Cleaning of Shell and Tube Heat Exchangers Keeping it Clean: Getting the Most Out of Shell and Tube Heat Exchangers
Why do some heat exchangers fail while others run reliably year after year? Although hardly any process plant can do without heat exchangers, the apparatus itself is far too often a black box during operation. There is another way: If you understand which heat exchangers are fouled and why, and what needs to be done, you can save money, energy and cleaning costs.
Whether in refineries, flue gas treatment or chemical plants: Heat exchangers are among the indispensable workhorses in plant engineering. However, continuous use in production takes its toll when deposits, crusts or even blockages and closures block the tubes. Even thin layers of less than 0.5 millimeters can result in a "heat transfer reduction" of 40 to 90 percent in the worst case. As a result, numerous studies have concluded that the additional energy cost of dirty heat exchangers could be responsible for one to two and a half percent of global CO2 emissions. For industrialized countries, the additional costs can amount to a quarter of a percent of GDP.
With suitable maintenance procedures, shell-and-tube heat exchangers could remain reliable and efficient for a long time. But this unpleasant task is usually entrusted to service providers in the hope that the equipment will be in good hands with the professionals. Often, there are not even uniform and meaningful standards for evaluating the success of the cleaning.
High-Pressure Cleaning for Heat Exchangers
Because cleaning is not just cleaning: In the meantime, a large number of processes have become established that promise thorough surface treatment. Whether using chemical agents, high-pressure water jets, ultrasonic baths or by drilling out the tubes, each method has its own strengths and weaknesses and must be selected to "suit" the situation of the plant operator.
Those who want to clean heat exchangers face the problem that the apparatus must be removed and opened. But even when the tube bundles have been pulled out of the shell, the fouling is by no means freely accessible. The inside of the tubes in particular is susceptible to blockages. For this purpose, maintenance engineers often rely on cleaning with high-pressure water jets, not least because most service providers are familiar with the method.
Loud and Energy-Hungry: Disadvantages of the Waterjet Process
But as is so often the case, the seemingly simple solution has pitfalls in practice: The effort is comparatively high and the process lengthy. Cleaning requires enormous amounts of energy and valuable water, and working is very uncomfortable due to the noise generated. For example, using a 480 kW/h motor for a 1,000-bar pump, high pressure consumes 256 liters of water every minute, with a fuel requirement of 24 liters per hour.
Moreover, handling the enormous pressures is not without danger. But, and this is perhaps the most important argument, the cleaning success is also only moderate, especially with hard incrustations.
Scraping, Drilling, Rubbing: Mechanical Cleaning
But there is another way: mechanical pipe cleaning uses brushes or scrapers, which are increasingly also guided automatically. The Cologne-based company Jänsch, for example, "shoots" a water-flushed round hollow drill through pipes up to eight meters long using compressed air. Alternatively, a flexible shaft can be used to bring flushing water or cleaning tools to the point of use in inner pipes that are difficult to access.
In the Raedler Tube Cleaning (RTC) process from the Austrian company AC Raedler, deposits are gently removed with a special drill head and the tube is polished by a brass guide. Due to the accuracy of fit, no residual deposits remain, so that the smooth surfaces form hardly any attack surface for new deposits. With RTC, it is thus possible to clean and regenerate a totally blocked pipe section three meters long within three minutes, explains the manufacturer. The electric motor used requires only 5 kW per hour and about 10 liters of water per minute. However, the process is not established everywhere, so appropriate machines and specialists must first come to the plant. In addition, only straight inner tubes can be cleaned, which becomes a problem with the U-bend heat exchangers that are common in the chemical industry.
Ultrasound and Pyrolysis: Alternative Maintenance Methods
In such cases, ultrasonic cleaning promises a remedy: the tube bundle is pulled out of the heat exchanger shell and immersed in a basin with cleaning liquid. Ultrasonic waves cause the material to vibrate, which creates cavitation bubbles. The energy released removes dirt and residues from the surface. The process is new and comparatively complex, but achieves good results on the outside of the tube.
Ultrasonic cleaning of blocked pipes often requires an "opening" with high-pressure lances, according to studies at a refinery in the Netherlands. However, the operators were able to save more than 75 percent labor and 86 percent water compared to "pure" high pressure. Thus, despite higher investments, total savings of up to 50 percent can be achieved, according to the final report from Holland. It is also possible to clean the entire pipe bundle at once with pyrolysis furnaces, in which organic contaminants decompose in an oxygen-poor atmosphere at about 450 ° C and can be separated as gas or dust.
Measuring, but how? How Good is the Heat Transfer
The better and more complete the cleaning, the better the heat transfer. If this value continuously decreases, as is the case with pure waterjet processes, the apparatus must be cleaned at increasingly shorter intervals and still remains below its capabilities. The consequence: premature scrapping.
This is why criteria are needed to measure the degree of fouling or success of cleaning processes for heat exchangers. The latter is still comparatively simple: You divide the heat transfer before and after cleaning and obtain a characteristic value for the cleaning quality.
Insights From the Outside
To determine the degree of contamination, the manufacturer Talcyon uses Apris, an acoustic pulse method that uses characteristic echoes to localize contamination, blockages or erosion damage within seconds. With software-accurate evaluation and short measurement times, the method (awarded the Best Practice Award by Frost & Sullivan in 2021) is said to be ahead of spectroscopy, eddy current methods or "invasive" ultrasound measurements.
While mechanical cleaning is oftentimes available on short notice, it is also noisy, time consuming and by no means cheap. Its efficiency can be lacking, resulting in shorter service intervals and reduced heat transfer.
Whether forward-looking or strictly according to plan, all maintenance strategies have one thing in common: The goal should always be the "bare pipe," explains Hans-Jürgen Kastner, senior consulting engineer and member of the German Industrial Cleaning Association. The longtime industry consultant has made heat exchanger maintenance a matter close to his heart and advocates a change in awareness among aggregates: Since even seemingly small impurities have enormous consequences, he says, there is no other way to deal with the loss of transfer performance.
Since about 0.5 mm can worsen the heat transfer by 40 to 99.7 percent, a superficial cleaning hardly brings noticeable improvements. Worse still, if even a fine film of dirt remains in the tube, the almost complete failure of heat transfer is quickly achieved.
Is there another way? How Heat Exchangers Stay Reliable
The engineer knows that in a typical industrial plant with countless heat exchangers, only a small number are prone to crusting and plugging. It is these apparatuses, Kastner says, that justify monitoring outside of routine maintenance from an economic standpoint. "If the operator lacks experience with heat exchanger maintenance, heat exchangers are sometimes oversized by a factor of two," Kastner knows. It would make much more sense to include maintenance in the engineering process - even such simple measures as dispensing with U-tubes considerably expand the possibilities for cleaning. ●