Three Factors that Affect Air Displacement
Effectively mixing large volumes of air may seem as simple as specifying a fan with a high enough CFM. However, there are other factors that strongly affect air displacement.
Air Column Size
• Longer blades create a wider column of air.
• Large columns of air travel father than smaller columns. A large column has proportionately less periphery and therefore less drag because the friction between moving air and stationary air occurs at the periphery of the moving column. The air column from a 3-foot diameter fan has more than 6 times as much drag per cubic foot as the air column from a 20-foot diameter fan.
• HVLS fans produce more air movement that travels farther than smaller fans.
Horizontal Floor Jet
• The movement of air away from the downward air column is called the “horizontal floor jet.”
• The longer the fan blade, the deeper the horizontal floor jet will be.
• HVLS fans have a large downward air column and a deep horizontal floor jet
that impacts all areas adjacent to the downward air column.
• Obstructions such as pallet racks or room dividers act as airflow barriers and prevent air circulation.
• Smaller fans with weak air columns are particularly hindered by obstructions.
• HVLS fans have a large diameter of downward air that washes over and around
When considering an HVLS fan, it is important to first determine the overall goals for the project. Some common goals are:
• Energy Efficiency
• Health and Wellness
Temperature and humidity are critical components of occupancy comfort. As discussed, uncomfortable workplaces result in reduced productivity, increased errors, and employee complaints. The air circulation from an HVLS fan provides a cooling breeze and moderates humidity in the summer. During the winter months, the rotation of the blades is reversed so that air is pushed up where it mixes with the warm air trapped near the ceiling resulting in a warmer work environment.
Energy efficiency is now top-of-mind. We see this in the focus on LEED certification and “Green Buildings”. When evaluating a ceiling fan, its energy efficiency can be an important element in the product selection process.
There are two main ways to evaluate the energy efficiency of a fan:
1) Ratio of air moved to energy used.
2) Reduced energy consumption of an HVAC system when paired with an HVLS fan.
The common method for determining the energy efficiency of a fan is to examine the amount of energy the unit uses in relation to the amount of air it circulates. The ratio is often evaluated by comparing the air moved (measured in CFM) to the energy used (purchased in kilowatt hours). The greater the CFM per kilowatt ratio, the less electricity it takes to move air within the room.
This comparison becomes especially important when designing air circulation strategies for large warehouse spaces. Assume a warehouse is 50-feet wide by 100-feet long with 20-foot ceilings. There are 100,000 cubic feet of air in the space. To circulate the entire volume of air using traditional high-speed fans would require thirteen total fans, as opposed to a single
14-foot HVLS fan. Running all thirteen traditional fans for a day would require 11 kilowatt hours of electricity, where the single HVLS fan would only require 7 kWH. At the current cost of electricity ($0.12 per kWH), it would cost $347 a year to use the traditional fans and only $222 to use the HVLS fan—a savings of 36 percent.
To measure the reduced energy consumption of an HVAC system when paired with an HVLS fan, compare kWH used per day without the fan to kWH used per day with the fan. Adding an HVLS fan to support an HVAC system often results in a 20 percent reduction in kWH usage because the thermostat can be set 3 to 5 degrees higher (summer) or 3 to 5 degrees lower (winter).
The Valencia Airport saved $66,000 a year.
Health and Wellness
There are two syndromes that can occur if a building has inadequate airflow: Sick Building Syndrome and Sweating Slab Syndrome. Sick Building Syndrome occurs when pollutants and/ or toxins build up inside a building and cause the occupants to experience symptoms such as dizziness, nausea, and coughing. Running in reverse, an HVLS fan can pull the fumes up to the exhaust fans removing the pollutants from the building.
Sweating Slab Syndrome impacts humid environments. When air becomes saturated with water (dew point) it deposits moisture on surrounding surfaces, making them slick and therefore dangerous for building occupants and equipment. HVLS fans moderate humidity levels by circulating the air and preventing saturated air from depositing moisture on floors, equipment, and inventory.