energy recovery in air handlers delivered by:jason richwine 16 april 2012columbus, oh

Energy Recovery in Air Handlers

Delivered by: Jason Richwine16 April 2012 Columbus, OH

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Energy Recovery in Air HandlersIntro

• Standards and Definitions

• Three Popular Technologies

• Wheels

• Plates

• Pipes

• Summary

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Energy Recovery in Air Handlers Standards & Definitions

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• Requires outdoor air (OA) be used for ventilation when it is not polluted

• Defines amounts of OA needed for various applications

• What’s the problem with ventilating?

ASHRAE 62.1-2010

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• 90.1 defines limits for building energy consumption

• ASHRAE plans to develop tools to design net-zero energy buildings (NZEB) by 2020, so NZEBs will be standard by 2030

• How can we have good ventilation and limit energy use?

ASHRAE 90.1-2010

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Exhaust air (EA) energy recovery is required when:

• Many commercial comfort applications don’t require energy recovery (<30% OA)

• Energy recovery may still make economic sense

*See Exceptions 6.5.6.1.a – 6.5.6.1.j

ASHRAE 90.1-2010

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• Voluntary Standard

• Defines test, rating, and minimum data requirements for published ratings, marking and nameplate data

• Helps you compare performance among manufacturers

• AHRI Directory provides list of manufacturers that comply

• Some manufacturers choose not to participate

AHRI 1060

http://www.ahridirectory.org/ahridirectory/pages/home.aspx

Solution YC - YORK Custom Air Handlers8

Energy Recovery in Air HandlersThree Popular Technologies

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Feature Wheel Plate HX Heat Pipe

Performance Best Very good Good

Base Price (media only) Middle range Lowest Highest

Price (packaged in AHU) Moderate across all unit sizes Lowest on smaller units, moderate-high on larger units

Highest in smaller units, moderate on larger units

Performance/Price Best Very good Good

Heat Transfer Effectiveness

Sensible: 50-80%Total: 55-85%


Sensible: 45-65%Total: N/A

Exchanger Size Most compact Largest Compact, flexible shape

Cross-contamination Levels

Highest cross-contaminationMinimized with purge

No cross-contamination up to 3” w.g.

No cross-contamination

Application Temperature Up to 150 deg F Up to 392 deg. F Up to 450 deg. F

Corrosion Resistance High - polymersMedium - aluminum

High - polymersMedium - aluminum

High

Cleaning Requirements Dry: Vacuum or compressed airWet: Low pressure rinse

Dry: VacuumWet: High pressure rinse

Wet: High pressure rinse

Maintenance Bearings, belts, motors, cleaning

Cleaning Cleaning

Airflow Limitation ~ 35,000 CFM No limit No limit

Energy Recovery in Air HandlersWheels

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What Are They?

• Rotating heat exchangers that transfer sensible and latent heat to precondition ventilation air

Material types

• Composite (polymer, fiber)

• Aluminum

Benefits

• High effectiveness

• Low pressure drops

Applications

• Many commercial applications where isolation of airstreams is not required

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Aluminum Polymer or FiberHeat transfer High thermal conduction Low thermal conduction

Why it matters Requires thermal mass

More material means thick (typ. 4”-12”), heavier wheel

Less material means thin (typ. 3”-6”), lighter wheel

Desiccant type Molecular sieve or polymer coating

Impregnated Silica gel

Why it matters Sprayed or dipped coatings may separate from substrate

Molecular sieve recovers latent heat well at low %RH

Desiccant is integral with substrate

Latent heat transfer increases as %RH increases


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30%-35% RH is typical for winter-time non-humidified indoor air

Desiccant Effectiveness

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Cross-contamination and Purge

• Cross-contamination occurs when air leaks past separators and seals, or is carried in the wheel as it rotates from the EA to the OA

• We want to limit leakage from EA to OA

• A mechanical purge directs OA to the EA section to flush air out the exhaust, limiting cross-contamination

• RA that entered wheel prior to the purge section has time to exit wheel on the exhaust side

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Blow-thru SupplyBlow-thru Exhaust

Advantage

Minimal leakage when supply and exhaust pressures are similar

Precaution

Design for appropriate pressure differences to manage leakage

Blow-thru SupplyDraw-thru Exhaust

Advantage

Minimal leakage from exhaust to supply

Precaution

OA can short-circuit to EA wasting fan energy

Draw-thru SupplyDraw-thru Exhaust

Advantage

Minimal leakage when supply and exhaust pressures are similar

Precaution

Design for appropriate pressure differences to manage leakage

Draw-thru SupplyBlow-thru Exhaust

Advantage

None

Precaution

Don’t do it. Too much EA will leak into OA.

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Frost Control

• Frost can form on wheels in the winter when warm moist return air contacts very cold wheel media

• Four types of control

• On/Off – least expensive and least complicated, but turns the wheel off when you most want to recover heat

• Bypass – some OA bypasses wheel. This reduces wheel capacity preventing EA from reaching saturation. Requires good mixing to prevent temperature stratification.

• Preheat – install heating coil in RA or OA. Heating RA decreases RA %RH to reduce frost risk, and increases temperature differences to improve wheel performance.

• Variable speed control – slowing the wheel reduces heat transfer rate, requires sophisticated controls, and may not work well on certain wheels

*See ASHRAE Journal March 2012 page 46 for good article on energy wheel control

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High





Cleaning Cleaning


Energy Recovery in Air Handlers Plate Heat Exchangers

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Energy Recovery in Air HandlersPlate Heat Exchangers

What is a plate heat exchanger?

• Fixed plate heat exchangers transfer sensible and sometimes latent heat

• Cross-flow design with airstreams passing at right angles

• Keeps air streams segregated under normal conditions

Construction Types

• Composite (polymer)

• Aluminum

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Benefits

• High transfer efficiencies

• Segregated airstreams

• No moving parts

• No electrical connections

Applications

• Commercial and industrial applications where segregation of airstreams is desired – pools, labs, processes, etc.

• Do not use when EA contains dangerous contaminants

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Configurations

• Vertical or horizontal orientation

• Various blow-thru and draw-thru fan combinations

• Bypass arrangements

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Configurations

• Multiple Heat Exchangers

• Counter-flow arrangements

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High





Cleaning Cleaning


Energy Recovery in Air Handlers Heat Pipes

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Energy Recovery in Air HandlersHeat Pipes

What is a heat pipe?

• Refrigerant filled heat exchangers that transfer sensible heat

• Hollow cylinders/tubes filled with a refrigerant

How heat pipes work

• Heat is absorbed in the evaporating section and boils the liquid refrigerant

• Vapor moves to the condensing portion via convection

• Heat is released from the condensing part of the pipe and vapor condenses

• Liquid returns – by gravity, wick, or pump – to the evaporating section

http://heatpipe.com/images/HP%20Stucture2.png

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Heat Pipes for Dehumidification

• Two sections: pre-cooling and reheat

• Warm air passes through first section and evaporates refrigerant

• Refrigerant vapor moves to the condenser

• Pre-cooled air passes through cooling coil which cools it and removes moisture

• Over-cooled air passes through condenser and is reheated to comfortable temperature

• Refrigerant condenses and flows back to evaporator

• Entire process accomplished using minimal additional energy

• Result is ability to remove 50% - 100% more moisture than a cooling coil alone

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Energy Recovery of EA to SA

• Side by side heat pipes

• Fluid moves in a continuous one directional flow through individual three dimension circuits

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Benefits

• Phase change provides increased efficiency compared to glycol runaround

• No cross-contamination of airstreams

• Accommodates air streams located further apart

Applications

• Commercial and industrial applications where plates are impractical

• When exhaust air contains dangerous orhighly obnoxious contaminants

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Price (packaged in AHU) Moderate across all unit sizes Lowest on smaller units, moderate to high on larger units







Exchanger Size Most compact Largest Very compactVery flexible shape

Cross Contamination Levels

Highest contaminationMinimized with purge

No contamination up to 3 in. wg.

No contamination



Medium High


Wet: High pressure rinse Wet: High pressure rinse

Maintenance Bearings, belts, motors None None


Energy Recovery in Air HandlersTechnology Comparison

energy recovery in air handlers delivered by:jason richwine 16 april 2012columbus, oh

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