Friction is everywhere. Even though friction finds itself in just about every system we design, it is one of those concepts that is not easily understood. Why do some designs accomplish the transfer of material and energy the way we expect them to while other designs fall short? At home, a drop of oil on a squeaky hinge could help a door close more smoothly. In piping and duct designs, understanding how the system will operate and choosing the right size pipe or duct will assure the correct amount of fluid is delivered at the correct pressure.
I like to visualize friction loss as rush hour traffic on a busy highway or the packed shuffling of a crowd leaving sports and concert venues. There are only so many cars you can pack on a highway, and only so many people you can get through a coliseum corridor (otherwise known as “egress” in architectural circles). Think back to our flow formula, Q=VA. For the two real life examples above, the area A is constant. As the vehicular or foot traffic slows down, so does the overall flow rate.