What Is Constant Air Volume?

The variable air volume system is the same as the supply air state, and the air volume is changed to adapt to the load change. The change of air volume can be realized by a special variable air volume terminal device. The terminal device can be divided into a throttle type, a bypass type and an induction type. Corresponding to this is the constant air volume system. ^[1]

The Variable Air Volume System (VAV system) was born in the United States in the 1960s. It maintains a constant supply air temperature based on changes in indoor load or changes in indoor required parameters, and automatically adjusts the air supply volume of the air-conditioning system to make indoor parameters Meet the requirements of all-air conditioning systems. VAV system pursues to meet the requirements of indoor air environment with less energy consumption. After the advent of the VAV system, it was not quickly promoted. At that time, the United States still dominated the Constant Air Volume (CAV) system plus end reheating and dual air duct systems. The oil crisis that broke out in the West in the 1970s caused VAV systems to be widely used in the United States, and continued to develop in the next 20 years. It has become the mainstream of American air-conditioning systems and is also used in other countries.

Figure 10-13 shows a typical VAV variable air volume system. With regular

Several problems in the design of VAV regulation system

1. Load and air volume problems: The maximum air volume of the winter and summer systems is calculated based on the maximum cold load or maximum thermal load of the system. The maximum cooling and heating load is not the sum of the maximum loads in each zone. The simultaneous load rate of the system should be considered. The cooling capacity provided by the air-conditioning equipment can automatically adjust with the load change within the building. The minimum air volume of the system can be calculated according to 40% -50% of the maximum air volume. The minimum air volume must meet the minimum requirements in terms of air distribution, and must be greater than the fresh air volume required for hygiene. ^[3]

In the VAV system, in addition to sending and returning (exhausting) fans, the larger noise sources also have VAV end devices. Pressure-independent VAVs are equipped with wind speed measurement sensors at the ends. These sensors generally require that the wind speed be higher than a certain value to ensure accurate measurement. Therefore, the wind speed flowing through the end inlet is relatively high, which is one of the reasons for the high noise generated by the end devices. Generally, the throttle end changes the air volume by adjusting the opening of the valve plate. Therefore, when the valve plate is closed, the wind speed passing through the valve plate also increases. Therefore, valve adjustment is also a source of noise.

The noise generated by the end device is introduced into the room through the supply air and the housing. The former is called Discharge Noise, and the latter is called Radiated Noise. In the end device product samples, detailed noise data are listed for designers' reference. Generally, the noise generated by the end device increases with the model, and increases with the increase of the back pressure difference. As the operating conditions of the VAV system change, the sound pressure level in the room is bound to change accordingly. Generally speaking, the human ear has a certain ability to adapt to the noise environment with a stable sound pressure level. After a long time, the human feeling will not be obvious. However, when the change in sound pressure level reaches 5dB, the human ear can feel it more clearly. This is why in some VAV systems, indoor personnel can sometimes hear noise, but sometimes they cannot feel it.

The air outlet of a general air-conditioning system has a constant cross-section, and the angle of the guide vanes rarely changes. Therefore, when the air volume decreases, it will inevitably affect the indoor air flow organization.

Foreign countries usually use the air distribution characteristic index ADPI to evaluate the airflow organization performance of a room. This index takes into account three factors, namely air temperature, air velocity and human comfort. If ADPI = 100%, it means that all the staff in the room feel comfortable; if ADPI reaches 80%, it can be considered as a satisfactory airflow organization effect. The relevant air flow organization test results show that: in the case of variable air supply, the slot diffuser and the lamp diffuser can maintain ADPI above 80% within a large air volume change range, indicating that these two types of air supply The performance of the tuyere is ideal.

Therefore, in the VAV system, ordinary square or circular diffusers are generally not used, but slit diffusers are used, and the side air outlet is rarely used.

1) Economical operation. Because the air volume decreases with the load, the cooling capacity and fan power are close to the actual needs of the building air conditioning load. In the transition season, you can also take advantage of outdoor fresh air cooling.

2) The indoor temperature of each room can be adjusted individually, and the air volume adjustment of each room is directly controlled by the thermostat installed in the room.

3) It has the advantages of general low-speed centralized air-conditioning system. For example, it can perform better air filtration and noise reduction, and is beneficial to centralized management.

4) Unlike other systems, the indoor ventilation frequency, airflow distribution, and fresh air volume can always be guaranteed. When the airflow is too low to affect the airflow distribution, the end reheating can only be used to further reduce the airflow.