Selecting the right fan for a new air handling unit (AHU) or AHU retrofit can be tricky as there are many fan options in the market. In this article, we will discuss which and why certain fans are better suited and more energy-efficient for your AHU.
What is an Air Handling Unit (AHU)?
In buildings with central air-conditioning systems (e.g. commercial buildings), air is treated in AHUs and then ventilated to various parts of the building. The AHU is made up of various components, including the fan, cooling coils, filters and more. Of these components, the fan consumes the most energy. Hence, to optimize the energy efficiency of the AHU, selecting the right fan is crucial.
ACMV systems account for 60% of the energy consumption of a typical commercial building [1]. Therefore, by retrofitting the AHU fans, there can be a significant reduction in energy consumption.
Type of AHU fans
The conventional option for AHU fans is the alternating current (AC) belt-driven centrifugal blower, which is a large fan system made up of the AC motor, impeller, belt and (AC) plug fan that requires variable frequency drive (VFD) for speed control. These are the least efficient option and have been around for decades.
There are various challenges when utilizing this option. This includes but is not limited to:
• Inherent energy losses: Components are typically manufactured separately and are not optimized for each other.
• Limited speed control: The lowest possible speed it can be set to is 25Hz, which is 50% of the speed range. This limits efficiency when operating at lower frequency, leading to energy wastage.
• Noise emissions: The outdated design could lead to increased noise emissions, turbulence and higher losses.
• No operational redundancy: As each AHU only contains one fan, there is no fail-safe in the event of system failure.
• Frequent maintenance: Regular maintenance of components (e.g. belt replacement, motor greasing) is expected, which contributes to overall costs.
In recent years, more AHU fan options have emerged. AHU retrofits or fan replacements can be carried out to replace the AC centrifugal belt-driven fans with newer and more energy-efficient fans.
AHU fan options include the electronically commutated (EC) centrifugal fan and the AC axial fan. They can be briefly described as such:
EC centrifugal fan
• Motor: EC
o Known as brushless DC motors
o AC input is converted to high DC supply to the motors
• Components: Integrated
• Size: Compact
• AHU set-up: Multiple fans in AHU
• Airflow: Diagonally to axis
• Works best with: Duct applications that require high static pressure
AC Axial Fan
• Motor: AC
• Components: Not integrated
• Size: Large and heavy
• AHU set-up: Typically a single fan in AHU
• Airflow: Parallel to axis
• Works best with: Applications that require non-ducted straight-line flow, high-volume flow
To determine which of these fans are more suited as an AHU fan, we will need to dive deeper and take a closer look. We will compare how the above fan types can perform in an AHU.
Motor type
EC centrifugal fan: In EC motors (or brushless DC motors), with integrated electronics, efficiency is higher than a DC motor.
AC axial fan: AC axial fan uses a typical AC motor. The only way to improve the efficiency is to use a fan with a higher IE motor classification.
Fan design
EC centrifugal fan: The EC centrifugal fan is an integrated unit (motor, electronics and impeller) where its components are optimised for each other. There is no need for an external VFD as well. As a result, there is minimal energy losses.
AC axial fan: The AC axial fan’s components are not integrated. Thus, they are not optimised for each other. Due to inherent losses, it is less efficient in moving air through the ducts.
Air velocity
EC centrifugal fan: By nature, centrifugal fans have a low velocity. The lower velocity increases the flexibility in the mounting direction of the supply air duct. In addition, a lower velocity translates to higher static pressure and lower turbulence loss.
AC axial fan: Axial fans have a higher velocity compared to a centrifugal fan running at the same revolutions per minute (RPM). Higher velocity translates to high dynamic pressure and reduces flexibility in mounting direction in a plenum.
Static Pressure
EC centrifugal fan: Centrifugal fans are able to produce the higher static pressure needed for moving air efficiently in ducting for the AHU application.
AC axial fan: Although an axial fan impeller can reach a high of 92% in total efficiency, installation in a ducted environment reduces this efficiency dramatically because it produces a higher dynamic pressure.
Airflow
EC centrifugal fan: Airflow through the system will be uniform. The aerodynamic design of the impeller directs the airflow diagonally away from the impeller to prevent the collision of airflow streams from parallel fans.
AC axial fan: In contrast, the airflow in the AHU unit with an AC axial fan will not be uniform for top throw supply air ducting. The air will be concentrated at the bottom. This also implies that the cooling coil will not be fully utilized.
Conclusion
Based on the motor and fan characteristics, it is clear that EC centrifugal fans will be a better choice for your AHU or AHU replacement. However, we have yet to discuss other practical details such as installation requirements, maintenance needs and noise emission levels. We will discuss this in the next article. Click on the button below to read part two of choosing the ideal AHU fan.
Article by
Marketing Department of ebm‑papst SEA Pte Ltd
References:
[1] “Super Low Energy Building Technology Roadmap”, Building and Construction Authority, 2018, Retrieved from https://www.bca.gov.sg/greenmark/others/SLE_Tech_Roadmap.pdf