1 stanley a. mumma, ph.d., p.e. professor emeritus penn state university, @ univ. park, pa...
TRANSCRIPT
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Stanley A. Mumma, Ph.D., P.E. Professor Emeritus
Penn State University, @ Univ. Park, [email protected]
Web: http://doas-radiant.psu.edu
Active and Passive Desiccants in
DOAS: Selection and Control
ASHRAE Seminar 11
Dallas Meeting 2007
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Presentation Outline DOAS defined, Selecting the Supply air Conditions:
flow rate, DBT, DPT, Equipment available to achieve the
desired SA thermodynamic state point,
Touch on a few Control Issues, Mumma preferred equipment choices, ASHRAE Headquarters Building
Renovation.
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DOAS Defined for this presentation
20-70% less OA,than VAV
DOAS Unit W/ Energy Recovery
Cool/Dry Supply
Parallel Sensible Cooling System
High Induction Diffuser
Building With
Sensible and Latent
cooling decoupled
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Key DOAS points:1. 100% OA delivered to each zone via its
own ductwork.2. Flow rate generally as spec. by Std.
62.1-2004 or greater (LEED, Latent. Control)
3. Employ TER, per Std. 90.1-2004.4. Generally CV.5. Use to decouple space S/L loads—Dry.6. Rarely supply at a neutral temperature.7. Use HID, particularly where parallel sys
does not use air.
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Key DOAS points
8. EW control to achieve a degree of air side economizer, to be discussed later.
9. Terminal reheat?—allowed by Std. 90.1.10.What about space heating?11.DOAS and displacement ventilation
(DV) are not competitors!
12.Other configurations? Never put other AC equipment in series with DOAS!
13.Terror resistance?
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How should the OA be introduced in DOAS-FCU applications?
Parallel, Good Series, Bad
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“SMART” Ceiling DOAS-Fan coil arrangement.
DOAS and FCU’s back in
parallel for SMART cost
effective performance
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How do DOAS’s perform under the threat of terrorist
activities?
10,000 ft2 Interior Zone 3
1,000 ft2
ExteriorZone 1
9,000 ft2 Exterior Zone 2
Facility floor area and zoning
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1 2 3
OAQ
OAC inC 1Q 2Q 3Q
recQ1V 2V 3V1C 2C 3CexhC
Filter, f
VAV System Schematic
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1 2 3
OAQ
OAC inC 1Q 2Q 3Q
1V 2V 3V1C 2C 3C
Filter, f
DOAS Schematic
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How do DOAS’s perform under the threat of terrorist
activities?VAV vs. DOAS, VAV filter-80%, DOAS-98%release at the OA inlet
0.00
0.05
0.10
0.15
0.20
0.25
0 10 20 30 40 50 60
time, min
dim
en
sio
nle
ss
co
nc
en
tra
tio
n DOAS =98
VAV Z1&2-80
VAV Z3-80
External release, filter efficiency VAV-80%, DOAS-98%
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How do DOAS’s perform under the threat of terrorist
activities?
External release, filter efficiency for equal conc or exposure
DOAS filter Eff. needed to match VAV performance, release near OA inlet
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90 100
VAV filter Efficiency
DO
AS
fil
ter
Eff
icie
ncy
DOAS Filter Eff. Equal C
DOAS Filter Eff Equal Exp.
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How do DOAS’s perform under the threat of terrorist
activities?VAV vs DOAS release in large interior space, Zone 3
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 10 20 30 40 50 60
time, min
Dim
en
sio
nle
ss
co
nc
en
tra
tio
n
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
DOAS
VAV Z3-80
VAV Z1&2-80
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Qlatent=0.68*scfm*w (grains).
If all latent load from people @ 205 Btu/person, then
or w=15 gr/lb with 20 scfm/person, requires 48F DPT if space 75F 50% RH
w=10 gr/lb with 30 scfm/person, requires 51F DPT if space 75F 50% RH (W=65 gr/lb)
Selecting the supply air DPT
or w=30 gr/lb with 10 scfm/person, requires 39F DPT if space 75F 50% RH
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or w=60 gr/lb with 5 scfm/person, requires -2F DPT if space 75F 50% RH
Selecting the supply air DPT
i.e. SA has only 5 gr/lb (65-60) , or is extremely dry! Not even a consideration for commercial applications—Mechanical or Desiccant!
16DRY BULB TEMPERATURE (F)
80
40
40
6050
50
60
70
70
80 90 100 120
90
.004
.016
.012
.008
HUMIDITY RATIO (Lbv/Lba)
.028
.024
.020
OA
EW
RA
1 2 3 4
5
PH CC
Space
w=15 grains, with 20
cfm/person
4, 48F DPT, 50 grains
5, 75F, 50%, 65 gr/lb
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140
168
196
112
84
56
Hu
mid
ity r
atio
(g
rain
s/lb
)
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DOAS equipment on the market today
4850 gr
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DOAS equipment on the market today
48-55+50
4850
varies
DRY BULB TEMPERATURE (F)
80
40
40
6050
50
60
70
70
80 90 100 120
90
.004
.016
.012
.008
HUMIDITY RATIO (Lbv/Lba)
.028
.024
.020
28
140
168
196
112
84
56
Hu
mid
ity r
atio
(g
rain
s/lb
)
OA
EW
RA
1 2 3 4 5
PH CC
Space
SW67
Dual Wheel
Arrangement
2
3
4 56
7
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DOAS equipment on the market today Type 3Desiccant added for 3 reasons:
1. 45F CHWS still works2. achieve DPT < freezing3. reduce or eliminate reheat
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Process on the Psych Chart
30
40
50
60
70
80
90
100
110
120
130
140
150
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46
48
50
52
54
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58
60
62
64
66
68
70
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76
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80
82
84
86
DBT, F
W,
gr/
lbm
2
1
4
5
6
721
3
4
567
3
Enthalpy 4 > 3 DOAS needs
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DOAS equipment on the market todayType 3
Desiccant wheel
EW
EW effec. Control’d, w/ bypass damper
RA when unoccupied,
EA when occupied
84 F DBT148 gr/lb
7558
7891
7498
7698
5443
44 DPT
Heater7891
5051
82129
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Process on the psych chart
30
40
50
60
70
80
90
100
110
120
130
140
150
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40
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48
50
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60
62
64
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68
70
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76
78
80
82
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DBT, F
W, g
r/lb
m
2
1
3 4
56
7
8
2 13
4 5 6 7 8
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Type III Desiccant Wheel
Heating
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DOAS equipment on the market today
76.5 F50 gr/lb
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Psychrometric Process
2030405060708090
100110120130140150
44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94
DBT, F
W,
gr/
lbm
2
1
3
4
21 34
2
27
56
7
9
8
Cold Deck
Neutral Deck
1
2
4
0
0
3 100% OA
Return
Zone, Typ.
Terminal Heating,
Typ.
Schematic of the IECDDVAV System
2
0 Outdoor Air1 Indirect Evap Cooler2 Filter3 Cooling Coil4 Direct Evap Cooler5 D-D VAV box6 Heating Coil7 Air to air Heat exchanger8 Building Return air9 Scavenger or Vaporizing air
Region of interest
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Psychrometric process, IEC vs EW
30
40
50
60
70
80
90
100
110
120
130
140
150
34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86
DBT, F
W,
gr/
lbm Significant chiller
size reduction with EW at design
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A few control issues
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80% - 85% of OA cooling load could be saved if wheel on: in this case almost 50% of coil load
Reheat adds significant cooling load, beside wasting heating energy.
h=35.7 btu/lbm
h=27.5 btu/lbm
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EW operation when OA below 54F
50
52
54
56
58
60
62
64
66
68
70
-18 -14 -10 -6 -2 2 6 10 14 18 22 26 30 34 38 42 46 50 54 58
OA Temperature, F
SA
Te
mp
era
ture
, F
EW Duty cycle between 54 and 40F to hold SAT,
then EW on.
No duty Cycle, EW on below 54F OAT
EW speed modulated between 54 and -18F to hold the desired SAT
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What has ASHRAE sponsored research found?
censored
Office: 1 story 6,600 ft2
Retail: 1 story 79,000 ft2
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Base Case: DX, 350 cfm/ton
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DX (400 cfm/ton) with Desiccant
Outdoor
Exhaust
Outdoor Supply
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DOAS w/ Desiccant +DX
350 cfm/ton
400 cfm/ton
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DOAS w/ EW +DX
CC
CC
350 cfm/ton
400 cfm/ton
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Performance for office, based upon 62.1-2004
ventilation req’dLocation Miami Hous Shrev Ft. Wor Atlant DC St. Lo NY Chic Port
DX w/ Desiccant 0 0 0 0 0 0 0 0 0 0
DOAS w/ Des. +DX 0 0 0 0 0 0 0 0 0 0
DOAS w/ EW +DX 0 0 0 0 0 0 0 0 0 0
DX w/ Desiccant 52% 23 18 12 9 1 -2 1 -8 -1
DOAS w/ Des. +DX 48% 18 14 8 8 -3 -5 -6 -14 -8
DOAS w/ EW +DX -18% -21 -20 -19 -19 -23 -26 -19 -26 -14
DX w/ Desiccant 51 45 43 45 40 44 41 59 41 38
DOAS w/ Des. +DX 54 48 46 48 44 47 45 63 45 42
DOAS w/ EW +DX 35 35 33 37 33 37 35 52 37 36
Annual Op Cost vs Base DX
Humidity Control (Occ. Hours >65% RH)
LCC: Equipment 1st + 15 yr Gas and Electric $, 1,000’s 2004 dollars
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Performance for retail, based upon 62.1-2004
ventilation req’dLocation Miami Hous Shrev Ft. Wor Atlant DC St. Lo NY Chic Port
DX w/ Desiccant 0 0 0 0 0 0 0 0 0 0
DOAS w/ Des. +DX 0 0 0 0 0 0 0 0 0 0
DOAS w/ EW +DX 0 1 6 0 0 0 0 0 0 0
DX w/ Desiccant 169 79 75 47 61 18 14 6 -11 -2
DOAS w/ Des. +DX 137 53 44 20 20 -9 -11 -14 -30 -15
DOAS w/ EW +DX -39 -42 -41 -42 -41 -51 -54 -44 -55 -28
DX w/ Desiccant 322 250 235 226 210 209 189 247 174 148
DOAS w/ Des. +DX 313 245 228 220 203 205 189 242 174 153
DOAS w/ EW +DX 88 91 90 104 92 100 90 138 100 106
Annual Op Cost vs Base DX (%)
Humidity Control (Occ. Hours >65% RH)
LCC: Equipment 1st + 15 yr Gas and Electric $, 1,000’s 2004 dollars
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NIST findings
July, 2005
NIST Finding: The more complex DOAS system modeling still showed latent cooling being provided by the WSHPs in the zones.Reason: Condensation will occur if the cooling surface temperatures (WSHP’s surface temp = 45F) are below the space DPT. This is a design/modeling problem, not a DOAS problem!
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Mumma Preferred equipment choices Always consider dual path DOAS to the
spaces, and use where it makes sense. I have yet to find a DOAS application where
EW’s should not be used, when controlled properly.
In most situations, use mechanical refrigeration to dehumidify, even if it means increasing the ventilation rate above the Std. 62.1 minimums. Choice is supported by the ASHRAE research project results discussed above.
To achieve the low temperature chilled water economically, use OPAC where cost effective.
Can these preferences be seen in practice? One potential case follows!
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How is ASHRAE HDQ renovation “walking the
talk”?
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How is ASHRAE HDQ renovation “walking the
talk”? Single wheel Dedicated Outside Air System (DOAS) supplying cold and dry (50 gr/lb) ventilation air throughout with >75% effective EW.
Off Peak AC used with favorable utility rates. Reduces demand changes and facilitates low CHWS temperature operation.
Chilled ceiling used throughout the 2nd level.
Dual Path, low temperature VAV serving the first floor.
Learning Center, DOAS with PTAC.
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