T E C H N I C A L
R E S E A R C H
R E P O R T
Condensation Evaluation of Permeable
and Impermeable Materials for
Air Distribution.
R. Brown, K.Gebke, N. Paschke
S. Ford (BESS Lab)
Theory
When specifying a duct system design, an
important consideration is the potential of
condensation on the exterior surfaces. As
metal duct is most commonly used, single
wall metal duct is applied where condensation is not an issue, and double wall or insulated metal duct would be used to prevent
condensation or heat gain/loss. Designers of
fabric duct systems also have options to control gathering of moisture and condensation
on the outer walls of the ducts.
Common fabrics are available in permeable
or impermeable constructions. Impermeable
fabrics typically are either manufactured as a
solid film material or a woven construction
with a coating on one or both sides. While
the coating offers fabric stability for cutting
and construction - it yields a defined barrier
offering little thermal benefit. The temperature gradient from duct surface to room air is
due to natural convection, therefore very
narrow - as shown in Figure 1.
Permeable fabrics are generally a woven
construction and are processed to a specific
permeability. In theory, conditioned air passes through the fabric and creates a thin layer
around the duct wall of tempered air. This
boundary prevents the warm, moist room air
from contacting the duct wall and generating
condensate. With this concept in mind, the
permeable fabric ducts can be considered a
direct alternative to insulated or double wall
metal ducts. Figure 2 reveals the expanded
gradient for the permeable fabrics due to
controlled outward airflow.
Figure 1:
Temperature Gradient
for Impermeable Fabrics
Figure 2:
Temperature Gradient
for Permeable Fabrics
Test Apparatus and Procedure
To prove the theory, metal duct and several fabrics were tested at the Bioenvironmental and
Structural Systems (BESS) Laboratory at the University of Illinois. Environmental chambers
were designed to produce conditions yielding condensation on duct surfaces. The first chamber provided a constant air supply of 300 CFM (½” of H2O) at 55°F. The second chamber was
used to model an unusual, but possible, hot and humid environment with temperature at 90°F
and the relative humidity held between 92.5 - 98% throughout testing. The testing samples
were 8” diameter and
30” long with no air
outlets. Prior to testing, each sample was
Warm Chamber
weighed in a plastic
Supply Chamber
90°F ± 5°
55°F ± 2.5°
bag to determine dry
95% RH ± 5%
weight.
Fan
1
300 cfm
8"Ø Test
Duct Sample
Figure 3: Test Chamber Apparatus
PAGE
© DuctSox Corp. 2005
The supply air looped
back into the supply
chamber once the air
passed through the
test duct section.
DSWP01_1005A
Once the sample was installed and chamber conditions achieved, testing was initiated
and the chamber conditions were logged at 10, 20 and 30 minute intervals per sample. A
total of 24 temperature readings were taken between 2 stations of 4 radial locations with
three test points each - distanced 0 -¼”, ½”, and 1” away from the test sample surface
(See Fig. 4). After the 30 minute test duration, each sample was returned to the plastic
bag and weighed a second time to determine the weight gain due to condensate formation and retention. Condensation that dripped from PolyTex
and Metal Duct was not collected or weighed with samples.
12:00
The described test was performed on each of the following samples:
MATERIAL
WEIGHT*
DESCRIPTION
Metal Duct
Galvanized Steel (Std. 18 Ga.)
PolyTex
Polyethylene, Woven & Coated
DuraTex
POROSITY**
0
5
0
Polyester, Woven & Coated
5.75
0
TufTex
Polyester, Woven & Coated
8.2
0
Sedona1
Polyester, Woven & Calendered
6.75
1
Sedona2
Polyester, Woven & Calendered
6.75
2
Microbe-X
Polyester, Woven
3.2
6
*Material Weight/Area: ounces/yd2
RESULTS:
1" from Duct Surface
1
2"
0-1 4"
9:00
6:00
Figure 4: Surface Temperature
Reading Locations
**Porosity: CFM/ft2 at 0.5” w.g. static pressure
2
Evaluations included three methods:
- Visual: Observation of Condensation
- Weight: Measuring Added Moisture Gains
- Temperature: Air Temperature Surrounding Test Samples
VISUAL
Condensation observation after the 30 minute test period.
MATERIAL
OBSERVATION
Metal Duct
Water beads had formed on entire surface, droplets joined and dripped
PolyTex
Water beads had formed on entire surface, droplets joined and dripped
DuraTex
Mist-like, miniature water droplets on surface, no dripping
TufTex
Mist-like, miniature water droplets on surface, no dripping
Sedona1
No change
Sedona2
No change
Microbe-X
No change
WEIGHT
The weight gains were substantial on
the impermeable fabrics due to the
development of moisture on the outer
duct surface. The permeable fabrics
picked up only a slight amount of moisture from the humidity. PolyTex and
Metal Duct weights include moisture
on surface only - as drippage was not
collected or weighed.
Microbe-X
1.52
Sedona2
1.99
Sedona1
1.95
PAGE
40.70
TufTex
46.00
DuraTex
53.63
PolyTex
58.20
Metal Duct
0
10
20
30
40
Weight (grams)
3:00
50
Figure 5: Net Weight Gain per Sample
60
TEMPERATURE
REDUCE INFLATION DYNAMICS
When
the fans
shut off,fabrics
fabric yielded
duct systems
Dependingair
onaround
suspension
As
predicted,
permeable
a film ordeflate.
halo of conditioned
the duct
surface
the warm,
to contact
theequipment
surface. This
halo
of conmethod,- preventing
deflation varies
fromhumid
17%ambient
to nearair
100%.
Upon
start
up,
ditioned
air
acts
as
a
thermal
barrier
extending
the
temperature
gradient.
Results
systems that are fully deflated have no airflow resistance thus resulting in afor the
impermeable
fabrics
were slightly
better
than,of
butduct
comparable
theno
single
wall metalThe
duct
large air mass
traveling
down the
length
with littletoor
resistance.
with
temperatures
near
the
duct
surface
being
close
to
the
ambient
air.
Comparing
%
momentum can cause excess noise due to fabric movement and inflation pop.dif-
PAGE
PAGE
% Difference between Supply and
Moist Room Air
3
ference yields clear graphical representation of the extended gradient.
90%
80%
70%
60%
Metal
x
PolyTe x
e
T
Dura ex
u
T fT
Fig. 8: Systems without AFD’s offer little resistance
to inflation airflow.
Adding50%
an AFD, whether one or two locations along the length, will stage the
inflation and reduce the noise and fabric movement. As the air flows towards
the AFD,
the added resistance creates a pressure that stages inflation. Since
40%
some of the initial air
mass
1 is already dispersed, a lesser amount of air makes it
edona
S
to the endcap.
The last
sections of fabric duct are then pressurized and start
30%
dona2
dispersing air. Se
be-X
Micro
20%
0-1/4"
1/2"
1"
Distance from Outer Duct Surface
Figure 6: Temperature Difference (%) around Sample Surface
CONCLUSIONS
While test conditions could apply only to extreme applications, building startup or during periods of shutdown - test results reveal a significant reduction in condensation on permeable fabrics. These consistent
test conditions and various evaluations clearly identify concerns of condensation for impermeable materials (metal or impermeable fabrics). Fabric permeability rate has little effect - as tests results are similar
from 1 cfm/ft2 to the 2 & 6 cfm/ft2 fabrics.
Fig. 9: AFD resists inflation velocity, resulting
in smoother start-ups.
Results from visual inspections reveal the effects of exterior texture for impermeable materials. Smooth
surfaced metal duct and PolyTex yielded drippage, while the woven exterior surface of DuraTex and
learnonly)
more,produced
or review only
othermist
technical
provided
by
TufTex (coated on To
inside
size documents
droplets and
no drippage.
DuctSox Corporation, please check out www.ductsox.com.
For applications in which condensation is a concern, permeable fabrics yield the lowest possibility for condensation. For impermeable materials, a woven exterior finish reduces risks of drippage.
DuctSox Corporation
4343 Kapp
Chavenelle
9866
Court Road
Dubuque, IA 52002
Peosta,
IA 52068
866-382-8769
/ Fax 563-589-2754
www.ductsox.com
866-382-8769
/ Fax 563-588-5330
www.ductsox.com
© DuctSox Corp. 2005
Specifications subject to change without notice
DSWP01A_1005A
© DuctSox Corp. 2006
Specifications subject to change without notice
DSWP03A_0806B