Combat fouling in plate heat exchanger by understanding the causes.

Fouling of heat exchangers is a significant problem in industry. How significant? The cost of dealing with fouling in the United States has been estimated at more than $4 billion per year.

How does it affect us? It robs heat exchangers of their effectiveness, causing process energy costs to rise. It increases equipment downtime, requiring expensive plant outages. And, to compensate, we deliberately oversize the heat exchangers we design, adding to the installed cost.

The common solution is to add a fouling factor and stop there. Design engineers who specify plate heat exchangers frequently will tack on a percent to the required heat transfer surface area. This is a commonly used solution, but it is limited. Oversizing a heat exchanger beyond a small percentage above the best design solution results in the opposite of what was intended; in other words, it results in a significant decrease in the velocity over the heat transfer surface, thereby reducing heat transfer.

And that’s not all. Misunderstanding the types and causes of fouling can result in time and money being spent on solutions that are ineffective.

Instead, the smart design engineer understands the causes of fouling in a particular application and acts accordingly to reduce the problem of fouling. Is it easy? Hardly. The causes of fouling are complex and interrelated. Simple solutions rarely exist, and in the end, fouling rarely can be completely eliminated. But is it worth pursuing? Absolutely. Imagine how much you could reduce operational costs if you could cut the preventive maintenance (PM) schedule in half.

There is increasing understanding of the causes and interrelationships of fouling, but there is no consensus on how to group them in explanations. This is one way of naming and explaining them, but there are others. Having said that, generally speaking, fouling causes can be sorted into these six types.

• Sedimentation. Imagine a creek with rapids and calm spots intermixed. See all the silt in the calm spots? That is sedimentation. Particulate settles out in the places of low turbulence.
   
• Crystallization. If you live in an area with hard water, look at the lime buildup on your plumbing fixtures. That is crystallization. The most common crystallization is calcium carbonate, which crystallizes out of hard water onto the heat transfer surface when the temperature rises above a certain point. Many know calcium carbonate crystallization fouling as scaling. Other materials may precipitate out (causing crystallization fouling) when the temperature drops.
   
• Chemical. This is best understood as “baking out” although there are numerous examples and special types. In processing of certain hydrocarbons it is called coking, but the best known (and probably most hated) is the baking out of milk proteins in pasteurizers. Because of the risk to food safety, this requires extremely rigorous preventive maintenance schedules for dairies and milk processors.

• Freezing. Freezing sometimes is viewed as the opposite of chemical fouling. Media enters the heat exchanger as a liquid or gas, and part of it solidifies by freezing, building up on the heat transfer surface. This frozen material affects heat transfer just as other forms of fouling do.
   
• Biological. This can be like a bad science fiction movie, where the lead actor shouts “Something’s growing in there!” In the case of many heat exchangers, that something can be mold or algae. The best-known example, however, is zebra mussel growth on equipment that uses water drawn from the Great Lakes. It was biological fouling in early shell-and-tube pasteurizers, with the resulting difficulty in cleaning, that led to the development of gasketed plate heat exchangers.
   
• Corrosion. This fouling occurs where the heat transfer material corrodes due to the action of the media flowing over it, and a layer of corrosion product builds up, providing an insulating layer. In some situations, the product that forms from corrosion is dislodged from the surface and can then re-adhere through sedimentation. What’s worse, corrosion roughens the heat transfer surface, making it easier for other fouling types to occur.

To address the issue of fouling, no matter the type, first ask yourself the right questions. This will point you toward the best ways to reduce fouling in your application. Important questions to answer include:

• What is the media?
   
• If it is water, what is dissolved in the water? Do you have a water test report?
   
• Is there particulate in the media? What size and concentration of particulate are present?
   
• What are the design temperatures and flow rates?
   
• What are the actual temperatures and flow rates on the existing heat exchanger?
   
• What did you find in the previous heat exchanger, if this is a new installation?
   
• What is the source of the media?
   
• What media treatment currently is being done?

Once you have a good grasp of the factors that may contribute to fouling, look to some combination possible solutions. Because usually multiple types of fouling are present, and they interact with each other, use the following proposed solutions only as a rough guide. In most cases, there are going to be two sets of possible solutions. Look into the first when you select or design the heat exchanger. Examine the second for the preventive maintenance plan. Once you’ve developed a preliminary plan, consult with experts to be sure of an optimal solution.

Possible solutions you can design into your engineered system include:

• Heat exchanger plates designed using computer modeling to minimize dead spots.
   
• Elevated pressure drop through the plate gaps compared to the pressure drop through the distributor.
   
• Reduced temperature differential between the two media.
   
• Lower operating temperatures.
   
• Elevated Reynolds numbers to maintain minimum shear stress levels.
   
• Smoother material surface for the heat transfer surface.
   
• Filtration upstream of the heat exchanger.
   
• Backflush system.
   
• Electropolishing of the heat transfer surfaces.
   
• Installation of a duplicate heat exchanger to allow one to continue to operate while the other is being serviced.

Possible solutions once the heat exchangers are installed include:

• Biocides in the media.
   
• Chemical treatment of the media.
   
• Clean-in-place (CIP) cleaning of the heat exchanger.
   
• Disassembly and cleaning of the heat exchanger.
   
• To shorten operation downtime, have a fully inspected set of gasketed plates on hand.
   
• Send the plate pack or complete heat exchanger unit to your service location for surface cleaning, dye penetrating and regasketing.

Understanding the causes of fouling in a particular application can help determine appropriate measures and reduce the crippling effects of fouling in an application. If you are not sure what type of fouling is prevalent in your application, contact the manufacturer for assistance.

Links

• GEA PHE Systems North America Inc.