VAV system(1)

VAV system(1)

Summarize

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 indoor requirement parameters, automatically adjusting the supply air volume of the air conditioning system, thus achieving the required full-air conditioning system for indoor parameters. The VAV system aims to meet the requirements of indoor air environment with less energy consumption. After its emergence, the VAV system did not spread rapidly. At that time, the dominant system in the United States was the Constant Air Volume (CAV) system combined with terminal reheating and dual duct system. The oil crisis that broke out in the West in the 1970s prompted the widespread application of the VAV system in the United States, and it continued to develop over the following 20 years, becoming the mainstream of air conditioning systems in the United States and also being applied in other countries.

merit and demerit

The VAV system has the following advantages:

1. Since the VAV system adjusts the air supply volume to the room to adapt to changes in load, and can also consider certain simultaneous usage conditions when determining the total air volume of the system, it can save the energy consumption of the fans and reduce the installed capacity of the fans. Relevant literature indicates that compared with the CAV system, the VAV system can save fan energy consumption by approximately 30% to 70%, and the simultaneous usage coefficient for different buildings can be around 0.8.

2. Since the terminal of the VAV system can adjust the air supply volume according to the deviation between the indoor temperature and the set value, it has certain independent control performance compared to the CAV system. During partial load conditions, it can effectively reduce reheat and may even completely avoid end reheat.

3. The system has good flexibility and is easy to modify and expand, especially suitable for buildings with variable layouts, such as rental office buildings. When indoor parameters change or partitions are reconfigured, it may only require replacing branch pipes and terminal devices, moving the air outlet positions, or simply reconfiguring the indoor thermostat settings.

4. The VAV system belongs to the full air system, and it has some advantages of the full air system, such as being able to utilize fresh air to eliminate indoor cold loads during transitional seasons and winter, being able to respond quickly to load changes, and having no problems of fan coil water accumulation and mold growth in the indoor area.

Although the VAV system has many advantages, with the advent of the VAV system, most systems have inevitably exposed the following problems to varying degrees.

From the user's perspective, the main issues are:

1. Lack of fresh air, causing people in the room to feel suffocated;

2. Excessive positive or negative pressure in the room, resulting in a large amount of outdoor air infiltrating and making it difficult to open the door;

3. Excessive indoor noise.

From the perspective of operation and management, the main problems are:

1. The system is unstable in operation, especially for systems with "Economizer Cycle";

2. The energy-saving effect is not obvious.

In addition, the VAV system also has some inherent disadvantages:

1. The initial investment of the system is relatively large;

2. For situations with significant changes in indoor moisture load, if a room temperature control method is adopted without a terminal reheat device, it is often difficult to ensure the indoor humidity requirements.

3. For a system, the problems may not always exist all the time; they may occur in one working condition and disappear in another.

From the surface, it seems that the VAV system merely adds some terminal devices and air volume regulation functions compared to the CAV system. However, due to the changes in air volume and the additional terminal equipment of the VAV system, the VAV system has special characteristics that are different from the constant air volume system in terms of scheme design, equipment selection, construction drawing design, and construction and commissioning. The problems and defects of the VAV system are caused by various factors. Some may require certain technical support to solve; while others may be avoided through the efforts of the air conditioning system designers.