Centrifugal fans include Housed centrifugal fans and Unhoused centrifugal fans. The commonly used type is a centrifugal fan with a snail shell, so the term 'with snail shell' is omitted from the name. The full name of a volute free fan is a volute free centrifugal fan, also known as a plug fan or a Plenum fan in different situations.

The centrifugal fan is covered with a snail shell to increase its static pressure head and fan efficiency in the high-pressure section. Generally speaking, after removing the volute, the static pressure head of most of the air volume range (high-pressure zone) of a centrifugal fan with a volute is lower than that of a centrifugal fan with a volute, and the efficiency of the fan in the high-pressure zone is therefore lower than that of a centrifugal fan with a volute. And the static pressure head of a portion of the airflow range (low-pressure area) is higher than that of centrifugal fans with volutes, and the efficiency of fans in low-pressure areas is also higher than that of centrifugal fans with volutes. After removing the snail shell, the maximum wind capacity of the centrifugal fan also increased. Given this characteristic of centrifugal fans, a special design of the impeller of the centrifugal fan resulted in a volute free centrifugal fan with high efficiency.

The outlet wind speed of a volute centrifugal fan is directional and uneven. If the air duct turns before the static pressure is fully recovered, it will result in significant energy loss. If the non volute centrifugal fan is placed at this wind direction turning point, this energy loss can be avoided. That's why non volute centrifugal fans are widely used as floor exhaust fans for cabinets in data centers.

Another main application of volute free centrifugal fans is to form fan walls. Replace one large-diameter centrifugal fan or axial flow fan with multiple volute free centrifugal fans.

The speed of large-diameter centrifugal fans and axial fans cannot be very high, so the noise generated is often low-frequency noise. It is very difficult to eliminate low-frequency noise. Cinemas, hotels, and office buildings all need to spend a lot of money to eliminate these difficult to eliminate low-frequency noises.

The emergence of fan walls without volutes has solved the most difficult problem in the air conditioning industry.

Non volute centrifugal fans are generally connected directly to the motor. Therefore, not only does it avoid energy consumption caused by belt transmission, but it also saves the operating cost of belt loss.

For variable air volume systems, non volute centrifugal fans often use electronically ventilated motors (EC motors). Among them, the average efficiency of IPM (Interior Permanent Magnetic) motors is over 90%.

In theory, there should have been centrifugal fans without volutes first, followed by centrifugal fans with volutes. A centrifugal fan with a snail shell should be considered an upgraded version of a centrifugal fan without a snail shell. But with the advancement of technology, nowadays in many applications, there is a return to non volute centrifugal fans, which have become upgraded versions of volute centrifugal fans. In many energy-saving renovation projects, volute centrifugal fans have been removed and replaced with non volute centrifugal fans.

But we cannot simply say that snail shell centrifugal fans are outdated just because of this. The largest AHU (Air Handling Unit) manufacturer in Japan claims to use a non volute centrifugal fan while also adding an incomplete volute. From this, it can be seen that the pressurization effect of the snail shell is still effective. In some occasions, it is still *.