In simple terms, fans are used to establish airflow wherever it is needed and does not occur naturally. For this reason, fans are fundamental in HVAC systems. However, the physical construction of fans varies and each type is intended for different applications.
The three types of fans most commonly found in HVAC systems are the following:
- Axial fan
- Forward curved centrifugal fan
- Back-tilted fan
This article will describe each type of fan and its applications, along with advantages and disadvantages. Many fan manufacturers offer integrated units that include a motor and controls, reducing space requirements and simplifying maintenance.
Keep in mind that ventilation systems in New York City are subject to the New York Mechanical Code, and a building can only be approved for intended occupancy if its internal systems comply with the code.
Axial fan
Axial fans have the simplest construction, with their blades arranged in a helix shape around a rotating axis. These fans are designed for applications where airflow and static pressure are relatively low.
Axial fans typically have the lowest cost of the three types described in this article, but remember that they are not suitable for applications with resistance to airflow (high static pressure).
Some common applications of axial fans include cooling towers, outdoor air conditioning condensers, cooling electronic components, and cooling combustion engines. All of these devices require airflow to release their heat, but there is little or no resistance to air movement, allowing the use of low-cost axial fans.
Centrifugal fan
These fans are characterized by having an appearance similar to water wheels or hamster wheels. Assuming the same airflow, a centrifugal fan will establish a higher pressure than an axial fan, consuming additional energy.
Consider that centrifugal fans produce more noise than axial fans, therefore their use is only recommended if the application requires it. They can be directly or belt driven, and can also be enhanced with a frequency inverter.
In HVAC systems, one of the main applications of centrifugal fans is in air handling units, which are required to blow air into ducts. Unlike open discharge fans in cooling towers, these AHUs must overcome the static pressure of the air duct system. In this application, the use of an axial fan would likely result in poor airflow.
Backward-tilted fans
These fans are characterized by the unusual shape of the blade, giving the impression that the device is installed incorrectly. Backward inclined fans can be classified into curved-blade and straight-blade fans and can be driven like a centrifugal fan.
These fans are commonly used in industrial applications with high airflow and variable resistance, such as dust collection, incineration systems, process cooling, and glass tempering. In commercial HVAC systems, backward-tilted fans are typically used in energy recovery ventilation (ERV).
Controlling fan speed
When a fan's full airflow is not required, its average airflow can be reduced with intermittent operation. However, this approach is not viable for applications that require controlled and consistent airflow. Fans are unable to change their speed on their own, which means an external speed variation device is required. Speed control can be achieved mechanically or electronically:
- For example, belt-driven fans can be equipped with pulleys of increasing diameter to have multiple speeds available. The same effect can be achieved with a gearbox.
- Variable frequency drives (VFD) condition the voltage and frequency supplied to the fan to achieve speed modulation, allowing a seamless transition between different rpm values. For fractional horsepower fans, brushless DC motors can achieve the same result as VFDs on larger motors.
Multi-speed motors are practical in open discharge applications with axial fans. In these cases, speed control is directly related to airflow (cubic feet per minute), since there is no resistance to airflow. The same cannot be said for ducted installations, where static pressure variations need to be taken into account – precise airflow control can only be achieved with a VFD.
Our experienced HVAC engineers can select the best fans to ventilate your building.
Importance of Cost-Benefit Analysis
Centrifugal and backward-inclined fans have an additional cost, but offer superior performance in applications where an axial fan would be ineffective. For best results, the professional opinion of HVAC design engineers is highly recommended. Keep in mind that ventilation system design not only influences fan power consumption, but also the load on space heating and cooling systems.
