Xin Feng Classroom | Ventilator Series: Types and Structures of Fans

01  

Types of Ventilators  

I. Classification by Total Pressure Generated  

Fans are categorized into three types: ventilators (total pressure ≤ 15 kPa), blowers (total pressure 15–340 kPa), and compressors (total pressure > 340 kPa). Ventilators are further subdivided as follows:  

Low-pressure ventilators: total pressure ≤ 0.98 kPa, medium-pressure fans with total pressure 0.98–2.94 kPa, and high-pressure fans with total pressure 2.94–14.71 kPa.

II. Classification by Working Principle Fans are categorized into two types based on their working principle: impeller-type and positive displacement-type. Impeller-type fans are further divided into centrifugal, axial flow, and mixed flow types. Positive displacement-type fans are classified into reciprocating and rotary types, with rotary fans subdivided into three categories: vane blowers, Roots blowers, and screw blowers.

02

Basic Structure of Ventilators

I. Inlet Duct The inlet duct, also known as the bell mouth, serves as the fan's intake. Its function is to uniformly guide gas into the impeller with minimal energy loss. Commonly used inlet duct types, as shown below, include: cylindrical, conical, arc-shaped, conical-cylindrical, and conical-arc (hyperbolic).

Cylindrical: Vortex zones form at the impeller inlet, resulting in poorer performance when drawing air directly from the atmosphere. Conical: Superior to cylindrical, but its short length yields suboptimal results. Curved: Outperforms the previous two types (widely adopted in practical applications). Conical-curved: Optimal design, adopted by virtually all high-efficiency fans.

The impeller should be fitted to the collector with a sleeve-type clearance. A face-to-face clearance configuration is generally less common. II. Impeller The impeller is the primary component of a fan, and its dimensions and geometry significantly impact the fan's performance. The impeller of a centrifugal fan consists of a front plate, rear plate, blades, and hub.

The blades are welded to the front hub. The welded impeller is lightweight with smooth flow passages. The rear hub is riveted to the wheel hub.

The configurations of the impeller front plate include flat front plates, conical front plates, and curved front plates, as shown in the figure.

 Centrifugal fan impellers can be classified into three types based on the installation angle of the blade outlet, as shown in the figure above: forward-curved, radial, and backward-curved.

At the same peripheral speed of the impeller, a larger blade outlet angle generates higher pressure. Therefore, for two centrifugal fans of identical size and rotational speed, the pressure produced by a forward-curved impeller is higher than that of a backward-curved impeller. However, backward-curved impellers generally exhibit better flow efficiency than forward-curved ones. Consequently, under normal conditions, fans equipped with backward-curved impellers consume less electricity than those with forward-curved impellers.     Simultaneously, the performance curves of the three impeller types reveal that beyond a certain flow rate, the shaft power of backward-curved impeller fans exhibits a decreasing trend, indicating their overload-resistant characteristics. In contrast, radial and forward-curved impeller fans show increasing shaft power with rising flow rates, suggesting a greater susceptibility to overload conditions. During abnormal operating conditions in ventilation and dust removal systems, the backward-curved impeller fan's overload protection prevents motor burnout. In contrast, the other two fan types may experience overload incidents leading to motor failure.

III. Housing

The performance and efficiency of a fan are primarily determined by the impeller, but the shape and size of the housing, as well as the configuration of the inlet, also influence these characteristics.

The casing is the outer shell surrounding the impeller, typically helical in shape. Its cross-section gradually expands along the impeller's rotational direction, reaching maximum width at the airflow outlet. Casing materials include steel plates, plastic sheets, and fiberglass-reinforced plastic. Cross-sections may be square or circular. Generally, low- and medium-pressure ventilators feature square cross-sections, while high-pressure ventilators predominantly use circular cross-sections.     The casing's function is to collect the airflow ejected from the impeller, reduce the velocity of the high-speed airflow, increase its static pressure, thereby overcoming external resistance to deliver the airflow.  Spiral casing configuration:

Archimedes' spiral. Volute outlet diffuser: Since the airflow exiting the volute diverges in the direction of impeller rotation, the diffuser is typically designed to expand toward the impeller side. Its expansion angle θ is usually 6° to 8°. The outlet of a centrifugal fan's volute features a tongue-shaped structure commonly called the volute tongue. This tongue prevents gas recirculation within the casing. Composition of the volute tongue: 1.  Pointed tongue: Used in high-efficiency fans, which generally produce higher noise levels. 2.  Deep tongue: Primarily used in low-speed fans. 3.  Short tongue: Primarily used in high-speed fans. 4.  Flat tongue: Used in low-efficiency fans, which produce lower noise levels.

 The clearance s between the tip of the spiral tongue and the outer diameter of the impeller significantly affects noise levels. A smaller clearance s results in higher noise, while a larger clearance s reduces noise. Typically, s is set to (0.05–0.10)D². The radius r of the arc at the tip of the spiral tongue has no significant impact on the aerodynamic performance of the fan but greatly influences noise levels. A smaller arc radius r increases noise levels. Typically, r is set to (0.03–0.06)D².

The discharge direction of a centrifugal fan housing can be oriented in any direction. During operation, it is typically determined by the combined indication of the fan impeller's rotation direction and the housing discharge position.

Inlet Diameter: Low pressure has the largest diameter, medium pressure is intermediate, and high pressure has the smallest diameter. Number of Blades: The higher the pressure, the fewer the blades; the lower the pressure, the more blades. 4. Drive Components The drive components of a centrifugal fan include the shaft and bearings, and some also include a coupling or pulley. These components connect the fan to the motor. The fan impeller is secured to the shaft using keys or countersunk screws. The shaft is mounted in bearings within the base frame and then connected to the motor. Rolling bearings are most commonly used for fan bearings. There are six methods for connecting centrifugal fans to motors.

5. General Structure of Axial Flow Fans The typical structure of an axial flow fan is shown in the figure. The impeller is mounted within a cylindrical casing. When the impeller rotates, air enters through the inlet collector and flows into the impeller. Under the action of the blades, the air pressure increases and moves in a direction close to axial flow before being discharged through the outlet.

 Axial-flow fans, like centrifugal fans, have six drive methods:

The air inlet and outlet positions of axial flow fans are categorized into two types: air inlet and air outlet. These are typically indicated by the angle of the outlet (or inlet):