William P. Bahnfleth, PhD, PE, FASHRAE Department of ...€¦ · William P. Bahnfleth, PhD, PE,...

William P. Bahnfleth, PhD, PE, FASHRAEDepartment of Architectural Engineering

The Pennsylvania State University

1KC ASHRAE Tech 2008 Seminar 4/17/2008

Bacteria◦ Tuberculosis◦ AnthraxVirus◦ Rhinovirus◦ SARSSources◦ Infected humans◦ Biological warfare/

terrorism

Characteristics◦ μm and sub-μm◦ Carrier particles

Droplet residueDust

Transmission◦ Airborne◦ Fomite

Cause/aggravate◦ Allergies◦ Asthma◦ Opportunistic

infectionsGrow in presence of food (organic material) and waterTypes◦ Aspergillus◦ Stachybotris◦ Penicillium

Characteristics◦ Surface growth—

mycelium◦ Spores, O(1-10 μm)◦ VOCs◦ MycotoxinsIn HVAC systems◦ Cooling coils◦ Damp filter media

Pathogens◦ Remove from air◦ Remove from surfaces◦ Limit person to person transmission with good

hygiene◦ Deactivate/destroy in air or on surfacesFungi◦ Control sources of moisture◦ Treat surfaces that cannot be kept dry◦ Remove spores from air◦ Deactivate/destroy in air or on surfaces

All airborne biological agents are filterableEase of filtration depends on particle size—MERV 6 not effective against μm-sized particlesHEPA and near-HEPA filters have high pressure dropsFungi can grow on filter media

Outside air can be used to dilute any airborne indoor contaminantConditioning of outside air is a major energy consumerLarge amounts of outside air conditioning in hot/humid climates may lead to moisture control problems

UVC, UVB radiation (~200–320 nm) damages DNA, RNA of microorganisms

1880s Finsen uses UVB to treat skin diseases1920s Studies of UV effect on microorganisms1930s First air treatment applications1940s Studies of surface mold disinfection1950s Use of UV in A/C described as

“standard” application in GE literature1980s First cooling coil disinfection

Philips UV lamp application guidance1990s Growth of commercial UVGI

Renewed scientific interest in UVGI2005 ASHRAE TG 2.UVAS formed2007 TG 2.UVAS becomes TC 2.92008 ASHRAE Handbook-S&E chapter on UV

Major laboratory study documents ability to deactivate microorganisms in moving air (RTI, 2002)Double blind office building study shows reduction of sick building symptoms and sampled microbial levels (Menzies, et al. Lancet2003)EPA ETV tests of nine commercial products show effectiveness against 3 standard microorganismsBut…much more is needed

To a first approximation:

◦ S = surviving fraction of initial population◦ I = UV fluence (µW/cm2)◦ t = duration of exposure (s)◦ k = decay rate constant (cm2/µW-s)Single pass efficiency of UVGI = 1-S

( )expS kIt= −

KC ASHRAE Tech 2008 Seminar 4/17/2008

k varies widely for different microorganismsRepresentative values (cm2/μW-s)◦ Bacillus anthracis 0.000031◦ Influenza A 0.0019◦ Mycobacterium tuberculosis 0.002132◦ Streptococcus pneumoniae 0.006161Accurate measurement of k is difficult and a weakness of existing design data

-Filter may be more effective for some microorganisms-Consider multiple modes of air treatment

Multi-Stage◦ Superimposed

exponentials for susceptible and resistant populations

ItkItk feefS 21)1( −− +−=

0.00001

0.0001

0 20 40 60 80 100 120

Time, minSu

ItkItk feefS 21)1( −− +−=

“Shoulder”◦ Slow no response

until threshold dose is reached

0 5 10 15 20 25 30

Time, min

0.000001

0.00001

0.0001

0 5 10 15 20 25 30 35 40

Time, min

Low-pressure Hg vapor lamps with quartz tubes produce nearly pure 253.7 nm UVCUVC output ~20-30% of input powerLifetime depreciation typically15-20% over 9000 hr life, but some lose 50% in 6000 hr life with moderate switching rate

Variety of sizes and shapesOutput Level◦ Standard output (425 ma)◦ High output (800-1200 ma)◦ High output lamps operate at

higher temperature than standard output lamps

Cathode◦ Hot cathode

Coated filament, thermo-ionic effectHigher output than cold cathodeStarts affect life

◦ Cold cathodeHigh voltage potential ionizes gas in lampLow power/outputLong life, not affected by starts

4/17/2008KC ASHRAE Tech 2008 Seminar 16

0 2000 4000 6000 8000Hour

0.1 1 10Switching rate (#/3hr)

Hot Cathode

0 20 40 60 80Lamp Surface Temperature [οC]

Maximum output when cold spot T = 40°C (109°F)

1 m/s = 196 ft/min, 15.6°C = 60°F, 35°C = 95°F

Effects are independent, multiplicativeImplies that lamp selection should be based on worst ambient conditions and lamp output just before changeoutRated capacity typically measured after 100 hr burn-in under favorable environmental conditions

Temperature-controlled variable flow lamp test duct

Philips TUV 25W – G25T8 in cross-flow test

Must correlate lamp output with surface cold spot temperatureUse quantitative infrared thermographyFeasible because quartz lamp tubes are ~opaque to infrared

Center (flow left to right) Socket End (hot spot at cathode)

Raw Data Output Contours

Negligible effect on◦ Lamp output (heat transfer)◦ Attenuation of UVGI in airPossibly significant effect on microbial susceptibility to UVGI—may increase or decrease, depending on the organism

Number and configuration of lampsReflectivity of enclosureAir flow conditions◦ Impact on lamp output◦ Impact on dose distribution

UV above occupied zone irradiates circulating airCommon rule of thumb for sizing (Riley, HSPS): 30 W per 200 ft2

Modern fixtures are 36 W (12 UV W) and cost ~$600First cost: $2.50/ft2

Operating cost: $0.13/ft2-yr for continuous operation @ $0.10/kWh

Environment for lamps is relatively stableStandard lamps perform wellDepreciation and failure are more serious concerns

Deactivate airborne microorganisms “on the fly”May do dual coil/filter cleaning dutySizing methods vary greatly among manufacturers—from rules of thumb to simulation based on specific disinfection targetsInstalled cost per 60W fixture ~$300Replacement lamps—$25-35 standard vs. $75-$125 proprietary

Typical recommendation: one 60W lamp per 6 ft2

duct cross section, mount within 3 ft of coil surface and allow at least 0.25s exposure timeAt 500 fpm, one lamp treats 3000 cfm →$0.10/cfm first cost, so ~ $0.10/ft2 for a typical all-air systemAt $0.10/kWh, annual cost for continuous operation ~$0.018/cfm-yr, also $0.018/ft2-yr ($52.56/yr per 60W lamp)Minimum clearance for 0.25s exposure @ 500 fpm is ~2 ftFull flow temperature rise ~0.06°F

Cooling effects on standard output lamps in-duct applications can be severeUse of high output lamps (“windchillcompensated) improves output relative to maximum but does not eliminate output variation as conditions change

0 150 300 450 600 750 900 1050

air velocity (ft. per min.)

TUV PLL 36W 70 F/21 C

Single-zone VAV with 10ºC (50ºF) supply airSystem running 9 a.m. – 5 p.m.0.8 μm bacteria, k=0.001cm2/µW-sVentilation per ASHRAE 62.1, MERV 6 filter (η = 10%) or noneUVGI sized for 99% kill with no correction for ambient effects

Temperature entering UVGI device is constant, but flow variation causes substantial output variation

-Ventilation, MERV 6 filter are ineffective-Constant output “99% kill” UVGI device nets ~60% peak reduction-Neglect of ambient effects causes non-conservative 30% error

Continuously irradiate coil or filter surfaces to control growth--Upstream/ downstream/bothMost well-accepted HVAC applicationClaimed to improve coil heat transfer and air-side pressure drop—proof neededClaimed to clean dirty coilsWide range of opinions on requirements: 5 μW/cm2 on opposite side of coil, 200-2000 μW/cm2 on irradiated face—proof neededGSA standards (P100, 5.9)—downstream of coils, above drain pans

Lamps and fan in a moduleCombines performance issues of in-duct device and portable air cleaner (i.e., ability to turn over air in treated space)

Particle filtration and UVGI are complementarySmaller microorganisms (viruses, bacteria) are generally difficult to filter but relatively easy to deactivate with UVGILarger microorganisms (spores) are relatively easy to filter and hard to deactivate.Moderately high efficiency filtration + UVGI may be optimal

UVC degrades many organic materials commonly found in HVAC systems◦ Synthetic filter media◦ Gaskets◦ Electrical insulation◦ Plastic pipeProblem for retrofitsRule of thumb-shield all organic components within 4-5 ft (1.5 m) of UV lamps

UVB and UVC cause skin and eye irritationOverexposure is a concern◦ Due to leakage from imperfectly sealed devices◦ Improper maintenance procedures and/or

malfunction of safeties◦ Upper room systemsNIOSH RELs for 253.7 nm UVC◦ 1 min: 100 μW/cm2

◦ 1 hour: 1.7 μW/cm2

◦ 8 hours:0.2 μW/cm2

Standards for rating◦ Components◦ AssembliesRigorously based sizing methods for all system typesDesign and application guidanceGenerally available modeling capability, including full project economicsManufacturer-provided data to support selection and designMore and better k value dataMore field studies to establish practical performance and identify problems

UVGI is not new and there is a pile of evidence to support its effectivenessAs an industry, UVGI is still developing, needs more standardization and openness, but that is not a reason to avoid using it todayAs with any IAQ technology, UVGI is most effective when applied in an environment where benefits can be quantified (e.g., healthcare)

William P. Bahnfleth, PhD, PE, FASHRAE Department of ...€¦ · William P. Bahnfleth, PhD, PE,...

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