145
Chapter
6
Design Procedures: Part 4
Fluid-Handling Systems
6.1 Introduction
All air-handling units (AHUs) and many terminal units, if they are
not self-contained, require a source of heating and/or cooling energy.
This source is called a central plant, and the means by which thermal
energy is transferred between the central plant and the AHU is usu-
ally a fluid conveyed through a piping system. The fluids used in
HVAC practice are steam, hot or cold water, brine, refrigerant, or a
combination of these. The equipment used to generate the thermal
energy is described in Chap. 7. In this chapter we discuss the trans-
port systems.
6.2 Steam
Steam is water in vapor form. Because it expands to fill the piping
system, steam requires no pumping except for condensate return and
boiler feed. The specific heat of water vapor is quite low, but the latent
heat of vaporization is high. As a result, steam conveys heat very
efficiently.
Steam may be used directly at the AHU (in steam-to-air, finned-
tube coils), or a steam-to-water heat exchanger may be used to provide
the hot water used in AHU coils or in radiation. Steam radiation is
also employed. When used directly, steam pressures are usually 15
lb/in
2
gauge or less. When used with a heat exchanger, steam pres-
sures up to 100 lb/in
2
gauge are common. Higher pressures allow

. At higher altitudes (and lower atmosphere pressures),
the boiling temperature decreases until in Albuquerque, New Mexico,
or Denver, Colorado, 1 mi above sea level, it takes 4 or 5 min to boil
a 3-min egg.
The steam property of greatest interest to the HVAC designer is
enthalpy, particularly the enthalpy of evaporation, or the latent heat
of vaporization h
fg
. This is the amount of heat, in Btu per pound, which
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Design Procedures: Part 4 147
must be added to change the state of the water from liquid to vapor
with no change in temperature. This same amount is removed and
used, in a heat exchanger, when steam is condensed. Note that while
liquid water has an enthalpy change of about 1 Btu /lb per degree of
temperature change and steam has much less than that, the change-
of-state enthalpy is 970 Btu/lb at 212ЊF. This is what makes steam so
efficient as a conveyor of heat. In calculating the steam quantity
(pounds per hour) required for a specific application, use the latent
heat h
fg
.
Steam quality refers to the degree of saturation in a mixture of
steam and free water. As indicated in Table 6.1, there is a saturation
pressure (or ‘‘vapor’’ pressure) corresponding to each absolute temper-
ature. When the pressure and temperature match, the steam is said
to be saturated, with a quality of 100 percent. When steam flows in a

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148 Chapter Six
Figure 6.1
Pressure-reducing station.
Figure 6.2
Two-stage pressure-reducing station.
through orifices. If greater pressure reductions are required, it is nec-
essary to use two or more stages, as shown in Fig. 6.2, or to use an
oversized PRV, preferably sized by the manufacturer.
6.2.3 Steam condensate
Condensate is usually returned to the boiler for reuse. In small sys-
tems, this can sometimes be done by gravity. In most systems, pump-
ing is required. The condensate flows by gravity to a collecting tank
from which it is pumped directly to the boiler or to a boiler feed sys-
tem, as described in Chap. 7.
Condensate is basically distilled water. It often includes dissolved
carbon dioxide, making a weak but corrosive carbonic acid. The cor-
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Design Procedures: Part 4 149
rosive character of condensate must be addressed in condensate piping
material selection.
6.3 Water
Water is used extensively in modern cooling and heating practice be-
cause it is an effective heat transport medium and because it is con-
sidered simple to deal with. Because the water system can be essen-

8, 10, and 20ЊF for cooling (resulting in a divisor of 4000, 5000, and
10,000, respectively) and 20 to 40ЊF for heating (a divisor of 10,000 to
20,000).
Another measure of water quantity is gallons per minute per ton-
hour of refrigeration. Because 1 ton ⅐ h equals 12,000 Btu/h, a 10ЊF
rise in the chilled water temperature works out to 2.4 gal/(min Ϫ ton).
An 8ЊF rise requires 3 gal/(min Ϫ ton), and a 20ЊF rise is 1.2 gal /
(min Ϫ ton). On the condensing water side, it is assumed that heat
rejection in a vapor compression machine is approximately 15,000
Btu/(ton Ϫ h) and a 10ЊF rise requires 3 gal /(min Ϫ ton Ϫ h). The
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150
TABLE
6.2 Properties of Water, 212 to 400؇F
SOURCE
: Reprinted by permission from Thermodynamic Properties of Steam, J. H. Keenan and F. G.
Keyes, published by John Wiley and Sons, Inc., 1936 edition. Subsequent editions have equivalent data.
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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 Ϫ h), except
that for absorption refrigeration, rejection will be 20,000 to 30,000
Btu/(ton Ϫ h).
6.4 High-Temperature Water

6.5 Secondary Coolants
(Brines and Glycols)
Brine is a mixture of water and any salt, with the purpose of lowering
the freezing point of the mixture. In HVAC practice, the term is also
applied to mixtures of water and one of the glycols. Brines are used
as heat transfer fluids when near- or subfreezing temperatures are
encountered. Ice-making systems for thermal storage often use a brine
solution as part of the scheme. Brines may be used directly in cooling
coils of air-handling units or, through heat exchangers, may be used
to provide chilled water. Brines are also commonly used in runaround
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152 Chapter Six
Figure 6.3
HTW end use, with cascading.
heat reclaim systems (see Chap. 7). Heating systems exposed to sub-
freezing air may use a glycol solution as a circulating medium.
6.5.1 Properties of secondary coolants
Calcium and sodium chloride solutions in water have been the most
common brines. Properties of pure brines are shown in Tables 6.3 and
6.4. For commercial-grade brines, use the formulas in the footnotes to
the tables. Note particularly that the specific heat decreases as the
percentage of the salt increases. Thus, a 25% solution of calcium chlo-
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153

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 Ϫ21ЊF and will
decrease the specific heat to 0.689 Btu/(lb ⅐ ЊF). 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.,

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

1
4
1
2
1
2
3
4
1
2
1
4
*Volume in cubic feet of water per foot of pipe length, standard weight. Also 8-, 10-, and 12-in pipe is
made with thinner walls, 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, have nominal size equal to
outside diameter but are not part of this standard.
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the manufacturers. PVC is often used for equipment drain lines.
Galvanized-steel piping is used occasionally. The dimensions are the
same as for black-steel pipe.
Occasionally cast iron, but more often ductile iron, has some HVAC
applications. Ductile iron can be grooved to accept gasketed iron cou-
plings.
Cast-iron piping is seldom used in sizes less than 4 in, although
cast-iron fittings are available down to 1-in size. Wrought-iron piping
has been used extensively in the past for steam condensate, but it is
seldom used anymore because of the extra cost.
Some regions of the country have a well-developed stainless steel
market. On a local basis, stainless steel piping may be found to be
cost competitive with other piping materials.
6.6.2 Pipe fittings
Pipe fittings include elbows, tees, wyes, couplings, unions, reducers,
plugs, caps, and bushings. Elbows may be 45Њ,90Њ, or even 180Њ, re-
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Design Procedures: Part 4 161
ducing or nonreducing, with short or long radius. Tees and wyes may
also be reducing types. Special fittings are available to prevent elec-
trolytic corrosion when dissimilar piping materials are joined. A stan-
dard manufacturer’s catalog can be consulted for dimensions and
types of fittings.
6.6.3 Joining methods
Steel pipe joints may be welded, threaded, grooved, or flanged. Weld-
ing is typical on piping 3 in in diameter and larger and should be done
in accordance with ASME power piping standards,

1.52 in. If left unrestrained, the pipe may move in unacceptable ways.
If the pipe is restrained without provision for expansion, large forces
will be developed and either the pipe or the restraints may break. The
expansion must be compensated by means of expansion joints, loops,
or elbows.
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162 Chapter Six
Figure 6.7
Expansion joint, bellows type. Left: plain; right: with equalizing rings. (Cour-
tesy of Adsco Manufacturing Corp.)
Figure 6.8
Expansion loop and expansion elbow.
Expansion joints may be of the bellows (Fig. 6.7), slide, or flex-joint
type. Joints are simpler than loops, but slide joints may develop leaks
over time unless packing is maintained or replaced. Bellows joints
need no packing but may eventually fail due to fatigue. Expansion
may also be controlled by means of loops or elbows. Figure 6.8 shows
a simple piping system with an expansion loop and expansion elbow.
The design provides for flexibility so that the pipe can bend without
exceeding the allowable stress of the pipe material. Information on
the design of expansion loops and elbows can be found in many ref-
erences (see Ref. 4) as well as from some pipe fitting manufacturers.
Note that using loops, offsets, and elbows to compensate for expan-
sion and contraction results in a system with little required mainte-
nance. Ball joints and slip joints have packing which must be main-
tained. Bellows joints tend to work harden over time.
For the expansion to be properly controlled, it is necessary to pro-