HVAC systems rely on ventilation equipment for space heating and air conditioning, since chillers and boilers by themselves cannot deliver the heating or cooling effect where it is required. In addition, ventilation systems ensure a constant supply of fresh air for indoor spaces. Based on the pressure and airflow requirements of each application, either a fan or a blower is used.
Before discussing the main types of fans and blowers, it is important to understand the difference between both concepts. The American Society of Mechanical Engineers (ASME) defines fans and blowers based on the ratio between discharge pressure and suction pressure.
Fan: Pressure ratio up to 1.11
Blower: Pressure ratio from 1.11 to 1.2
Compressor: Pressure ratio exceeds 1.2
Fans and blowers are necessary for air to overcome the flow resistance caused by components like ducts and dampers. There are many types available, each suited for certain applications. Choosing the right type helps optimize HVAC performance, while poor selection leads to energy waste.
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Types of Fans
Fans can be classified into centrifugal or axial based on how they establish airflow. In turn, there are several subtypes in each category, and choosing a fan that matches the application is critical for a high-performance HVAC installation.
The following table summarizes the main types of centrifugal fans: radial, forward curved, backward curved and airfoil type.
-High pressure and medium flow -Tolerates dust, moisture and heat, making it suitable for industrial use -Power consumption increases significantly along with airflow
-Medium pressure and high flow -Suitable for HVAC systems with relatively low pressure, such as packaged rooftop units -Tolerates dust, but is not suited for harsh industrial settings -Power consumption increases significantly along with airflow
-High pressure and high flow -Energy efficient -Does not experience a dramatic increase in pressure with airflow -HVAC and industrial applications, also forced draft systems
-High pressure and high flow -Energy efficient -Designed for applications with clean air
On the other hand, axial flow fans are classified into propellers, tube axial and vane axial.
-Low pressure and high flow, low efficiency -Suited for moderate temperatures -Airflow is reduced drastically if static pressure increases. -Common applications include exhaust fans, outdoor condensers and cooling towers
-Medium pressure and high flow -Cylindrical housing and small clearance with fan blades to enhance airflow -Used in HVAC, exhaust systems and drying applications
-High pressure and medium flow, high efficiency -Physically similar to tube axial fans, integrating guide vanes at the intake to improve efficiency -Common uses include HVAC and exhaust systems, especially where high pressure is required
With such a wide selection of fans, there is a solution for almost any application. However, variety also means there is higher 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 they 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 with centrifugal pumps. They normally include a gear system to achieve speeds well over 10,000 rpm. Centrifugal blowers can have a single-stage or a multi-stage construction, where the single-stage design offers a higher efficiency, but the multi-stage design provides a wider airflow range at steady 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 rapidly when an obstacle raises pressure, making them unsuitable for applications with a high chance of clogging.
Positive-displacement blowers have a rotor geometry designed to capture pockets of air, 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 clogging the system. Another important difference with centrifugal options is that positive-displacement blowers are typically driven by belts instead of gears.
Fans and blowers are normally specified based on the pressure and airflow requirements of each application, as well as site-specific conditions such as dust and temperature. Once the right type of fan or blower has been specified, performance can normally be enhanced with control systems. For example,variable frequency drives (VFD)can drastically reduce the electricity consumption of fans that operate intermittently.
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