For an optimal cabin environment

Expenditure in resources significantly reduced

For passengers on board commercial aircraft a cabin environment that is both pleasant and constant comes high on the wish list. The heat exchangers located in the area at the transition between fuselage and wings play an important role in making this possible. Heat exchangers enable thermal energy to be transferred from one medium to another without the two media – water, gas etc. – actually coming into contact with each other.

In the lamella heat exchanger found in a commercial aircraft the two media are the air heated up by the engines and the cold ambient air from outside. The two airflows flow in separate circuits through vertical and parallel aluminum plates that constitute the core of the heat exchanger. By mixing appropriate proportions of these airflows it is possible to control precisely the temperature of the cabin air and to raise or lower it to the desired level.

To free the aluminum plates inside the heat exchangers, i.e. in the core, of deposits and dirt accumulated during flying operations, they have to be cleaned at regular intervals. This is very difficult because the housing and the core cannot be dismantled and the sensitive plates are very difficult to access. Moreover, it is only possible to determine very roughly the extent of the dirt accumulation based on past experience of the build-up typically found at a given location or after a given number of aircraft flying hours. Due to the exacting safety requirements regarding any possible residue and the high standard of cleanliness required, the heat exchangers have to be cleaned to a reproducible high standard. This is all the more important because defective cleaning could result in an unacceptable loss of pressure in the heat exchanger.

Under the "Heat Exchanger Cleaning" research project Lufthansa Technik joined forces with the Process Technology department of Helmut Schmidt University Hamburg to create the technical conditions for the development of a new method of cleaning heat exchangers. In the course of the research project, which ran from April 2010 to October 2011, the principles were established for a high-quality, reproducible cleaning procedure that would be economical in its use of resources and would also be gentle on the components. The fundamental knowledge gained as a result of this project, which was supported by the Federal Ministry of Education and Research, will serve Lufthansa Technik as the basis for planning future cleaning systems.

Engineers conducted extensive investigations into the dirt accumulation and physical/chemical surface analysis of the aluminum plates in the core, on the basis of which they succeeded in learning a lot that would help in the development of an improved cleaning process. Succeeding work packages were aimed at finding a suitable cleaning agent and at optimizing the process engineering for the dirt accumulations previously analyzed. For this purpose heat exchanger cores were cut up into cubes five centimeters by five centimeters, producing around 800 samples, sufficient to conduct all the relevant test series.

From the technical point of view the primary project objectives were accomplished. Thus, from a large number of candidate substances an optimal cleaning agent that not only cleaned thoroughly but was also gentle on the surface was identified. At the same time many parameters for the cleaning procedure currently employed were improved. Thus, water and energy consumption was cut by 50 percent and the time taken for cleaning and drying was also reduced significantly. Thanks to the differentiated test series the temperature of the rinse water was optimized. Furthermore, it will also be possible in the future to use the immersion bath on the heat exchangers a lot less; this used to consume a lot of time and energy. Based on the insights gained from the research project it has been possible to significantly reduce the resources consumed by cleaning, so that in the future it will be possible to clean heat exchangers not only a lot more economically but also – in line with the principles of environmentally friendly maintenance ("Green MRO") – more ecologically as well.