Page 39 - HVAC Pump Handbook

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Piping System Friction
36 The Basic Tools

3.1 Maximum Velocity in Pipe

There are a number of conflicting tables on the maximum allowable
water velocity in HVAC piping. The failure of many of these tables of
maximum velocity is their lack of consideration of the hydraulic radius
of commercial pipe. The hydraulic radius of a pipe is the cross-sectional
area of a pipe divided by the circumference of its inner surface. It is
calculated as follows:
d 2
Area
4
Circumference d
area d
Hydraulic radius (3.1)
circumference 4

where d inside diameter, in
Obviously, the hydraulic radius increases with pipe diameter, and
therefore, the allowable velocity should increase with the pipe diame-
ter. Hydraulic radii for commercial pipe are shown in Table 3.1. It is
quite clear that 36-in inside diameter (ID) pipe with a hydraulic radius
of 9.0 in must be rated velocity-wise differently than 3-in schedule
40 pipe with a hydraulic radius of 0.8 in.
Hydraulic radius may introduce a new guideline for the reevalua-
tion of friction for flow of water in piping and pipe fittings. The cur-
rent information on pipe friction and recommended velocities in pipe
are too dependent on testing done on small pipe; often the data are
then extrapolated for larger pipe. It is very difficult to test large pipe
fittings such as those with diameters greater than 20 in.
There are several recommendations for allowable velocity in HVAC
pipe; some are based on a maximum friction loss per 100 ft. Actually,
as indicated previously, final pipe velocity is within the province of
the designer who is responsible for first cost as well as operating
costs. Here is an excellent point at which the designer can use com-
puter capability in sizing piping. Several computer runs at different
pipe sizes can be done to achieve the economically desirable pipe
size. This should be done for the major piping such as loops and
headers. The size of smaller branches and coil connections will fall
more into the realm of the designer’s experience. Table 3.2 provides
an elementary example of this program comparing 12-, 14-, and 16-in
diameter commercial pipe.
The operating cost decreases while maintenance and amortization
of the first cost increase with the pipe size. The economic pipe size is
at the minimum point of the sum of the two values or curves.

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