How does hose diameter affect friction loss at a given flow?

• A. Larger diameter reduces friction loss for the same flow.
• B. Smaller diameter reduces friction loss for the same flow.
• C. Diameter has no effect on friction loss.
• D. Larger diameter increases friction loss for the same flow.

Answer: (A)

Friction loss comes from the energy the water must spend to slide along the hose walls. For a fixed discharge, increasing the hose diameter increases the cross-sectional area, which reduces the fluid velocity (V = Q/A). Since velocity drops with a larger diameter, the wall friction that converts flow energy into heat is reduced. In practical terms, doubling the diameter makes the area four times bigger, so the velocity is about a quarter for the same flow, and friction losses fall significantly (roughly with the square of the velocity, all else equal). That’s why a larger diameter hose carries the same flow with much less friction loss. Smaller diameter hoses push the water faster, increasing wall friction and thus the losses.