A SUPPLY OF CLEAN AIR (2/2007)

The commercial vehicle market is booming. This will also enhance the sales prospects for cabin air filters in lorries, transporters and building vehicles. Air conditioning systems with suitable filters have become all but standard equipment in the meantime. However, when a vehicle is used with a clogged cabin air filter, severe consequences for heating and air conditioning systems may be the result. The replacement intervals stipulated by the vehicle manufacturers must therefore be strictly observed. Generally this means: about every 12,000 to 15,000 km. For a lorry that travels an annual distance of 250,000, this would be at least every 3 weeks.

While some years ago only passenger cars were equipped with cabin air filters, there is practically no commercial vehicle today that is not fitted with a filter that protects drivers from polluted air entering the cabin. And that is a good development. Without cabin air filters, drivers in their cabins would be subjected to pollutant concentrations 6 times higher than at the roadside. This is mainly due to the so-called tunnel effect, which means that the exhaust fumes of the vehicles in front are directly drawn into the vehicle interior. But also irritant gases such as ozone, toxic gases such as fertilizers and diesel exhaust particulate and other irritants such as pollen and annoying smells would get straight into the interior without cabin air filters. MAHLE Filter and Knecht cabin air filters are both available as particulate and combination filters. Particulate filters protect from fine dust, but also from pollen, industrial fallout and other very fine particulates that can reach the lungs. In addition, active carbon filters absorb smells and gases and reduce up to 99 % of ozone.

THE STRUCTURE OF A CABIN AIR FILTER WITH ACTIVE CARBON
A layer of special active carbon granules is sandwiched between two fleece layers. The unpleasant smells are deposited in the pores of the active carbon. Active carbon consists mainly of carbon (mostly > 90 %) with an extremely porous structure. The pores are interconnected like in a sponge. Just 2 grams of active carbon are roughly equivalent to the surface area of a football pitch – a standard filter containing about 200 g carbon has therefore a filter surface of about 100 football pitches.

CABIN AIR FILTERS FOR COMMERCIAL VEHICLES: THE FINE AND THE COARSE
In contrast to passenger vehicles, commercial vehicles are often used in dusty environments such as building sites. Their cabin air filters must therefore not only eliminate smallest particulate from the cabin, but also coarse particulate. However, the finer the filtration, the more dirt is removed – and the faster the dirt absorption capacity is exhausted. It is the task of the R&D engineers to combine the required fine filtration with a long service life. When the coarse particulate load is extremely high as, for instance, at a building site or a quarry, the filter must be replaced more often – or there needs to be a way of cleaning it. During the development of the LA 43, the decision was made in favour of cleaning.

The fleece of the LA 43 is supported and protected by a grid on both sides.

FOR EXTREME COARSE PARTICULATE LOAD: THE CLEANABLE MAHLE FILTER
The higher the degree of air pollution, the sooner the filter will clog. In extremely dusty environments, the cabin air filter can become so dirty in a few hours that the ventilation system practically ceases working. For such extreme situation, MAHLE has developed the filter element LA 43 in cooperation with Mercedes- Benz. The filter can be cleaned by knocking out the dirt manually and is used as original equipment in the building site versions of the Actros and Actros II models.

The design of the filter has been modified to make it resilient to the cleaning process that is required several times daily under extreme dust loads. From the outside, the special filter differs from other MAHLE cabin air filters in its more robust construction, with stable aluminium side members and massive end-mouldings made from plastic. Optimum sealing is achieved with a so-called blade location in the end pleat of the fleece and a plastic foam seal all around. A corrosion-resistant wire grid at both sides supports and protects the filter fleece. The filter medium consists of a synthetic filter fleece that has been designed for a significantly higher dust take-up capacity for this application.

The LA 43: robust design with side members of plastic (1) and aluminium (2) as well as a foam seal all around (3).

SERVICE INTERVALS: ONLY A GUIDELINE
Cabin air filters are mostly located in front of the vehicle's heating or air conditioning system, where the polluted outside air is drawn in and blown into the vehicle interior after cleaning – this can be up to 150 l air per second when the filter is in optimum condition. For clogged filters, this air volume is reduced. This can be noted clearly, when only a limited amount of air reaches the passenger compartment, although the blower is set to maximum (if in doubt, a visual check of the filter will help). When the filter is clogged, it is high time for replacement, or in case of the LA 43 for cleaning – regardless of the distance covered since the last filter change. If this is not done, expensive consequential damage of heating or air conditioning can result.

The cleaning process for the LA 43 can be repeated up to 80 times. Tests in laboratory and practice have shown that even after 80 cleaning procedures (tapping out and blowing out) the initial conditions of a new filter are almost reached again. However afterwards, the filter should be replaced as the performance can deteriorate rapidly.