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INDOOR AIR QUALTY / PLUMES

The most recent article dealing with Indoor Air Quality Issues dealt with investigations of air quality problems involving allergies and discomforts caused by the use of certain adhesives, which are akin to perfumes. It also touched on the use of mobile TV cameras to investigate ductwork for mold and fungus which can produce odours. It further described briefly the gas chromatograph and the TO14 method of analysis and also the NIOSH 6011 method used to detect chlorine and chlorine compounds.

'Big Smoke'

This article will deal with plumes because the persistence of plumes is often not recognized as being a problem. The photograph accompanying this article is one of the smoke stack belonging to International Nickel in Sudbury. Even against a leaden sky, this plum is clearly visible.

Plume is a general term and describes a transient condition where two fluids have not mixed. The obvious example is smoke from a stack which eventually will dilute and finally mix or, if it is heavy enough, fall to the ground before dissipating itself. When that happens, one speaks of a point of impingement (POI) which can be several city blocks in area.

Plumes are sometimes liquid such as when sewer water having a high biological oxygen demand (BOD) can be detected in receiving waters for long distances. Another example would be hot cooling water from a power station spreading for a considerable distance into Lake Ontario.

Leaking Underground Storage Tanks (LUST)

A leaking underground storage tank will pollute the ground water. Gasoline plumes which partly dissolve and partly float as free product are common polluters. In time, the soluble components may out distance the free product effectively giving two plumes from one source, one dissolved and one of free product.

Odour Plumes

Odours form plumes also and this is our subject. Odours could come from many sources; they are often heavier than air and the molecules tend to cling together. Examples of heavy odours might be cleaning fluids such as the odour of varsol, or the solvents used in waxes and polishes and paint thinner.

Some designs for modern homes and buildings involve air returns at floor level and the air supply in the ceiling. This means that it is possible for a heavier than air odour to be directly picked up in the return system and, if not diluted sufficiently or removed by some filtering agent to be returned through the air supply. Recent fires in highrise apartment buildings have demonstrated the efficiency of the air conditioning system in distributing the smoke to all parts of the building.

If the flow in the duct is at a high enough velocity, the original plume will mix and the concentration of odour or the other material will be virtually uniform.

Laminar Flow

However, some air handling systems are designed with large ducts and low air velocities. Some times this is done to minimize duct sound, but when this happens the flow in the duct tends to be streamline or laminar. Walters Forensic Engineering was involved in a recent situation where the fresh air system fan had openings on the suction side almost at floor level which sucked in odours, such as varsol fumes, into the fresh air supply. Since the duct was only on the verge of being turbulent, the resulting plume travelled through the fan blades without mixing and formed a layer along the bottom of the supply duct. It stayed intact and remained detectable despite a number of bends, turns and changes in elevation. It finally flowed directly into one particular suite when the main duct turned the rest of the gas.

In the HVAC field, the problem of persisting plumes of unwanted odour or other gases may not have been fully recognized. Some mysterious cases might need to be reopened.

Hints

One useful solution to such a problem might be to caulk, seal and tape the fan housing and low pressure return ducts to prevent unwanted odours from being sucked into the systems. Also HVAC designers should be wary about oversizing duct work for the amount of air to be handled.

 
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