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Decoupling the BuildingLatent and SensibleLoads Using 100%

Outside Air Systems

ASHRAE Chicago Seminar 39 January 24, 2006

Stanley A. Mumma, Ph.D., P.E.Professor of Architectural EngineeringPenn State University, Univ. Park, PA

email: sam11@psu.edu

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 OAthan VAV

DOAS Unit W/ Energy Recovery

Cool/Dry Supply

Parallel Sen. Cooling System

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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• 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%

How Much is $208 Billion/yr.---in light of the ~ $13,000 Billion/yr 2006 USA GDP (value of goods and services)

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Fan Coil UnitsFan Coil Units

Air Handling UnitsAir Handling Units Unitary ACsUnitary ACs

Parallel Terminal Systems

Radiant Cooling PanelsRadiant Cooling Panels

Chilled Beams

DOAS air

Induction Nozzle

Sen Cooling Coil

Room air

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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.

VAV problems solved with DOAS plus Radiant or Chilled Beam

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Additional benefits of DOASBeside 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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Outdoor air unit with TER

OA

Space 3, DOAS in

parallel w/ CRCP

Radiant Panel

DOAS with Parallel Radiant

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Sample DOAS pitfalls?

• 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

14DRY 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

45

PH CC

Space

1

2

3 4,5

wet

dry

Other regions?

Next slide

15DRY 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

)

wet

dry

EW control for various OA conditions

EW on whenOA h > RA h

EW off

EW offEW to modulate or duty cycle to hold SAT SPwhen OA < SAT SP

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

28

140

168

196

112

84

56

Hu

mid

ity r

atio

(g

rain

s/lb

)

Example of an incorrect EW control logic

EW on

EW off

EW on

Cleaning cycle when off!

Frost protection when cold outside!

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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.

CC load ~ 10 tonEW off, huge control error when it could significantly reduce the CC load if operating.

Neutral temperature a huge energy waste!

CC Control based upon 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

EWeff, 0.8

RAT, 72F

SAT=OAT when EW off (40F)

SAT=OAT+(RAT-OAT)*EWeff (65.6F)

OAT=40F

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

50

52

54

56

58

60

62

64

66

68

70

-20 -16 -12 -8 -4 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56

OA Temperature, F

SA

Tem

per

atu

re, F

No duty cycle SAT

OPP duty cycle SAT

Revised duty cycle SAT

VFD EW SAT

Heating Coil begins modulation at -18F to maintain 54F SAT

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Sample of the duty cycle operation as the OA DBT rises through 40F

15 min. cycle

EWoff

EWon

Duty cycle off, EW on 100%Not used when OA < 40F to prevent tripping the freeze stat during EW off part of cycle

Beginning of EW duty cycle

Target avg. duty cycle temperature

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Conclusions

• 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. EngineeringPenn State University

• 814-863-2091• e-mail: sam11@psu.edu• Snail Mail:

214 Engineering Unit AUniversity Park, PA 16802

• Visit the: http://doas-radiant.psu.eduweb site.