Whenever an internal combustion engine is run,those in the immediate vicinity will be exposed to dangerous exhaust fumes.
In the limited space that is typical of the vehicle workshop or service station, as well as in loading bays at distribution centres, these fumes can present a major health hazard for the workforce.
In recent years the long time effects of vehicle exhaust gas on the humanbody have become well documented.
As a consequence most countries now have laws and regulations governing the permissible levels of exhaust fumes in the workshop and also recommendations as to how this problem should be tackled.
Swedish recommendations, for example, require that the vehicle exhaust fumes should be extracted directly from the exhaust pipe through hoses and ducting connected to an extractor fan. Airflows are also specified.
They are at present:
For lorries, trucks, buses: 1080 m³/h
In countries where no regulations exist, specifically for exhaust extraction, there are, nevertheless, limits for various pollutants usually expressed in parts per million (PPM).
Where extractor fans are located outside the buil-ding, the system inside the building will normally be under negative pressure or suction.
Where a fan is installed indoors the associated ducting must be leak proof so there can be no escape of fumes.
Generally speaking most users prefer small diameter hoses because they are easier to handle.
As the required airflow is known, a fan should be selected according to the pressure drop in thehose.
As a basic rule,the smaller the diameter of the hose chosen, the more powerful the fan required will need to be and vice versa.
Thelength of the hose is also an important factor, since this has a direct effect on pressure drop.
It will be seen from this that for any given work shop a suitable compromise
needs to be found.
In theory a decrease in hose diameter of 50% willcorrespond to a four-fold
increase in flow velocity which in turn leads to a sixteen-fold increase in pressure drop.
Since the power consumption of the fan motor is proportional tothe pressure it follows that the fan would need to be increased by sixteen times.
As a result of this, best practice indicates that hoses less than 75mm in diameter for cars and 125 mm for lorries should not be selected.
In normal circumstances use 100 mm for cars and 150 mm for trucks.
Experience has also shown that the nominated flow rate of 1080 m³/h is insufficient when heavy trucks, construction machinery, large farm vehicles, military vehicles etc.
or longer periods of running at high engine RPM are undertaken.
In these cases it will be necessary to choose larger hose and ducting diameters in order to avoid excessively large fans.