Chapter hvac engineering Fundamentals: Part 1 Introduction

168
T
ABLE
6.7
Equivalent
Length
of
Pipe
for
90
ⴗ
Elbows,
ft
3
4
1
4
1
2
1
2
1
2
1
2
S
O
URCE
:
Copyright
2001,
American
Society
of
Heating,
Refrigerating
and
Air
Conditioning
Engineers,
Inc.,
www
.ashrae.org.
Abstracted
by
permission
from
ASHRAE
Handbook,
2001
Fundamentals,
Chap.
35,
T
able
8.
Design Procedures: Part 4
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Design Procedures: Part 4
169
TABLE
6.8 Iron and Copper Elbow Equivalents
See Table 6.7 for equivalent length of one elbow.
SOURCE
: Copyright 2001, American Society of Heat-
ing, Refrigerating and Air Conditioning Engineers,
Inc., www.ashrae.org. Reprinted by permission from
ASHRAE Handbook,
2001 Fundamentals,
Chap. 35,
Table 9.
branches, as shown in the pressure profile in Fig. 6.16, so that the
minimum amount of balancing is required. This is accomplished by
beginning the return main at the location of the first takeoff from the
supply and continuing the return main parallel to the supply main
with flow in both pipes in the same direction. The return main in-
creases in size as the supply main decreases. Finally, the return main
goes back to the central plant.
The loop system (Figs. 6.17 and 6.18) consists of supply and return
mains which are closed loops, with a constant pipe size. The loops are
fed at one point from the central plant, flow goes in both directions on
the loop, and branch takeoffs may be located at any point on the loop.
At some point in the loop, there will be no flow, depending on the
geometry and demand of the branches. Because the loop is closed, it
is self-balancing; i.e., as branch takeoffs are added or removed, the no-
flow point will move until balance is restored. For an overall pressure
loss equivalent to that in a reverse-return system, the loop pipe size
(diameter) should be about 40 to 50 percent of the main size from the
central plant.
The pressure profiles for the three systems are based on full design
flow. If flow is reduced, the profiles will change to less main slope and
greater available head at each branch.
In comparing the three configurations, note that the system geom-
etry, especially the relationships among the central plant and the var-
ious points of use, will influence cost and efficiency greatly. In general,
Design Procedures: Part 4
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Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.

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