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F. PRESSURE CLASSIFICATION AND LEAKAGE

F. PRESSURE CLASSIFICATION AND LEAKAGE

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CHAPTER 2

Figure 2-3 RELATIVE OPERATING
COST VS ASPECT RATIO
(based on equal duct area)

Figure 2-2 RELATIVE INSTALLED COST
VS ASPECT RATIO

tural strength to meet the pressure classifications in
SMACNA standards, but will keep initial duct system
construction costs as low as possible. Each advancement to the next duct pressure class increases duct
system construction costs.

The comparison in Table 2-5 is made on the basis of
galvanized sheet metal ductwork, and all ductwork
being sealed in accordance with the minimum classifications as listed in the SMACNA "HVAC Duct Construction Standards-Metal and Flexible", First Edition 1985.
The amount of duct air leakage now may be determined in advance by the HVAC system designer, so
that the estimated amount of leakage can be added
to the system airflow total when selecting the system
supply air fan. The amount of duct air leakage, in
terms of cfm per 100 square feet (I/s per square

Table 2-5 RELATIVE DUCT SYSTEM COSTS
(Fabrication and Installation
of Same Size Duct)

Since the installed cost per system varies greatly,
depending on local labor rates, cost of materials,
area practice, shop and field equipment, and other
variables, it is virtually impossible to present definite
cost data. Therefore, a system of relative cost has
been developed. Considering the lowest pressure
classification, 0. to 0.5 in w.g. (D to 125 Pa) static
pressure as a base (1.0), the tabulation in Table 2-5
will give the designer a better appreciation of the
relative cost of the various pressure classes.

2.5

ECONOMICS

metre), is based on the amount of ductwork in each
"seal class". Additional information may be found in
Chapter 5 of the SMACNA "HVAC Air Duct Leakage
Test Manual", First Edition-1985, and in Chapter 32
of the 1989 ASHRAE "Fundamentals Handbook" It is
important to note that a one percent (1%) air leakage
rate for large HVAC duct systems is almost impossible to attain, and that large unsealed duct systems
may develop leakage well above 30 percent of the
total system airflow . The cost of sealing ductwork
may add approximately 5 to 10 percent to the HVAC
duct system fabrication and installation costs, but
these costs may vary considerably, depending on job
conditions and contractor plant facilities.
system

GCOST
OF
FITTINGS
Chapter 14-"Duct Design Tables and charts contains fitting loss coefficients from which the HVAC

DUCT

SYSTEMS

designer may select the one best suited for
the situation. However, the fitting that gives the lowest, i.e. efficient dynamic loss, may also be the most
,expensive to make. A higher aspect ratio rectangular
ductfitting might cost very little more to make than a
square fitting, and much less to make than some
round fittings. Variables apply here, probably more
than in all previous discussions.
Without trying to develop a complete estimating procedure, using a 5 foot (1.5m) section of ductwork as
a base, the relative cost of a simple full radius elbow
of constant cross-sectional area is approximately
from 4 to 8 times that of the straight section of ductwork. The relative cost of a vaned, square-throated
elbow of constant size might even be greater.
The HVAC system designer should bear in mind that
much of the ductwork fabricated today is done from
automated equipment, whereby fabrication labor is
reduced to a minimum by the purchase of an expensive piece of capital equipment. However, many fittings are still handmade, which results in very high
labor to material costs.

Table 2-6 ESTIMATED EQUIPMENT SERVICE LIFE (2)

2.6

OF

CHAPTER 3

ROOM AIR DISTRIBUTION
COMFORT
CONDITIONS
An understanding of the principles of room air distribution helps in the selection, design, control and operation of HVAC air duct systems. The real evaluation
of air distribution in a space, however, requires an
affirmative answer to the question: "Are the occupants comfortable?" The object of good air distribution in HVAC systems is to create the proper combination of temperature, humidity and air motion, in the
occupied zone of the conditioned room from the floor
to 6 feet (2m) above floor level. To obtain comfort
conditions within this zone, standard limits have been
established as acceptable effective draft temperature. This term includes air temperature, air motion,
relative humidity, and their physiological effects on the
human body. Any variation from accepted standards
of one of these elements causes discomfort to occupants. Lack of uniform conditions within the space
or excessive fluctuation of conditions in the same part
of the space may produce similar effects.
Although the percentage of room occupants who object to certain conditions may change over the years,
Figures 3-1 and 3-2 provide insight into possible objectives of room air distribution. The data show that
a person tolerates higher velocities and lower temperatures at ankle level than at neck level. Because
of this, conditions in the zone extending from approximately 30 to 60 inches (0.75 to 1.5 m) above the floor
are more critical than conditions nearer the floor.
Room air velocities less than 50 fpm (0.25 m/s) are
acceptable: However, Figure 3-1 and 3-2 show that
even higher velocities may be acceptable to some
occupants. ASHRAE Standard 55-1981R recommends elevated air speeds at elevated air temperatures. No minimum air speeds are recommended for
comfort, although air speeds below 20 fpm (0.1 m/s)
are usually imperceptible.
Figure 3-1 shows that up to 20 percent of occupants
will not accept an ankle-to-sitting-level gradient of
B

about 4°F (2°C). Poorly designed or operated sys-

tems in a heating mode can create this condition,
which emphasizes the importance of proper selection
and operation of perimeter systems.
To define the difference (0) in effective draft temper-

ature between any point in the occupied zone and
the control condition, the following equation is used:
Equation 3-1
0 = (tx -to) - a(V, - b)
where (U.S. Units):
0 = effective draft temperature, °F
tx = local airstream dry-bulb temperature, F
tc = average room dry-bulb temperature, F
Vx = local airstream velocity, fpm
a = 0.07
b = 30
where (Metric Units):
0 = effective draft temperature, °C
tx = local airstream dry-bulb temperature, °C
tc = average room dry-bulb temperature, °C
Vx = local airstream velocity, m/s
a=8
b = 0.15
Equation 3-1 accounts for the feeling of "coolness"
produced by air motion and is used to establish the
neutral line in Figures 3-1 and 3-2. In summer, the
local airstream temperature, tx, is below the control
temperature. Hence, both temperature and velocity
terms are negative when velocity, Vx, is greater than
30 fpm (0.15 m/s) and both of them add to the feeling
of coolness. If, in winter, tx is above the control temperature, any air velocity above 30 fpm (0.15 m/s)
subtracts from the feeling of warmth produced by tx.
Therefore, it is usually possible to have zero difference in effective temperature between location, x,
and the control point in winter, but not in summer.

AIR DIFFUSION

PERFORMANCE INDEX
(ADPI)
1. Comfort Criteria
A high percentage of people are comfortable in sedentary (office) occupations where the effective draft
temperature (0), as defined in Equation 3-1, is between -3 Fand + 2°F (-1.7°C and + 1.1°C) and the

3.1

ROOM

AIR

DISTRIBUTION

Figure 3-1 PERCENTAGE OF OCCUPANTS
OBJECTING TO DRAFTS IN AIR-CONDITIONED
ROOMS (U.S. UNITS) (2)

Figure 3-2 PERCENTAGE OF OCCUPANTS
OBJECTING TO DRAFTS IN AIR-CONDITIONED
ROOMS (METRIC UNITS) (2)

air velocity is less than 70 fpm (0.35m/s). If many
measurements of air velocity and air temperature

humidity. These and similar effects, such as mean
radiant temperature, must be accounted for separately according to ASHRAE recommendations.
ADPI is a measure of cooling mode conditions. Heating conditions can be evaluated using ASHRAE Standard 55-1981 R guidelines or the ISO Standard 773083, "Comfort Equations."
The following cooling zone design criteria for the various air diffusion devices maximize the ADPI and
comfort. These criteria also account for airflow rate,
outlet size, manufacturer's design qualities, and dimensions of the room for which the system is designed.

were made throughout the occupied zone of an office,
the ADPI would be defined as the percentage of locations where measurements were taken that meet
the previous specifications on effective draft temperature and air velocity. If the ADPI is maximum (approaching 100 percent), the most desirable conditions
are achieved.
ADPI is based only on air velocity and effective draft
temperature, a combination of local temperature differences from the room average, and is not directly
related to the level of dry-bulb temperature or relative

3.2