What are the noteworthy issues in central air conditioning design

What are the noteworthy issues in central air conditioning design

1. Variable air volume ratio

　　 The air-conditioning system runs under partial load conditions most of the year, that is to say, the fans, ducts and air volume of the end of the variable air volume system are mostly between the maximum and minimum air volume limits.

　　According to experience, if no problems occur under these two extreme states, then the system can basically be guaranteed to operate normally most of the time.

The ratio of the minimum design air volume to the maximum design wind is defined as the variable air volume ratio (KV). Generally, the room’s variable air volume ratio KV value z* should not be less than 0.4 ~ 0.5, otherwise it will easily lead to room airflow organization deterioration, noise and ventilation problems ；

　　The KV value of the system should not be less than 0.4 ~ 0.5, otherwise it will cause serious shortage of system fresh air and unstable control.

　　 Whether it is a room or a system, the VAV ratio is a relatively important dynamic characteristic parameter that characterizes the VAV system;

　　 2. Fresh air problem

　　 In the transition season, by adjusting the opening of the three valves of fresh air, return air and exhaust air, the fresh air volume is changed to maintain a mixed air temperature. This approach is to shorten the opening time of the chiller, let alone whether the economic circulation system can operate normally in the quantitative system.

　　 For air-conditioning systems using mixed air, whether it is a constant air volume system or a variable air volume system, the fresh air volume is distributed in each room according to the load. In other words, even if the total fresh air volume meets the requirements, some rooms will have insufficient fresh air.

　　 For a variable air volume system, since the air volume sent into the room changes, the fresh air volume of the room must also change. If in order to meet the requirements of these rooms, the total fresh air volume will increase, and may even exceed the required air supply volume in some cases.

　　 Therefore, the ASHRAE standard proposes that under a certain amount of fresh air, the content of carbon dioxide in the total return air may not exceed the standard, and the return air can be used to reduce the total fresh air volume.

　　ASHRAE standard only answers the question of how to determine the total fresh air volume. However, for a variable air volume system, the air volume sent into the room changes, and the fresh air volume of the room must also change. The problem of fresh air is even more prominent.

　　 Therefore, in occasions with high requirements for fresh air, a new air duct may have to be laid separately. In this way, the space occupied by the air duct will increase.

　　 Therefore, in the variable air volume system, there are three main problems with fresh air: the control of the total fresh air volume, the determination of the total fresh air volume, and the distribution of fresh air.

　　 Three, noise problem

　　 In the variable air volume system, the relatively large noise source is in addition to the sending and returning (exhaust) fans, there are also variable air volume terminal devices. The wind speed flowing through the terminal entrance is relatively high, because the pressure-independent variable air volume terminal is equipped with wind speed measurement sensors. These sensors generally require the wind speed to be higher than a certain value to ensure accurate measurement.

　　 This is one reason why the end device produces higher noise.

　　The general throttling type end changes the air volume by adjusting the valve opening. When the valve plate is closed, the wind speed flowing through the valve plate also increases, so valve adjustment is also a source of noise.

　　 Fourth, airflow organization

　　The air supply outlets of general air-conditioning systems are fixed and cut back, and the angle of the guide vanes is rarely changed, so when the air volume decreases, it will inevitably affect the indoor airflow organization.

　　 5. Small pressure between fingers

　　 Since the new exhaust air volume of the variable air volume system and the return air volume of the room are variable, the positive pressure of the room also fluctuates, and it is not certain that the air volume system is so stable.

　　 If this problem is not dealt with properly, problems such as difficulty in opening the door and serious wind leakage between door and window cracks will occur.

　　 Six, the choice of terminal device

　　 There are many types of terminal devices, and products from different manufacturers have their own characteristics. When selecting, the acoustics, control performance and room function requirements of the terminal device should be fully considered.

　　 When selecting the size, generally based on the design of the maximum air volume, a certain margin must be considered to meet the needs of future development. However, the end type selection should not be too large. Excessive selection will reduce the adjustment range of the air valve and reduce the adjustment ability, which will easily cause the end air valve to oscillate when the air volume is small.