Transcript Slide 1

Decoupling the Building
Latent and Sensible
Loads Using 100%
Outside Air Systems
ASHRAE Chicago Seminar 39
January 24, 2006
Stanley A. Mumma, Ph.D., P.E.
Professor of Architectural Engineering
Penn State University, Univ. Park, PA
email: [email protected]
http://doas-radiant.psu.edu
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Outline
• What are 100% OA systems?
• What is the benefit of decoupling the space
sensible and latent loads?
• What parallel sensible cooling equipment is
available when dry ventilation air is
delivered to the space?
• What are the operating benefits of these
decoupled systems, including op. cost?
• A few pitfalls to avoid when applying these
systems?
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100% OA Systems
1. 100% OA, but at a low flow rate,
approximately the rate required by
ASHRAE Std. 62.1-2004: i.e. DOAS.
Also needed: Parallel sensible cooling
only system to meet the thermal loads.
2. 100% OA, at the flow rate required to
meet the entire space sensible load—
frequently up to 5 times the flow needed
for ventilation alone.
Will not be discussed here—but could
be during the question time if interested.
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DOAS Systems
20-70%
less OA
than VAV
DOAS Unit
W/ Energy
Recovery
Parallel Sen.
Cooling System
Cool/Dry
Supply
High
Induction
Diffuser
Building With
Sensible and
Latent
cooling
decoupled
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Why decouple sensible and latent
space loads with DOAS?
Tight humidity control minimizes the
potential for IAQ problems and related
sick-building illnesses*, and improves
thermal comfort and productivity.
*Which are caused largely by biological
contaminants breeding in damp ducts, ceiling tiles,
insulation, behind vapor barriers and carpet.
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Potential Health/productivity
Related Economic benefits of DOAS
The significance of DOAS is illustrated by
estimates that US companies lose as much
as $48 Billion annually to cover medical
expenses and $160 Billion annually in lost
productivity as a result of sick-building
illnesses.
Source: ASHRAE Literature.
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How Much is $208 Billion/yr.--in light of the ~ $13,000 Billion/yr 2006
USA GDP (value of goods and services)
• Medical & productivity cost (loss) to
US business as % of GDP:
(208/13,000)*100 = 1.6%
• National debt annual increase as a
% of GDP: (300/13,000)*100 = 2.3%
• Katrina Gov. appropriations as
%of GDP: (150/13,000)*100 = 1.2%
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Parallel Terminal Systems
DOAS air
Induction Nozzle
Sen Cooling Coil
Radiant Cooling Panels
Room air
Chilled Beams
Fan Coil Units
Air Handling Units
Unitary ACs
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VAV problems solved with DOAS
plus Radiant or Chilled Beam
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Poor air distribution.
Poor humidity control.
Poor acoustical properties.
Poor use of plenum and mechanical shaft space.
Serious control problems, particularly with tracking
return fan systems.
• Poor energy transport medium, air.
• Poor resistance to the threat of biological and
chemical terrorism, and
• Poor and unpredictable ventilation performance.
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Additional benefits of DOAS
Beside solving problems that have gone
unsolved for over 30 years with
conventional VAV systems, note the
following benefits:
• Greater than 50% reduction in
mechanical system operating cost
compared to VAV.
• Equal or lower first cost.
• Simpler controls.
• Generates up to 80% of points needed for
basic LEED certification.
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DOAS with Parallel Radiant
OA
Outdoor air unit with TER
Radiant
Panel
Space 3,
DOAS in
parallel w/
CRCP
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Sample DOAS pitfalls?
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•
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Wrong supply air temperature.
Wrong controls for the enthalpy wheel.
Wrong controls for the heating coil.
Wrong controls for the cooling coil.
Wrong location for the heating coil.
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Quick Review of DOAS
system psychrometrics
13
90
EW
5
.0 196
28
4
RA
Space
.0168
24
80
wet
W
B
et
.0 2140
0
40%
l
ub
)
(F 7 0
.0 1112
6
2
.084
12
60
3
4,5
20%
.0 56
08
50
dry
40
.0 028
4
40
50
60
HUM D
I T
I Y RA T O
I (Lbv /Lba )
Other regions?
Next slide
1
60%
CC
80%
PH
3
70
80
90
100
D R Y B U LB T EM P E R A T U R E (F )
Humidity ratio (grains/lb)
2
1
OA
120
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EW control for various
OA conditions
90
.0 196
28
80
60%
EW on when
OA h > RA h
80%
.0168
24
40%
W
.0 1112
6
.084
12
60
20%
.0 56
08
50
dry
40
EW to modulate
or duty cycle to
hold SAT SP
when OA
4 0 < SAT SP
50
EW off
60
70
80
90
100
D R Y B U LB T EM P E R A T U R E (F )
.0 028
4
Humidity ratio (grains/lb)
wet EW off
B
et
l
ub
)
(F 7 0
HUM D
I T
I Y RA T O
I (Lbv /Lba )
.0 2140
0
120
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Example of an
incorrect EW
control logic
90
EW on
60%
40%
)
(F 7 0
Frost protection when
.0 1112
6
cold outside!
.084
12
60
20%
.0 56
08
50
40
.0 028
4
40
50
60
70
80
90
100
D R Y B U LB T EM P E R A T U R E (F )
Humidity ratio (grains/lb)
W
bl
Cleaning cycle when .0off!
2140
0
HUM D
I T
I Y RA T O
I (Lbv /Lba )
EW off
80%
.0168
24
80
Bu
et
.0 196
28
EW on
120
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> 3 ton cooling
lost with wheel off
Reheat adds 2.8 tons of
cooling load, plus the
heating energy wasted in
the 1,710 cfm OA sys.
Neutral temperature a
huge energy waste!
EW off, huge control error when it
could significantly reduce the CC
load if operating.
CC Control based upon
CC load ~ 10 ton
maintaining a SA DPT,
what happens if the OA
is hot and dry ?
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Failing to minimize the use of a
chiller when it is cool outside —
can be a pitfall for DOAS systems.
• EW binary control—a duty cycle saves
chiller operation.
• EW using a VFD, maximizes the free
cooling of a DOAS.
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EW Duty cycle defined
RAT, 72F
OAT=40F
SAT=OAT when EW off (40F)
SAT=OAT+(RAT-OAT)*EWeff (65.6F)
EWeff, 0.8
By adjusting the EW ON time (54.7% or 8.2 min) in 1 period (15 min) can get
an avg. temperature equal to the desired SAT (54F). Duty cycle changes to
100% ON at 40F OAT to avoid tripping freeze stats. NOTE: HC must be off
since when EW off, DAT < DATSP and the CC must be off when the EW on!
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EW operation when OA below 54F
70
No duty cycle SAT
68
OPP duty cycle SAT
66
SA Temperature, F
Revised duty cycle SAT
64
VFD EW SAT
62
60
58
56
54
52
Heating Coil begins
modulation at -18F to
maintain 54F SAT
50
-20
-16
-12
-8
-4
0
4
8
12
16
20
24
28
OA Temperature, F
32
36
40
44
48
52
56
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Sample of the duty cycle operation
as the OA DBT rises through 40F
15 min.
cycle
Duty cycle off, EW on 100%
Not used when OA < 40F to
prevent tripping the freeze stat
during EW off part of cycle
EW
on
EW
off
Beginning of EW duty cycle
Target avg. duty cycle temperature
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Conclusions
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100% OA DOAS systems discussed.
IAQ cost benefits of decoupling discussed.
A few parallel systems introduced.
Operational benefits explored.
Humans are still capable of falling into pits. A
few pitfalls discussed.
I am convinced DOAS is the future since it
solves VAV problems at lower first and operating
costs, while providing improved IEQ and safety!
More information is available on the DOAS web
site noted on the first page.
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If Questions or need more on the
control logic, feel free to contact:
• S. A. Mumma, Prof of Arch. Engineering
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•
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Penn State University
814-863-2091
e-mail: [email protected]
Snail Mail:
214 Engineering Unit A
University Park, PA 16802
Visit the: http://doas-radiant.psu.edu
web site.
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