Fan Law 2: Total Static Pressure changes with the square of CFM (or RPM). What it means: A 10% increase in CFM will result in a 21% increase in static pressure. Think about that. A small increase in airflow creates a significant increase in duct pressure.
Pressure (P, Pa) varies as the square to the ratio of the rotational speed (U, u/min) of the impeller. Therefore if the propeller speed is increased by 10%, the total static pressure will increase by 21%.
The static pressure is controlled by increasing or decreasing the speed of the blower. As your CFM increases, the static pressure will decrease. Modulating supply fans typically controlled by a VFD are best used in a system for regulating the static pressure.
CFM (Cubic Feet per Minute) is calculated by multiplying the velocity (Feet per Minute) with the area (Square Feet).
1) Airflow is directly proportional to fan speed. If fan speed is reduced by 10%, the airflow rate will decrease by 10%. 2) Pressure is proportional to the fan speed squared, if the fan speed is reduced by 10%, pressure will decrease by 19%. 3) Fan energy consumption is proportional to the fan speed cubed.
To summarize these 3 fan laws, flow changes proportionately to speed. Static pressure changes as a function of the change in speed squared. And brake horsepower changes as a function of the change in speed cubed.
The ideal gas law states that PV = nRT, or, in plain English, that pressure times volume equals moles times the gas law constant R times temperature.
CFM = (fpm * area), where fpm is the feet per minute. At the place of the FPM value, input the area after it is squared. You will get the result.
In compressed air terms, pressure delivers the force, yet horsepower delivers the flow. These two measures are inversely related: as pressure (PSI) increases, the flow rate (CFM) decreases, and vice versa.
FPM is turned into CFM by multiplying the average FPM in the duct by its area. Depending on the duct shape one of the following can be used: Using square area for simplicity.
Multiply the specified operating static pressure by the correction factor to determine the standard air density equivalent static pressure. (Corrected static pressure = 3.0 x 2.00 = 6”. The fan must be selected for 6 inches of static pressure.)
As the static pressure increases, the CFM delivered decreases. This is even true of variable speed or ECM motors; they only produce their rated airflow up to a maximum static pressure before CFM drops off.
The same goes for static pressure, but it is expressed in inches of water. A 0.5” reading corresponds to a blood pressure of 120/80, which is right where HVAC systems should be. A 1.0” reading, however, indicates excess airflow restriction pressure in the equipment by a factor of 2.
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.
According to Pascal's Law, “The external static pressure applied on a confined liquid is distributed or transmitted evenly throughout the liquid in all directions”. The static pressure acts at right angles to any surface in contact with the fluid.
The first fan law relates the airflow rate to the fan rotational speed: Volume flow rate (CFM) is directly proportional to the fan rotational speed (RPM). If the fan RPM is increased, the fan will discharge a greater volume of air in exact proportion to the change in speed.
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.
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.
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.
CFM Formula
Use the formula below to calculate CFM: CFM = (Room Volume in cubic feet) x (ACH) / 60.
This entirely depends on the application. For a residential fan you might have in your living room, a CFM of 5,000-6,000 might be totally sufficient. For large warehouses, you might want a fleet of large-diameter HVLS fans that each have a CFM of over 300,000.
If you need to quickly determine how much cfm you need to deliver to a space, this is a great place to start. If the cooling load is relatively small, the system may require closer to 1 cfm/sq ft.
The pressure law states that pressure is proportional to temperature, for a fixed amount of gas at constant volume. The pressure law is the basis of the Kelvin scale. When gas molecules stop moving, the pressure becomes zero as there are no collisions with the walls.
Since pressure is defined as the force per unit area, its formula is expressed as P = F/A, where P is pressure, F is force, and A is the area by which the force is applied perpendicularly. In fluid pressure, force is equivalent to fluid weight, making the pressure equation P = (rho)gh.
Boyle's law, published in 1662, states that, at a constant temperature, the product of the pressure and volume of a given mass of an ideal gas in a closed system is always constant. It can be verified experimentally using a pressure gauge and a variable volume container.