The performance of the different kind of fans and their operational performance is essential to decide the type of fan to be used for a specific requirement. Designing a proper duct system is also necessary to optimise air flow and distribution.

1. Introduction

Describing a fan’s performance is a bit like showcasing a high-performance sports car – it’s not a one-size-fits-all scenario. Unlike traditional horsepower measurements for cars, fans have their own unique metrics: volumetric flow rate and pressure rise. These two factors, while variable, always have a special relationship with each other.

This relationship is depicted on a graph – the fan’s unique signature. For optimal performance, this curve is ideally plotted at a fixed speed, creating what is called “inherent-speed” curve. On this graph, the volumetric flow rate is located on the baseline (x-axis), while the fan’s pressure rise sits on the y-axis. This is the essential information, but additional parameters can be added, like absorbed power, efficiency, and noise level.

Figure 1: Sample Graph for Fan Performance

 

Fan’s peak efficiency – its peak moment on the curve, known as the “best efficiency point” (b.e.p.). It is the point where fan performs with the least power consumption and the least noise for its design. Operating at this point provides the ideal outcome for energy efficiency.

Fans can operate at various points on their characteristic curves. You can choose a smaller fan at a higher speed if you’re okay with a bit more noise and slightly lower efficiency. On the other side, an oversized fan to the left of the b.e.p. may experience issues such as “stalling,” leading to heightened noise, vibration, and unsteady flow.

In fan design, the major task is to find the sweet spot on the curve – the right balance of performance, efficiency, and noise tailored to required needs. 

2. Centrifugal Fans

Figure 2 :  Animation from

 

A centrifugal fan is a mechanical device designed to move air or gases in a specific direction, typically at an angle to the incoming fluid. These fans, often referred to as blowers, utilize a ducted housing to guide outgoing air in a desired direction or across a heat sink. 

The primary function of centrifugal fans is to move air from the rotating inlet to an outlet, commonly used in ducted applications to either draw air through ductwork/heat exchangers or push air through similar impellers. Notably, they offer comparable air movement to standard axial fans but from a more compact package, making them suitable for overcoming higher resistance in air streams.

Centrifugal fans harness the kinetic energy of impellers to propel the air stream, enabling it to counter resistance from ducts, dampers, and other components. These fans displace air radially, altering the airflow direction typically by 90 degrees. They are characterized by their sturdiness, quiet operation, reliability, and versatility in diverse conditions.

Unlike positive-displacement blowers, which maintain a constant mass of air, centrifugal fans are constant-volume devices. This means that, at a consistent fan speed, they move a relatively constant volume of air. The actual mass of air, however, varies based on factors like air density influenced by temperature changes and elevation.

Comparatively, centrifugal fans are more efficient than positive-displacement blowers. But the blowers may have a lower capital cost and achieve higher compression ratios. When compared to axial fans, centrifugal fans are more suitable for ducted applications in residential, industrial, and commercial settings. Axial fans, on the other hand, are preferred for high-volume air movement, such as warehouse exhaust or room circulation, thanks to their ability to operate at higher volumes and lower static pressures with higher efficiency.

The centrifugal fan’s distinctive drum shape consists of fan blades arranged around a hub. The hub rotates on a driveshaft within the fan housing, causing the gas to enter from the side, turn 90 degrees, accelerate due to centrifugal force over the blades, and exit the fan housing. This design ensures effective and directed airflow in various applications.

3. Axial Fans

The development of axial flow fans has experienced significant progress since the Second World War, largely attributed to the introduction of high-strength aluminium alloys. These alloys enable the fans to operate at rotational speeds necessary for generating substantial pressure. Axial fans closely adhere to classical theory and generally require less specialized knowledge compared to centrifugal fans.

Photo 1: Axial Fans

 

Axial fans can be broadly classified based on how the flow of air is constrained:

  • Ducted Fan:
    • The air is directed to flow through a duct, encouraging it to enter and exit the impeller in an almost axial direction.
    • This configuration is effective in applications where a controlled and directed flow is desired.
  • Diaphragm or Ring Mounted Fan:
    • Air is transferred from one relatively large air space to another.
    • This design is suitable for scenarios where a controlled transfer of air between spaces is required.
  • Circulator Fan:
    • The impeller rotates freely in an unrestricted space.
    • Examples include pedestal or ceiling fans, where the fan circulates air throughout a room without being confined by a duct or specific channel.

4. Ducting

An air distribution system can range from a simple setup involving a fan with ductwork connected to either the inlet or discharge, or both, to a more complex system that includes additional components. In a more intricate system, you might find a combination of a fan, ductwork, air control dampers, cooling coils, heating coils, expansion joints, diffusers, turning vanes and more. The fan acts as the central “air pump” in the system. It provides the energy needed to overcome the resistance to airflow imposed by the various components.

Photo 2: Ducting

 

Essentially, the fan is responsible for propelling the airstream through the system. It ensures that it can effectively navigate and circulate through the ductwork and associated components. The complexity of the system is determined by the inclusion of various elements that contribute to the overall functions.

The construction of ducts involves the use of various materials, with an emphasis on specific requirements based on the application’s nature. Material selection for duct systems should undergo careful consideration, similar to other components within the system. Different materials significantly impact overall system performance, and while each material has its advantages, it is crucial to evaluate their limiting characteristics before making a selection.

Some common materials used for ducts include galvanized steel, black carbon steel, aluminium, stainless steel, fiberglass-reinforced plastic (FRP), concrete, fibrous glass (duct board). 

It’s crucial to note that special material selection and construction may be necessary for designing systems serving nuclear projects, earthquake-prone areas, and projects with other unique requirements. These considerations highlight the importance of tailoring material choices to specific environmental, structural, and functional demands in order to ensure optimal performance and compliance with specialized project needs.

A comprehensive duct system specifications and detail drawings should meet the following detailed requirements:

  • Local Standard/Code Requirements
  • Duct System Static Pressure Classifications
  • Duct Material Selection
  • Allowable Duct Leakage
  • Insulation Requirement
  • Sound Control Devices and Methods
  • Outlet and Inlet Performance
  • Filters
  • Dampers
  • Duct Mounted Equipment
  • Special Duct Suspension System

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