Dust-free workshop pressure difference: dust-free workshop to be operated on a variety of objects, such as electronic products, drugs, health products, microorganisms, viruses, etc. In the process of operation, some objects (such as viruses) will produce harmful or even biologically dangerous aerosols. Therefore, whether the production of products in a dust-free workshop or virus research, it is necessary to protect the products from external contamination as well as to keep the pollution generated during operation. In addition to the use of well sealed dust-free workshop barrier protection, also need to use the role of pressure difference, so that the flow in accordance with the specified direction. For example, the static pressure of the positive pressure dust-free workshop is greater than that of the adjacent room or the environment. By using this differential pressure, the air in the dust-free workshop can only flow outward, which prevents the pollution of the adjacent room or outdoor environment to the dust-free workshop. Negative pressure dust-free workshop is just the opposite, in the dust-free workshop to do some biologically dangerous operations, or in the operation process will produce a strong sensitization of drug dust, etc., it is necessary to use this pressure difference, so that the pollutant control in a specific range. Therefore, through the pressure difference and the dust-free workshop of the sealing envelope structure of the joint action, the formation of a barrier protection system, to achieve the purpose of pollution control.

No vehicle workshop differential pressure detection

The essence of differential pressure is the balance between fresh air volume and exhaust air volume + differential pressure air volume

Dc system: fresh air volume = exhaust air volume + differential air volume

Circulating system: fresh air volume + return air volume = return air volume + exhaust air volume + differential air volume

The essence is: fresh air volume = exhaust air volume + pressure differential air volume

Therefore, the adjustment of pressure difference in clean room should reflect the balance between fresh air volume and exhaust air volume + differential air volume. (1) for the dc system, a certain amount of air is sent in, and the exhaust air is adjusted to the establishment of pressure difference. (2) for the circulation system, a certain amount of air can be 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 exhaust air in some clean rooms, adjust the air volume to the establishment of pressure difference. The air volume into the clean room is required to be constant and therefore cannot be used as a control variable.

I. external disturbance factors causing the fluctuation of pressure difference in clean room and common basic countermeasures are as follows

(1) the resistance change of HVAC system, mainly the change of the filter resistance, will cause the change of air supply, affect the fluctuation of indoor pressure difference. It is easy to control, as long as the system resistance changes to a certain value after running to a certain period, manual timely adjustment can be. Also commonly used in constant air volume valve control pressure difference, can automatically maintain the air volume unchanged, also have the use of variable speed blower, to filter pressure difference for fan speed control, maintain a certain air volume.

(2) the change of outdoor wind pressure (caused by outdoor wind direction and wind speed, etc.) will affect the pressure difference air volume in the clean room and cause the fluctuation of indoor pressure difference. It is also easier to control. Requirements: the envelope structure and the air tightness of the door are good, the pressure difference between the clean room and the outdoor is maintained ≥10Pa, and the pressure difference between the adjacent clean rooms is maintained ≥5Pa. When the outdoor wind pressure is adverse to the cleanliness, a certain pressure difference in the clean room can be maintained without problems.

(3) the opening and closing of cleanroom door will cause the fluctuation of cleanroom pressure difference. When opening the door, two differential pressure can't maintain clean room, air is a two-way exchange, the exchange capacity is associated with the air temperature difference between the two room, is also associated with the move of person, foreign studies have considered in Δ t = 0 ℃, two-door air exchange capacity of 0.19 m3 / s. Δ t = 2 ℃, 0.24 m3 / s. To prevent the occurrence of reverse flow, the air velocity through the gate should be greater than 0.2m/s(the air velocity required to maintain the low pressure difference mentioned in ISO 14644-4); In engineering, the airflow velocity of 0.25m/s~ 0.5m/s is often used abroad. It is not easy to control, so closed-loop control is needed. When using quantitative control, buffer chamber or airlock should be set.

(4) the change of exhaust air in clean room process inevitably brings the fluctuation of pressure difference, while the change of exhaust air volume is sometimes large. At this time, closed-loop control is needed to avoid the fluctuation of indoor pressure difference and the reverse flow.

(5) the start-stop of HVAC system and biological cleanroom kaori, steaming, disinfection and sterilization of the air valve operation is bound to lead chamber pressure fluctuations, but this is temporarily appeared in the process of abnormal operation, to no effect on the normal operation, so need to fan, valves and other operation of the interlock system and valve operation such as specified, do not need to be considered in differential pressure control system.

The differential pressure control mode and the selection of the cleanroom system should be suitable for the requirements. There are two types of pressure differential control methods commonly used in clean room. One is the so-called static control, that is, the constant air volume control, and the other is the "direct pressure" control of the closed-loop control system and residual pressure valve control.

Ii. Fixed air volume control mode

1. Features and requirements: when the door of the clean room is closed, the air supply (fresh air + return air)= return air + exhaust air + pressure differential air volume, and the pressure differential value is established; In addition to the pressure differential air volume, the rest of the air volume is maintained.

The premise is :(1) the amount of exhaust air is required to be a certain, or at least a certain period of time, after the change of readjust. (2) envelope structure and door require good air tightness. (3) use buffer chamber or airlock, if necessary, increase the number of air change, so that airlock faster recovery and maintain its required cleanliness. There is no guarantee that there will be no backflow when it is opened to the door.

2. Fixed air volume control system: (1) manual control: after manually adjusting the pressure difference value, lock the air valve such as feed, return and exhaust, and then readjust it when the system resistance changes. It is often used in clean rooms in pharmaceutical factories. (2) fixed air volume valve control: air volume valve is set on the clean room air supply pipe, return pipe and exhaust pipe, there are only set on the air supply pipe, exhaust pipe, and return pipe is not set, of course, the former is better, but the investment is greater. The constant air volume valve is a machine composed of spring and valve body, which can compensate the change of the static pressure in the air duct by driving the displacement of the valve body with the spring to maintain a certain air volume, it can automatically maintain a certain air volume into the HVAC system when the resistance changes, and the air volume control is more accurate. At present in the isolation ward, operating room, laboratory, etc.

3. Energy saving measures of fixed air volume control mode :(1) the fixed air volume valve for energy saving can be bistable, that is, the clean room can operate with rated air volume when working, and operate with low air volume to save energy when not working. Bistable setting can be manual (watchman) setting, also can be light control, often used in the operating room and some laboratories. (2) in order to save energy, some pharmaceutical factories also use return air and local exhaust valve to close, at this time the linkage return air valve open, with the same air volume for interlock inversion, in order to maintain the indoor pressure difference. At this point, it is appropriate to use the return air fan and is variable speed, exhaust fan is also appropriate to be variable speed, can control the speed of the fan with the static pressure of their respective duct.

4. Determination of pressure difference air volume under constant air volume control: when the constant air volume is controlled, the pressure difference air volume is related to the air tightness of building envelope and door.

Iii. Closed-loop control mode

At present, the closed-loop control of industrial clean room often adopts "direct pressure" control, that is, the maintenance control of indoor pressure difference of fresh air or return air or exhaust air as a variable. Closed-loop control can cope with :(1) pressure fluctuations caused by exhaust changes. (2) reverse flow caused by an open door. (3) pressure fluctuations caused by outdoor wind pressure and system resistance. The main control is not the reverse flow, but there will be a certain range of pressure fluctuations, which can meet the requirements of the clean room. At present, this control method is often used in the clean room with the change of exhaust air and the clean room with door opening and closing frequently and not suitable for buffer room or airlock.

Common control systems:

(1) to indoor positive pressure to maintain control of the fresh air as variables: applicable to a HVAC system of a clean room, control more directly, when using fresh air units, general appropriate USES variable speed fan in order to realize the energy saving, especially when new air volume change is bigger, the circulation fan can generally constant air volume operation, do not make automatic adjustment. At present, the electronic industry, semiconductor, integrated circuit factory USES this kind of control system more.

(2) indoor pressure difference maintenance control return air as a variable: this system is generally applicable to one HVAC system for multiple clean rooms, due to the layout of the air duct and connection, the control of return air is more reasonable than the control of fresh air. In general, feed, return fan or single fan can not be automatically adjusted, fresh air by the system static pressure and fan characteristics to balance the air volume, of course, when necessary, the use of automatic control valve and fan air volume, pressure difference can be more stable.

(3) take the maintenance control exhaust of indoor pressure difference as the variable: this system is mostly used for the constant exhaust air volume, while the frequent door opening is used to prevent the reverse flow when the door is opened. Characteristic is to send, return air system can be a constant air volume operation, generally can not set up automatic adjustment fan air volume device; Exhaust fan should be used according to the situation to save energy.

(4) independent positive pressure delivery air pressure differential control system: the positive pressure supply air system is used for the pressure differential control in the operating room. The positive pressure fresh air is filtered by two levels and sent to each clean room through the regulating valve. The positive pressure air volume of this system is completely independent of the air volume of the HVAC system, which is easy to control, especially for the operating room which is not working, as long as the required pressure differential air volume of the positive pressure fresh air, HVAC system does not work. The variable speed fan should be used in the independent positive pressure wind machine to save energy.

Four, residual pressure valve control

The residual pressure valve installation is simple, the price is low, the control sensitivity can also be. But the disadvantage is: after long-term use, the closure will be lax; Destroy indoor flat clean, smooth surface, more than a dust accumulation. When using HVAC system, it may be equivalent to using chamber pressure to maintain control air volume or return air volume. When controlling return air, corridors are usually used as return air duct. Cross contamination should be paid attention to.

V. clean room differential pressure control mode and the selection of the system shall be in accordance with the requirements

At present, for industrial clean rooms and general (+) pressure biological clean rooms, fixed air volume control and closed-loop control systems are adopted as required. Its closed-loop control system generally adopts "direct pressure" control. The measures to adjust the air volume matching of the feed and return fans or single fans in the HVAC system are as follows: (2) for the closed-loop control system, manual adjustment is generally also adopted. The static pressure of the system and the self-balancing of the characteristics of the fan are utilized, and no automatic air volume adjustment device is set. (3) air volume changes and requirements are high, there are also the use of static pressure control corresponding wind valve for automatic adjustment of matching linkage or flow tracking control for air volume regulation, but generally not much.

Laboratory, biological safety laboratory, by contrast, the negative pressure control system of dc system, the need for energy saving in the variable air volume control, tend to require more reliable, sensitive and rapid response of the control system, such as venturi adopts variable air volume control valve, and the corresponding flow tracking combined with a "direct" pressure control of integrated system, in order to protect security.