Letters to the editor
Steve Bird, C ENg, CIBSE, ASHRAE, got in touch
I do not in any way advocate that my understanding of certain issues is the final certainty in any way what so ever. I do believe Covid infection is subject building engineering services engineers are being exercised over. Another challenging and informative article this month – MYTH BUSTING.
The whole issue of bacterial and virus “carry over” in central air handling plant is fundamentally valid. The article shows that it is not “critical” if plant is correctly selected, operated and maintained. The small amount of infection transmission between supply outdoor and room exhaust air is shown to be low and well controlled with the right design / install / maintain approach. The likely ratio source contaminant “carry over” in fresh outdoor from space exhaust air is small indeed. This aspect would apply to both central all air and primary ventilation air plants. What the “carry over” risk in all air systems with a proportion of recirculated space exhaust air via motorised dampers remains an issue to be determined by manufacturing test standards. The recirculation of space exhaust air into fresh outdoor supply air is an issue of design. The recirculation air path can give a direct and significant mixing of the two air streams.
An intriguing statistic would be what proportion of systems in operation are designed on an “all air” supply principle. In terms of operating these such as to minimise cross contamination by closing recirculation dampers without original seasonal design peak load of recirculation or energy recovery from exhaust air, plants are likely to fall short on thermal performance.
Would it be reasonable to presume, without the benefit of collective data that could show otherwise, a large proportion of comfort cooling and air condition systems designed, operated and maintained are of primary air format serving room terminal units – fan coil, active and passive chilled beams as examples? This format intends a large proportion of air to recirculate within the occupied space. Terminal units providing significant space cooling and heating capacity. Is it likely this proportion of recirculated air could carry and spread contamination across the occupied spaces?
The recent health guidelines are clear that if one has to work in an office space face covering must be worn. Direct fresh air ventilation is also recommended in the opening of windows or similar. A large proportion of modern offices do not allow for opening windows on the basis of environmental control and peak energy optimisation. If this is a correct scenario the additional fresh air ventilation is not available.
Can it be stated that building services engineers are now entering upon a planning in a new and changing perspective? It is understood that installations have been tested where in space independent recirculation units room air passes through high level UV-C filtration shielded from the occupants. UV-C carries a risk to those in its vicinity as it is a very high level of radiation exceeding that of the sun rays. Not being specialist in this area of filtering / decontamination the subject of its application would suggest there is much investigating and determination to be undertaken.
ASHRAE has published a recent article on the effects of increased ventilation rates have on virus cross contamination in occupied spaces. It suggests a palpable improvement but does not completely eliminate the source or spread.
As it has been clearly defined that the current pandemic has change the pattern of office / home working. With suggestions of an ongoing change in office occupancy and use of office space. Is the building engineering services sector such that interaction and propagation of ideas are greatly assisted by informal interaction between those involved? Close space working aids this principle. It appears the industry has some basic assessments and developments in the future design, install and maintenance provisions in what is very much a service industry in support of business and living standards.
It is certain discussion will continue in efforts to develop a meaningful and economic solutions to what are unique and uncertain conditions.
