In the process of modern manufacturing and scientific research, cleanroom workshop is a common facility to ensure the cleanliness of production and experimental environment, especially for some processes and experiments with high cleanliness and low pollution requirements. HVAC system is the key to ensure the stability and reliability of key environmental parameters such as temperature, humidity and differential pressure in cleanrooms of pharmaceutical factory, and pressure control is a key link of HVAC system. This paper discusses and studies the pressure control of the room where all kinds of large-scale exhaust equipment are located, which is the key and difficult control object of the industry, combining with various schemes of room pressure control.

1. Room pressure control principle and mainstream control scheme

1.1 Room Pressure Construction Principle

Clean air treated by temperature and humidity regulation, filtration and other processes is sent into the controlled room, and the proportion of air supply, return and exhaust volume in the room is adjusted, and the leaked air volume is controlled, so as to form the residual air volume capable of establishing stable pressure in the room, that is, to form stable room pressure. The differential pressure or pressure gradient between the related controlled room and its adjacent room is formed immediately.

Therefore, the residual air volume is a key factor in establishing room pressure.  When the residual air volume of the room is positive after the air supply volume minus the air return and exhaust volume, and leaked air volume, the room is a positive pressure room; when it is negative, the room is a negative pressure room.

1.2 Summary of Room Pressure Control Scheme

1.2.1 Traditional manual valve adjustment scheme

Conduct the air balance commissioning for cleanroom air conditioning system. When each room has enough residual air volume, you can directly adjust the hand valves of air supply, return and exhaust of relevant rooms; when the air balance of the whole air conditioning system is adjusted, the pressure gradient of relevant rooms will be established.

This scheme has the advantages of simple control and low cost, but the disadvantages are also obvious. If the filter is blocked in the later stage, the balance is easy to be broken, frequent manual adjustment is required, and the anti-interference ability is poor. It is used more often in production environments with low costs where cleanliness levels are not high, and less often in biopharmaceutical projects with high costs.

1.2.2 Customized delivery, scheduled and adaptive solution CAV+CAV

This scheme is to install a mechanical constant air volume valve, namely CAV box, on the air supply, return and exhaust pipes of cleanroom. No matter whether the system filter is blocked or not, how the resistance changes, it can always ensure that the volume of air supply, return or exhaust is relatively constant based on the its pressure-independent principle, thus ensuring that the residual air volume in the room meets the requirements. 
The scheme has the advantages of simple control, relatively low cost, strong anti-interference ability, stability and reliability, and is widely used in various cleanrooms in the pharmaceutical industry, but it is not suitable for rooms with large exhaust equipment.

1.2.3 Fixed delivery plus bi-stable scheduled automatic control solution CAV+e-CAV Box

The scheme is to install a CAV box on the air supply pipe of cleanroom, and a bistable electric constant air volume valve, namely e-CAV box, on the air return or exhaust pipe
, which is suitable for rooms with large exhaust equipment. It increases the cost of electric actuator and related control, and belongs to the scheme with relatively low cost of automatic control.

1.2.4 Delivery variable discharge automation control and pressure bias control solution CAV + VAV

The scheme is to install a CAV box on the air supply pipe of cleanroom, and a variable air volume valve, namely VAV box, on the air return or exhaust pipe.

This is a cascade room pressure control scheme which can automatically track and mainly control the room residual air volume, and take an auxiliary and precise control of the pressure. The air exhaust volume can be monitored. When the total air exhaust volume of the room changes, the automatic control system calculates the current setting value of variable air volume valve of the air return or exhaust in real time according to the that of the residual air volume to maintain stable room pressure, and takes this as the main control link. At the same time, by monitoring the change of room pressure, the system will do PID operation for the current value and set value of room pressure, that is, the PID output value will be multiplied by a certain influence factor (10%, 20%, etc., determined by field commissioning), and superimposed on the current set value of variable air volume valve in the main control link. The set value will be set for secondary fine adjustment, so that the pressure control is more accurate [1].  If the air exhaust volume of equipment cannot be monitored, PID operation can be directly performed with the current value and set value of room pressure, and the PID output value directly controls the variable air volume valve to maintain the room pressure.

The scheme integrates the advantages of stability of automatic tracking control of residual air volume and accuracy of pressure directly participating in offset control.  Therefore, this scheme is suitable for rooms with high pressure control requirements.  This solution is relatively expensive, due to the use of a fast-response type VAV Box, but the control instantaneity effect is better.