Today’s most common method for the cleaning of heat exchangers with (partly) organic pollutions is high-pressure water cleaning. Everybody knows it and almost everybody uses it since it is easily accessible. Whilst this method of cleaning can boast many advantages, it also comes with a couple of disadvantages like less cleanliness inside the bundle or between tubes and shell and also roughening of the surface as a result of the enormous pressure.

Both disadvantages create the same problem: a negative influence on the production process. Is it a necessary evil? No, there has already been an alternative for years: cleaning by pyrolysis. Thermal cleaning provides a very good cleaning solution, which approaches and even equals in many cases the production parameters of new heat exchangers.

What Is Thermal Cleaning?

The theory is simple. By using heat in combination with a low oxygen level, the organic pollution – like plastic, oil, paint, rubber, food remains – will be converted from a solid substance into a dust remnant. In other words, 1 kilo of pollution will be reduced to 50 grams of dust, which is easy to remove! Moreover, and this is one of the biggest advantages of this cleaning system, the heat reaches every place in the contaminated part. As a result the conversion into dust will take place everywhere in the parts to be cleaned.

In practice it is therefore possible to perfectly clean both the inside of the tubes (even with static mixers), as well as around the tubes or between tubes and shell. Even when the bundle is mechanically blocked in the housing the thermal cleaning will remove the pollution so that both parts can be separated.

Brief Introduction to the Technique

The process

Pyrolysis is the thermal conversion of organic materials in an oxygen-poor environment. At a temperature below 450°C the organic materials are converted into a homogeneous residue, ready for further controlled processing.

At such high temperatures higher hydrocarbons are decomposed into components with a much lower molecular mass, resulting in pyrolysis gases – examples are ethane, ethylene, propane, propylene – pyrolysis oil which contains aromatic components and a carbon-rich residue.

The pyrolysis gas as well as the oil is transformed into carbon dioxide and vapour because of partial oxidation. This phase is exothermic, at which 40 percent of the released energy is re-used to decompose the organic material.

A very important factor in the process, together with a steady heating and cooling, is maintaining a constant temperature, in order to prevent damage to the parts that are to be cleaned.

General functioning

The pyrolysis oven consists of an operating room of 1 to 120 m³, depending on the type. The common maximum dimensions currently are 10 x 3 x 2,7m, but note that bigger dimensions are also possible.

The objects to be processed are put on a loading cart, which is pulled into the furnace chamber. After closing the door this chamber is made inert by lowering the oxygen level to 8 percent. Then the temperature is slowly increased to 450°C, depending on the character of the objects and the kind and amount of pollution.

When the temperature required for vaporisation is reached, a slight overpressure leads the released gases towards the afterburner chamber. Here they are processed at high temperature, after which they are removed. It is perfectly possible to regenerate this heat to recover a part of the energy.

As a result of the fact that all organic components are gasified because of the heat, only a residue consisting of pigments and inorganic fillers remains after cooling. This is in general 1 to 5 percent of the original pollution volume and can be easily removed by various techniques.

Because the heat is able to reach every spot, thermal cleaning is extremely suitable for heat exchangers. This technique makes it possible to decompose the pollution at highly unapproachable spots into easy to remove dust, which is impossible with only high pressure cleaning.

Special Conditions for the Thermal Cleaning

To be sure that the heat exchanger gets the best possible thermal treatment, a number of essential matters have to be respected. Besides proper oxygen control, there exists a method to monitor the exchanger itself by means of thermocouples at various spots. It is important to utilize object temperatures instead of chamber temperatures in order to be able to follow the process in the oven correctly.

Another important technical aspect is the presence of an excellent internal circulation unit, which ensures proper circulation in the oven chamber. Due to this there is a homogeneous temperature on all sides, which is essential to avoid temperature differences inside the object to clean.

The newest systems dispose of an internal heat exchanger and cool with air instead of water. Through this, temperature control has increased considerably; it is almost possible to control temperature accuracy to the degree. Moreover, not having to inject water into the chamber involves less trouble with volatile rust on the parts to be cleaned.

To draw up a proper temperature protocol it is important to know the exact composition of the material as well as the type of pollution. In case of doubt a reliable laboratory analysis can give a decisive answer. The weight of the exchanger, the type of material, the geometry and the type of pollution determine the heating up and cooling down curve and can largely give an indication of the total necessary time in the oven.

Also the pollution itself strongly influences the temperature program to follow. Does the pollution liquify before it gasifies – like some plastics – or does it remains solid until complete gasification? A laboratory analysis or practical research can also give a decisive answer on this matter, which enables a more correct assessment of the program.

If the pollution is of a type that liquifies during heating, it will do so during the program when the melting point is reached. This can be seen since this phase enables the pollution to melt in large measure out of the object to be cleaned. Subsequently the gasification of the remaining pollutant can more easily take place and the total cleaning time will be reduced.

Step 1 - Before thermal cleaning.

Step 2 - After thermal cleaning.

Step 3 - HP after-cleaning.

Step 4 - Final result.

Also the type of material used for the construction of the heat exchanger is very important; after all, this material determines the maximum temperature for the thermal treatment. Tests have even revealed that Duplex material can properly be cleaned using this method.

Besides all these conditions, the know-how of the oven operator is also very important. The operator has to know exactly which program to follow in order to have the correct heating up and cooling down process. During the thermal cleaning of a heat exchanger it is imperative that the material is heated up and cooled down simultaneously to avoid internal tensions. The operator has to make sure that the temperature is steered is such a way that it increases or decreases at the same time on the inside as well as on the outside.

Some Examples of Results Achieved

A good example is the heat exchangers from a naphtha cracker with so-called popcorn pollution. With high-pressure water it was cleaned every two months. After the first thermal cleaning the equipment could be used without problems for two years – a huge improvement.

An oil refinery was eager to compare the effect of thermal cleaning with their traditional high pressure cleaning method. They therefore used two side-by-side operating trains of four connected heat exchangers. One train was cleaned thermally, the other one with high pressure. The test revealed that the thermally cleaned heat exchangers produced such good results that the next scheduled stop could be postponed. According to the customer this resulted in huge savings!

As a last example I would like to cite another case of a customer with a historical non-removable contamination between the tubes and the fixed housing. With traditional cleaning techniques no good results could be achieved. The thermal treatment was able reduce the present pollution to easily removable dust from anywhere within this huge heat exchanger. After the customer took the exchanger back into service, it turned out that it came up with practically the same performance in comparison to its original performance values, resulting in huge savings on steam costs.

Which Dimensions Are Possible?

Currently heat exchangers of 60 tons with a maximum length of 10m can be cleaned thermally. However this does not mean that the method itself is limited to these dimensions. Bigger is definitely possible!

It may seem strange to build a thermal cleaning installation especially for the cleaning of one heat exchanger with exceptional dimensions; but practice has already proven that this is often a budgetary interesting solution. A new pyrolysis oven is much cheaper, or available much more quickly than many heat exchangers. It definitely pays off to consider thermal cleaning for the real big boys!

Certainly more types of products can be cleaned with the thermal cleaning method like big vessels, pumps, extruder parts, filters, pipelines, reaction vessels and many more. Also all kinds of organic pollution like PP, PE, PS, PC, SAN, PET, PA (also reinforced), PBT, PU, carbonized material, cokes can be removed, which shows that the thermal cleaning method has a very high potential as a substitute or even an improvement for today’s cleaning techniques.

In fact in a lot of cases the thermal cleaning technique offers you the possibility to make your polluted heat exchangers or other parts like new again. This surely is an advantage that cannot be ignored.

Robert Mol,

General Manager, Thermo-Clean Group,

Robert.mol@thermoclean.com