Essentials
SELELE MASHILO
Selele Mashilo has a mechanical engineering diploma from Tshwane University of Technology and
a refrigeration and air-conditioning diploma from Unisa. His experience includes over a decade in
government as deputy-director building services before rejoining the private sector in 1998 as HVAC&R
project engineer. He is the former chairperson of the Refrigeration and Air Conditioning Empowerment
Forum of SA (RAEFSA), the Air Conditioning and Refrigeration Industrial Council of SA (ACRICSA), and
Black Energy Services Companies (BESCO).
CONTROL OF
GASEOUS CONTAMINANTS
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.
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.
COMMERCIAL EQUIPMENT
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. RACA
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.
ASHRAE
Figure 1: Typical pressure drop ranges
46
RACA Journal I September 2020
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