Thermal Cleaning of Heat Exchangers

Cleaning of heat exchangers polluted with (partly) organic contaminations like cokes, plastic, oil, paint, rubber, food etc. still is a major problem for many companies. The need of parts’ cleaning is found in each and every industrial branch. Some take it as a daily necessity; others consider it an incidental procedure in times of calamity, turnaround or shutdown. Apparently it is just part of the game, but nevertheless it should be done as quickly as possible and as best one can. In most cases this is very often related to the available knowledge of the known cleaning techniques.

Photo 1: Heat exchanger soiled with plastics

As a result of the ever increasing quality of final products, the search for longer running times, the need for better cleaning results and cutting down costs or due to more stringent environmental laws and procedures, most of the traditional methods of cleaning, like chemical or high-pressure cleaning, are sometimes no longer adequate. Maybe the thermal cleaning method can offer the desired solution for your cleaning problem?

Thermal cleaning methods

1. Advantages of thermal cleaning

Vaporizing organic elements only yields some 5 to 10% of the original pollution, which can be easily removed. Moreover, thermal cleaning very intensively removes the broadest range of pollutants in a relatively short period of time, without causing damage to the substrate.

2. Brief introduction to the technique

2.1. The process

Pyrolysis is the thermal conversion of organic materials in an oxygen-poor environment. At a temperature below 400°C (750°F) 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 (ethane, ethene, 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% 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. This is also important in order to avoid unwanted (waste) gaseous fractions.

3. Pyrolysis ovens

As stated before, this is the only correct thermal cleaning technique for the treatment ofsoiled heat exchangers. To give you of proper idea, I will describe the general functioning of this type of installation, after which the conditions necessary to guarantee a safe and perfect heat exchanger cleaning will be further examined.

General functioning

The pyrolysis ovens consist of an operating room of 1 to 75 m³, depending on the type. The common maximum dimensions currently are 8m x 3m x 2,5m, but note that even 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%. Then the temperature is slowly increased to 420°C (788°F), depending on the character of the objects and the kind and amount of pollution.

The oxygen content of the cheapest oven systems is even not controlled. It is taken for granted that the oxygen will leave the chamber automatically during the gasification. If the temperature increases after all, a lot of water is injected so that the temperature has to fall. These are of course the real paint stripping ovens which are note suitable for the “real” job.

When the temperature required for vaporizing 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. Occasionally this air current is used for heat recycling in order 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 5% of the original pollution volume and can be easily removed by various techniques.

Because the heat is able to reach every spot – also in the middle of tube heat exchangers or between the tubes and the shell of a heat exchanger with fixed shell -thermal cleaning is extremely suitable for heat exchangers. This is impossible with for example only high-pressure cleaning.

Special conditions for the thermal cleaning of heat exchangers

To be sure that the heat exchanger gets the best possible thermal treatment, a number of essential matters have to be respected. Besides a proper oxygen control, there exists a method to monitor the exchanger itself by means of thermocouples at various with the owner agreed 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 takes care of a 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 to cool down with air instead of water. Through this, temperature control has increased considerably; it is almost made possible to steer accurate to a degree. Moreover not having to inject water in the chamber involves less trouble with volatile rust on the parts to clean.

To draw up a proper temperature protocol, it is important to know the exact composition of the material as well as the pollution. In case of doubt, a reliable laboratory analysiscan 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.

Photo 12: After thermal cleaning, note that also the paint will be gone afterwards

Also the pollution itself strongly influences the temperature program to follow. Does the pollution liquefies 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 liquefies during the heating up, there will be a foreseen phase in the program around the melting point of the pollution. This phase enables the pollution to melt in large measure out of the object to clean. Subsequently the gasification of the remainders can more easily take place and the total cleaning time will be reduced. In case of extreme pollutions of thousands of kilos, even a two-phase treatment can be chosen. A great part of the pollution will than be melted during a first thermal treatment at low temperature. Afterwards the remaining part will be gasified completely during a second treatment to remove the pollution entirely.

Also the type of material used for the construction of the heat exchanger is very important; after all the material determines the maximum temperature for the thermal treatment. Although there are differing opinions concerning the possible thermal cleaning of duplex steel bundles, various tests have revealed that this type of heat exchangers can be properly cleaned using this method. Only the maximum delta T between the different thermocouples is much lower than with exchangers made out of regular steel types, due to the expansion differences between the metals used in duplex steel.

Besides all conditions concerning the correct installation technique, a proper knowledge of the used material and an analysis of the pollution, also the know-how of the oven operator is 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 imminent 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 in such a way that it increases or decreases at the same time on the inside as well as on the outside. This can be done in large measure automatically by means of thermocouple steering in the newest installations, although supervision is still necessary.

Advantages of thermal cleaning for the different types of heat exchangers

The traditional methods for the cleaning of heat exchangers are often very effective and therefore used for many years. Nevertheless there are a number of very specific problems which everyone recognizes. The problem with high-pressure cleaning is the deterioration of the surface as well as the accessibility of the pollution. Remainders involve that your exchanger has to be taken out of production sooner to be cleaned again.

Using chemical cleaning can sometimes take a long time to dissolve the pollution. The availability of the bundle will be postponed. Thermal cleaning on the other hand is very effective and reaches every spot. The results proof that a degree of cleanliness of almost 100% can be reached which results in longer operating times and consequently cost savings.

To be quoted as a good example are the heat exchangers from a naphta cracker which were cleaned with high-pressure every two months. After the first thermal cleaning, they could be used without problems during 2 years; a huge improvement. And there are more.

Hereunder you will find a definitely not complete overview of the different types of heat exchangers, together with the typical advantages and disadvantages of thermal cleaning for each.

Pipe bundles

· With this type of heat exchanger, the contamination can be in or/and around the tubes. High-pressure cleaning has the problem that the water jet can barely reach the very inside of the exchanger to remove the pollution around the tubes. This means that some bits are left behind so the contamination happens more quickly during the production, resulting in a shorter operating time.

· Thermal cleaning destroys the pollution everywhere and therefore gives a much better level of cleanliness which results in a longer operating time. For extreme contamination, often a high-pressure cleaning is used for the first rough cleaning of the bundle, after which thermal cleaning can take place.

· In case of hairpin bundles, thermal cleaning can remove the pollution inside the corners of the tubes easily, where most of the times the problems occur.

Bundles with a fixed shell

· The problem with classic high-pressure cleaning is that there is practically no access to do a cleaning around the tubes. Often chemical cleaning by flushing is used to clean the soiled inside, but the effectiveness is often very poor. Very good results were seen using the thermal cleaning method to remove the pollution around the tubes.

· To get a good temperature control on the inside of the heat exchanger often a blow through system is used to allow a good air ventilation on the inside of the shell. Besides of the heat, you will need a bit of oxygen to get a good oxidation of the pollution. Otherwise you will be confronted with carbonized material after the thermal treatment which is very difficult to remove.

· Like always all organic parts are reduced to ashes, but in this case it is better to use air to remove already a great part of this dust. Using water will leave the remaining ashes very sticky and therefore difficult to remove all of it. After the cleaning by air, a rinsing with water is done, but there still can be the possibility that during the first running hours, dust particles coming from remaining ashes can disturb your process. This normally goes away after a short time. In some cases an endoscopy is done to verify the result of the cleaning on the inside.

Bundles with static mixers (fix or non-fix)

· High-pressure water cleaning has no use here, the penetration degree is zero. Chemical treatment can give results depending on the possibility of a flow on the inside of the used chemical to dilute the pollution enabling it to be removed. In most cases however both traditional methods gave a very poor result, which immediately shows the necessity of the thermal cleaning method.

· First the thermal treatment will be executed to destroy the organic pollution on the inside of the static mixers. Then a combination of water and air is used to remove the dust particles from the inside of the mixers. This is done until the water is clear again, which indicates that most of the ashes are gone.

· No guarantee can be given after the treatment that it is 100% clean, because in case of the use of fix static mixers, no control method can be used to inspect the inside of the tubes. The only control you will have is to see that the water flows trough normally, so the tube is not blocked.

· After the cleaning procedure is done, there also might be the possibility of dust particles in your flow during the first running hours. This is not the case with removable static mixers, because they are removed after the thermal treatment and worked on separately to get them 100% clean.

Spiral or plate heat exchanger

High-pressure does not give an optimal result here either, and chemical cleaning generates an awful lot of waste. With the classic methods, there are always deposits left on the walls, which means the exchangers become dirty more quickly during production; that's why thermal cleaning is also the most feasible solution here.

Compablocs

· Traditional water cleaning is, depending on the pollution, in some cases very difficult even impossible.
Like with all other heat exchangers, thermal cleaning offers a good possibility to make your Compabloc almost new again using controlled heat and a good rinsing afterwards.

Twisted tube heat exchangers

· Because of the twist in the tubes, the accessibility needed for a good cleaning by water is very poor. You only can clean the outside like this.

· Thermal cleaning offers very good results because of its technical characteristics. All dirt will be removed also from the very inside of this kind of heat exchangers.

Plate heat exchangers

· In most of the cases a chemical treatment is used and has been proven to be very effective to clean the soiled plates of this kind of heat exchangers. High-pressure cleaning is sometimes done, but the risk of damaging is very high.

· Thermal cleaning can be used to remove all kinds of pollution on the plates, like glue, and a polymerized or carbonized pollution, which are difficult to remove with chemicals.

· Thermal cleaning however can not remove scale, while this is inorganic!

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 etc. 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 soiled heat exchangers or other parts like new again. This surely is an advantage which can not be ignored. Therefore I hope the information given to you in this article helps you to make the right decision next time a cleaning problem occurs.

For more information, please contact Robert Mol (robert.mol@thermoclean.com)

Posted in: Chemical Engineering Processing

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