In many cases, faults such as noise and low efficiency of centrifugal fans are related to their volute tongues, and in many fan renovation projects, the renovation of volute tongues is also very common, and sometimes small changes can To improve the efficiency of the fan to a certain extent, or change the performance of the fan, what part is the volute tongue of the centrifugal fan? What is the impact on performance?
1) Spiral tongue of centrifugal fan
There is often a “tongue-like” structure formed by the shell wall at the connection between the volute and the air outlet of the centrifugal fan, called the volute tongue, which is used to prevent part of the gas from circulating in the volute. When the impeller blades are rotated, the airflow at the outlet of the passage passes near the volute tongue, and the tongue of the volute tongue divides them into two: most of the airflow flows along the passage to the outlet of the fan; a small part of the airflow passes through the volute tongue and the impeller. The gap between them flows back to the volute, and after rotating with the impeller for one week in the volute, it returns to the volute tongue to participate in a new shunt.
Different centrifugal fans have different shapes and sizes of volute tongues. Generally, volute tongues include flat tongue, short tongue (shallow tongue), deep tongue and pointed tongue:
1. When the centrifugal fan adopts sharp tongue, the upper limit of efficiency is higher, but the efficiency curve is steep, the economic working area is small, and the noise is large.
2. The deep tongue is close to the sharp tongue, and is mostly used in centrifugal fans with low specific speed.
3. When the short tongue is used, the efficiency curve is flat and the economic working area is wider.
4. The tongue of the flat tongue is tangent to the volute and perpendicular to the direction of the airflow outlet. It is mostly used for low-noise centrifugal fans.
5. The efficiency of the centrifugal fan with flat tongue is somewhat lower than that of the above three types.
2) The influence of the worm tongue on the performance of the centrifugal fan
The shape and geometric size of the volute tongue and the gap (t) between the outer circumference of the impeller have a great influence on the noise of the fan.
1. Influence of the cochlear-lingual gap
Regarding the clearance t between the top of the volute tongue and the impeller, generally take the following two values:
t=(0.05~0.10) D2 (applicable to backward impeller);
t=(0.07~0.15) D2 (applicable to forward impeller).
(1) The gap value has a great influence on the noise. If the gap value is too small, the pressure in the performance curve of the centrifugal fan will increase when the flow rate is large, but the efficiency will decrease, the noise will increase, and the fan will make a screaming whistle. .
(2) As the gap value increases, the noise decreases. If the gap is too large, although the noise is reduced, the pressure and efficiency are reduced.
For the same centrifugal fan, when it works near the suitable working point, the difference in the sound pressure level of the noise can reach 15dB (A) due to the difference in the size of the gap.
2. The influence of the shape of the snail tongue
Regarding the shape of the volute tongue, it is mainly related to the radius r of the arc at the top of the volute tongue, generally r=(0.03~0.06) D2, the lower limit is used for large centrifugal fans, and the upper limit is used for small centrifugal fans. Not much, but the efficiency will drop. The size of the arc radius at the top of the volute tongue also has a significant impact on the noise of the centrifugal fan:
(1) As long as the gap is large, when the radius is greater than 12mm, the aerodynamic noise of the fan will not be affected much;
(2) When the radius is too small, the noise of the fan will still increase.
For the same centrifugal fan, if the radius value is changed, the noise difference is about 6dB(A).