Aspect ratio is the ratio between the width and height of a duct. The greater the perimeter (distance around) of a duct, the more friction loss there will be.Īspect ratio is a way to determine the best practical perimeter for a duct.
This means that if the quantity of air and the area of the duct remain the same: For the same air quantity and velocity, a duct with a greater surface for the air to rub against will develop more friction loss. Choosing the best aspect ratio of a duct can reduce friction loss.įriction loss in a duct is the result of the air molecules rubbing against the inside of the duct. Understanding aspect ratio is important for the equal friction loss method. By maintaining the same friction loss per 100 feet, it is only necessary to determine the total length of the duct run to determine the total friction loss. The equal friction loss method varies the velocity but maintains the same friction loss per 100 feet of duct run. This gives the equivalent duct size based upon maintaining the same friction loss. The industry has generally adopted the equal friction loss method of sizing duct. Understanding the relationships of this equation is essential to understanding airflow in duct. This is not to say that the equation Q = A X V is not an important equation. Using Q = A X V to calculate duct size changes with each change in CFM maintains the same velocity, but the friction loss for each size will not remain the same. The static pressure at the fan outlet must be equal to the resistance of the duct system. The duct designer needs to know what the total pressure loss is for a duct run in order to select the proper size fan. The equation Q = A X V does not take this into account. However, the pressure loss changes each time the duct size changes.