Ductwork distribution systems become contaminated at greatly varying rates ranging from extremely slow in the case of ductwork downstream of High Efficiency Particulate Arrestance (HEPA) filters to rapid in extract/return ductwork.
Modelling and empirical studies are underway to assess the effects of dust challenge, filter efficiency and maintenance, duct configuration, velocity, temperature and relative humidity on deposition rates. Recent study showed that annual deposition rates varied in the tested buildings from 0.2 to 2.3 g/m2, however, the measurement methods vary and are clearly an important factor in the variance of these results. Whilst fouling rates and determinants are difficult to establish, it is clear that filter specification and critically filter maintenance are crucial in the case of supply air systems. Filter maintenance can be neglected and in the Indian environment filter medium and frames are more prone to wetting, clogging and collapse due to high humidity in high or low dry bulb conditions, and to seasonal rain water ingress.
Extract/Return
Extract or return air duct systems are naturally more prone to fouling, since the air they carry is obviously more fouled and is typically not filtered. For comfort HVAC systems contaminants produced within the served area are usually dominated by people and their activities e.g. skin flakes, textile fibres from clothing, bedding and soft furnishings. It is interesting to note that humans give off about 500,000 particles per minute due to natural skin exfoliation whereby the outer skin layer is completely replaced over a period of two or three days. The fouling of quasi-industrial extraction systems in non-industrial premises is normally dominated by the material being extracted; typical examples include kitchen grease extracts and laundry lint extracts.
Naturally, industrial extract systems can be affected by a wide variety of contaminants taken from the workplace, ranging from flours to volatile chemicals.
Internally-generated contaminants
Internally-generated contaminants, i.e. contaminants sourced from within the HVAC system, can be important. There are three major types:-
Corrosion products can be generated at plant, typically local to outside air intakes, cooling coils and humidifiers as well as further downstream within ductwork and intermediate plant where condensation, long-term high humidity or inward water leakage has occurred. Central plant rust sources can be remarkably prolific in spreading flake or dust throughout systems.
Internal insulation, typically man-made mineral fibre (MMMF) or foam plastic, at central plant, ductwork or intermediate plant can break down due to mechanical abrasion (eg. at a filter or fan chamber where man access occurs); to erosion or fluttering/vibration of MMMF scrim covers due to simple air velocity or turbulence: or to ageing of foam plastic. Standing moisture, associated with cooling coil condensation, humidification and with condensing-out of moisture on cold duct surfaces, can rapidly attain microbiological significance.
Airborne micro-organisms along with dust can be stripped out of the air stream by spray humidifiers and wet coil fins to accumulate and grow in nooks and crannies, imperfectly draining sumps or drain pans and downstream due to droplet carry-over or compartment leakage. The author has found examples of bio-mass slime accumulated at up to 100mm thickness in systems operating in sub-tropical/tropical (de-humidification) climate zones.
Effects of contamination
Dirty HVAC systems can have a wide range of potentially deleterious effects, including discomfort, odour, ill-health, visible dirt emissions, reduced airflow, increased energy consumption and the special case of fire risk.
Effects of dirt
The table, taken from the Building Services Research and Information Association (BSRIA, UK) Technical Note 18, summarises some of the chief effects of dirt. These are:
- Health risks to building occupants
- Deposition of dust particles on to dust-sensitive electronic equipment such as computers and VDU’s
- Reduced air flow causes reduced plant efficiency leading to increased energy consumption and shorter operating life
- Automatic fire dampers can become clogged up and ultimately rendered inoperable
- Increase fire risks caused by build-up of deposits of combustible material in ductwork and filters and particularly in kitchen extract systems from fluff, grease and fat in a hot atmosphere
- Damage to furnishings and décor
- Cross contamination risks in hospitals
- Increased risk of corrosion
- Large banks of cells in electrostatic precipitors can collect dust particles at the rear of the collector and result in short-circuiting and plant shut down
Human Effects
There are generally only infrequent reports of acute ill-health, such as legionella pneumophila associated with contaminated HVAC systems. The greater cause for concern is for more widespread, chronic, low-level ill-health or discomfort, and in a workplace its associated effect on productivity. Some epidemiological studies have shown the prevalence of sick building symptoms to be higher in mechanically ventilated than in naturally ventilated buildings and more complex HVAC systems appear to be more vulnerable. The Danish “Town Hall Study” was a major research project in this area, evaluating 28 buildings, and found that the risk of developing ‘sick building’ symptoms increased with rising dust levels. A follow-up study analysed dust from the same buildings to find relations between the components of the dust and the presence of “sick building” symptoms.
In this study the concept of dust as ‘carrier’ for micro-organisms and for adsorbed Volatile Organic Compounds (VOC), such as formaldehyde, benzene and hydrocarbons, is employed. Smaller particulates, such as simple construction dust and man-made mineral fibres can be directly and mechanically irritating to skin and mucous membranes such as the eyes and upper respiratory tract.
Contaminated systems can also cause sensory discomfort i.e. odour. This can explain direct ‘musty, stuffy, dirty’ experiences and it is believed that these sensations may lead an individual to more general dissatisfaction with the environment.
Airflow Effects
Ventilation system flow can be reduced by the accumulation of material within ductwork systems by increasing surface friction or by constriction of cross-sectional area. In one study of a hospital extraction systems lint had built up to a depth of 70mm in one system and reduced airflow to one-third of normal value; at another flow had reduced by half after only two and a half years operation. Whilst these cases are likely to be extreme examples, smaller reductions in efficiency may be significant especially to tightly designed systems with energy economy in view. At supply systems, such gross fouling is less likely, however, intermediate equipment such as zone heating/cooling batteries, VAV and mixing boxes are regularly found to be blocked.
A study over three years by the Danish Building Research Institute into apartment/flat block ventilation by a combination of warm air heating and kitchen/WC extraction showed that the supply and extract main volume flows fell by 5.4% and 1.2% respectively per annum. Cleaning of the systems showed that between 29% and 77% of the reduction was due to fouling of the systems. Specific electrical consumption of the fans was measured and found to rise by 5% per annum and of this, between 55% and 100% was due to fouling of the systems. The knock-on effects of reduced airflow include failure to provide designed ventilation rate, insulation of heat exchangers, reduced carry-through of heating or cooling, and increased energy cost. Perhaps most noticeable at galley and cabin bathroom systems is the reduction in effectiveness of odour control.
Microbiological Effects
Micro-organisms and microbial derivatives are parasites, viruses, bacteria and fungi and their products such as antigens, mycotoxins and microbiological volatile organic compounds. Micro-organisms are ubiquitous in the environment and most are not harmful to health in most situations. Unfortunately there is very little agreement between microbiologists regarding acceptable standards, health effects of various species and the significance of particular airborne or surface colonisations. Furthermore there is a wide variety of non-comparable measurement techniques. Engineers tend therefore to rely on general principles such as the avoidance of elevated or concentrated colonisation, good housekeeping to provide good general hygiene and removal of clear risk factors such as dirt and moisture.
Some authoritative general guidance is provided that there are two types of health effects from micro-organisms:
- allergies (ranging from allergic rhinitis to potentially serious conditions such as extrinsic allergic alveolitis)
- infection (ranging from the common cold to Legionnaires’ Disease)
Fire
Fire hazards are overwhelmingly related to specific extraction systems, notably kitchen and laundry systems, although there is some experience of infrequent occurrences of in-duct combustion in comfort system extract/return ductwork. Grease and lint in ductwork provide a hidden combustion load which is difficult to inspect and clean. This has led to a spate of high-profile fires in buildings in the UK, where fortunately injury has been avoided but disruption and financial loss has been severe. In the author’s home town of Eastbourne in the UK, at the District General Hospital, electrical arcing at an automated ironing machine caused first a rapid burn dust explosion, followed by ignition of settled dusts.
Craig Booth, Triventek, Denmark
