Friction head loss is directly proportional to which of the following factors?

Friction head loss is primarily influenced by the roughness and length of the pipe through which a fluid flows. As fluid moves through a pipe, it experiences resistance due to the internal surfaces of the pipe, which is affected by the roughness—smoother pipes generally reduce friction loss. Additionally, longer pipes contribute to greater friction loss because the fluid has to overcome more resistance over a greater distance. Thus, both the roughness and length of the pipe play crucial roles in determining friction head loss.

While other factors like temperature, pressure, fluid viscosity, and density can affect fluid behavior and flow characteristics, they do not directly correspond to friction head loss in the same proportional way as the pipe's roughness and length do. For example, while fluid viscosity influences how easily a fluid flows, it is not the primary determinant of frictional losses in pipes; rather, it mainly modifies the flow regime and can influence other aspects of the system.