22.1
CHAPTER 22
PIPE SIZING
Pressure Drop Equations
.........................................................
22.1
WATER PIPING
.......................................................................
22.5
Flow Rate Limitations
..............................................................
22.5
Hydronic System Piping
...........................................................
22.6
Service Water Piping
................................................................
22.8
STEAM PIPING
.....................................................................
22.12
LowPressure Steam Piping
...................................................
22.13
HighPressure Steam Piping
..................................................
22.13
Steam Condensate Systems
....................................................
22.13
GAS PIPING
..........................................................................
22.20
FUEL OIL PIPING
................................................................
22.21
HIS CHAPTER includes tables and charts to size piping for
T
various fluid flow systems. Further details on specific piping
systems can be found in appropriate chapters of the ASHRAE
Handbook.
Two related but distinct concerns emerge when designing a fluid
flow system: sizing the pipe and determining the flowpressure rela
tionship. The two are often confused because they can use the same
equations and design tools. Nevertheless, they should be determined
separately.
The emphasis in this chapter is on the problem of sizing the pipe,
and to this end design charts and tables for specific fluids are pre
sented in addition to the equations that describe the flow of fluids in
pipes. Once a system has been sized, it should be analyzed with
more detailed methods of calculation to determine the pump head
required to achieve the desired flow. Computerized methods are
well suited to handling the details of calculating losses around an
extensive system.
PRESSURE DROP EQUATIONS
DarcyWeisbach Equation
Pressure drop caused by fluid friction in fully developed flows of
all “wellbehaved” (Newtonian) fluids is described by the Darcy
Weisbach equation:
(1)
where
'
p
=
pressure drop, lb
f
/ft
2
f
=
friction factor, dimensionless (from Moody chart, Figure 13 in
Chapter 3)
L
=
length of pipe, ft
D
=
internal diameter of pipe, ft
U
=
fluid density at mean temperature, lb
m
/ft
3
V
=
average velocity, fps
g
c
=
units conversion factor, 32.2 ft·lb
m
/lb
f
·s
2
This equation is often presented in head or specific energy
form as
(2)
where
'
h
=
head loss, ft
g
=
acceleration of gravity, ft/s
2
In this form, the density of the fluid does not appear explicitly
(although it is in the Reynolds number, which influences
f
).
The friction factor
f
is a function of pipe roughness
H
, inside
diameter
D
, and parameter Re, the Reynolds number:
(3)
where
Re
=
Reynolds number, dimensionless
H
=
absolute roughness of pipe wall, ft
P
=
dynamic viscosity of fluid, lb
m
/ft·s
The friction factor is frequently presented on a Moody chart
(Figure 13 in Chapter 3) giving
f
as a function of Re with
H
/
D
as a
parameter.
A useful fit of smooth and rough pipe data for the usual turbulent
flow regime is the
Colebrook equation
:
(4)
Another form of Equation (4) appears in Chapter 3, but the two
are equivalent. Equation (4) is more useful in showing behavior at
limiting cases—as
H
/
D
approaches 0 (smooth limit), the 18.7/Re
term dominates; at high
H
/
D
and Re (fully rough limit), the 2
H
/
D
term dominates.
Equation (4) is implicit in
f
; that is,
f
appears on both sides, so a
value for
f
is usually obtained iteratively.
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 Spring '11
 range
 Fluid Dynamics, ........., ASHRAE

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