Page 53 - HVAC Pump Handbook

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Piping System Friction
50 The Basic Tools

For example, assume that 50°F water is flowing through 4-in
schedule 40 steel pipe at 200 gal/min. From Table 3.5, the velocity is
5.04 ft/s, and the diameter of the pipe is 4.026 in, or 0.336 ft. From
2
Table 2.2, the kinematic viscosity for 50°F water is 1.41 10 5 ft /s.
The Reynolds number is
5.04 0.336
1.20 10 5
1.41 10 5

From the Moody diagram shown in Fig. 3.2, the friction factor f is
0.0198.
The Moody diagram, named after its originator, is described in
Fig. 3.2. This diagram generates the friction factor f of the Colebrook
equation (Eq. 3.4) for a broad variety of Reynolds numbers and, there-
fore, velocities and viscosities. This particular diagram accepts a con-
stant relative roughness factor of 0.00015 and provides curves for
all popular sizes of steel pipe. This is adequate for most HVAC appli-
cations of steel pipe.
Another diagram that is useful for work with a number of liquids
where the kinematic viscosity is known is shown in Fig. 3.3. The
Reynolds number can be selected from this diagram once the pipe
diameter, water velocity, and viscosity of the liquid are developed.
Also, if the temperature of the liquid is determined, the kinematic vis-
cosity of that liquid can be determined from this figure.
The practical use of Reynolds number and the Moody diagram can
be demonstrated by the following example. Assume that

1. A condenser water system is used for cooling computer air-
conditioners employing a propylene glycol solution of 50% where
the liquid temperature can vary from 30 to 130°F.
2. The flow rate is 750 gal/min at 30°F in a 6-in-diameter steel pipe.

Question. What is the variation in friction loss in feet per 100 feet of
length as the liquid varies from 30 to 130°F?

Factual data
1. From Fig. 2.4, the viscosity of the glycol solution is 8.5 cP at 30°F
and 1.4 cP at 130°F.
2. From Fig. 2.5, the specific gravity of the glycol solution at 30 °F is
3
1.09 water at 60°F, or 67.95 lb/ft . Likewise, the specific gravity
3
at 130°F is 1.06 water at 60°F, or 66.08 lb/ft .
3. From Table 3.5, the inside diameter of 6-in steel schedule 40 pipe
is 6.065 in, or 0.505 ft. Also from this table, with 750 gal/min

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