AlfaTech HVAC System Strategies for Airborne Diseases

HVAC SYSTEM STRATEGIES For Ai rborne Di seases

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01

Executive Summary

02

Airborne Diseases and Covid-19

03

Increased Ventilation

Air Filtration

04

Air Disinfection

05

Temperature and Humidity

06

Control of Air Direction

07

Restroom Exhaust Control

08

Sequence of Operation Updates

09

Facility Operations and Maintenance

10

HVAC SYST EM STRAT EG I ES F o r A i r b o r n e D i s e a s e s

E XECUT I VE SUMMARY

Suggested Solutions: Improvements can be made to HVAC equipment, controls programing and facility operations. The following options are discussed in more detail throughout this document.

Situation : The Transmission of airborne disease is a concern in commercial buildings. There are many uncertainties with the current Covid-19 pandemic which is not confirmed to be airborne yet. However, the World Health Organization continues to recommend airborne precautions for circumstances and settings according to risk assessment. Objective : To reduce the risk of transmitting airborne diseases from infected individuals who may or may not have symptoms through specific modifications to both the operation of and equipment for HVAC.

• Increased Ventilation • Air Filtration • Air Disinfection (UVGI) • Temperature & Humidity Control • Air Direction & Space Pressurization Control • Restroom Exhaust Control

• Sequence of Operations Updates • Facility Operations & Maintenance

Conclusion: These recommendations should be followed based on building risk assessment, and in conjunction with the Building Pandemic Management Plan. Impacts of these suggestions on the occupant comfort, building pressure control, energy usage and other systems should be considered and analyzed before implementation.

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AIRBORNE DISEASES AND COVID -19

• Large droplets (around 100 m) released from an infected person

will not move further than 2 meters (6.5 feet).

• “Droplet nuclei” is less than 5 m in diameter and small enough to

be transported by air motion, including HVAC systems.

• It is known that the main routes of Covid-19 transmission are

through droplets and physical contact.

• It has not been confirmed if the virus transmits through small

airborne particles and aerosols. However there has been a case in

a restaurant where the HVAC system is in question.* There is also

a study on the aerodynamic analysis of the virus suggesting that

Sars-CoV-2 can be airborne and aerosol concentration is relatively

higher in restrooms.**

* https://wwwnc.cdc.gov/eid/article/26/7/20-0764_article ** https://www.nature.com/articles/s41586-020-2271-3

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The Wells-Riley Equation A MATHEMATICAL MODEL OF AIRBORNE INFECTION

Nc: Number of new infections

S: Number of susceptibles

I: Number of infectors

Increased ventilation (Q) will reduce the number of new infections in the building

q: Number of infectious doses

p: Pulmonary ventilation rate per susceptible

t: Exposure time

Q: Flow rate of uncontaminated air

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INCREASED VENTILATION

• Outside air has high bacterial diversity and low average

pathogenicity.

• Indoor air has low bacterial diversity and high average

pathogenicity.

• Increased building ventilation rates will help dilute droplet

concentration.

• Required ventilation rate is not clear, however maintaining a CO2

level of 550 ppm or lower could be a good indicator.

• Buildings located near pollutant sources and freeways should take

additional measures on ventilation air with higher efficiency filtration

systems.

• Higher ventilation rates will increase building cooling and heating

loads. Existing system capacity should be analyzed to avoid comfort

issues. Considering lower occupant density in buildings, it is

anticipated that majority of the systems could handle higher rates.

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AIR FILTRATION – CENTRAL

The fraction of particles removed from air passing through a filter is

termed “filter efficiency” and is provided by the Minimum Efficiency

Reporting Value (MERV) under standard conditions.

• MERV 13 or greater are efficient at capturing airborne viruses.

•

MERV 14 filters are preferred.

• HEPA filter efficiencies are higher than MERV 16 but will have

higher pressure drops.

• Carbon filters are effective with chemical gasses but cannot be

utilized for capturing airborne viruses. Utilization of carbon filters on

outside air can be beneficial in areas with poor outside air quality.

Courtesy of ASHRAE Standard 52.2-2017

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AIR FILTRATION – LOCAL

Portable filtration systems can be placed at locations with higher

contamination & high traffic areas. Unit placement and position

should be determined in accordance with the existing HVAC

system.

• HEPA filters and UV Air Purification Systems can be included.

• Added humidification function can also be included in dry climates.

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AIR DISINFECTION – ULTRAVIOLET GERMICIDAL IRRADIATION (UVGI)

Ultraviolet germicidal irradiation (UVGI) uses short-wave ultraviolet

(UVC) energy to inactivate viral, bacterial, and fungal organisms so

they are unable to replicate and potentially cause disease. In short,

UVGI does not kill viruses, it just deactivates them.

• UVGI system is a cost-effective and a flexible solution.

• Can be placed within ducts, Air Handling Units (AHU), plenum spaces, or in rooms above occupied areas.

• Low electrical consumption if installed centrally within a unit-- approximately 2-3 kw for a 100k sf office building.

• Ideal conditions are, 200-400 fpm air velocity & 80-95F ambient air dry bulb temperature.

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AIR DISINFECTION – UVGI ADVANTAGES

Coil after UVGI

Coil before UVGI

There are additional benefits with inside AHU installations, such as:

• Prevents fungus & mold build up on the coil(s)

• Coil pressure drop reduction by 10-15%

• Increased heat transfer efficiency due to cleaner coil surface

• No safety concerns with occupant exposure.

• Increased Irradiance capacity allows higher predictive measures.

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AIR DISINFECTION – UVGI DISADVANTAGES

Precautions with Inside AHU Installations are:

Ozone

• Ozone production occurs with wavelengths below 240nm.

• Titanium Doped (Doped Quartz - Ozone Free) quartz tubes prevent ozone production while natural and synthetic quartz materials still

allow transmission.

Courtesy of Helios Quartz

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AIR DISINFECTION – UVGI DISADVANTAGES

Human Exposure

• Occupational exposure guidelines for UVGI have been developed by CDC/NIOSH. In room applications are usually limited to 0.2 W/cm2. Exceeding

the permissible exposure time can cause short- and long-term damages.

• In-unit applications prevent human exposure and allow much higher irradiance.

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AIR DISINFECTION – UVGI DISADVANTAGES

Material Degradation

Some materials degrade under UVC. Sealants, filter material and filter

supports can be impacted on HVAC system.

• Organic filter fibers and Hepa filters have an average of 5-6% weight loss versus fiberglass filter weight loss of 25%

• Other than structural damage, polyester filters may crumble to the touch and lofted fiberglass may irritate eyes (fibers becoming

airborne within ductwork) under UVC exposure.

• UVC resistance ratings shown on Table 8 per ASHRAE RP-1509: A – No effect (aluminum tape, all organic materials

experience mass loss)

B – Minor Effect (mainly cosmetic changes)

C – Moderate Effect (some cracking/pitting)

D – Severe Effect (structural damage, not recommended)

Courtesy of Table 8, ASHRAE RP-1509

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TEMPERATURE AND HUMIDITY

• Higher space humidity levels slow the evaporation of large droplets.

• Controlled humidity reduces transmission of certain airborne infectious organisms including influenza.

• Breathing dry air can cause desiccation of nasal mucosa, which can make occupants more susceptible to respiratory virus infections.

• Maintain dry-bulb temperatures per ANSI/ASHRAE Standard 55- 2017.

2018 study: Humidity decreased Influenza A illness in pre-school

• Maintain RH level between 40% and 60%. This could require humidification during dry & cold and dry & hot days.

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CONTROL OF AIR DIRECTION - OVERHEAD DISTRIBUTION

The majority of buildings in USA are served by “Forced Air” systems.

These systems can have either overhead or underfloor distribution

systems.

With overhead distribution, the conditioned air enters the space from

supply ducts/diffusers installed over the occupied zone. The return air

exits via return air plenums or diffusers positioned in the ceiling or full

height walls.

Overhead air distribution is designed to mix the entire air volume in an

enclosed space and the air will circulate for longer and then reach

occupants.

Any suspended pollutants or particles will linger for greater periods of

time, resulting in lower air quality.

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CONTROL OF AIR DIRECTION - OVERHEAD DISTRIBUTION

• This system is not very efficient with providing occupant isolation.

• Workstation placement should be reconfigured to allow for supply and return air zones. This may result in further reduced occupancy.

• HVAC system ductwork and diffusers should be placed accordingly so that workstations are not located within airstream.

• Supplementary exhaust systems (ducted or unducted) can be considered to guide the air away from occupants at all times.

• Portable filtration systems can be utilized in high traffic areas.

• Personalized ventilation systems can be considered where other measures are not feasible. This option will require drastic

infrastructure upgrades.

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CONTROL OF AIR DIRECTION - UNDERFLOOR DISTRIBUTION

Displacement ventilation or underfloor installations are more efficient for

pollutant control.

Fresh air enters the space from floor level and takes advantage of

natural thermal buoyancy which sees warmer air rise towards the

ceiling. Along the way, the air currents will force pollutants up and away

from occupants, resulting in a much higher air quality.

In spaces with T-bar ceilings additional return grilles will reduce return

air movement within the occupied space.

With this system the workstations can be reconfigured to create a layout

similar to “hot aisle-cold aisle” configuration. Common supply air will be

provided to the rows back to back and the return air will rise up from the

middle rows. Diffusers placed in return rows will be shut closed.

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RESTROOM EXHAUST CONTROL

• Increase restroom exhaust rates as much as exhaust and make-up system capacities allow.

• Maintain continuous exhaust (24/7).

• Provide fresh outside air to restrooms in lieu of passive ventilation. Utilize reheat coils to prevent overcooling.

• Investigate make up / transfer air sources. install dedicated systems if necessary.

• Lower seat lid while flushing, if possible.

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RECOMMENDED SEQUENCE OF OPERATIONS UPDATES

❑

Increase the duration of building purge mode / flush mode to

increase dilution.

❑

Run restroom exhaust continuously 24/7. Increase restroom

exhaust rates if exhaust and make up system capacity allows.

❑

Monitor and control building humidity. Revise dehumidification

and humidification sequences as necessary.

❑

Maintain equal space pressures on all floors in multi-story

buildings to avoid cross contamination between different floors.

❑ Maximize supply air reset sequence for buildings equipped with

this sequence. This would allow more outdoor air and more air

changes within the building.

❑

Disable demand-controlled ventilation and unoccupancy /

vacancy sequences during scheduled business hours where

possible. Use CO2 sensors for particle measurement only.

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FACILITY OPERATIONS AND MAINTENANCE RECOMMENDATIONS

❑ HVAC unit components (filters, coils etc.) can be contaminated.

Take necessary precautions while servicing equipment.

❑

While working on roof, keep distance from sewer vent pipes

where possible.

❑

Replace filters more frequently.

❑

Run restroom exhaust continuously 24/7. Increase restroom

exhaust rates if exhaust and make up system capacity allows.

❑ Ensure CO2 sensors are working properly (all zone and system

level).

❑ If UVGI systems are applied, control human exposure. In duct /

AHU systems should be provided with dedicated disconnects to

service.

❑ Ensure UVGI compatible sealants and filtration system is utilized.

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