Xin Feng Classroom | Ventilator Series: Fundamental Theoretical Knowledge

01

Working Principle and Theoretical Equations of Ventilators

I. Working Principle of Ventilators The working principle of ventilators is identical to that of centrifugal pumps. Both rely on the rotational motion of impellers to impart energy to gas, thereby increasing its pressure and velocity to achieve the purpose of gas transportation.

 The impeller is housed within a spiral casing. When the impeller rotates, fluid enters axially, then turns 90° into the impeller flow channel and exits radially. Continuous impeller rotation creates a vacuum at the inlet, enabling fluid to be drawn in and discharged in an uninterrupted cycle.

II. Theoretical Equations for Ventilators The theoretical equations for centrifugal fans are also derived from the momentum theorem based on the velocity triangle. Since the ventilator is single-stage and the gas is not highly compressible, it can be assumed that the gas density at the inlet and outlet is identical. This makes the equations identical to those for centrifugal pumps.

 Therefore, centrifugal fans also exhibit the theoretical characteristics of centrifugal pumps. Axial fans conform to the theoretical equations of centrifugal fans and adhere to aerofoil theory, with their design and manufacturing primarily based on aerofoil theory.

III. Ventilator Blades 1. Blade Types Based on ventilator theoretical equations and the impeller velocity triangle principle, ventilator blades also come in three types: 

- Forward-curved blades when the blade installation angle β₂ > 90° 

- Backward-curved blades when the blade installation angle β₂ < 90°

 - Radial blades when the blade installation angle β₂ = 90°

2. Performance Comparison of Three Blade Types 

① Forward-curved blades: Maximum static pressure, smallest blade area, lowest efficiency. Suitable for applications requiring high static pressure where rotational speed (n) and impeller diameter (D) are constrained.

② Backward-curved blades: Highest efficiency, largest blade area, lowest static pressure. Suitable for high-power fans. 

③ Radial blades: Offer intermediate values for pressure, blade area, and efficiency among the three types. However, their simple manufacturing process and resistance to fouling and wear make them commonly used in medium- and low-pressure fans.

02 

Similarity Conversion for Fans (Similarity Law)

I. Pressure Conversion Formula

II. Flow Conversion Formula

III. Power Conversion Formula

03 

 Specific Speed ns of Fans   

     I. Specific Speed

   The characteristic parameter representing the optimal operating condition of a fan—specific speed (nS).

II. Applications of Specific Speed 1. Classification of Fans by Specific Speed ns: 

— Centrifugal Fans, ns = 11–90 

      ① High-pressure Centrifugal Fans, ns = 11–30

     ② Medium-pressure Centrifugal Fans, ns = 30–60

     ③ Low-pressure Centrifugal Fans, ns = 60–90 — Mixed-flow fans, ns = 90–110 

— Axial-flow fans, ns = 110–500 

2. Selecting Fans to Meet Operating Conditions Based on Specific Speed ns: Fans are designated and categorized by their specific speed. For example, in the 4-72 fan model, the "4" denotes the pressure coefficient, while "72" represents the fan's specific speed ns. Therefore, by first calculating ns based on operating conditions, one can identify suitable fans.  

3. Using specific speed for similar design of new fans: Similar design principles leverage the fact that two similar fans must share identical specific speeds ns to design new fans. If the design parameters of the new fan are specified

—such as flow rate Q, total pressure P, density ρ, and rotational speed n—first calculate the specific speed ns. Then, select an existing fan with proven performance through testing or long-term operation that has an identical or similar specific speed ns as the reference model. Finally, scale the reference model proportionally to obtain the geometric dimensions of the newly designed fan.