Chapter hvac engineering Fundamentals: Part 1 Introduction

Design Procedures: Part 4
177
Figure 6.21
Tank at pump suction, pumping away from boiler.
Figure 6.22
Tank at pump discharge, pumping into boiler. (Not recommended.)
Design Procedures: Part 4
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178
Chapter Six
Figure 6.23
Tank at top of system.
E
V
⫽
(6.3)
t
P
/
P
⫺
P
/
P
a
f
a
o
where
E
⫽
net expansion of the water in the system when heated from
minimum to maximum temperature. Expansion
E
equals the system
volume times the percentage increase indicated in the graph of Fig.
6.24. For chilled water, the minimum design temperature is used, com-
bined with the maximum anticipated temperature during a summer
shutdown.
Expansion tanks for high-temperature water systems are always
provided with a cushion of inert gas (usually nitrogen) or high-
pressure steam, which is continuously maintained by an automatic
control system with rapid response. Maintenance of system pressure
is critical to proper operation of the HTW system. HTW systems are
usually large and have a wide temperature difference from supply to
return. For details on handling expansion in HTW systems, see the
relevant ASHRAE handbook chapter.
5