Stop me if you’ve heard this one before: Fans are constant volume machines. While this is not a joke, it often requires an explanation. Imagine the fan wheel being a series of scoops. As it rotates, each scoop gets filled with material. The volume of material is always the same, whether it is feathers or rocks. Of course the rocks weigh more, so it takes more effort (or brake horsepower [BHP]) to move them. Remember, a scoop is a scoop!