User Guide
This User Guide is intended to give users further insight into the calculations underlying the CBE Fan Tool, and provide a worked example.
This tool allows designers to quickly select and layout ceiling fans in a given room to meet their airspeed requirements and other constraints.
The tool uses models to predict the airspeed distribution in a given combination of room and fan related characteristics, along with knowledge about constraints related to these inputs. The models are based on full-scale laboratory testing described in this article. The tool generates solutions using the following steps.
For the room length and width defined by the user, the tool calculates a set of fan layouts such that:
- the absolute size of an individual fan cell is within a reasonable range (see Cell size slider in the Advanced Constraints for default values) given the underlying laboratory test dataset for fans between 1.2 and 4.3 m (4 and 14 ft) in diameter.
- the resulting fan cells are reasonably square, such that is the aspect ratio (max(length/width, width/level)) is close to 1 (see Maximum cell aspect ratio slider in the Advanced Constraints for maximum default value).
- the fan diameter to cell size ratio is within a reasonable range of typical applications (see Diameter to cell size ratio slider in the Advanced Constraints for default values) based on existing best-practice guidance.. Here the cell (or 'room') size is calculated using the square root of the floor area to account for non-square cells.
- the number of fans in either direction is within the acceptable range defined by the user (see # fans in X or Y direction slider in the Advanced Constraints for default values).
Given the set of candidate fans selected by the user, the tool calculates whether or not the fan will 'fit' in the ceiling height defined, and at what height it will be mounted. There are several components to this.
- all fans must be mounted such that the blades are at least 2.1 m (7 ft) from the floor.
- fans that meet UL 507 tip speed and thickness criteria may be mounted at or above 2.1 m (7 ft).
- fans that do not meet the UL 507 criteria must be mounted at or above 3.05m (10 ft).
- fans should be mounted at a distance from the ceiling that avoids 'starving' the fan. This distance depends on the fan diameter, but once it is larger than 0.2 times the fan diameter there is little effect. (see minimum mount distance slider in the Advanced Constraints for default minimum value).
- in addition to the above, the tool allows the user to constrain the blade height. The fan must be mounted within the acceptable range of blade heights defined by the user (see blade height slider in the basic Constraints for default values).
The tool then calculates the minimum and maximum allowed blade height, and only retains solutions which have a blade height that fits within all of the above constraints - i.e. the minimum allowed height does not exceed the maximum allowed height. For the retained solutions, the tool places the fan in the middle between the min and max allowed heights.
The tool calculates the minimum, area-weighted average, and maximum airspeeds in the room using the experimentally derived equations described in this article. See the Appendix in that article for a worked example of the calculation for a seated occupant. There are two modifications made to account for the fact that these experiments all took place in square cells, whereas the fan cells in the tool are not typically square.
- the tool divides the minimum air speed estimate by the cell aspect ratio to account for the lower air speeds that can be expected from non-square rooms. Until more experimental data is available, this is our best estimate for this effect.
- the tool uses the square root of the floor area as the characteristic room width.
Once the tool calculates the airspeeds, it only retains solutions with results that fit within the constraints defined in the 'Which design air speed ranges?' input drop-down.
In addition to the above, the tool also calculates additional metrics:
- the Uniformity of the air speeds in the space, using the same method as described in the laboratory experiments: 1 - (difference between max and min airspeed)/(max airspeed). The tool only retains solution with uniformity results within the range defined by the Uniformity slider (under Advanced Constraints).
- the fan air speed, calculated by dividing the rated airflow by the circular area swept by the fan blades.
- the cooling effect associated with the minimum, area-weighted average, and maximum air speed in the space according to the ASHRAE 55 definition.
The underlying models are based on full-scale laboratory tests. Though these tests covered a wide range of fans and room sizes, for all test cases, the fans were centered in a square room so as to provide the most coverage. Cases with non-square rooms an/or off-centered fans will yield less accurate results. As this is a commonly asked question, see the FAQ regarding 'How accurate are the air speed predictions' for a more detailed discussion.
The tool itself provides an example for new users - simply click the 'Show me an example' button. The tool then loads an example design case for a 13 x 16 m space with a 3.7 m high ceiling. The example also has pre-selected 4 different fan types as design options that are being considered. The tool automatically calculates these solutions using the calculation method, and one of these has been pre-selected. Clicking on a different solution allows users to explore the different choices they have that meet their design criteria and constraints.
There are several different ways to display the results for a specific solution. The default selection is the 'Floor plan'.
Clicking 'Floor plan' will show the optimal layout for the selected solution for that space, along with other basic information about the layout such as fan center-to-center spacing in each direction. The example also assumes a default grid to overlay on the floor plan. The grid settings can be modified by clicking the 'Display settings' button. The 'Display settings' dialog also allows users to override the default assumptions for number of fan blades.
Clicking 'Cell plan' will show a close-up view of the cell around a single fan, and display the following information:
- Approximate locations where the minimum and maximum air speeds will occur (pink and blue areas respectively).
- Estimates for the min, avg, and max airspeed that will occur, and the cooling effect associated with each (right of the fan)
- Characteristics of the selected fan (left of the fan)
- Basic information about the solution such as the cell dimensions, the fan density, and the assumptions made by the underlying model (see top and bottom left of display)
Clicking on 'Cell section' will display a schematic of the fan cell with some occupants (to scale) for perspective.