Pressure Loss in Pipe Runs !

Pressure Loss in Pipe Runs !

Where there is energy, there is inefficiency; that's a law of life. And life wouldn't possibly exclude chilled water piping networks, and piping in general, from this law.

As fluid flows inside a pipe, it rubs against the surface of the pipe, and among itself. This friction causes a dissipation of flow energy into thermal energy.

Loss of flow energy means loss of pressure. The more fluid flows inside a pipe, the more pressure it loses.

To calculate pressure loss, multiple formulas have been presented. The three most commonly used (being the most accurate) formulas are:

1. Darcy Weisbach

2. Hazen Williams

3. Manning

The three equations share one thing in common: Pressure loss = Function of (Pipe diameter, Length and Flow Rate)

Hazen Williams and Manning equations were developed empirically, meaning from experimentation, rather than from direct theory. They're used in situations where the diameter of a pipe is to be calculated from a fixed pressure loss value.

However, this doesn't have to be the case in chilled water systems. Most of the time, you'll be calculating pressure loss in a pipe of known diameter, length and flow.

Darcy Weisbach is the most accurate formulation, but it requires the use of a diagram called Moody's diagram.

In the Darcy Weisbach's formula, there's a factor called "friction factor", or f. f is a function of pipe material, diameter, fluid properties and flow velocity.

f is obtained from Moody's diagram based on a fixed Reynold's number and relative roughness.

Re = density * velocity * diameter / dynamic viscosity

ε/D = pipe roughness / diameter

f = function of (Re, ε/D)

Hf = Head loss due to friction

= function of (D, L, V, f)

D: Pipe's internal diameter

L: Length

V: Flow Velocity

f: Friction factor, obtainable from Moody's diagram