Blowers and fans are essential devices in the world of engineering. These electro-mechanical or mechanical devices are commonly used where gas flow is required, such as in aspirating, conveying, cooling, exhausting, ventilating, etc. Different types of blowers and fans improve airflow on engines, cool electronic enclosures and induce drafts in boilers. In this guide, you'll learn about different varietes and how they work.
What are HVAC Fans?
A fan is a major component of an HVAC unit – it’s like the workhorse of any ventilation system! An air conditioning system won’t work effectively without the right fans. These devices establish the much-needed consistent airflow, especially in areas where airflow is needed.
Types of HVAC Fans
There are various types of HVAC fans, each uniquely suited for different applications. The most common HVAC fans include:
- Axial fans
- Centrifugal fans
- Backward-inclined fans
1. Axial Fans
These kinds of fans have the simplest construction. They come with propeller shaped-like blades around a rotating shaft. Axial fans work the same way as airplane propellers – they produce an aerodynamic lift which then pressurizes the air. As the name suggests, these fans move air along the fan’s axis. They are ideal where the static pressure and the airflow are low (they won’t work well in high static pressure or high resistance).
Axial fans are light, compact, and inexpensive (they are the cheapest among the three types). However, they are noisy. To abate the noise, you can insulate the duct using a spring isolator or by mounting the fan on soft materials such as rubber to reduce vibration transmission. Installing baffles or dampening materials can also help in noise reduction.
Some of the applications of axial fans include ceiling fans, electronic devices cooling (a desktop computer, for instance), combustion engine cooling, box fans, outdoor air-conditioning condensers, and cooling towers.
2. Centrifugal Fans
These types of fans are known by their distinctive blade shape…they are radial, forward-curved, or backward inclined. You have probably seen hamster wheels or waterwheels, right? – they resemble centrifugal fans. They are designed with a rotating impeller that works to increase the air stream speed.
Centrifugal fans will establish higher pressure compared to axial fans and, in turn, consume much more energy. If you have ever closely looked at vacuum cleaners or home furnaces (if not, please do – you’re a handyman!), you must have seen the centrifugal fans.
Unlike the axial fans, the centrifugal fans can work against environments of high resistance, making them ideal for ventilation and exhaust systems. They also produce high pressures, making them suitable for adverse conditions such as moist conditions, high-temperature environments, or dirty air streams.
These fans make more noise than the axial fans, and they should be used when necessary. Centrifugal fans can either be driven through a belt or directly. Additionally, you can use a variable frequency drive to enhance it in particularly demanding applications. These fans can be used in air-handling units in HVAC systems (blowing air into duct systems).
3. Backward-Inclined Fans
The backward-inclined fans are characterized by their unusual blade shapes – from an untrained eye, they seem to have been incorrectly installed. These fans can further be sub-grouped into two: straight blade and curved blade fans. They are driven in the same way as the centrifugal fans.
The fans are designed to handle large volumes of clean air required in HVAC systems. They can also be used in the exhaust and industrial air supply applications. Backward-inclined fans are used in settings with variable resistance and high airflows such as glass tempering, dust extraction, incineration plants, and process cooling. In commercial HVAC systems, they are used in ERV (Energy-Recovery Ventilation).
Quick Tip: Back-inclined and centrifugal fans are costly compared to the axial ones, but they offer superior performance in terms of their applications. You could say…they succeed where the axial fans have failed.
How to Control the Speed of Fans
Sometimes, the full airflow of a fan might not be required, which calls for the reduction of its average airflow. This can be achieved through an intermittent operation. Fans cannot control their speeds by themselves, and they require an external speed variation device. The speed of a fan can be controlled either electronically or mechanically.
For instance, for a belt-driven fan, you can use pulleys of increasing diameters to regulate the fan's speed. The same concept will also apply to a gearbox. VFDs (Variable Frequency Drives) can also be used to condition the frequency and the voltage of a fan for speed modulation. This way, you can have a seamless transition between various rpm values. Brushless DC motors will also have the same effect on fractional horsepower fans in larger motors.