1 decoupling the building latent and sensible loads using 100% outside air systems ashrae chicago...
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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: [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 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.
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: [email protected]• Snail Mail:
214 Engineering Unit AUniversity Park, PA 16802
• Visit the: http://doas-radiant.psu.eduweb site.