appropriate to be variable speed, can be in accordance with the static pressure of their respective air duct to control the speed of the fan.


2.4 Adaptability of constant air volume control method to door opening and closing

As mentioned above, the control method of constant air volume cannot avoid the reverse airflow when the door is open and closed.At present, some operating rooms adopt the interlock of door opening, closing and exhaust fan to maintain the low pressure difference without reverse flow.It should be noted that: if the clean room exhaust air volume itself ≥ the door opened to maintain the air pressure difference, the effect is positive, otherwise it is difficult not to appear reverse flow, especially when the door opened a little longer.

In addition, at present, many operating rooms adopt the differential pressure control method of constant air volume valve, which is mainly based on the centralized uniform air supply above the operating table in the operating room, which can better prevent the entry of pollutants in the surrounding area, and the microbial concentration in the surrounding area is not as good as the clean corridor.

2.2 Constant air volume control

2.2.1 Characteristics and application requirements

Its characteristics are as follows: when the clean room door is closed, make the air supply (fresh air * return air) + return air * exhaust air * differential pressure air volume, and establish the differential pressure value;All air volumes are maintained, except differential pressure air volume.

Constant air volume control method can not solve the process exhaust air volume change and the door opening and closing of the interference on the pressure difference, that is, in these cases to appear reverse flow.Therefore, the premise of its adoption is:

(1) a certain amount of exhaust air is required, or at least a certain amount of air is required for a period of time, and it should be adjusted when it changes later;

(2) good air tightness of enclosure structure and doors;

(3) buffer chamber or airlock shall be adopted, and the number of air changes shall be increased if necessary, so as to restore the airlock quickly and maintain the required cleanliness;Even if there is a reverse flow in the adjacent space without buffer room, the processing or processing object will not be significantly affected.

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