Decoupling the Building Latent and Sensible Loads Using 100% Outside Air Systems

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Decoupling the BuildingLatent and SensibleLoads
Using 100Outside Air Systems
ASHRAE Chicago Seminar 39 January 24, 2006
Stanley A. Mumma, Ph.D., P.E.Professor of
Architectural EngineeringPenn State University,
Univ. Park, PAemail sam11_at_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 loadfrequently up to 5
   times the flow needed for ventilation alone.
   Will not be discussed herebut could be during
   the question time if interested.

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DOAS Systems
20-70 less OAthan VAV
High Induction Diffuser
Cool/Dry Supply
DOAS Unit W/ Energy Recovery
Building With Sensible and Latent cooling
decoupled
Parallel Sen. Cooling System
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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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How Much is 208 Billion/yr.---in light of the
13,000 Billion/yr 2006 USA GDP (value of goods
and services)

• 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

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Parallel Terminal Systems
Radiant Cooling Panels
Chilled Beams
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VAV problems solved with DOAS plus Radiant or
Chilled Beam

• 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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Additional benefits of DOAS

• Beside 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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DOAS with Parallel Radiant
OA
Outdoor air unit with TER
Radiant Panel
Space 3, DOAS in parallel w/ CRCP
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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
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EW
196
4
5
RA
Space
2
3
1
OA
168
CC
PH
1
140
Other regions?Next slide
112
wet
2
84
3
4,5
Humidity ratio (grains/lb)
56
dry
28
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EW control for various OA conditions
196
168
EW on whenOA h gt RA h
140
112
wet
EW off
84
Humidity ratio (grains/lb)
56
dry
EW off
EW to modulate or duty cycle to hold SAT
SPwhen OA lt SAT SP
28
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Example of an incorrect EW control logic
EW on
196
168
Cleaning cycle when off! Frost protection when
cold outside!
140
EW off
112
EW on
84
Humidity ratio (grains/lb)
56
28
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Reheat adds 2.8 tons of cooling load, plus the
heating energy wasted in the 1,710 cfm OA sys.
gt 3 ton cooling lost with wheel off
Neutral temperature a huge energy waste!
CC Control based upon maintaining a SA DPT, what
happens if the OA is hot and dry ?
CC load 10 ton
EW off, huge control error when it could
significantly reduce the CC load if operating.
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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 controla duty cycle saves chiller
  operation.
• EW using a VFD, maximizes the free cooling of a
  DOAS.

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EW Duty cycle defined
RAT, 72F
OAT40F
SATOAT when EW off (40F)SATOAT(RAT-OAT)EWef
f (65.6F)
EWeff, 0.8
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 lt DATSP and the CC must be off when the
EW on!
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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
Duty cycle off, EW on 100Not used when OA lt 40F
to prevent tripping the freeze stat during EW off
part of cycle
EWon
EWoff
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_at_psu.edu
• Snail Mail214 Engineering Unit AUniversity
  Park, PA 16802
• Visit the http//doas-radiant.psu.eduweb site.