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Theory of Modelling IAQ - Coggle Diagram
Theory of Modelling IAQ
Relationship between Sources, Ventilation, and Concentration
Concentration [mg/m3] = (source rate - remove rate) [mg/h] / ventilation rate [m3/h]
Excluding factor:
Outdoor source
For removal by sinks or indoor air chemistry
Contaminant removal mechanism
Filtration and air cleaning
Sinks or Deposition on surfaces
Deposition on surface
Filters designed to remove particles function
must be able to be tightly bound to the filter media
Gases are removed by adsorption on charcoal or other media or by chemical transformation through contact with catalytic surface materials
The rates vary with the nature of the gas or particle, the surfaces, the airborne concentrations
Size of particle factor
fine particles deposit equally on horizontal and vertical surfaces
larger particles fall to the horizontal surfaces below them due to
gravitational forces
Sink effect
Definition
: process of removal pollutant by deposition on indoor surfaces
The sink effect serves to buffer very high concentrations by allowing chemicals to deposit on surfaces when concentrations are high
Disadvantage
: the extension of the pollutant
residence time in indoor air over much longer periods
Different gas phase chemicals and different surface materials produce different sink dynamics based on variable adsorption and desorption rates
Example
: Pesticides
That other large or heavy molecules tend to be less volatile and, therefore, are found on surfaces more than in the air
This works well to maintain a desired level of a pesticide in air but also results in some unwanted prolongation of the presence of pesticides and other toxic chemical substances (dioxins, furans, etc)
As the air concentration diminishes, the chemical tends to leave the surface and re enter the air
Example
: ETS
Some of the heavier molecules in ETS tend to be both the most
toxic and the most difficult to eliminate
Mechanism for removal pollutant
Passive removal
the pollutant is removed by settling, condensation or sorption
Active removal
Chemical transformation
Ventilation (dilution or exhaust)
Dilution control
The contaminated air is turned over and replaced quickly enough to minimize potential exposure and related odors.
Disadvantage
: not effective for removing dust, controlling air movement is needed to be considered
Strategies for removing indoor air pollutants
Source control
Steps
: identification source, elimination, reduction or isolation
The most effective strategy
Example
: encapsulating a particle board sheet material can reduce the emission rate of formaldehyde and other volatile organic chemicals from product
Performance of room air distribution systems
Ventilation Efficiency
Definition
: The ventilation system's ability remove pollution originating from a source in a room
Characteristics
Relative ventilation efficiency
how the system's ventilation ability varies between different parts of a room
It measures of dispersion and does not take into account absolute concentration levels or concentration changes from the initial concentration level
mean concentration in the whole room
Absolute ventilation efficiency
the ability of the ventilation system to reduce a pollutant concentration in relation to the feasible theoretical maximum
represents change in concentration (as a result of change in ventilation rate)
Generation Rate
Rate at which the contaminant is produced indoors
Impact on indoor concentration
Constant
Exponential decay
Time dependent
Air Exchange Efficiency
an indication of the degree or rate at which ‘old’ air is replaced by ‘new’ air within the space
described as the ratio between the shortest possible mean age of air and the actual mean age of air
mean age of air
: how long the air has spent in the room