Specific to air handling units, there are a number of ways to improve the performance of fan systems. In HVAC systems, fans use roughly 40% of all electricity. The problem is that even by following all of the proper procedures in selecting fans for HVAC use, low total efficiency persists. From 2005 to 2009, the Energy Concept in Sweden conducted performance measurements on 767 fans in HVAC systems. While a few of the fans had an efficiency between 50% and 60%, the majority reached only 33% for total efficiency.
For the most part, there is tremendous potential for improving on energy consumption of fan systems. Whether commercial, residential, or industrial fans in Canada, as much as 50% electricity could be saved with a more efficient design, as well as better strategies for control. Below are some examples of how the performance of fan systems can improve.
- Variable Frequency Drive – A Variable Frequency Drive (VFD) is designed to adjust the speed of an electric motor based on load. Therefore, the motor speed is reduced and energy is saved. The exact efficiency of the VFD alone varies since the nominal output along with partial load cause the drive to lose between 2% and 5% of torque and speed. However, by implementing a Variable Frequency Drive in many applications, performance improves and energy consumption drops.
- Belt Drive – With a belt drive, there is a considerable loss of efficiency. The actual efficiency depends on the type of belt, complete gear adjustment, and the calculation of the belt gear. Under normal circumstances, efficiency is 90% when running at medium power, but with an incorrect gear adjustment, efficiency slips to 70% and even 60%. To boost performance, newly designed AHUs should avoid fans that are belt-driven, instead using direct driven-type fans that offer 100% transmission efficiency.
- Aerodynamic Design – In a fan, dynamic pressure loss is expected. However, the aerodynamics on the fan hood is what dictates the actual amount of loss. As long as the fan hood is well-designed, it will give less dynamic pressure loss compared to a chamber.
- Fan Wheel – For commercial and industrial fans in Canada, performance can be bolstered based on the type and design of the blades on the fan wheel. Efficiency up to 85%, which is the highest, can be achieved by using wheel type B. In this case, the blades curve backward. For greater airflow at smaller sizes of the wheel, a forward type wheel is often used. Unfortunately, this results in lower efficiency. In AHUs, it is important to always select backward curved blades.
The bottom line, the type of fan systems, as well as size, plays a key role in overall performance. For an HVAC system, the required performance of the fan includes size restrictions, airflow requirement, ambient temperature, and pressure, among other things. Therefore, the first thing you need to do is choose the right type of fan.
Typically, centrifugal fan systems in casing offer an efficiency slightly higher than fans without casing. In an AHU, there is more vulnerability to the effect of installation. Because this reduces the level of efficiency, a centrifugal fan without casing with backward curved blades provides the best performance.