//What is High Volume Low Speed Fan?

What is High Volume Low Speed Fan?

By Mr Teng Kok Wai, AIG

Heard of Big Ass Fans? Big Ass Fans is a fan manufacturer but it is frequently associated with the large fans that we can sometimes see in warehouse, mall atriums and airports. The more suitable terminology is high volume low speed fan (HVLS). A high-volume low-speed (HVLS) fan is a type of mechanical fan used in commercial area and industrial premises. HVLS fans are generally ceiling fans although some come as pole mounted fans. Typically residential type of ceiling fans have diameter up to 4.5 feet while HVLS fans have relatively large diameter ranging from 8 feet (2.4 m.) up to 24 feet (7.3 m.). Unlike the standard high speed fans (such as bifurcated fans for kitchen or axial fans for underground carpark), HVLS fans generally move very slowly. With very low rotational speed, HVLS fans move large amounts of air, henceforth the name, high volume, low speed.

HVLS fans can be found in warehouse (particularly picking areas), distribution centers, shopping malls (at atriums) and school assembly halls to name a few. Owing to the rising cost of energy, HVLS fans are often used to supplement air conditioning & mechanical ventilation (ACMV) systems as they provide stronger cooling effect while helping to maintain a constant temperature and humidity.

How do they work?
HVLS fans work on the principle that a breeze moving across your skin on a very hot humid day. The cooling moving air breaks up the moisture-saturated boundary layer (also known as sweat) surrounding the body and accelerates evaporation to produce a cooling effect.

No matter their size, ceiling fans produce a column of air as they turn. This column of air moves down and out along the floor. Called a horizontal floor jet, this deep wall of horizontal moving air is relative to the diameter of a fan, and to a lesser degree, the speed of a fan. Once the floor jet reaches its potential, it migrates outward until it meets a side wall or other vertical surface. Please refer to below image.
Under ideal conditions, an 8 foot (2.4 m) fan produces a floor jet of air approximately 36 inches (91 cm) deep. A 24 foot (7.3 m) fan produces a floor jet 108 inches (274 cm) deep, tall enough to engulf a human standing on the floor.

Why we care so much about the air flow?
This air flow effect can cause problems when it comes to fire protection and automatic sprinkler operation. The downward air-flow can delay the operation of sprinklers by NOT allowing the heat from a fire to rise through the air flow and operate the sprinkler heads in the required area over the seat of the fire. In addition to that, the fan (i.e. motor and the blades) itself can also create obstruction to the sprinkler water pattern. Both of these factors, they can have a negative impact to sprinkler operation and effective fire suppression.

Owing to the above, the immediate direct impact will be a higher normal loss expectancy (NLE) which are contributed by larger fire areas and also larger water damage. Larger fire areas are caused by delay of the sprinkler operations meanwhile a much larger water damage caused by accidental activated of other sprinkler heads in non fire area. The impact will be much more signification for warehouses or distribution centers as their combustible loads are pretty substantial in such occupancy.

What are required in such situation as per NFPA 13?
Based on the findings through numerous large scale tests using both standard spray sprinkler and Early Suppression Fast Response (ESFR) sprinklers, it was determined that the HVLS fans do have a negative impact on the effective operation of sprinkler systems provide the fans are interlocked to shut down upon activation of the first sprinkler head of sprinkler water flow. Consequently, the installation of HVLS fans in buildings equipped with sprinkler, including ESFR sprinklers, shall comply with the following:

  1. The maximum fan diameter shall be 24 feet (7.3 m).
  2. The HVLS fan shall be centered approximately between four adjacent sprinklers.
  3. The vertical clearance from the HVLS fan to sprinkler deflector shall be of minimum 3 feet (0.9 m).
  4. At a minimum all HVLS fans shall be interlocked to shut down immediately upon receiving a water flow signal from the automatic sprinkler system in accordance with the requirements of the NFPA 72. Ideally, the fans should be interlocked to a fire alarm device such as smoke detector or heat detector in addition to the waterflow device. A fire alarm device will likely respond much sooner than the waterflow signal. This will help to ensure that the fans are shut down immediately. Connection to multiple devices will also improve reliability of fan shut down.

So, the next time a risk engineer or a property underwriter visit warehouse, shopping malls or airports, it’s time look into the small details of HVLS fans.