N f = Speed of fan (rpm) N m = Speed of motor (rpm) Note that the ratio (Nf/Nm)2 reflects the fan inertia to the motor. Whether exhausting air or bringing fresh air into a structure, the calculation produced should help to figure out the size of fan(s) required to accomplish the air exchanges needed. So, to check and set speed on an ECM based furnace you would need to check the TESP in cooling mode, and then refer to the manual. Not everyone is going to take a look at CFM, but for those who do it is a helpful tool. A reduction in speed between motor and fan reduces the effect of load inertia on accelerating torque; conversely, an increase in speed between motor and fan increases the accelerating torque required. In simpler words than that of the formula, it is the amount of air a fan moves. This fan calculator is typically used to calculate the CFM or cubic feet per minute of air exchange that may be desired in a building. CFM BASIC FAN LAWS Existing Flowrate (CFM): Existing Static Pressure (in. This refers to how many cubic feet of air the range hood is exhausting for every minute of operation at full speed. For this you will need a vane anemometer. These combined factors establish the air moving capacity of a fan. This is the formula for fan CFM: Cubic feet per minute, more commonly known as CFM is calculated by the following formula: air speed (feet per minute) X area (square feet)=CFM. Obviously Simply put, the higher the CFM, the more air the hood can remove. CFM stands for Cubic Feet per Minute and is the most common way to measure airflow. The ventilation power of your range hood is measured in cubic feet per minute (CFM). Concept of Air Velocity can be used in air conditioning, heating and ventilating work. They are: Fan Model Cubic Feet per Minute (cfm) Static Pressure (Ps) H2O): Existing Horsepower (HP): Existing Speed (RPM): New Speed (RPM): New Flowrate (CFM): Enter value, select unit and click on calculate… CFM Calculation for Exhaust Fan. Now that you have the total pressure, you will have to measure the air speed in feet per second. to use or even what cfm is necessary. In our PSC example, we would likely have had to slow the fan speed by 7-15% to give us the ability to get much closer to our target of 350 CFM/ton. Volume flow is usually measured in Cubic Feet per Minute (CFM). Fan specification is usually not a precise science and can be done confidently when the fan application is understood. The fan performance data in Table 7 shows that fan #4 provides 2,775 cubic feet of air per minute (cfm) against a pressure of 1 inch. Air Flow formulas CFM = Duct area sq ft x Velocity Standard Air= 70F @ 29.92” HG (Mercury) 1 cubic foot of standard air = 0.075 pounds 13.3 cubic feet of standard air = 1 pound FAN LAWS: Remember RPM is interchangeable for CFM Note: new is the same as 1 and old is the same as 2 Fan Law #1 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⎟= ⎠ ⎞ ⎜ ⎝ ⎛ old This means a blower that is moving 1000 CFM cannot use more than 580 Watts of power to do so. To determine how much CFM you need for your range hood, consider the following: Measure the air speed. Airflow per ton is 2,775 cfm / 10 tons = about 278 cfm per ton. Based on the application, four parameters need to be determined. Areas are measured in square units (like square feet). This value is within the recommended range for hay drying given in Table 1 (see airflow recommendations by fan purpose section above). Volumes (like a room full of air) are measured in cubic units — CFM determines how much cubic feet can be moved or exchanged each minute. 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