Some noticeable problems in the design of the hott

2022-08-05
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Several noticeable problems in air conditioning design

I. variable air volume ratio

the air conditioning system operates under partial load for most of the year, that is, the fan, air duct and terminal air volume of the VAV system are between the two limit states of maximum air volume and minimum air volume for most of the time. According to experience, the relevant standard: BBT 0024 ⑵ 004 if no problems occur under these two limit states, It can basically ensure that the system operates normally most of the time. The ratio of minimum design air volume to maximum design air volume is defined as variable air volume ratio (kV). Generally, the Kv value of the room should not be less than 0 5. Otherwise, it is easy to cause deterioration of air distribution, noise and ventilation problems in the room; The Kv value of the system should not be less than 0 5. Otherwise, it will lead to serious shortage of fresh air in the system and unstable control

generally speaking, it is easy to determine the maximum design air volume of a room, but it is relatively difficult to determine the minimum design air volume for places with uncertain load changes such as conference rooms, cinemas and restaurants. In fact, in determining the minimum air volume, in addition to considering the characteristics of load changes, the room air distribution and indoor air quality requirements should also be considered. Too small air supply volume in the room will cause problems such as cold air sinking, insufficient fresh air, insufficient ventilation times, etc. In order to ensure the measurement accuracy of wind speed, pressure independent terminal devices also have minimum air volume requirements

in addition, for rooms with lamp return air, part of the lighting load is not directly entered into the room, but taken away by the return air, which increases the temperature difference between supply and return air. The omniscient service load cannot be included in the calculation of air volume. Therefore, when determining the design air volume, the influence of room return air mode should be considered

VAV ratio is an important dynamic characteristic parameter of VAV system in both rooms and systems

II. Fresh air problem

a. Figure 2 is a typical economic cycle system of CAV system. In the transition season, adjust the opening of the three valves of fresh air, return air and exhaust air to change the fresh air volume to maintain a mixed air temperature. This practice is to shorten the start-up time of the refrigerator. Let's not talk about whether the economic circulation system can operate normally in the constant air volume system. In the current operation, the operation is simpler and more humanized. However, simply doing as shown in Figure 2 can not guarantee the fresh air volume in the VAV system. Figure 3 shows the pressure distribution diagram of a system. The solid line represents the design condition, and the dotted line represents the condition when the design air volume is 50%. It can be seen that if no measures of constant fresh air are taken, when the total air volume decreases, the pressure (absolute value) at the mixing point of fresh return air will decrease, resulting in the decrease of fresh air volume

for the air conditioning system with mixed air, whether CAV system or VAV system, the fresh air volume is distributed among rooms according to the load. In other words, even if the total fresh air volume meets the requirements, some rooms will have the problem of insufficient fresh air. For VAV system, since the air volume sent into the room is variable, the fresh air volume of the room must also be variable

therefore, in VAV system, there are three main problems in fresh air: control of total fresh air volume, determination of total fresh air volume and distribution of fresh air (the control of fresh air volume is introduced in § 5 of this article)

b. determination and distribution of fresh air volume

as mentioned earlier, whether CAV system or VAV system, fresh air volume is distributed among rooms according to load. Even if the total fresh air volume meets the requirements, some rooms will have the problem of insufficient fresh air. 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. For this purpose. ASHRAE standard proposes that under a certain amount of fresh air, the CO2 content in the total return air does not necessarily exceed the standard, and the return air can be used to reduce the total fresh air volume. The standard gives the calculation formula of the corrected total fresh air volume

where y - the ratio of the corrected total fresh air volume to the total air supply volume

x - ratio of total fresh air volume and total air supply volume of uncorrected demand

z - the maximum value of the ratio of fresh air volume to air supply volume in each room

ashrae standard only answers the question of how to determine the total fresh air volume. However, for VAV system, the air volume sent into the room is variable, and the fresh air volume of the room is bound to change. The problem of fresh air is even more prominent. Therefore, in the occasions with high requirements for fresh air, the new air duct may be laid separately. In this way, the air duct occupies more space in the building

the problem of fresh air is related to many factors such as building load characteristics, system form and outdoor meteorological conditions. The above methods or assumptions may be feasible from the control logic, but they are not applicable to any system in practice. For a particular building, it is necessary to analyze the summer working condition, winter working condition and transition season economic cycle working condition of the system

III. according to the statistics of the National Bureau of statistics

in the VAV system, in addition to the supply and return (exhaust) fans, there are VAV terminal devices. Pressure independent VAV terminals are equipped with wind speed measurement sensors. These sensors generally require that the wind speed be higher than a certain value to ensure accurate measurement. Therefore, the wind speed passing through the terminal inlet is relatively high, which is one of the reasons for the high noise of the terminal device. Generally, the throttle type terminal changes the air volume by adjusting the opening of the valve plate. Therefore, when the valve plate is turned down, the wind speed flowing through the valve plate also increases. Therefore, valve adjustment is also a source of noise

the noise generated by the terminal device is introduced into the room through the air supply and the housing. The former is called discharge noise, and the latter is called radiated noise. Detailed noise data are listed in the product samples of terminal devices for designers' reference. Generally, the noise generated by the terminal device increases with the increase of the model and the rear differential pressure. As the operating conditions of VAV system are changing, the indoor sound pressure level is bound to change accordingly. Generally speaking, the human ear is sensitive to the noise environment with stable sound pressure level

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