Chapter hvac engineering Fundamentals: Part 1 Introduction

153
T
ABLE
6.3
Pr
operties
of
Pur
e
Calcium
Chloride
Brine
*Mass
of
w
ater
per
unit
volume
⫽
Brine
mass
minus
CaCl
2
mass.
†Specific
gra
vity
is
solution
at
60
⬚
F
referred
to
w
ater
at
60
⬚
F.
S
O
URCE
:
Copyright
2001,
American
Society
of
Heating,
Refrigerating
and
Air
Conditioning
Engineers,
Inc.,
www
.ashrae.org.
Reprinted
by
permission
from
ASHRAE
Handbook,
2001
Fundamentals,
Chap.
21,
T
able
1.
Design Procedures: Part 4
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154
T
ABLE
6.4
Pr
operties
of
Pur
e
Sodium
Chloride
Brine
*Mass
of
commercial
NaCl
required
⫽
(mass
of
pure
NaCl
required)
/(
%
purity).
†Mass
of
w
ater
per
unit
volume
⫽
brine
mass
minus
NaCl
mass.
‡Specific
gra
vity
is
solution
at
59
⬚
F
referred
to
w
ater
at
39
⬚
F.
S
O
URCE
:
Copyright
2001,
American
Society
of
Heating,
Refrigerating
and
Air
Conditioning
Engineers,
Inc.,
www
.ashrae.org.
Reprinted
by
permission
from
ASHRAE
Handbook,
1989
Fundamentals,
Chap.
21,
T
able
2.
Design Procedures: Part 4
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---

Design Procedures: Part 4
155
Figure 6.4
Viscosity of calcium chloride brine. (
SOURCE
: Copyright 2001, American So-
ciety of Heating, Refrigerating and Air Conditioning Engineers, Inc., www.ashrae.org.
Reprinted by permission from ASHRAE Handbook,
2001 Fundamentals,
Chap. 21, Fig.
3.)
ride will lower the freezing point of the mixture to
⫺
21
⬚
F and will
decrease the specific heat to 0.689 Btu / (lb
䡠
⬚
F). This means that the
solution will transport only about two-thirds of the heat transported
by pure water at the same mass flow rate and temperature difference.
The volumetric flow rate is partially offset by the increased mass of
the mixture in pounds per gallon.
Note that the viscosity of the brine increases (Fig. 6.4) while the
thermal conductivity decreases (Fig. 6.5) as the percentage of salt
increases. Compared to pure water, this results in a higher pumping
head and lower heat transfer rate. These brines are less effective than
water as a heat-conveying medium. The tables indicate a percentage
solution at which a minimum freezing temperature is obtained. This
is the
eutectic point.
Brine solutions are corrosive, particularly when
exposed to air or carbon dioxide. Inhibitors are recommended. Chro-
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156
Chapter Six
Figure 6.5
Thermal conductivity of calcium chloride brine. (
SOURCE
: Copyright 2001,
American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc.,
www.ashrae.org. Reprinted by permission from ASHRAE Handbook,
2001 Fundamen-
tals,
Chap. 21, Fig. 4.)
mate solutions are now typically prohibited. Other chemicals, such as
sodium nitrite or sodium borate, may be used. A qualified water treat-
ment expert should be consulted.
Solutions of ethylene glycol or propylene glycol in water are used
extensively. With proper inhibitors to prevent corrosion, these solu-
tions can lower the mixture’s freezing point to well below 0
⬚
F (Fig.
6.6). As with the salt solutions, the thermal conductivity and specific
heat of the mixture decrease and the viscosity increases with an in-
crease in the percentage of glycol. Inhibitors must be checked and
maintained periodically. While both HVAC and automobile glycols are
formulated from the same base compounds, the additives are different,
and automobile glycols are typically not suited for HVAC use.
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Design Procedures: Part 4
157
Figure 6.6
Properties of sodium chloride brine solutions, and freezing points of aqueous
solutions of ethylene glycol and propylene glycol. (
SOURCE
: Copyright 2001, American
Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., www.ashrae.org.
Reprinted by permission from ASHRAE Handbook,
2001 Fundamentals,
Chap. 21, Figs.
5, 6, 7, 8.)
Common refrigerants may also be used as a secondary coolant. That
is, liquid refrigerant may be pumped through distribution piping. Re-
frigerants have the advantages of low freezing points and low viscos-
ity, but also have low specific heats.
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158
Chapter Six
6.6
Piping Systems
A piping system is the means by which the thermal energy of a fluid
is transported from one place to another. The type of fluid and its
temperature and pressure influence and limit the choice of piping ma-
terials. Most systems are closed; i.e., the fluid is continually recircu-
lated and no makeup water is required except to replace that lost due
to leaks. Steam systems are partly to completely open—as when the
steam is used for a process or humidification—and require continuous
makeup water. Cooling-tower water systems are open and need
makeup water to replace the water evaporated in the tower.
Closed systems require some means of compensating for the changes
in volume of the fluid due to temperature changes. Expansion (com-
pression) tanks are used.
Piping must be properly supported, with compensation for expan-
sion due to temperature changes and anchors to prevent undesired
movement.
6.6.1
Piping materials
By far the most common material used in HVAC piping systems is
black steel (low-carbon steel). Table 6.5 covers dimensional data for
steel pipe. Pressure ratings vary with the pipe size (greater for smaller
pipes), but in general, standard-weight pipe can be used for working
pressures up to 300 lb / in
2
gauge, extra-strong pipe to 450 lb / in
2
gauge,
and double-extra-strong pipe to 1000 lb / in
2
gauge or more. Pipes of
14-in and larger outside diameter (OD) are made with thinner walls
for the lower pressures which are often acceptable, as well as with
thicker walls for higher pressures.
Another standard defines pipe sized by
schedule number.
In this
system, schedule 40 is the same as standard weight, and schedule 80
is the same as extra-strong, up to 6 in in size. Sizes of 8, 10, and 12
in standard weight are the same as schedule 30.
Black steel is often preferred because it is strong, is readily avail-
able, can be used over a wide range of temperatures and pressures,
and is easy to assemble and join by several common methods. If proper
inhibitors are used in the steam, water, and brine, black steel corrodes
very little; and in closed systems it will tend to stabilize in a neutral,
noncorrosive state. Unfortunately, very few systems remain com-
pletely closed for very long, so at least some water treatment is nec-
essary.
Another popular piping material for HVAC systems is copper, usu-
ally in tubing form. Copper pipe has the same outside diameter as
steel pipe, with slightly thinner walls. Dimensions of copper tubing
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159
T
ABLE
6.5
Steel
Pipe
Dimensions
and
W
eights
1
4
1
2
1
2
3
4
1
2
1
4
*V
olume
in
cubic
feet
of
w
ater
per
foot
of
pipe
length,
standard
weight.
Also
8-,
10-,
and
12-in
pipe
is
made
with
thinner
w
alls,
but
these
are
nonstandard.
Intermediate
sizes
such
as
3
1
⁄
2
in
are
also
made,
but
seldom
used.
And
1
⁄
8
and
3
⁄
8
in
are
also
made.
Larger
sizes,
14
to
30
in,
ha
ve
nominal
size
equal
to
outside
diameter
but
are
not