What is the difference between fans and blowers?

HVAC systems rely on ventilation equipment for space heating and air conditioning, as refrigerators and boilers alone cannot provide the heating or cooling effect where it is needed. Furthermore, ventilation systems ensure a constant supply of fresh air to interior spaces. Based on the pressure and airflow requirements of each application, a fan or blower is used.

Before discussing the main types of fans and blowers, it is important to understand the difference between the two concepts. The American Society of Mechanical Engineers (ASME) defines fans and blowers based on the relationship between discharge pressure and suction pressure.

• Fan: Pressure rate up to 1.11
• Fan: Pressure ratio 1.11 to 1.2
• Compressor: Pressure ratio exceeds 1.2

Fans and blowers are necessary for air to overcome resistance to flow caused by components such as ducts and dampers. There are many types available, each suitable for certain applications. Choosing the right type helps optimize HVAC performance, while poor selection leads to wasted energy.

Improve indoor air quality with a professional ventilation design.

Types of fans

Fans can be classified as centrifugal or axial based on how they establish airflow. In turn, there are several subtypes within each category, and choosing a fan that matches the application is critical to a high-performance HVAC installation.

The following table summarizes the main types of centrifugal fans: radial, forward-curved, backward-curved and airfoil type.

On the other hand, axial flow fans are classified into propellers, axial tubes and axial vanes.

With such a wide selection of fans, there is a solution for almost every application. However, the variety also means there is a greater chance of selecting the wrong fan without proper guidance. The best recommendation is to avoid “rule of thumb” decisions and instead get a professional design that meets the needs of your project.

Types of blowers

As previously stated, blowers operate with a pressure ratio of 1.11 to 1.2, which makes them intermediate between a fan and a compressor. They can produce much higher pressures than fans and are also effective in industrial vacuum applications that require negative pressurization. Blowers are divided into two main categories: centrifugal and positive displacement.

Centrifugal blowers have some physical similarity to centrifugal pumps. They typically include a gear system to reach speeds well above 10,000 rpm. Centrifugal blowers can have single-stage or multi-stage construction, where the single-stage design provides greater efficiency, but the multi-stage design provides a wider range of airflow at constant pressure.

Like fans, centrifugal blowers have applications in HVAC. However, thanks to their superior pressure output, they are also used in cleaning equipment and automotive applications. Their main limitation is that airflow decreases quickly when an obstacle increases pressure, making them unsuitable for applications with a high probability of clogging.

Positive displacement blowers have a rotor geometry designed to capture air pockets, driving flow in the intended direction at high pressure. Although they rotate at lower speeds than centrifugal blowers, they can produce enough pressure to blow away objects that clog the system. Another important difference with centrifugal options is that positive displacement blowers are typically driven by belts rather than gears.

Conclusion

Fans and blowers are typically specified based on the pressure and airflow requirements of each application, as well as specific site conditions such as dust and temperature. Once the right type of fan or blower has been specified, performance can usually be improved with control systems. For example, variable frequency drives (VFD) can drastically reduce the electricity consumption of fans that operate intermittently.