What is Pressure loss?
The pressure a fluid loses to friction and disturbed flow as it passes through pipes and equipment. It is not recovered downstream.
It sets the pressure a fan or pump must supply, the pipe size, and how flow divides between branches. Ignore it and the design flow simply does not arrive.
Drinking through a thin straw takes effort because friction inside the straw eats pressure; a longer straw makes it harder still.
The Pipe block in Chemical CAD computes straight-pipe friction and local losses of bends, reducers and valves. Each element's loss can be switched to 0 Pa. Static pressure changes from elevation and velocity changes are kept separate and remain when a loss is switched off.
Straight-pipe friction uses the Darcy–Weisbach equation:
\Delta p_f = f\,\frac{L}{D}\,\frac{\rho u^2}{2}
The friction factor f is 64/Re for laminar flow (Re < 2300) and the Colebrook equation for turbulent flow (Re > 4000); in between, the flow is transitional and unpredictable, so f is interpolated and the result says so. Gas density changes with pressure and temperature, so the pipe is divided into segments and the momentum and energy balances are solved for each:
p_2 = p_1 - \Delta p_f - \rho g\,\Delta z - G\,(u_2 - u_1)
The constant-density approximation is used only while the pressure change is at most 10 % of the inlet pressure and the Mach number at most 0.3. Near-sonic (choked) flow is not modeled.