Page 59 - HVAC Pump Handbook

Rishel_CH03.qxd 20/4/06 5:35 PM Page 56

Piping System Friction
56 The Basic Tools

The figures and tables of this chapter provide data that can be
referred to during a piping design procedure. Software is also avail-
able that implements the formulas and eliminates much of the
drudgery in calculating pipe friction. Software packages are available
from various sources at reasonable costs. It is urged that the design
basis for such software be fully understood before it is used on a pro-
ject. Designers with knowledge of software and computers can devel-
op some of their own computer programs for pipe friction by using the
aforementioned equations.

3.4 Steel and Cast Iron Pipe Fittings
A significant part of the friction loss for HVAC piping is caused by the
various fittings that are used to connect the piping. There have been
some very unacceptable practices used for the calculation of fitting
losses. For example, some people recommend that the pipe friction be
calculated and a percentage of that loss be added for the pipe fitting
and valve losses. This is a very poor and inexact method of computing
friction losses for valves and fittings. The proper method of computing
fitting loss is to determine that loss for each and every fitting and
valve. Again, the Hydraulic Institute’s Engineering Data Book and
Ingersol Rand’s Cameron Hydraulic Data are excellent sources for fit-
ting and valve losses. Interesting data on the possible variations in
the friction losses for fittings and valves are included in Table 32(c) of
the Hydraulic Institute’s Engineering Data Book; variations from 10
percent to as high as 50 percent are described.
2
Most fitting losses are referenced to the velocity head V /2g of the
water flowing in the pipe. Table 3.5 for steel pipe friction lists the veloc-
ity head at various flows in the pipe. A K factor has been developed for
many of the popular pipe fittings so that the loss through fittings Hf is
V 2
Hf K (3.7)
2 g
The Hydraulic Institute provides various K factors for valves and
fittings in their Engineering Data Book; these are listed in Tables 3.7
through 3.12 as approximate values. Unfortunately, at this writing,
many popular fittings such as reducing elbows or tees have no data.
Also, welded steel pipe reducers have significant losses, and there are
no reliable data on them.

Miscellaneous K factors
Couplings and unions. Couplings and unions depend on the quality of
manufacture. Assume an average K factor of 0.05 for them.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.