Background Information

Cooling effect

ASHRAE 55-2017 “Thermal Environmental Conditions for Human Occupancy” is the benchmark for thermal comfort calculations. ASHRAE 55-2017 describes how to calculate the cooling effect of air velocity over skin.

It should be noted that air movement does not reduce the air temperature, but rather cools the skin through evaporation and convection.

The amount of skin cooling (or cooling effect) is dependent on a number of factors.

These are:

• Air temperature
• Relative Humidity (Higher humidity reduces the evaporative cooling effect)
• Occupant activity level (Higher activity levels generally produce more perspiration and therefore more cooling.)
• Amount of Clothing (The more clothing worn, the less skin is exposed for cooling)
• Air Velocity
• Other minor factors

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Destratification

If the environment is heated, the hot air will rise and a temperature difference between the floor and ceiling forms. (The temperature difference between floor and ceiling increases by 1.4 deg C for every 1metre of building height, when the building is heated).

Therefore, in a building with a 10m high ceiling, the temperature would be approximately 14 deg C warmer at ceiling level than at floor level (assuming the roof is insulated and exhaust fans are not removing the heat).

HVLS fans can be used to bring the warmer air at ceiling level down to floor level and mix it. This will increase the temperature at ground level similar to that at the roof. This is called Destratification.

In order to achieve good destratification the building requires at least 0.5 Air Changes per hour of air movement. It is also important to ensure velocities do not exceed 0.2m/s so that the occupants don’t experience drafts.

Fantech’s analysis has shown that if fans are initially selected to provide a cooling effect, they will also provide good destratification in all cases if the fans are run at 10% of full speed. This will give more than 0.5 Air Changes per hour and will also ensure that the velocities at ground level (and at head height) are less than 0.2 m/s.

Therefore Fantech recommends that HVLS fans are selected for cooling effect and that the coverage and position of these fans will also provide very good destratification.

While destratification can work with the air flow blowing down or up from the HVLS fans, we recommend it blows down towards the floor in most scenarios. This ensures the fan blades are operating efficiently and the warmer air is getting down to floor level.

The fans could technically operate in reverse (blowing up) at a higher speed and still avoid drafts at floor level. However, more fan energy is required and there is also a risk that the warm air does not flow naturally to the floor because of obstructions (purlins)

Introduction to the Selection tool

The Fantech HVLS selection tool is available in 2 versions; basic and advanced. Both versions use the calculation code from ASHRAE55-2017 to predict the cooling effect of air velocity.

The advanced version is designed for engineers and selects HVLS fans to achieve an exact cooling effect based on all the input parameters entered into the selection tool.

The basic version uses the same ASHRAE 55-2017 code, but makes certain assumptions.

It assumes

• Clo (clothing level) = 0.57 (Building occupants wear trousers and a short-sleeve shirt).
• MET (Activity level) = 2 (Building occupants walk on a level surface at an average speed of 3.2 km/hr).
• It also assumes that we have standard atmospheric pressure, no external heat sources, and no hot radiant surfaces nearby.

As the cooling effect is dependent on relative humidity, the basic selection tool calculates the cooling effect at both 90% and 10% humidity. The lower value shown in the desired cooling effect drop down box relates to 90% humidity and the higher value is for 10% humidity.

How to use Fantech’s basic HVLS selection tool

1. Enter the Building width, length and height in meters. This will only work for simple square or rectangular buildings.
2. Enter the ambient air temperature you would like to analyse. The selector will use this value to populate the desired cooling effect drop down box with a series of values.
3. Select the desired cooling effect from the drop down box.

The selector determines the coverage diameters for each fan in order to achieve the desired cooling effect. This is the average cooling effect between floor level and a height of 1.5m within the specified diameter (It should be noted that the cooling effect will be greater than we predict inside the coverage diameter, provided that there are no obstructions).

Based on the coverage diameter, the selector then calculates the number of fans required and spacing of the fans.

Energy savings for air-conditioned buildings

If a building is air-conditioned, HVLS fans can work in conjunction with air-conditioners and potentially reduce the energy used by the air-conditioning unit.

As an example, If we have a building with no HVLS fans, and a set point of 25 deg C, the occupants will experience 25 deg C thermal comfort. However, if HVLS fans are used a cooling effect of 3 deg C, in conjunction with the same air-conditioned building, the occupants will have the same thermal comfort when the air-conditioners set point increases to 28 deg C (28 – 3 = 25)

Therefore HVLS fans can be used to raise the set point of air-conditioners and achieve the same level of thermal comfort. Increasing the air conditioners set point saves energy usage and operational costs.

The “Set point increase” value is the amount that the set point can be increased, while maintaining the same thermal comfort. In the above example the set point can be increased by 3 deg C and still maintain the same thermal comfort.

The % energy saving are approximate amounts for Australian conditions and also allow for the energy consumption of the HVLS fans.

Heating savings

If the building uses a heater, this section estimates the % heating savings by destratifying air in a heated building.