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Stanley A. Mumma, Ph.D., P.E. Professor Emeritus

Penn State University, @ Univ. Park, PAsam11@psu.edu

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

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40

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90

100

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120

130

140

150

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48

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62

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70

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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

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90

100

110

120

130

140

150

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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

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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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