Air volume: the number of cubic meters of air conveyed by the fan per minute, SI: m³/h.

Full pressure: all the energy possessed by the gas, equal to the dynamic pressure + static pressure, SI: Pa.

Dynamic pressure: the pressure required to accelerate the gas from zero speed to a certain speed, SI: Pa.

Static pressure: the difference between the absolute pressure of a fluid at a certain point and the atmospheric pressure, SI: Pa.

Fan speed: the number of revolutions per minute of the fan impeller, SI: RPM;

Shaft power: the actual power transferred to the fan shaft by the motor excluding external loss factors, usually considered to be the actual power required by the fan, SI: KW.

Noise: the noise formed by the superposition of aerodynamic and mechanical noise during normal operation of the fan; most manufacturers publish the A-recorded weighted noise (dBA) at 1.5 m. SI: dBA

Full pressure efficiency: air volume X full pressure / shaft power / 1000 / 3600 * 100%

Power supply: 380/50/3, 220/50/1, 220/50/3, 690/50/3, etc.

Outlet air velocity: the air velocity of the fan outlet cross-sectional area, controlling the outlet air velocity can indirectly control the noise. si: m/s

Parameters to be provided for selection

• fan form, type and application
• installation method
• gas composition (including special temperature, humidity, corrosiveness and impurity)
• direction of air outlet
• indoor or outdoor installation
• other limited conditions (e.g. noise less than 60dBA, etc.)
• accessories and special requirements

Fan performance curves

That downward curve on the graph represents the working point of the fan, the vertical axis is the wind pressure, and the horizontal axis is the air volume. Selection should avoid the working point immediately below the maximum pressure point and the point 40% below the maximum pressure.

Axial fan recommended selection area in the curve began to fall smoothly in the region, the working area of 65 ~ 90%:

Backward tilting fan air volume axial power curve has the highest point, not overload characteristics. Working area 40~85%:

Forward curved fan air volume wind pressure curve is steeper, working area 35~80%:

In general the larger the air volume, the smaller the wind pressure. When designing the duct, according to the pipeline resistance calculation and air volume requirements, determine the total air volume and static pressure loss of the duct system, and it is best to retain 30~50 Pa residual pressure at the air outlet. The result thus obtained is the basis for your choice of fan. For example, design a pipeline. The most unfavorable loop under the static pressure loss of 300 Pa, the total air volume required is 5000 m³/h, then your fan should be able to choose the work curve to meet the 5000 air volume, static pressure of 330~350 Pa of the model.

Selection according to the sample

Fan manufacturers will print the factory’s fan product samples and catalogs. In the fan product samples and catalogs, usually according to the series, machine number listed in a variety of rotational speeds under the selection of performance table, the performance parameters in the table is the fan’s highest efficiency point of 90% of the range of values, and to take the value of 6 to 8 performance points for selection.

05Selection of fan type and model

·Fans are categorized by impeller form

Centrifugal fan, axial flow fan, mixed flow fan, cross-flow fan, etc;

·Fans are classified according to the installation location or according to the installation form

Roof fans, side-wall fans, duct fans, fan boxes, etc;

·Classification of fans according to use

Exhaust fans, supply fans, filter fans, dust removal fans, smoke exhaust fans, etc;

·Combination of fan classification

Roof centrifugal exhaust fans, sidewall axial flow exhaust fans, smoke exhaust mixed flow fans, etc.

Selection of fan types by specific rotation number

Specific revolutions ns is a factorless parameter that reflects the combined characteristics between flow rate, full pressure and speed of different types of ventilators. It is usually the specific speed for a single stage with single suction, taking the value at the point of highest efficiency.

To determine the required specific rotation speed of the fan, the speed of the fan must first be selected. The geometry of the selected fan should not be too large, and the circumferential velocity of the impeller should not be too high, so that if the initial rotational speed is unqualified, it can be adjusted and calculated from scratch.

Example: Q = 23612 m³/h P = 5761 Pa

As the motor speed is generally 2900 r/min, 1450 r/min, 960 r/min, 730 r/min several, try to take a large speed, which can reduce the external dimensions of the fan, the other from the fan pressure this is a high-pressure fan, so choose 2900 r/min and 1450 r/min two kinds of speed for the selection of shape.

ns1=62.26 (n=2900 r/min)

ns2=31.28 (n=1450 r/min)

Therefore, it is possible to select a 4-62 type or 9-26 type of forward curved centrifugal fan, backward curved centrifugal fan, or mixed flow fan. (The number “4” in front indicates the pressure coefficient, and “62” indicates the airflow coefficient, 72 large airflow, 62 medium airflow, 26 low airflow, 19 small airflow, and 12 small airflow.)

Determine the outer diameter of the impeller of the fan (D) according to the formula of the pressure coefficient of the fan:

Picture

P – full pressure Pa, D – impeller diameter m, n – impeller speed r/min, ρ – medium density kg/m³

When n = 2900 r / min can choose 4-62 type seat number 15 fan, when n = 1450 r / min can choose 9-26 type seat number 10 fan. Then according to economic considerations, the choice of 9-26-10 fan.

Determination of fan shaft power:

Picture

Starting power Ne=1.15N=54.28 kW   

The main principle of fan selection: reasonable organization of airflow, to complete the required functions

1. Selection, first find the static pressure

Example 1: a heat treatment workshop, an area of 4000 square meters, plant height of about 6m, no air conditioning, the highest average temperature in the workshop in summer can reach 50 ℃, in order to reduce the temperature in the workshop, so that the staff feel comfortable, the use of mechanical air supply and exhaust way to introduce the outside world of cold air. For the first time, 10 sets of side-wall exhaust fans were used, in the form of louvered air supply, but the louvered installation position was high (about 4m). After use, the workshop surface temperature reduced by 5 ℃, 5.5 meters at the traveling car, the temperature reduced by 10 ℃, the staff is not very satisfied with its effect. After the transformation, the original fan position and the number of units are unchanged, increase the area of air supply louvers, the height of the louvers will be reduced to 0.5m from the ground. After the transformation, the total amount of air supply and exhaust in the workshop is basically unchanged, but the surface temperature in the workshop is reduced by 9 ℃, and the staff think that the effect has improved significantly.

Reason analysis: heat treatment equipment is the main heat source of the workshop, the air is heated to the roof above the gathering, after convection cycle, the temperature in the whole workshop increases. The first program, the use of mechanical air supply and exhaust is not wrong, but should not be installed too high louvres, so that the cold air into the room is quickly heated by the hot air mixture, not to the role of personnel cooling. In the second program, the height of the louvers was lowered, so that the cold air first flowed through the surface of the staff, and then mixed with hot air to reduce the surface temperature in the workshop at most. Achieved the design purpose.

Example 2: a workshop sitting in the north to the south, due to topographical reasons, perennial southeast winds, resulting in the workshop airflow is mainly from south to north. Because of the heavy acetic acid smell in the workshop, so the owner wants to increase the mechanical exhaust, and then install a row of fans on the south wall. After use, the effect is very unsatisfactory. After remodeling, the exhaust fan on the south wall is installed on the north wall, and motorized louvers are added to the original fan position on the south wall. The effect is very obvious after remodeling.

Reason analysis: the original program mechanical exhaust and natural wind direction is opposite, so the exhaust effect is very unsatisfactory. After remodeling, mechanical exhaust and natural wind to form a synergy, greatly improving the exhaust effect, in addition to increasing the louvers, but also strengthen the effect of natural ventilation. So the effect will be more obvious.

Similar occasions:

a. Need to exhaust heat or hot steam, should try to give priority to the installation of roof exhaust fan;

b. When heating, cooling or sending fresh air is needed, warm airflow or cold airflow should be allowed to flow through the staff’s location as much as possible, so duct fans or side-wall fans are mostly used;

c. For fire-fighting smoke exhaust, priority should be given to roof fans or suspended air ducts, so duct fans are mostly used;

d. Utilize natural airflow as much as possible (the location and form of fans should be set reasonably).

2. Control of airflow stratification / sub-area

Airflow stratification can be used not only in the clean room, can also be used in many other situations. Airflow stratification only need to consider and control the airflow within a certain space or a certain height range.

Example: A cement filling workshop requires 20 times/hour of air exchange if the whole air exchange is done, but only 5 times/hour of air exchange is required if airflow layering technology is used. Therefore, it can save a lot of equipment cost and operation cost.

Similar occasions: operating room, bacterial culture room, flour mill filling workshop, cafeteria. Welding workshop, etc.

3. Local air supply and exhaust

In some cases, the workshop as a whole clean, but there are a few individual sources of serious pollution (or serious heat), then you need to use the local exhaust.

In some cases, only need to take care of the fixed position of the staff of the airflow, it should be taken to send local fresh air.

Example: an open space, the outside temperature is very low (-30 ° C), but the staff need to heat. If the overall heating is used, it can only be a waste of energy. In this case, heat radiation heating should be considered first, or the use of local heating fan can be.

06Whether less noise is better for selection

Noise always accompanies the operation of the fan and cannot be eliminated. Recent studies have shown that as long as the wind speed exceeds 0.75 m/s, noise is generated. Of course, the lower the wind speed, the less n   oise is generated.

Noise is harmful pollution. In our design, we always want to minimize the noise of the fan equipment as much as possible. With the development of fan technology, the noise pollution we can do is getting smaller and smaller.

But isn’t it true that when selecting a fan, the less noise the better?

Small noise is of course good, but must take into account its economy. The lower the required noise level, the higher the cost of the whole unit. Approximately for every 10 dB reduction, the cost of the fan rises by a factor of 1 (empirical value, non-linear). Most fans are unlikely to have a minimum noise level below 35 dBA.

So when choosing a fan, the noise “ enough is good”, unnecessary must pursue low noise.

For example, if the turbine is located in an uninhabited area, it is sufficient to consider that the noise level does not exceed the “red line”.

Fan equipment is located in the area of the existence of higher noise equipment, fan equipment noise can be set to “the highest equipment noise – 6dBA”, after the synthesis of the noise is at most 1 dB higher, and the cost of the most economical. If the “highest equipment noise -10dBA”, the synthesized noise is still that of the highest equipment, and the lower noise has been “obliterated”.

If the equipment in which the fan is located is soundproof or sound-absorbing, it is sufficient to consider the effect of noise transmission.

07 Noise control

Fan noise sources are divided into aerodynamic noise and mechanical noise, aerodynamic noise has eddy current noise, rotating noise, radiation noise, mechanical noise has bearing noise, vibration noise. According to different sources of noise to choose the appropriate noise control methods. Usually there are the following practices to control noise:

1. Selection of suitable fans and piping forms

In the case of the same air volume and pressure, the noise of axial fan is higher than that of backward tilting impeller, backward tilting impeller is higher than that of forward curved impeller, and the radiated noise of round pipeline is lower than that of rectangular pipeline.

2. Control the duct wind speed (dynamic pressure)

3. Away from the attenuation method

Place the fan at a location far away from the target to minimize the impact through the natural attenuation of sound.

4. Addition of noise barriers

Separate the fan equipment from the target area, through the reflection and absorption effect of the isolation barrier to achieve the effect of noise reduction. For example, set up equipment room/equipment layer, soundproof box, soundproof glass cover, etc.

5. Adding sound deadening devices

The use of silencing materials to eliminate noise. The use of loose porous, convex surface materials, so that the sound drilled into the holes continue to reflect attenuation, wave peaks and valleys superimposed attenuation, thus playing a role in reducing the effect of noise. For example, set up silencer, muffler box, muffler cover, sound-absorbing cotton, etc.. You can also increase the muffler to change the direction of airflow noise transmission.