Basically, the airflow of a fan is proportional to the rotational speed, and the static pressure is proportional to the square of the rotational speed. In other words, doubling the rotational speed doubles the airflow and quadruples the static pressure.
While static pressure confronts obstacles head-on, airflow operates with the philosophy of maximum volume with minimal effort. Fans optimized for high airflow are often used to move larger amounts of unrestricted air across a broader area.
For starters, it helps to know the difference between pressure and flow. Air pressure is measured in pounds per square inch or PSI, while air flow (also referred to as volume) is measured in cubic feet per minute or CFM.
Air Flow Volume (CFM) – Simply put, it is how much air a fan moves. Static Pressure (SP) – The resistance of the fan pressure has to blow against to move air in the desired direction.
CFM represents the volume of air we move per minute, so static pressure is the pressure this movement exerts on our ventilation ducts. To illustrate this phenomenon, let's imagine we're blowing into a small straw. Our cheeks swell because too much air wants to pass through the straw at the same time.
The static pressure is controlled by increasing or decreasing the speed of the blower. As your CFM increases, the static pressure will decrease.
CFM (cubic feet per minute) is a flow unit while Pascal is a pressure unit. There is no direct relation between the two.
Many HVAC contractors agree that 0.5 inches of static pressure is ideal. At this level, HVAC systems aren't subjected to accelerated wear. Also, noise levels remain moderate when heating or cooling systems are on. Maintaining static pressure at 0.5 inches is also beneficial for HVAC ductwork.
450 cfm and 600 cfm are directly comparable, with 600 cfm being 33% more air by volume than 450 cfm. This measurement down not take into account the velocity of the air coming out of the nozzle.
The number depends on a few things: the fan's motor, the blade pitch and the length and/or shape of the fan blades. Ultimately that means the greater the CFM, the more air a fan moves.
Static Pressure From CFM Formula
To calculate the static pressure from CFM, divide the CFM by the area, divide the result by 4005, square this result, then finally, subtract this from the total pressure.
If the measured ESP is greater than 0.5” WC, or if the measured ESP is beyond the maximum allowable of the blower performance curve this MAY indicate a restrictive system due to undersized duct, dirty components and/or closed branch ducts.
As previously mentioned, the relation between the flow and the pressure is directly proportional. So, as pressure increases, the flow rate increases.
Airflow can be measured in liters per minute. This means that the higher the pressure difference between two sites, the more air flowing between them.
While fans with more blades can be quieter and aesthetically pleasing, they often move less air than fans with fewer blades. Three-bladed fans are generally found to be the most effective in terms of airflow.
Static pressure is the pressure exerted by a fluid when it is at rest. It is measured perpendicular to the fluid surface and is independent of the direction of flow. Dynamic pressure is the pressure exerted by fluid movement. It is measured parallel to the direction of flow and depends on the velocity of the fluid.
What is a good CFM for a leaf blower? A good CFM for most yards ranges between 400-700, depending on the size of the area you need to clear.
To determine your ideal CFM, multiply your kitchen's cubic feet by 15. For instance, 1,500 cubic feet x 15 air exchanges = 22,500 cubic feet per hour. Now, divide your total by the number of minutes in an hour: 22,500 cubic feet / 60 minutes = 375.
Air Quality Issues
If static pressure is too high in an individual room, it might cause an air leak that could lead to dirty air being drawn into the rest of your home's ventilation system.
In general, a good static pressure for an HVAC system falls within the range of 0.5 to 0.8 inches WC. This range ensures that the system is operating efficiently without exerting undue stress on the components.
Static pressure is the resistance to airflow in ductwork. You want the right balance of static pressure in your system because too much or too little can cause problems, including premature system failure and energy inefficiency.
To sum it up and not bore you with the equations, the static pressure changes squared with the cfm. That means, if you don't change anything in a system and double your cfm, you will quadruple your static pressure.
For every horsepower, a compressor delivers 4-5 cfm, at 100 psi pressure. In other words - a 1 horsepower compressor will output around 4 to 5 cfm at 100 psi pressure. A 10 HP unit will output around 40 to 50 cfm at 100 psi.
The higher the CFM, the more air the fan circulates, making it more effective at cooling your room. For a standard room with an 8-foot ceiling, a fan with a CFM rating of 4000 to 5000 is generally sufficient. However, for larger rooms or rooms with higher ceilings, you might need a fan with a higher CFM rating.