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 1st 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 < = 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)

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 - 1968Inactive 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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THANK YOU FOR YOUR TIME !

Questions/Comments ?

Dunlap Consulting Services LLCBirmingham, Ala.

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