Page 95 - Industrial Ventilation Design Guidebook

60 CHAPTER 4 PHYSICAL FUNDAMENTALS

Final pressure is

4.1.5.6 Dimensioning of a Duct with Liquid Flow
This example demonstrates the dimensioning of a duct with a frictional
incompressible fluid flow. Now the Bernoulli equation can be written as

where

To determine the pressure losses, we have to find out whether the flow is lam-
inar or turbulent, because £ = f(Re, k/d). In practical dimensioning, Eq.
(4.66) and the Moody chart are used.

Empirical tables are used to determine the value of the single resistances.
The determination of the desired volume flow V and the pressure differ-
ence p 2 - p\ leads to an iterative procedure in a turbulent case, due to terms £
m
and XC • a laminar flow

so no iteration is needed. This is due to the fact that the pipe diameter d is re-
duced from its value in the formula for the pressure loss caused by friction, be-
cause Re = 4q v/TT dv.

Example 3
A pipe of diameter d 0 conveys water at a volume flow rate q v as shown in
Fig. 4.6. The lateral branches 1-2 and 3-4 are each required to have volume
flows of q v/6. The pressures at points 0, 2, and 4 are known; the water leaves
the system at points 2 and 4. Determine diameters d 4 and d 2.