Chapter hvac engineering Fundamentals: Part 1 Introduction

172
T
ABLE
6.9
Retur
n
Main
and
Riser
Capacities
for
Low-Pr
essur
e
Steam
Systems,
lb
/
h
1
2
1
32
1
24
2
3
1
16
1
8
1
4
1
2
1
2
3
4
1
4
1
2
1
2
1
2
3
4
1
4
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
16.
Design Procedures: Part 4
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Design Procedures: Part 4
173
Figure 6.13
Out-and-back piping system.
Figure 6.14
Pressure profile for out-and-back piping.
tank which has a watertight flexible diaphragm to separate the air
and water.
Open expansion tanks are sometimes used (Fig. 6.20). Because this
tank is open to air, the maximum water temperature is limited. The
tank must be above the high point of the piping system, and the cor-
rosion problem created by the air interface must be recognized. The
possibility of freezing also exists if the tank is in an unheated space.
The point at which the expansion tank connects to the piping system
is the point of no pressure change when the pump is started or
stopped. It is preferable that this point be on the suction side of the
pump to minimize the total pressure. If the connection were on the
discharge side of the pump, the total pressure would have to be greater
in order to prevent cavitation at the pump inlet. An exception to this
is an attic location for the tank, which would result in a smaller tank
than one at the bottom of the system. This arises out of the pressure
relationships in the sizing equation, as described below.
The size of a closed expansion tank is determined by (1) the volume
of water in the system, (2) the range of water temperatures expected
in normal operation, (3) the air pressure in the expansion tank when
Design Procedures: Part 4
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
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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