While the occupiers and developers plan to restart their workplace and some of them are planning for new workplace fit-outs, the following questions should be kept in mind w.r.t air conditioning. For instance:
- How effective is the air conditioning system?
- What could be done to ensure safer environment at the workplace?
- How to ventilate more and dilute airborne containment's?
- What are the tools needed?
Let us decipher and simplify some basic HVAC terminologies along with associated facts, which can help creating strategies to evaluate and enhance the presently installed air conditioning system.
Understanding virus transmission routes: Size is virus is 80-160 nanometres and transferred via microscopic air borne particles and contaminated aerosol droplets. It is the smaller particles size <5 microns, which can stay airborne and can be transported over long distances. They are troublesome specially in dry air as these can remain suspended in air.
Relative humidity: Moist air helps virus to stick to the surfaces and retard free flowing nature of these small particles. Studies indicate that 80% relative humidity and above tends to neutralize the virus. While higher humidity >80% would also activate growth of fungi & dust mites and can be harmful for respiratory ailments like asthma. Considering above, a relative humidity level of 40%-70% is most suitable environment for humans.
Temperature: Comfort temperature is normally considered between 24-30 degrees after accounting air velocity & relative humidity. Studies have shown the viral growth occurs more at lower temperature (7-8 degrees) but decreases progressively as the temperature goes up.
Indoor air circulation: In commercial workplaces, we normally have central systems where AHU’s (Air handling units) are serving several rooms through air ducts and return air through ventilation air ducts. In circulation, we circulate 90% of indoor air and bring 10% of outdoor air (fresh air) which is mixed at AHU. During ventilation, exhaust through toilets discharge additional 10% air. We can also treat outdoor air through TFA (Treated Fresh air) unit, which helps to lessen the heat load and improves the quality of outdoor air. A minimum fresh air volume of 8.5 Cum/hr per person is recommended.
Also, there is a need to ensure that ventilated air is not mixed between the rooms or zones. Additionally, in some of the cases it is seen that there is NIL or inadequate outdoor air because of various reasons like design omission, damper might be closed, leakages in duct and faulty execution. Hence, this is the most crucial aspect and needs to be thoroughly checked. Components of air circulation system are Filters, Coil, Duct, Motor, and Fan unit.
Choice of filters: Generally, filters with MERV 5-12 grade are being used in the AHU’s, which are good enough to remove tiny dust particles and aerosols of size 1-10 micrometre. These days filters are available with MERV 13-16 grade, which can absorb even smaller particles like smoke, pigments, bacteria and droplets (sneeze), etc., which can effectively eliminate the virus.
UVGI: Ultraviolet germicidal irradiation (UVGI) is a disinfection method that uses short-wavelength ultraviolet (ultraviolet C or UVC) light to kill or inactivate microorganisms by destroying nucleic acids and disrupting their DNA so that they no longer grow or multiply in your space. This process is also called as UV sterilization. UV lights are typically installed near your primary AC coil or in your ductwork. Microorganisms that pass within the UV bulb's line of sight will be destroyed. When installed in duct, it is called Duct Zapper which is a kind of rectangular frame and can be installed inside duct lining. The germicidal action of UV irradiation leads to drastic reduction in the microbe count and VOC (volatile organic compound), thereby enhancing the air quality. This product finds wide range of application in hospitals and pharmaceutical industry.
Biophilic design: Indoor plants as a part of biophilic design provides a cleaner, healthier air for us to breathe, thus improving our well-being and comfort. In fact, it can help reduce carbon dioxide levels by about 10% in air-conditioned offices and 25% in buildings without air conditioning.
Portable Room Air cleaners: With regards to removing virus and bacteria, technologies vary from passive to active filtration. Some of the technologies used are Ionization, PCO, ESP, ozone generators etc. Passive technologies include HEPA filtration that can remove particles down to 0.1 micrometres or even smaller particles. An H13 certified or equivalent HEPA filter is effective to remove the viruses.
Having briefly understood on above mentioned topics, it is recommended to evaluate / design the HVAC system basis 5-fold strategy which are:
a) Indoor Air Quality (IAQ)
b) Air pattern
There are other Do’s and Don’ts which can be applied while operations and maintenance of the HVAC system:
- Fresh air system to start 2 hrs before the normal office hours and shall be continued throughout the day. Similarly, exhaust in the building shall never be turned off
- Never lower the temperature, if the indoor space is below 24 degrees and avoid thermal variations by avoiding ON/OFF machines
- Avoid multiple entry/exists to the building
- Sitting shall be avoided under the diffuser/grill to avoid the throw on the head
- Clean the filters every week
- Ensure that toilet exhaust doesn’t come in the other occupied areas
- Ensure that during duct installation all open ends are covered and sealed properly to avoid ingress of dust
- Maintain proper chilled water temperature to control the RH. Otherwise, dehumidification will not happen
- Ensure that areas around AHU’s and inside the ducts are cleaned properly. These days we have sophisticated system like ‘robo ducts’ which can effectively clean and disinfect the ducts
There are detailed operations and maintenance guidelines and additional recommendations for healthcare and industrial facilities, which are well-described in latest guidelines recently published by ISHRAE and should be referred.
By taking adequate precautions and measures, we can combat, eliminate, and prevent spread of COVID 19 virus.