Within engineering specification documents provided to detail exhaust fan systems for a particular project, one will commonly encounter both terms of either plume height or stack height. Understanding the difference between these two common terms could be a deciding factor to whether a project is won or lost.
Stack height is defined as the overall height of the exhaust fan system. In our condition, this would include the roof curb, plenum, fan housing and fan nozzle/wind band. The exhaust air that leaves the exhaust fan system through the wind band is considered to be the plume rise. This is defined as the height from the top of the wind band to the point at which the exhaust air reaches a near zero upward velocity. The term used to define the combination of these two heights is the effective stack height.
Many conditions are taken into account when determining the overall effective stack height. The ASHRAE/Briggs Equation is utilized to calculate the effective stack height while taking into consideration the following parameters. Wind speed is referred to as the most important determining factor. The cross wind velocity is commonly represented within the HVAC industry for calculations as 10mph and 15mph. Also important is the stack discharge velocity, which through ANSI is set as an industry standard minimum value of 3,000 fpm. The third factor taken into account within this equation is the effective stack diameter, which represents the diameter of the outlet at the top of wind band. These three key factors will be useful in calculating the plume height, which can be added to the known stack height to yield an effective stack height value.
ASHRAE/Briggs Equation:
Example: Fan Model TS2S075A12, 7,000 cfm and 2.5”WG yields a 4,421 fpm outlet velocity
Conventional fan/stack exhaust systems typically have a smaller effective stack diameter than the Tri-Stack unit, therefore making them less effective at producing an equivalent plume rise. In the following example, consider the same inlet conditions and the same outlet velocity, for comparison purposes.
Example 2: Conventional Fan, 7,000 cfm and 2.5”WG yields a 4,421 fpm outlet velocity
As seen in the above examples, the conventional fan would need a larger stack height to account for an equivalent effective stack height of the Tri-Stack unit. This leads to a more expensive option when going with a conventional fan system, because now a higher stack will be required, which could lead to the need for guy wires. Another negative aspect for the conventional fan with larger stack would be that now it produces a much larger visual obstruction on a roof line, which is commonly unattractive to customers within the industry. The following image illustrates the two examples provided above.
