Factors To Consider In Centrifugal Fan Selection

Centrifugal fans perform the basic functionality of moving air from one place to another. There are several centrifugal fan types and sizes available each with certain application benefits. Since there is a great diversity in fan applications such as drying, cooling, conveying, aerating and ventilation etc the specifications needed for selecting the fan system also vary. A fan with right design and material, chosen by considering process requirement will enable good efficiency and reliability. Proper selection of the fan type is dependent on certain factors like fan type & size, airflow rate, static pressure, density of the air to be moved, system resistance, temperature range, space constraints, material to be handled, variations in operating conditions etc.

Fan type & size
Centrifugal fans have varied design such as backward inclined, radial blade, forward curved, airfoil design etc depending on fan blade configuration. These fans deliver different airflow rates at different pressures. A right fan type has to be selected because each has its own application range and has varied ability to perform in the industrial process. Also choosing the right size of fan in system design is important to have good energy efficiency and system performance. When an under sized fan is used, it is difficult to meet the variable demand conditions of an industrial process. Similarly when industries use over-sized fans to avoid the under performing systems, it can increase capital & operating costs while decreasing fan reliability.

Airflow rate
The airflow rate will determine how well the fan system can handle the material against the system resistance. Wrong ducting, system effect or improper installation of dampers can have a negative impact on airflow. These issues have to be evaluated when selecting the fan system, specifically when fans are used for material handling in industrial process. If proper velocity is not maintained in the air by the fan, then it will not carry the material through the system to the destination point.

Pressure drop
All control components used at the inlet or outlet of a fan system will have an affect on the airflow passing through the system. Hence when determining the pressure requirements for a fan system, it is important to consider the restrictions to the airflow that is caused by the air moving through ducts, hose, filters, pipes, dampers and louvers and other air control components. While selecting the fan, the pressure drop has to be considered through all of the duct work and across other system components that restrict airflow.

Density
The standard air density at which fan has to operate is .075 pounds per cubic foot at 29.92″ barometric pressure and 70° F temperature. However, fan density may vary from standard for many reasons such as temperature or altitude changes, moisture content or mixture of gases other than air etc. At temperatures higher than standard (70°F) the density will decrease whereas at temperatures lower than standard the air density is greater. Similarly, the higher the altitude the less denser the air will be. If the fan is operating in a system where a gas or vapor with higher or lower density need to be handled, corrections must be made to the standard conditions. It is important to consider the affect of these factors that are influencing the fan air density while selecting a fan system.

Material to be handled
Several factors are to be considered when material to be handled is going through the fan system. Choosing a fan with proper blade type is important otherwise the material will be accumulated on the blades. For clean air applications backward curved and airfoil blades are efficient, whereas if the material is dirty and granular, radial wheels are appropriate. While handling moist or wet materials, fans with special coatings or material liners need to be used for wheels and/or housing to avoid material corrosion and abrasion. In case of high temperature applications, motor heat shields, shaft coolers, shaft seals, insulated housings are to be used in fan construction to avoid the affect of temperature on fan system.

Space constraints
While designing the fan assemblies, sufficient space should be left for duct work, maintenance access, foundation and structural support etc. A good location has to chosen based on the desired airflow pattern by considering the structural constraints. If a fan system has to mounted on a roof or wall it is essential to determine the fan weight, size and speed. To adjust the fan in a restricted space, many times fan construction use multiple elbows, compact or small ducts etc. This will create a pressure drop across the system affecting the fan performance because of non-uniform airflow. It is essential to select the most efficient fan with minimal noise that can fit within the space available in an application.