## 1. Introduction

The friction factor is used to calculate the pressure drop due to the
flow of a fluid in a pipe. It represents the interaction in between
the fluid and the pipe.

**There are different ways to calculate it,
one can be graphical, using a Moody graph, but for automating
calculation it is not practical, thus correlations are required. The
Colebrook correlation is usually
admitted as being accurate enough for most industrial applications
but presents a difficulty as it does not explicitly express the
friction factor, that is why it may be interesting to consider
Churchill equation that allows to directly calculate the friction
factor.**

**Note that the friction factor used here is
Darcy (also called Darcy-Wesibach or Moody) friction factor.**