By Selele Mashilo
Some of the spaces occupied are subjected to gaseous contaminants and odours.
Commercially available equipment and controls can be used to achieve clean air for inhalation. Equipment for filtration is manufactured to adhere to health standards based on the Occupational Safety and Health Act.
Gas measuring equipment that is manufactured nowadays is becoming more accurate and engineers in the air conditioning industry depend on their performance and control mechanisms.
Gaseous contaminants come from sources which determine the nature of the contaminants and their concentrations, as well as applicable standards and economics on controlling the gases. Sources of polluted gaseous air may be brought into occupied space as air conditioning make-up air which will also result in an emission to the environment.
The nature of contaminants helps to determine the control mechanism of the gases and selection of correct materials and systems. In air conditioning processes, recirculated air may be used which reduce equipment costs or 100% fresh air.
The filtration equipment depends on the occupied space activities. At some point, where high efficiency filters are used, the equipment costs may also rise to high levels. Remember, filtration and its pressure drops influence the equipment sizing; in particular the fan selection.
Economics can be a deciding factor since the costs of air cleaning in a system can vary greatly.
System design considerations
Contaminant gases do not exist with single properties, as such. Filtration brings many physical and chemical uncertainties when each contaminant has unique properties affecting their control.
The contaminant gases may vary in molecular weight, polarity, partial pressure or thermal conductivity. Temperature and pressure relationships according to Dalton’s partial pressure law must also be taken into account. Each contaminant in the mixture behaves individually. This poses a challenge in selecting the filtration material and the manufacturer takes that into consideration. Material may be tested in laboratories or measured on site under operations.
As indicated in the previous article, filters may be primary, secondary and tertiary. However, the most critical thing is the make and thickness of the pellet design which determines the pressure drop across the material. Effects of filters on air approach face velocity must be considered.
Figure 1: Typical pressure drop ranges