Several fresh air volume control methods in clean room purification engineering

Several fresh air volume control methods in clean room purification engineering

In the variable air volume air conditioning system, since the air supply volume changes continuously with the load change during the system operation, it will also affect the pressure condition in the air conditioning box, especially the pressure in the mixing section. When the load is dynamically reduced, the air supply volume will also decrease, and the fresh air volume...

Text label: clean room purification project, fresh air volume control method

Several fresh air volume control methods in clean room purification projects

In the variable air volume air conditioning system, since the air supply volume changes continuously with the load change during the system operation, it will also affect the pressure condition in the air conditioning box, especially the pressure in the mixing section. When the load is dynamically reduced, the air supply volume will also decrease, and the fresh air volume will also decrease proportionally with the reduction of the supply air volume. When the load is very low, it may cause insufficient fresh air volume in some partitions of the variable air volume air conditioning system. On the contrary, when the air supply volume increases with the increase of the load during operation, if no control is performed, the fresh air volume will also increase proportionally with the supply air volume, resulting in an increase in the energy consumption of the variable air volume air conditioning system. Therefore, it is necessary to effectively control the fresh air volume of the variable air volume air conditioning system, especially the automatic control of the small fresh air volume, so as to ensure that the fresh air volume of the variable air volume air conditioning system can meet the requirements of the system.

1. Fan tracking control method

This method refers to using the difference between the supply air volume and the return air volume to indirectly control the fresh air volume. The supply air volume of the supply fan minus the return air volume of the return fan equals the fresh air volume and maintains it as a constant. When this control method is adopted, air volume monitoring devices are installed at both the supply and return fans. During the operation of the variable air volume air conditioning system, no matter how the supply air volume changes dynamically, the return air volume can be tracked and adjusted to keep the supply air volume and the same air volume. The difference is unchanged, that is, the fresh air volume is maintained unchanged, and the indirect control of the fresh air volume is realized.

This method is reasonable in theory, but in actual operation, the existence of dry measurement error will bring a series of problems. The measurement accuracy of supply air volume and return air volume directly affects the engineering feasibility of this method and control accuracy. Due to the low measurement accuracy of air volume, a large error in fresh air volume may result, on the one hand, may increase the energy consumption of the system; on the other hand, it may cause insufficient fresh air volume of the system, causing indoor air quality problems.

2. New and return air mixing box differential pressure control method

When the variable air volume air conditioning system operates in the condition of small fresh air volume, the fresh air regulating air valve is closed, the fresh air valve and the exhaust air valve are opened and properly adjusted according to the initial settings. In the mixing chamber, fresh air and return air are mixed. The mixing chamber is also called a mixing static pressure box. A static pressure difference sensor is set in the mixing static pressure box, and the static pressure value of the outdoor air is used as the zero point of the static pressure sensor. Zhongjing Global Purification can provide clean room, purification engineering, clean room consulting, planning, design, construction, installation and transformation and other supporting services.

The system operates with variable air volume. If the air supply volume increases, the negative pressure value in the mixing static pressure box decreases, the opening of the smaller fresh air valve decreases, and the fresh air volume of the system decreases synchronously. DDC controls the return air valve valve. The opening degree decreases, and a certain negative pressure in the mixing static pressure box is generally maintained; if the air supply volume decreases, the negative pressure value in the mixing static pressure box increases, the opening degree of the smaller fresh air valve increases, and the system fresh air volume increases. Synchronous increase, DDC controls the return air valve opening to increase, the system fresh air volume increases simultaneously, DDC controls the return air valve opening to increase, and continues to maintain a certain negative pressure in the mixing static pressure box.

New and return air mixing box differential pressure control method is more suitable for large and medium VAV air conditioning systems.

3. Bypass fan method

A bypass duct is introduced in parallel next to the original fresh air duct, and the bypass fan is in operation at any time when the system needs fresh air to ensure the required smaller fresh air volume at any time. One way to control the amount of fresh air is the bypass fan method.

The design of the bypass duct can make the bypass airflow easy to be detected by the sensor more accurately. In some of the methods that have been discussed, it is difficult to accurately measure the fresh air volume during the dynamic change process, which has a great impact on the actual engineering application and also causes energy loss on the blower. Therefore, the bypass fan method controls the fresh air volume. The accuracy is greatly improved. However, this method also has shortcomings, such as the need to lay additional pipes, the structure is complex, and the existing system is not easy to retrofit.

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