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138
T
ABLE
5.6
Performance
of
Dry
Media
Filters
1
2
1
4
1
2
1
4
NA:
Indicates
that
test
method
cannot
be
applied
to
this
level
of
filter
.
S
O
URCE
:
Copyright
1983,
American
Society
of
Heating,
Refrigerating
and
Air
Conditioning
Engineers,
Inc.,
www
.ashrae.org.
Reprinted
by
permission
from
ASHRAE
Handbook,
1983
Equipment,
Chap.
10,
T
able
2.
Design Procedures: Part 3
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139
T
ABLE
5.7
Performance
of
Renewable
Media
Filters
(Steady-State
V
alues)
Description
Type
of
media
ASHRAE
weight
arrestance,
%
ASHRAE
atmospheric
dust-spot
efficiency
,
%
ASHRAE
dust-
holding
capacity
,
g/f
t
2
Approach
velocity
,
ft
/min
20
to
40
m
glass
and
synthetic
fibers,
2
to
2
1
⁄
2
in
thick
V
iscous
impingement
7
0–8
2
⬍
20
60
–
180
500
P
ermanent
metal
media
cells
or
overlapping
elements
V
iscous
impingement
7
0–8
0
⬍
20
NA
(permanent
media)
500
Coarse
textile
denier
nonwoven
mat,
1
⁄
2
to
1
in
thick
Dry
60
–
8
0
⬍
20
15
–
7
0
500
F
ine
textile
denier
nonwoven
mat,
1
⁄
2
to
1
in
thick
Dry
80
–
9
0
⬍
20
10
–
5
0
200
S
O
URCE
:
Copyright
2000,
American
Society
of
Heating,
Refrigerating
and
Air
Conditioning
Engineers,
Inc.,
www
.ashrae.org.
Reprinted
by
permission
from
ASHRAE
Handbook,
2000
Systems
and
Equipment,
Chap.
24,
T
able
1.
Design Procedures: Part 3
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Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
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140
Chapter Five
Figure 5.40
Vertical laminar-flow air distribution.
Figure 5.41
Horizontal laminar-flow air distribution with separate AHU for temperature
and humidity control.
and low across the entire room. Diffuser outlet velocities will be 30 to
40 ft / min.
For horizontal laminar flow, one entire sidewall is used for supply
and the opposite wall for return. Again, outlet velocities will be low,
usually less than 50 ft / min, which is the threshold level for human
sensing. Because a low
⌬
T
is needed in the room, a separate air-
handling unit for temperature and humidity control is sometimes
provided (Fig. 5.41). Separate fan and filter ‘‘modules’’ are used for
room air circulation.
5.9
Stratification
Stratification
is the separation of air into layers having different tem-
peratures. The HVAC designer is concerned with two kinds of strati-
Design Procedures: Part 3
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Design Procedures: Part 3
141
Figure 5.42
Typical mixing-box configurations.
fication: that which occurs in the conditioned space as a result of in-
correct air distribution, and that which occurs in AHUs and ducts as
a result of inadequate mixing.
The first affects comfort, and is discussed elsewhere in this chapter.
The second is a result of system geometry. Both can be corrected or
minimized by proper design.
The primary cause of stratification in an AHU is inadequate mixing
of outside and return airstreams. The typical ‘‘mixing box’’ does not
typically provide good mixing. Several situations are shown in Fig.
5.42. Figure 5.42
a
illustrates the worst case. The two airstreams enter
side by side, and essentially no mixing occurs. This lack of mixing will
carry through filters, coils, and even fans, creating problems with
sensing and control as well as causing nuisance trips of freeze protec-
tion sensors—which are not really nuisance trips, because otherwise
a portion of the coil might freeze.
Figure 5.42
b
shows a somewhat better condition but only if parallel-
blade dampers are used, resulting in poor control of volume flows.
Figure 5.42
c
gives still better results and allows the use of opposed-
blade dampers. However, if the distance between the inlets is more
than 4 or 5 ft, there will still be a great deal of stratification.
Design Procedures: Part 3
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