How to choose the air outlet And calculate Central air conditioning duct air speed method ?

Ducts, are duct systems used for air transport and distribution. There are two types of ducts: composite ducts and inorganic ducts. Ducts can be classified by cross-sectional shape and material.

A central air conditioning air outlet is an end device used for air supply and return in a central air conditioning system and is a kind of air distribution equipment. The air supply outlet sends the cooled or heated air indoors, while the air return outlet sucks back the dirty air indoors, forming a whole air cycle between the two, ensuring the indoor cooling and heating effect while also ensuring the cooling and comfort of the indoor air.

 The size of the air outlet depends on the size of the capacity of the indoor unit, if the air outlet is too large, the air duct is too long, the airflow speed will drop, thus affecting the effect of air conditioning; if the air outlet selection is too small, the airflow speed will become large, resulting in the wind blowing directly on the human body caused by discomfort, and may lead to excessive noise.

1) The wind speed in the air duct:

General air conditioning room on the air conditioning system of the limited noise allowed to control between 40 ~ 50dB (A), that is, the corresponding NR (or NC) number of 35 ~ 45dB (A). According to the design specifications, to meet this range of noise allowable value of the main air speed of 4 ~ 7m / s, branch air speed of 2 ~ 3m / s. Ventilator and anechoic device between the air duct, the air speed can be used 8 ~ 10m / s.

2) the size of the air outlet calculation:

In order to prevent air outlet noise, air supply outlet air speed should be 2 ~ 5m / s. The size of the air outlet and the calculation of the size of the wind pipe is basically the same, generally when the floor height of 3 ~ 4 meters in the room about to take the wind speed in 2 ~ 2.5m / s.

3) Back to the wind speed of the air intake:

When the air return is located in the upper part of the room, the suction speed is 4 ~ 5m / s, the air return is located in the lower part of the room, if not close to the place where people often stay, take 3 ~ 4m / s, if close to the place where people often stay, take 1.5 ~ 2m / s, if used in the corridor back to the air, take 1 ~ 1.5m /s.

4) Duct installation considerations and duct calculations:

In the case of duct design as small as possible to ensure that the main air speed 5m / s, branch air speed 3m / s.

(1) duct calculation formula: the selected equipment air volume ÷ 3600 ÷ wind speed = duct cross-sectional area;

Also note to ensure that the duct: long side ÷ short side ≤ 4, generally do not > 4, special circumstances special treatment;

(2) the choice of air outlet: the selected room air volume ÷ 3600 ÷ wind speed = diffuser throat cross-sectional area;

Note: The cross-sectional area of the double louvered air outlet is the area obtained from the above formula ÷ 0.7.

5, calculate the size of the air duct:

(1) equal damping method (equal pressure method) is a convenient calculation method, applicable to a variety of occasions;

(2) Determine the basic damping coefficient in the main duct according to the following table:

Because the return air duct is located in the suction part, mainly subject to external pressure, attention should be paid to reduce its duct burden. For the duct system, it is often used:

(1) air supply duct: 0.08 ~ 0.15mm H 2 O / m;

(2) return air duct: 0.06 ~ 0.1 mm H 2 O / m as a benchmark.

6） in the air duct machine air duct design, pay attention to the air duct machine inlet and outlet air with static pressure box, in order to equalize the wind pressure, reduce noise, and make the static pressure box to ensure that the flow rate in 3 m / s below, its length can be determined according to the actual situation.

7） wind pressure estimation:

(1) such as elbows, tees, reducers, etc. less case of loss of about 4pa per meter.

(2) such as elbows, tees, reducers, etc. more cases of loss of about 6pa per meter.

8） The air duct fan coil air outlet design:

(1) the first air supply and fan coil outlet distance to be appropriate;

(2) fan coil air supply and return air outlet distance should be appropriate. (≤ 5 meters)

9) The selection of air outlets:

(1) new air outlet, air supply outlet with double louvre air outlet;

(2) return air outlet with grille air outlet;

(3) exhaust air outlet with double louvers;

(4) fluorine system because the air volume is generally small, such as the requirement of winter heating needs, it is appropriate to use double louvers, should not use diffusers.

Table 1 Recommended air supply outlet flow rate

Table 2 Air velocity in the air duct

Table 3 Maximum allowable flow velocity of low-speed duct system (m/s)

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