The professional engineer must be able to properly size a fan. There are two main parameters that must be determined, (1) Volumetric Flow Rate [CFM] and (2) Static Pressures.
DETERMINING VOLUMETRIC FLOW RATE [CFM]
In order to find the volumetric flow rate of air that the fan must blow, will depend on the following factors, (1a) heat/cooling load, (1b) ventilation/exhaust or (1c) velocity.
(1a) First in the HVAC and Refrigeration field fans are used to provide cool/hot air to properly control the temperature of the space. The amount of air required is determined by the cooling/heat load and the desired temperature and the supply air temperature.
(1b) Second in the HVAC and Refrigeration field fans are used to provide ventilation to adequately remove noxious fumes, like carbon dioxide from occupied spaces. The amount of ventilation or exhaust is determined by researching ASHRAE 62.1 for the required factor. This factor could be a person factor, for example, “Provide 15 CFM per person” or it could be an area factor, for example, “Provide 1 CFM per square foot of area”.
(1c) Volumetric flow rate (CFM) can also be determined by the required velocity. This method is typically used in industrial ventilation situations and in kitchens. A high velocity is required in a system in order to keep particles suspended in the air so that they may be exhausted out of the space.
DETERMINING TOTAL STATIC PRESSURE [IN. WG]
The second parameter that must be found in order to size a fan is to determine the total static pressure. This is the total pressure that the fan must overcome in order to deliver the required CFM to the required location. The total static pressure is a function of the (2a) duct friction losses, (2b) duct fitting losses and (2c) miscellaneous equipment losses.
(2a) Duct Friction Losses: Straight lengths of duct incur friction losses on the airflow, which must be calculated by the engineer in order to properly size the fan. The amount of friction loss is a function of the velocity of air and the size of the duct. Another important tool that is required is the Standard Friction Loss in Standard Duct graphs, which can be found in the ASHRAE Fundamentals Handbook or the Mechanical Engineering Reference Manual.
(2b) Duct Fitting Losses: Each fitting also will have a friction loss associated with its construction. In order to find these fiction losses, the engineer will need the ASHRAE Fundamentals Handbook or the Mechanical Engineering Reference Manual. Duct fittings losses are dependent on the type of fitting and the velocity of the air through the fitting. The type of fitting will have a corresponding “K-factor” or “C-Coefficient”, which can be found in the ASHRAE of Fundamentals book and some typical fitting losses are also shown in the Mechanical Engineering Reference Manual. The “K-factor” or “C-Coefficient” is the multiplied by the velocity pressure in order to get the pressure loss due to the duct fitting. Remember that the velocity is found by first converting the rectangular or oval duct to equivalent diameter, then calculating the area.
(2c) Miscellaneous Equipment Friction Losses: In a duct system, there are also miscellaneous equipment losses due to different types of equipment, like filters, fans, diffusers, registers and grilles. The friction losses are given by the equipment manufacturer for different velocities and flow rates.