Elements of Fan Motor Sizing
Exhaust fans are a critical component in commercial kitchen ventilation systems, and the motor is a critical component of the exhaust fan. Selecting a properly sized motor for an exhaust fan ensures that the kitchen contaminants are effectively being carried through the hood and duct system and then expelled outdoors. Correct motor sizing also impacts overall cost, flexibility, and performance.
Example Situation: A direct drive fan has a brake horsepower of 0.6 HP but is equipped with a 1 HP motor. Why does the fan need a 1 HP motor? Based on the specific application need, there are many variables that should be considered during fan selection such as torque, horsepower, and fan laws to ensure you’re getting the proper performance from the unit.
Moving Air is a Variable Torque Process
Torque is the twisting force that causes rotation. All fan wheels or propellers spin, thus a fan requires torque to move. When a fan first turns on, it spins very slowly and moves very little air (CFM). The amount of torque required to turn the impeller (fan type) is also very low because there is little resistance or static pressure to overcome while the airflow is still slow. As the fan speed (RPM) increases, the airflow (CFM) increases, and more torque is required to spin the impeller. This increase in required torque is a result of the increased airflow (CFM), causing additional static pressure, or resistance that the fan must overcome.
Horsepower Fluctuates Depending on Need
While torque creates a twisting action on the impeller shaft, horsepower (HP) is the amount of work done over a specified period. In fans, HP is the measurement of torque multiplied by the speed (RPM) of the motor. This means that if either RPM, torque, or both increases, the HP it takes to run the fan will also increase. Therefore, moving more airflow requires more HP.
The Basics of Fan Laws
Several terms previously mentioned influence airflow. The terms RPM, CFM, static pressure, and HP have a relationship with one another, most commonly called the fan laws or affinity laws. Three basic fan laws address these terms.
The first fan law shows that the change in RPM is directly proportional to the change in CFM. (Figure 1). Example: A 10% increase in RPM results in a 10% increase in CFM.
Figure 1 – Fan Law
The second fan law indicates that as RPM increases, static pressure (Ps) also increases. However, this change is not directly proportionate. The Ps increases by the square of the change in RPM. Since the change in RPM is the same as CFM change, this law shows that as airflow increases, the resistance to flow increases significantly.
The third fan law illustrates how HP is affected by changes in airflow, stating that the amount of HP required to move air in a system, is multiplied by the cube of the change in RPM. If the RPM increases by 10%, the CFM will also increase by 10%. This same change will increase the Ps by 21% while the HP required to overcome the Ps and move the air will increase by 33%.
Your Application Matters
Looking back at the example situation: A direct drive fan has a brake horsepower of 0.6 HP. Why does the fan need a 1 HP motor?
One answer is flexibility. If you have a need to increase your design airflow by 10%, the HP of the fam will need to increase by 33%. With an initial brake horsepower of 0.60, a 33% increase would require a new brake horsepower of .798 which would be too large for a ¾ HP motor to accommodate. In this application, a 1 HP motor would support the increase in HP needed.
In some applications, you may not ever have a reason to increase the airflow for the entire life of the fan so you wouldn’t need a larger motor size. Having flexibility in the airflow range for the fan proves beneficial. In a kitchen application, you might decide to change out a griddle for a new char broiler which requires more airflow to contain the heat and smoke emitted. Or you may decide to utilize more efficient hood filters to prevent grease build up in your system which wasn’t initially a concern during the design of your restaurant. More efficient filters typically add static pressure to the system which then requires more fan horsepower to overcome.
Considering the needs of your application ahead of time will provide the necessary flexibility in your operation and alleviate additional costs for potential future design changes. Remember, it’s easier to increase the speed of your Vari-Green exhaust fan with the turn of a dial than it is to change out the motor later to get to the airflow you need for your commercial kitchen to function the way it should.
Connect with an Accurex expert to learn more about motor sizing at the onset of any kitchen ventilation project.
