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).
SURFACE TRANSPORTATION
AIR CONDITIONING
By Selele Mashilo
Many people spend most of their lives inside a vehicle so a comfortable
environment inside the vehicle becomes a necessity.
A
vehicle environmental control system becomes a
necessity particularly where pollution in and outside cities
is severe, and slow traffic flow subjects vehicle occupants to
uncomfortably high temperatures. The environmental control
system of vehicles may consist of a heater-defroster, ventilation,
and air-conditioning systems.
A typical human comfort-zone is an atmosphere that provides
comfort and causes less fatigue. This atmosphere can be achieved
by using an air-conditioning system which is well-designed and
controlled. Maintaining temperature to an average of 22⁰C in
summer and also controlling humidity inside a car is very important.
Heating can be achieved by using engine cooling water from the
radiator through a heat exchanger for winter conditions. There is no
need for a reverse cycle refrigeration system. vent windows may be used at the back of the vehicle to expel air to
the outside so that fresh air can be let in without keeping the cabin
under high positive pressure.
HEATING THE VEHICLE CABIN AIR CONDITIONING
Interior heating is generated by engine coolant as a heat source.
Outside air is passed through a heat exchanger into the cabin. Hot
air must not be recirculated internally as it will increase humidity
and may reduce visibility through windows due to fogging when air
temperature rises above dew point temperature on glass. Heater
capacity is achieved by controlling water flow or controlling the
amount of air through the heat exchanger. The refrigeration system of a car is basically similar to any other
refrigeration system for cooling. The system has a compressor,
condenser, expansion valve and evaporator. All the components
perform ‘normal’ refrigeration functions and follow the pressure
enthalpy process of a refrigeration cycle. The compressor is driven
from the engine power and this reduces engine efficiency as some
of the engine driving power is used by the compressor.
Further space to accommodate the condenser and evaporator
within the cabin or below the cowl must be created.
DEFROSTING
Defrosting is achieved by using electric heaters mounted on the
front or rear windscreen of the vehicle. Another method is to use
the air conditioning for the dehumidification process.
VENTILATION
Fresh air from outside may be introduced by using ram air or forced
air. In both systems, air enters the interior of the vehicle through
screened openings in the cowl in front of the vehicle. Sometimes,
www.hvacronline.co.za
GENERAL CONSIDERATIONS
The general considerations include ambient temperature and
contaminants, flexibility of the system, physical parameters,
durability of the air conditioning, electrical power to be used by the
air conditioning components, refrigeration capacity influenced by
components sizing, air filtration, refrigerant piping insulation and
the noise of the system. RACA
RACA Journal I January 2020
73