How to Mitigate Fouling in Heat Exchangers Using Hardware Technologies
In our previous three articles, we explained the causes of heat exchanger fouling and how to identify the fouling mechanism. However, let's debunk the myth that fouling is an unavoidable part of normal operation. There exist various strategies that can mitigate fouling in heat exchangers, and even, in certain instances, almost eradicate it. This article will delve into the technologies that can minimise fouling in functioning heat exchangers, which handle single phase liquids or gases. Each of these methods comes with a price tag: in one of the coming articles we will examine the costs and benefits of applying these solutions.
These techniques might not necessarily target the fundamental cause of the fouling, but the aim here is to reduce the fouling rate within the heat exchanger. Some of these are suitable for the tube side alone, while others can be applied on both sides. Certain methods can be implemented during initial design, while others are put into action as retrofits upon realising the extent of the fouling issue. The decision-making process for adopting these methods is influenced by the budget - cost and timing impact at the design phase, and cost justification based on observed fouling in operating heat exchangers. We will briefly outline these technologies and later present in a table format.
We have refrained from naming any specific providers to maintain an unbiased understanding, with no endorsements. The choice of technology that best suits a scenario is contingent upon numerous factors, including the severity of the issue, modification costs, expected gains, maintenance or refurbishment costs, the timing of execution, previous on-site experiences, and few other factors. Each issue should be considered individually and should be comprehensively discussed with the technology provider.
As we explained in the article “How Heat Exchangers Foul in Different Fluid Phases”, fouling in single-phase services occurs due to the deposition of particles, with some potentially undergoing a thermal conversion to a coke-like substance. The deposit rate is determined by the fluid shear at the wall and the affinity between the tube surface and the precursors.
Let’s look at how we can use hardware technologies to mitigate fouling in heat exchangers handling single-phase liquids or gases.
Tube Inserts
Inserts function in a couple of ways – by either disrupting the boundary layer with their own motion to minimise deposition, or by altering the flow pattern at the wall which redirects fluid, along with the solid precursors, away from the wall.
The first kind of tube insert could resemble Fig.1. Here, the extended spring vibrates as indicated by the arrow, disrupting the flow at the wall to create an effect of increased shear stress. These kinds of inserts can typically cut down the fouling rate by 50%. They need a minimum velocity to be effective as the flow drives the spring's motion, and they do increase the pressure drop. For the pressure drop increase, a rough estimate suggests an additional 0.5 bar for a 2-pass tube bundle and 1.0 bar for a 4-pass.
The second type of tube insert may look like the one depicted in Fig.2: a thick wire grid designed to foster radial fluid mixing. This pulls material away from the tube's wall, as depicted by the arrow, effectively reducing the time for a solid particle to settle. These inserts can potentially reduce fouling by over 70%. Although they can increase pressure drop roughly 2 to 3-fold based on equivalent shear stress, the actual numbers can vary significantly due to factors like matrix density and design factors, so it's best to consult the manufacturer for exact data.
Tube Bundle Vibrations
There exists a technology that applies high-frequency, minuscule amplitude vibrations to tubes, preventing material from settling. Transducers welded to the stationary tubesheet, connected to all tubes, facilitate this. However, power supply is needed at the heat exchanger.Its effectiveness is closely tied to flow velocity. A reduction in fouling rate by over 90% can be achieved with sufficient velocity. Below the required velocity, the fouling reduction may drop to as low as 50%. It leaves pressure drop unchanged and helps decrease fouling on both sides of the tube.
Tube Surface Adhesion
Modifying the tube metallurgy (for instance, replacing carbon steel with stainless steel) or applying a coating on one or both sides of the tube can curb the precursors' adhesion tendency on the tube surface, slowing down the deposition rate on the coated side.
Both methods do not affect the pressure drop, but the coating could add a small heat transfer resistance, roughly 2-3%, which is typically negligible. The effectiveness of these technologies relies on velocity, ranging from 50% to over 90% with better results at higher velocities.
Final Notes
From the authors viewpoint, there are a couple of crucial factors that can often be overlooked whilst dealing with fouling issues:
- While certain fouling mitigation techniques might be useful depending on the circumstances, others might not be. But, at least one of the methods is bound to be effective even in the most challenging of scenarios. It's not advisable to disregard a solution based on mere suppositions about the nature of the fouling. Suppliers are the best resource to guide you regarding the applicability of a method. Blend their suggestions with your personal expertise and expert advice. To illustrate, take the instance of proven success with spring-like inserts in residue service, which might be hard to imagine.
- When interacting with suppliers, provide comprehensive information. This allows them to offer the best possible advice. Data could comprise actual operational conditions such as flows and temperatures, anticipated variations in these parameters, the heat exchanger's history, including any mechanical breakdowns, existing and projected periods between cleanings, and the availability of a pressure drop. As mentioned above, most of these techniques perform effectively at higher velocities. Therefore, if the unit operates at reduced flow rates for substantial durations, the supplier could possibly devise a different plan or suggest other options.
Upcoming article
In the forthcoming article, our discussion on fouling mitigation will progress. We'll analyse two-phase services, modifications to the heat exchanger's geometry, and alterations in operational parameters, including velocity, temperature, and vapor formation.