The Fundamentals Of Fan Selection

December 19, 2002
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An HVAC system is only as good as the sum of its parts. If one component of the system is malfunctioning, the entire efficiency can be thrown off. The same can be said if the system has been designed with components that are wrong for the job.

For example, the effectiveness of a system, whether it is for residential, commercial, or industrial use, can be significantly altered if the incorrect fan is chosen.

According to one fan manufacturer, choosing the right fan for the job does not have to be difficult. In fact, the company says that the choice is all about the process of elimination.

VAST ARRAY OF VARIABLES

Since every HVAC installation is unique, there are thousands of different pieces of equipment and components to consider. There are fan models for almost every scenario. The key is knowing what variables to take into account when designing a system.

“There are so many variables involved in selecting a fan that if it’s not done properly, it can impact performance and price,” said Jay Haferbecker, senior application engineer for Greenheck Fan Corporation (Schofield, WI).

Although there are a number of things to consider when selecting a fan, Haferbecker explained that the most important things to think about could be summed up in six questions.

  • What is the application?

  • What is the cfm and static pressure of the system?

  • Where will the fan be mounted?

  • What are the sound constraints?

  • What is going through the airstream? (Does it include clean air, grease from kitchens, fumes from fume hoods, or air at high temperatures?)

  • What are the price constraints?

    When they answer these questions, designers can begin to narrow down fan choices.

    THE BASIC APPLICATION

    According to Greenheck, the basic purpose of ventilation is to replace stale air with clean, fresh air. The process of ventilating a building or space can change depending on the situation, but the fundamentals of proper airflow do not change. They include cfm and static pressure.

    Haferbecker explained that once you find out these two variables, the decision becomes much easier. First, the amount of cfm and static pressure will help determine the type of fan that will be used, including whether one chooses a propeller fan over a centrifugal fan or a direct-drive fan over a belt-drive fan.

    Systems that require moving large volumes of air (5,000-plus cfm) at low static pressures (0.50 inches or less) will benefit from the use of a propeller fan, said Haferbecker; centrifugal fans, on the other hand, are more efficient with higher static pressures.

    Haferbecker said that direct drives require less maintenance because they do not have pulleys, belts, or bearings that need to be greased. For lower air volumes and static pressures, Greenheck recommends a direct-drive fan. Belt drives can be better suited for larger amounts of cfm and static pressure.

    Fan selection can have a tremendous impact on system efficiency. (Photo courtesy of Greenheck Fan Corporation.)

    MORE TO THINK ABOUT

    Once the basic issues have been dealt with, a designer can begin to think of other issues that will help in fan selection. Cfm and static pressure will greatly dictate the fan choice, but other factors will come into play that will help in selecting a specific model.

    The demands of the application and the needs of the end user will help in making a better fan decision. Some of these other demands will include the sound level, price, maintenance, system setup, and fan access location.

    Some fans take sound into account more than others. In some applications, such as warehouses or other industrial situations, quieter fans are not a necessity. But for applications such as office buildings, libraries, and museums, fans with low noise levels are a must. Centrifugal fans are considered to be quieter than propeller fans.

    Price and maintenance will also have an impact on fan selection. For example, designers need to think about the initial cost of the fans. How much will it cost to purchase and install them? How much will these fans cost the end user over time? A larger horsepower fan can consume many times the original price of the fan in energy costs over its lifetime.

    It is also beneficial to think about the reliability of the fan. Maintenance can be more costly on some fans than on others. When fans do need to be repaired, how accessible are new replacement parts? If a fan does need to be replaced or repaired, how much downtime will occur?

    Haferbecker said that direct-drive fans can help to eliminate issues of maintenance and price. These fans need little to no maintenance, are easier to install, and are less expensive, but the belt-drive performance adjustability is sacrificed, unless a variable-frequency drive (VFD) is used.

    If price is an issue and applications need more high-end equipment and need to move larger volumes of air, it is possible to use more than one fan.

    But Haferbecker warns against oversizing fans. He said that some people believe that oversizing the fan will enhance efficiency, but in reality this will make the system less efficient.

    Designers must also think about what kind of air will be moved. Some industrial and commercial applications will require a system to move heated air or air with fumes. In applications that require the ventilation of a kitchen space, the exhaust air will have grease in it. Some production facilities may have dirtier air, which will require a special fan.

    In the majority of propeller fans, the motors are mounted in the airstream. This means that these fans work best when the indoor air is relatively clean. It also means that these fans cannot handle temperatures over 120 degrees F.

    But many centrifugal fans have motors that are out of the air-stream. This enables them to deal with more contaminated air and higher temperatures.

    Finally, fan location is very important. The most common locations to place fans include the roof, in a wall, or in a duct. The basic components will not change, but the fan housings will.

    TECHNOLOGY CAN HELP

    “The industry is somewhat changing,” said Haferbecker. “And the process of selecting equipment has changed.”

    He explained that in recent years technology has had an enormous impact when it comes to selecting the equipment and units used in a particular application, including fans.

    Haferbecker said that there are software programs that can help in planning and designing systems. Some programs can greatly reduce the amount of time it takes to select fans. They can also help in making recommendations for the fans that will be the wisest choice for particular applications.

    Haferbecker also said that while technology keeps changing, the fundamentals of physics do not. He said that the key factors that help in determining the selection of components will always remain the same, and designers should familiarize themselves with these issues.

    Publication date: 12/23/2002

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