Page 56 - HVAC Pump Handbook

Rishel_CH03.qxd 20/4/06 5:35 PM Page 53

Piping System Friction
Piping System Friction 53

flowing through this pipe, the velocity is 8.33 ft/s, and the velocity
head is 1.08 ft.

Computing the kinematic viscosity first in square feet per second from Eq. 2.2.
At 30°F,

6.7197 ⋅ 8.5 ⋅ 10 4

0.841 10 4
67.95
At 130°F,

6.7197 ⋅ 1.4 ⋅ 10 4

0.142 10 4
66.08
The data have now been collected to compute the Reynolds number
at the two temperatures. At 30°F,

V D 8.33 ⋅ 0.505
R 5.00 10 4

0.841 ⋅ 10 4
At 130°F,

8.33 ⋅ 0.505
R 2.96 10 5
0.142 ⋅ 10 5
The friction factors can now be selected from the Moody diagram
4
(Fig. 3.2). At 30°F and a Reynolds number of 5.00 10 , the friction
5
factor f is 0.022. At 130°F and a Reynolds number of 2.96 10 , the
friction factor f is 0.017.
The friction in feet per 100 ft can now be calculated from the Darcy
Weisbach equation (Eq. 3.3). At 30°F,

0.022 ⋅ 100 ⋅ 1.08
Hf 4.71 ft/100 ft
0.505
At 130°F,

0.017 ⋅ 100 ⋅ 1.08
Hf 3.64 ft/100 ft
0.505

This example demonstrates the use of Reynolds number and the
Moody diagram. It also emphasizes the variation in pipe friction with
viscosity. In this case, the friction at 130°F was 77 percent of that at
30°F. Also, this demonstrates that the friction for water at 60°F from
Table 3.5 of 3.57 ft/100 ft should not be used in calculating friction
losses for this glycol solution.

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