Contaminant Removal Effectiveness
As mentioned above, the traditional definition of ventilation efficiency or, in approved terms, “contaminant removal effectiveness,” is the ratio between contaminant concentration in the exhaust air and the concentration at a point in the occupied space, i. e.,
Ђcp= CV’Cp, (8.18)
Ecp is contaminant removal effectiveness in the zone of occupancy
Ce is contaminant concentration in the exhaust air
Cp is contaminant concentration at a point in the zone of occupancy
In other words, this contaminant removal effectiveness is a measure of how much cleaner the air is in the occupied spaces than in the exhaust. See Fig. 8.8.
When detailed information on heat and contaminant sources is available, assessment of design is improved by evaluating the effectiveness of contaminant removal achieved by space ventilation. The set of contaminant removal effectiveness indices in Table 8.5 is given in accordance with contemporary use of indices.
The basis of comparison is still the complete mixing scenario, where the concentration of the contaminant in question is homogeneous throughout the room, and equal to the value in the exhaust Ce. All concentrations are net values, i. e., rated above values at the supply opening. It can be shown that
Ec = Ce/<c:> = </<
Application of the “age of air” concept can be justified by the fact that the content of contaminants found in the exhaust air normally rises from the value found in supply air entering the room. On its voyage through the room, the air is likely to pick up more contaminants the longer it stays in the room. This is a very simple assumption. It can be argued, however, that using the age of air concept is the best way to evaluate ventilation design for scenarios where little or no information is available on use of the room and locations and emission rates for heat and contaminant sources.
In order to have effective exchange of air in important locations in a room, the age of the air in those locations should be low. The basis for comparison is the complete mixing scenario. That scenario gives the same age for any air volume selected in the room, identical to the nominal time constant for the ventilation airflow, . A steady-state scenario is assumed. See Sutcliffe for an overview of definitions related to age of air.6 The various air exchange efficiency indices are presented in Table 8.6.
The average age of air for all air molecules in the complete room can be found by performing a step-up tracer gas experiment, and by measuring tracer gas concentration Ce in the exhaust opening. The same procedure can be used for CFD simulations. The definition for average age of air in the room is
For ideal displacement ventilation, ea. Values for the other indices depend on the location of the zone in question.
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