Fouling mitigation in single phase services using hardware technologies
In this series of articles we will look at how heat exchangers foul, how to understand the root causes of fouling, and how to mitigate the impact of fouling. The material presented is based entirely on the author’s experience and analysis of operating situations in the Oil & Gas industry. However, many theories and varied experiences exist across the industry and amongst researchers.
By Himanshu Joshi – Heat exchanger specialist
In this third article of the series we will take the information from Parts 1 and 2 and see how to combine it with process conditions and feed testing to determine the dominant (or most likely) fouling mechanism. Once we know the mechanism, we will take the next step of determining the best mitigation strategy.
The first three articles showed how and why heat exchangers foul and how to know the fouling mechanism. However, fouling is not inevitable, and it doesn’t need to be accepted as part of normal operation. There are many methods available to reduce the rate of fouling (i.e., how fast it builds up), and in some cases bring it down to near zero. This part will look at technologies which can be used to reduce fouling in operating heat exchangers, with single phase liquids or gases. There is a cost to each of these methods, and in one of the future articles we will see how to analyze the cost vs. benefit of a mitigation method.
Use of these methods may or may not attack the root cause of fouling; we are only looking to minimize the fouling rate in the heat exchanger. Some are applicable to the tube side only, while others can be used on both sides. Some of these techniques can be implemented during the original design while others will be retrofits once the magnitude of the fouling problem is known. This decision is affected by cost – budgets available at the design stage vs. a justification based on observed fouling.
The different technologies are described below briefly and presented later in tabular format. Mention of any specific supplier has been avoided, to promote a general understanding without a recommendation. Which technology applies to a situation depends on many factors including the severity of the problem, the cost of the modifications, the expected improvement, whether there is a recurring cost, timing of implementation, previous experience at the site, and a few others.
Each problem should be considered individually and thoroughly consulted with the technology supplier. Remember from Part 1 that fouling in single phase services occurs due to deposition of particles, some of which may undergo a thermal conversion to cokelike material. The rate of deposition is controlled by fluid shear at the wall and the attraction between the tube surface and the precursors.
Inserts act in a couple of ways – either by disturbing the boundary layer with their own movement to minimize deposition, or by changing the flow pattern at the wall which moves fluid away from the wall and along with it the solid precursors. If the fouling precursors are relatively large particles, such as catalyst in a slurry, inserts are unlikely to be effective and the particles may even stick to the insert wires.
The first type may look like Fig.1. The stretched spring vibrates as shown by the arrow and disturbs the flow at the wall to create the effect of increased shear stress. A typical fouling rate reduction of 50% can be achieved with these inserts. They do require a minimum velocity to be effective because the flow drives the motion of the spring, and they do add pressure drop. For the latter, a rough estimate is an extra 0.5 bar for a 2-pass tube bundle and 1.0 bar for 4-pass.
The second type may look like Fig.2. The dense wire matrix is designed to promote radial mixing of the fluid, carrying material away from the wall, as shown by the arrow, in effect minimizing the time available for a solid particle to deposit. A typical fouling rate reduction of 70%+ can be achieved with these inserts. These inserts also add pressure drop, of the order of 2X to 3X on an equivalent shear stress basis. But there is a large variation depending on the matrix density and other design factors, so only the supplier can give you an accurate number.
Tube bundle vibration
A technology is available to induce high frequency but exceedingly small amplitude vibration to the tubes, which prevents deposition. This is done with transducers attached to the stationary tubesheet, which connects to all the tubes. The transducers require electric power to be available at the heat exchanger.
The effectiveness of this technology is dependent on the flow velocity. Fouling rate reduction of the order of 90%+ can be achieved if a minimum velocity can be maintained, but below that velocity the effectiveness could be as low as 50%. There is no impact on pressure drop, and the technology will reduce fouling on both sides of the tube.
Tube surface attraction (“stickiness”)
A change in tube metallurgy (e.g., from carbon steel to stainless steel) or applying a coating on either side of the tube, can decrease the tendency of the precursors to stick to the tube surface and thus minimize the rate of deposition (on both sides). Neither technique has an impact on pressure drop, but the coating may introduce a small amount of heat transfer resistance, of the order of 2-3%, which is usually inconsequential.
The effectiveness of these technologies is dependent on velocity and varies from 50% to 90%+, being higher at higher velocities.
In the author’s experience, two important aspects while trying to mitigate a fouling problem can be easily missed, as follows.
Some of these solutions may work for a given situation while others won’t, but one of these is highly likely to work for even the most severe situation. It is best not to dismiss a method based on perception of what the fouling might be. The suppliers are the best source to advise on where a method may or may not be applicable, and that advice should be combined with your own experience, and consultation with a specialist. For example, there is successful experience with the spring-like inserts in residue service, something that may be difficult to visualize.
When dealing with the supplier, provide them with complete information, so they can make the best recommendation. This could include actual operating conditions (flows and temperatures), expected fluctuations in those conditions, history of the heat exchanger including mechanical failures, current and desired durations between cleanings, and availability of pressure drop. For example, as stated above, most of these techniques work better at higher velocities, so if the unit runs at lower flow rates for a significant amount of time the supplier might be able to design differently or provide alternatives.
Catch up on fouling focus!
Have you missed the earlier installments of this multi-part series? All articles are available in our online archive: https://heat-exchanger-world.com/category/technical-articles/
Upcoming in parts 5
In the next article we will continue the topic of fouling mitigation, looking at two phase services, geometry changes to the heat exchanger, and changes to operating parameters such as velocity, temperature, and vapor generation.