User Guide

Plumbing Design Assistance Manual l Chapter 4 – Pipe sizing l 43
The C
v
method for calculating friction loss
C
v
is a function of ow rate versus pressure drop across the tting. It also accounts for the density of the uid.
Refer to the following friction loss calculation using the C
v
method.
P
ƒ
F
2
∆P =
(
––––
) (
––––
)
P
w
C
v
Where,
∆P = pressure drop across the tting (psi)
lb
P
ƒ
= fluid density
(
––––
)
ft
3
lb
P
w
= density of water at 60°F
(
––––
)
ft
3
The density of water at 60°F (15.6°C) is 62.38 pounds per cubic foot.
F = volumetric ow rate through the tting (gpm)
C
v
= known C
v
rating of the tting
This calculation yields a value in pounds per square inch which, for hydronic sizing,
needs to be converted to feet of head as 100 percent water.
The conversion to feet of water is shown below.
144in
2
• ∆P
h
ƒ
= ––––––––––––––––
1ft
3
• P
ƒ
Where,
hƒ = head loss due to friction across the tting (ft)
lb
P
ƒ
= fluid density
(
––––
)
ft
3
∆P = pressure drop across the tting (psi)
This value can be cumulatively added to the values from the other ttings in
the distribution line to calculate the total loss for the ttings in the system.
Example 2
For a given elbow with a C
v
of 6.7 and a ow rate of 3.5 gpm at a water temperature
of 160°F (71.1°C), what is the friction loss of the tting (in feet)?
Assuming the uid is 100 percent water, here is the calculation.
lb
P
ƒ
= 60.99 ––––
ft
3
F = 3.5 gpm
C
v
= 6.7
lb
60.99 –––
P
ƒ
F
2
ft
3
3.5 gpm
2
∆P
=
(
–––
) (
–––
)
=
(
––––––––––
)
(
––––––––
)
= 0.267 psi
P
w
C
w
lb 6.7
62.38 –– –
ft
3
Note: To make the units work in the C
v
formula, it must be assumed the C
v
has the units of gpm and
the result multiplied by 1 psi.
With the pressure differential known, the value found can be converted to head loss in feet as seen below.
144in
2
• ∆P 144in
2
• 0.267 psi
h
ƒ
= –––––––––––– = –––––––––––––––––––– = 0.63 ft
1ft
2
• P
ƒ
lb
1ft
2
•60.99 –– –
ft
3