Chapter hvac engineering Fundamentals: Part 1 Introduction

150
T
ABLE
6.2
Pr
operties
of
W
ater
,
212
to
400
ⴗ
F
S
O
URCE
:
Reprinted
by
permission
from
T
hermodynamic
Properties
of
Steam,
J.
H.
Keenan
and
F
.
G.
Keyes,
published
by
John
W
iley
and
Sons,
Inc.,
1936
edition.
Subsequent
editions
ha
ve
equivalent
data.
Design Procedures: Part 4
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Design Procedures: Part 4
151
actual heat rejection will vary with the refrigeration system efficiency
and will usually be somewhat less than 15,000 Btu / (ton
⫺
h), except
that for absorption refrigeration, rejection will be 20,000 to 30,000
Btu / (ton
⫺
h).
6.4
High-Temperature Water
High-temperature water
(HTW) systems operate with supply water
temperatures over 350
⬚
F and with a pressure rating of 300 to 350 lb/
in
2
gauge (psig). Maximum temperatures are about 400
⬚
F in order to
stay within the 300 lb / in
2
gauge limit on pipe and fittings. Medium-
temperature systems operate with supply water temperatures be-
tween 250 and 350
⬚
F, which allows the use of 150 lb / in
2
gauge rating
on piping systems. Table 6.2 lists properties of water at temperatures
up to 400
⬚
F.
Systems must be kept tight because water at these temperatures
will flash instantly to steam at any leak. Large temperature drops at
heat exchangers are typical—150 to 200
⬚
F is normal. The system must
be carefully pressurized to above the saturation pressure correspond-
ing to the water temperature, to prevent the water from flashing into
steam.
Heat exchangers are used to provide lower-temperature hot water
or steam for HVAC use. HTW may be used directly for generation of
domestic hot water. Most jurisdictions require double-wall heat ex-
changers to guarantee protection from tube failure and cross-
contamination. It is common to place user equipment in series, taking