100% Outside Air Systems for Light Commercial Applications

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100 Outside Air Systems for Light Commercial
Applications
Outside Air Delivery Strategies Direct Reheat
Reclaim (DX) Methods Passive Reheat
ASHRAE Luncheon March 8, 2006
by Steve Dunlap P.E. Dunlap Consulting Services
LLC Birmingham, Ala.
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OUTSIDE AIR DELIVERY Dedicated Outside Air Unit
NEUTRAL AIR 70 F to 75 F/DRY
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Tempering Air Unit - Restaurant
Application Provides 30 to 40 of the total
outside air to each host unit. STD Rooftops
distribute and mix the outside air Passive
Reheat Opportunity
Controls Sequence Capacity Steps on Outdoor
Conditions. Discharge Air Control Cycles Selected
Capacity Step as Required. Evaluate 1st Stage
Heat Capacity Ensure Std. Rooftops are not
Forced into Cooling Mode.
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Neutral Air Corridors
Application Low Space Sensible Load High
Outside Air Delivered Via Corridor. Constant
Reheat Required to Maintain Less than 60 RH
Hotels
Toilet Exhaust
Controls Unit Capacity Steps Sequenced on
Outdoor Conditions (Dry Bulb and
Dewpoint) Discharge Air Control with Space Reheat
Over-ride Options. Active Mechanical
Dehumidification and Reheat to 52 F Outdoor
Dewpoint
Guest Room
Corridor
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Sole Source
Application Treats High Outside Air and Provides
Space Conditioning
Replacement Air Unit - Kitchen
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Alabama Sole Source Air Unit
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Reheat Requirements
Primary Reasons for Reheat
Comfort Prevent Space Overcooling while
Dehumidifying 75 F 50 RH (55F Dewpoint gtgt55F
DB)
IAQ RH Control Reduces Risk of Mold and Mildew
growth (lt 60 RH).
Process Drying - Sometimes.Dehumidification
and RH Metrics are Not Necessarily the Same
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Power Allowances for Lighting/Appliances are
Being reduced gtgt Lower Space SHR gtgt Building
Balance Point However we can reclaim waste heat
and inject back into the space for
dehumidification.
Was Great Reheat ! 3watts/sqft
ASHRAE 90.1 1999 Simultaneous Heating and
Cooling Exception Zones where at least 75 of
the reheat energy is provided from a
site-recovered (including condenser heat) or site
solar energy source.
Lighting or Electric Heat Reheat COP
1 Reclaimed Refrigerant Reheat COP 4 to 6
However, There are No Rating Standards for the
Efficacy of Air To DX Reheat Systems..
So What Are You Specifying Anyway ??
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Typical Refrigerant Direct Reheat Methods
Mechanical Dehumidifier Operation Mild
Conditions
Combine this Simple Technology with More of the
Same Under Design Conditions.
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Typical Refrigerant Direct Reheat Methods
Disable Dehumidification Capacity/ Reheat -
Design Dry Bulb
Or We Can Add Condenser Surface and Controls to
take advantage of Dehumidifier Compressor with
reheat modulation capabilities..
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Typical Refrigerant Direct Reheat Methods
Full Condenser Reheat Added to The Cooling System
Reheat Capacity is Typically Modulated.
Reheat Modulation Requires Added Oil Return
Strategies. Dehumidification Reheat can be
compromised under lowered cooling demand.
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Typical Refrigerant Direct Reheat Methods
Hot Gas Reheat De-Superheater Added to The
Cooling System
50 of Total
Loss of Cooling Loads, Result in a Loss of
Reheat. Typically Applied Only in Low Cfm/Ton
Applications. Requires Added Oil Return
Strategies.
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Typical Refrigerant Direct Reheat Methods
Liquid Sub-Cooling Reheat Added to the Cooling
System
Recuperative Adds Additional Evaporator
Capacity when Enabled (allows higher leaving coil
temperatures for sensible loads). Reheat
Temperature Rise is a FCT of CFM/Ton. Typically 8
to 12 F. Simple Cost Effective Approach to Low
Cost Dehumidification and Reheat
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Morning Warm Up Strategies
Disclaimer This design concept was created by a
professional for fun, it should not be attempted
at home
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Specifying Reheat
Calculations Maximum Space Reheat Requirements
(lowest sensible heat gain with dehumidification
requirement..begin at the cooling coil) For
Nashville 24/7 Operation If ODB lt 65 455
Hours Above 55 F Dewpoint Both Design Dew Point
(Latent) and Dry Bulb Criteria (Sensible)

• Select Specify Equipment
• Application Tempering, Neutral Air, Sole Source
  (100 O.A., Mix, etc.)
• Reheat Type
• Dedicated Dehumidification Circuit with Reheat
• Full Condensing Reheat
• Hot Gas Reheat
• Liquid Sub-Cooling Reheat
• Supplemental Reheat (If reclaimed heat is not
  available at reduced loads)

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Specifying Reheat

• Submittals
• Reheat Layout Full Piping/Component Schematic
  Refrigerant Flow Diagram
• System Refrigerant Pressures With and Without
  Reheat Activated. Refrigerant Charge.
• Refrigeration Control Sequence Includes Low
  Ambient Criteria and Head Pressure Reqs
• Modulating Systems Description of Oil Return
  Strategy
• Physical Placement of the Reheat Coil
  (Performance De-rate for Cooling Coil Contact ?)

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PASSIVE REHEAT APPROACHES DX Dual Path Split
Face Coils Return Air Bypass Limited
Flexibility with DX
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Passive Reheat Layout with 100 Outside Air Unit
If the fraction of outside air (dehumidified to
55 F) to each std rooftop remains in the 25 to
35 range of the outside air, space conditions in
temperate climates will rarely require reheat (if
the ambient temperature is above 68F and internal
latent loads are low) as shown below 55 x . 3
16.5 (continuous dehumidification of outside
air) 74 x . 7 51.8 (return air at 74 F- no
call for space cooling)
Mixed Air Temp 68.3 F (the 74 F provides free
reheat while outside air is dehumidified

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Passive Reheat Layout with Split Face Coil
Air Handler - Split Face DX Coils - 1968
Inactive coil allows mostly return air bypass
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Upflow Rooftop With All The Popular Options
Split Face Coils
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expectations
Cold Shower Options
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THANK YOU FOR YOUR TIME !
Questions/Comments ?
Dunlap Consulting Services LLC Birmingham, Ala.
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