Field Experience Controlling a

Dedicated Outdoor Air System (DOAS)

Stanley A. Mumma, Ph.D., P.E., Prof.

Jae-Weon Jeong, Ph.D., Instructor

Department of Architectural Engineering

Penn State University, @ Univ. Park, PA

http://doas-radiant.psu.edu

ASHRAE Denver Symp #2June 26, 2005

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Presentation outline DOAS and Test Site Defined Controlled Devices, Instrumentation,

and Control The Why & How of Continual

Performance Monitoring Measured Air Diffusion Performance

Index (ADPI) of System:f (Effective Draft Temperature)

Measured Thermal Comfort, PPD Conclusions

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DOAS Defined for this presentation

100% OA No Recirc.

DOAS Unit W/ Energy Recovery

Cool/Dry Supply, CV

Parallel Sensible Cooling System

High Induction Diffuser

Building With

Sensible and Latent

cooling decoupled

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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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Building Site

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An Inside View

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Another inside view

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T7FM1

DOAS Schematic

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

)wet

dry

EW full Speed

EW Off

EW Modulate

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Continual PerformanceMonitoring—the Need

Dr. J Woods reports that 5-10% of entire non industrial building stock has building related illnesses.

And 10-25% of the Stock has sick building syndrome.

These are facilities that began their life with no known problems, then degraded.

Also, DOE reports that monitoring could save 0.45 Quads/yr of energy.

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Categories of performancedegradation

Insufficient diagnostic and alarm tools for early warning of degradation.

Management’s lack of awareness of the economic consequences.

Management’s Indifference.

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Avoiding Potential Degradation in a DOAS-

Radiant System Compromised SA quantity: equipment

problems, dirt, etc: FM 1 used to detect. Compromised building pressurization,

(infiltration): FM 5 used to detect. Compromised supply air temperature:

detect EW using T6-7-10, or CC T8. Condensation: Cond sensor to detect.Note: sensors color coded with next slide

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T7FM1

DOAS Schematic

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ADPI achieved w/ the DOAS system For the test facility, the air flow rate

was about 0.3 cfm/ft2, or about 30% of a VAV (at design). Some have expressed concern about satisfactory air motion.

Experiments were performed in the winter, when convective action was not supplemented by the overhead cooling panels (no panel cooling).

Even in the winter the space has a cooling load—so no convective impact from heating.

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ADPI Defined An indication of the %of the locations

in a space with a velocity of 70 fpm or less and an EDT between -3F and +2F.

Effective Draft Temperature (EDT):

=(TL-TR)-0.07*(VL-30)Where

, EDT, °FTL, local mean air stream DBT, °F

TR, average room DBT, °F

VL, local mean air stream velocity, fpm

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Figure 4, Effective Draft Temperature,

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

-5 -4 -3 -2 -1 0 1 2 3

T-Tc (F), [local - ambient air temperature]

Lo

cal M

ean

Air

Vel

oci

ty, f

t/m

in

=2=- 3

ADPI=(34/35)*100=97%, or 97% of the observations were between -3<2 F

=0

The mean velocity at the

35 stations ranged from 12 to 30 fpm (all

below 70 fpm). The EDT for 34

of the 35 locations ranged

from -3<EDT+2.

Therefore theADPI was

[34/35]*100=97%

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ADPI conclusion

DOAS can deliver exceptionally high ADPI’s; a very favorable finding!

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Thermal Comfort Thermal comfort is a function of the

following variables that influence metabolic heat transfer: 1. Dry-bulb temperature (DBT),2. Relative humidity,3. Mean radiant temperature,4. Air movement,5. Metabolism, and6. Clothing worn by the occupants.

Comfort, then, is almost completely a function of the space air distribution, provided there is sufficient heating or cooling to meet the thermal and humidity control requirements. (i.e. ADPI important).

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Measuring Thermal Comfort: Thermal

Comfort Meter

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Thermal Comfort Test Results

-0.01<PMV<+0.07

Where, PMV subjective scale: +3 hot+2 warm+1 slightly warm0 neutral-1 slightly cool-2 cool-3 cold

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Predicted Percent Dissatisfied (PPD)

Thermal Comfort Index

51525354555657585

-2.5 -2

-1.5 -1

-0.5 0

0.5 1

1.5 2

2.5

PMV(Predicted Mean Vote)

PP

D

(Pre

dic

ted

% D

issa

stif

ied

)

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Predicted Percent Dissatisfied (PPD) test

results The PPD for the tests:5.1%<PPD<5.4%

That means almost 95% of the occupants were satisfied.

ASHRAE’s accepted thermal comfort design guidelines permits PPD to be as high as 20%. Satisfying nearly 95% of the occupants is certainly far superior to the ASHRAE target of 80% satisfied .

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Conclusions Dedicated outdoor air systems (DOAS),

when properly designed and controlled, are capable of delivering very stable and comfortable environments (PPD = 5%).

The authors have experienced no difficulties making the system and controls perform as designed/desired.

Perhaps the keys to success are:– the proper control of the enthalpy wheel,

and – the control of the cooling equipment to

assure that the space latent loads are completely handled by the ventilation air.

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Conclusions It has been demonstrated that good air

motion is achieved (ADPI of 97%) with ventilation air flow alone (typically around 20-30% of that required for thermal control),

It is not necessary to deliver large quantities of primary air to provide thermal comfort.

As a result, there can be significant air movement energy savings when a CRCP hydronic parallel system is used to meet the balance of the space sensible load not met with the ventilation air.

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Conclusions Finally, because of the ability of the

DOAS to decouple the space latent control from the sensible control, space relative humidity levels are maintained at the desired design level.

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Inherent problems with VAV Systems

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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VAV problems solved with DOAS/Radiant

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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Consequences of system degradation—Ref: Dr. Jim

Woods 20% of US workers experiencing health related symptoms

Another 20% of US workers are experiencing hampered performance

50% of US workers have lost confidence in management’s ability to deal with the situation.

A major economic investment is needed to mitigate each problem and recover workers “goodwill”.

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Computing Occupancy From Measured CO2 Data Steady state vs transient

computations. Why count people in light of

ASHRAE Std. 62.1-2004?– Floor component.– Occupant component.

– Causes space CO2 concentration to change with occupancy.

– DCV made more difficult.

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Computing Occupancy From Measured CO2 Data

Transient equation in difference form:

Pep=(V*(N-N1)/+ SA*(N-Ci))/(G*1,000,000)where

Pep = number of occupantsV = the space air volume, ft3

N = the space CO2 concentration at the present timestep, ppmN1= the space CO2 concentration one time step back,

ppm = the time step, min.SA = the supply airflow rate, scfmCi = the CO2 concentration in the supply air, ppm

G = the CO2 generation rate per person, scfm

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Computing Occupancy From Measured CO2 Data How well does it work?

– As long as the temperatures remain nearly steady, the accuracy is remarkably good (within 2 people for a 40 person space).

– But when the OA temperature drops, error is introduced in the CO2 measurements.

When the SA flow is large (many people), the counts can be off by many people. For the test site, by about +5 people.

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Measuring Thermal Comfort A thermal comfort meter can measure

the influence the six variables. The instrument uses a heated ellipsoidal

transducer designed to simulate the thermal pattern of a human being. It contains a surface temperature sensor, and a surface-heating element whose power is adjusted automatically by the thermal comfort meter to bring the surface to a temperature similar to that of a thermally comfortable human.

The rate of heat production needed to attain this temperature is used as a measure of the environmental conditions.