Presentation on theme: "100% Outside Air Systems for Light Commercial Applications ASHRAE Luncheon March 8, 2006 Outside Air Delivery Strategies Direct Reheat Reclaim (DX) Methods."— Presentation transcript:
100% Outside Air Systems for Light Commercial Applications ASHRAE Luncheon March 8, 2006 Outside Air Delivery Strategies Direct Reheat Reclaim (DX) Methods Passive Reheat by Steve Dunlap P.E. Dunlap Consulting Services LLC Birmingham, Ala.
OUTSIDE AIR DELIVERY – Dedicated Outside Air Unit NEUTRAL AIR – 70 F to 75 F/DRY TEMPERING - 55 to 63 F SOLE SOURCE - Variable Supply Air Temp Dedicated Outside Air Unit – Treats both space and outside air loads.
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 1 st Stage Heat Capacity – Ensure Std. Rooftops are not Forced into Cooling Mode.
Neutral Air – Corridors: 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 Application: Low Space Sensible Load – High Outside Air Delivered Via Corridor. Constant Reheat Required to Maintain Less than 60% RH Guest RoomCorridor Toilet Exhaust Hotels
Sole Source: Application: Treats High Outside Air and Provides Space Conditioning Replacement Air Unit - Kitchen Surgical Unit Controls: > 8 ACH Discharge Air Control With Space Temperature Reset < 8 ACH Space Control with Discharge Air Control Options Supply Air Tempering Dehumidification Tempering Reheat Cycling or Modulating Morning warm up Heating Modulation
Alabama Sole Source Air Unit
Reheat Requirements Primary Reasons for Reheat: IAQ: RH Control Reduces Risk of Mold and Mildew growth (< 60% RH). Comfort : Prevent Space Overcooling while Dehumidifying 75 F & 50% RH (55F Dewpoint >>55F DB) Process: Drying - Sometimes…….Dehumidification and RH Metrics are Not Necessarily the Same
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. Power Allowances for Lighting/Appliances are Being reduced >> Lower Space SHR >> Building Balance Point However we can reclaim waste heat and inject back into the space for dehumidification. Lighting or Electric Heat Reheat COP = 1 Reclaimed Refrigerant Reheat COP = 4 to 6 Was Great Reheat ! – 3watts/sqft However, There are No Rating Standards for the Efficacy of Air To DX Reheat Systems….. So What Are You Specifying Anyway ??
Mechanical Dehumidifier Operation – Mild Conditions Combined AC & Dehumidifier – Design Dewpoint [dehumidifier 15 to 20% of total Capacity] Typical Refrigerant Direct Reheat Methods Useful Heat for Comfort ? Heatpump COP = 5 Combine this Simple Technology with More of the Same Under Design Conditions…………. Mis-Matched for 3 capacity Steps
Disable Dehumidification Capacity/ Reheat - Design Dry Bulb Typical Refrigerant Direct Reheat Methods Or We Can Add Condenser Surface and Controls to take advantage of Dehumidifier Compressor with reheat modulation capabilities…….. Full Utilization of Cooling Capacity with Dedicated Dehumidification Cycle Added Condenser & Controls Reheat Disabled and Cycled to meet Sensible Loads
Typical Refrigerant Direct Reheat Methods Full Condenser Reheat Added to The Cooling System 50% of Total Capacity Reheat Capacity is Typically Modulated. Reheat Modulation Requires Added Oil Return Strategies. Dehumidification & Reheat can be compromised under lowered cooling demand.
Typical Refrigerant Direct Reheat Methods Hot Gas Reheat – De-Superheater Added to The Cooling System Loss of Cooling Loads, Result in a Loss of Reheat. Typically Applied Only in Low Cfm/Ton Applications. Requires Added Oil Return Strategies. 50% of Total Capture only the Heat of Compression
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 Increased Cooling Capacity
Morning Warm Up Strategies Disclaimer: This design concept was created by a professional for fun, it should not be attempted at home
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 < = 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)
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 Req’s Modulating Systems – Description of Oil Return Strategy Physical Placement of the Reheat Coil (Performance De-rate for Cooling Coil Contact ?) Specifying Reheat
PASSIVE REHEAT APPROACHES – DX Dual Path Split Face Coils Return Air Bypass – Limited Flexibility with DX
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
Passive Reheat Layout with Split Face Coil Air Handler - Split Face DX Coils Inactive coil allows mostly return air bypass 2001 – Back by Popular Request: “The Split Face DX Coil Will Become A Factory Installed Option For All Product Lines”… Major OEM EXEC We listen to our customers…..
“Opposed Blade Dampers Designed To Mix Return and Outside Air” “ Free Reheat with full standing water options.” Upflow Rooftop – With All The Popular Options – Split Face Coils Development Engineer “ Our new Marketing initiatives continue to exceed customer expectations” “Cold Shower Options”
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