Section 4: Pipe and Fitting Sizing
In the previous section, we have looked at the different types of pipes which can be used in a
hydronic system. We have also looked at the various types of fittings used in conjunction with
the
piping.
In this section, we will examine the
friction losses
which occur when water flows through the pipes
and
fittings.
Friction losses are dependent on the following factors:
a. Water velocity
b. Pipe internal diameter
c. Pipe length
d. Pipe internal wall roughness
Generally, friction increases when:
-
Water velocity increases
-
Internal diameter decreases
-
Pipe length increases
-
Wall roughness increases
The basic formula to calculate pipe friction losses (Hf) is the
Darcy-Weisbach formula:
Hf = f * (L/D) * v
2
/2g
where f = friction factor
L = pipe length
D = pipe internal diameter
v = mean velocity
g = acceleration due to gravity
The friction factor is a function of the
roughness parameter
, e, which in turn
depends on the pipe material (e.g. steel, copper, PVC), and the Reynolds number,
Re.
Re = D * v
ρ
/ * µ
where
ρ
= density
µ = dynamic viscosity
Generally, the friction losses are presented in graphical form, for various pipe material.
See attached charts for steel pipe Schedule 40,copper pipe and PVC.
Note that Carrier has developed two different type of graphs for steel pipes, i.e. for
close
and
open
system. The friction losses for the open system is higher than the close system, for the
same parameters. This is to take into account the vulnerability of open systems to cause pipe
fouling and scaling on the internal surfaces. A factor of approximately 1.75 is used for this
purpose.
It is to be noted that the pipe friction loss charts presented are all for flows in the
turbulent regime
(Re > 10 000)
.
Section 4
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