FOULING IN HEAT EXCHANGERS

 

Fouling is the formation of unwanted material deposits on heat transfer surfaces during process heating and cooling.

It occurs in all industries and most heat exchanger designs, with impacts ranging from heat transfer degradation to flow resistance and pressure drops. By forming a kind of insulation on heat-transfer surfaces and reducing heat transfer, fouling undermines heat exchange efficiency.

In this article, we will outline:

• The costs, types, and causes of fouling
• How to detect fouling
• How to clean your heat exchanger
• How to prevent fouling

COSTS OF FOULING

Fouling prevention for heat exchangers is typically focused around the heat exchangers itself, however the fouling and heat exchanger performance can be affected by system characteristics that are present both before and after the heat exchanger. These characteristics can include varying product properties, product handling prior to the heat exchanger and operational performance of other equipment, such as pumps, valves and back pressure.

~ Melissa Fryer, Sanitary Heat Transfer Business Development Manager, Alfa Laval

In short, the costs of fouling are many and varied.

• Production inefficiencies from pressure drops and poor heat transfer
• Maintenance time and equipment for removal of heavy fouling deposits
• Cleaning equipment & chemicals
• Hazardous cleaning solution disposal
• Reduced service life and added energy costs
• Production loss from shutdowns
• Replacement of plugged equipment
• Decreased regeneration efficiency

• TYPES OF FOULING

  The most commonly occurring types of fouling and aging in hygienic processing fall into four main types:

  • Incrustation: the accumulation of a crust or coating of processed fluids, minerals, or cleaning agents on the surface of heat exchanger parts.
  • Scaling: a type of incrustation caused by calcium carbonate, calcium sulfate, and silicates.
  • Sediment: comes from corrosion products, metal oxides, silt, alumina, and diatomic organisms (microalgae) and their excrement.
• Biological growth: Sources of biofouling include bacteria, nematodes, and protozoa.
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CAUSES OF FOULING IN HEAT EXCHANGERS

Several variables contribute to fouling, including water pH, product viscosity, and the roughness of component surfaces, among many others.

Together, the variables can be expressed as a fouling factor that numerically represents resistance to heat transfer — or thermal resistance — in your system.

KEY FOULING FACTORS

Dairy applications include fats, sugars, and proteins that contribute to fouling. In food and beverage applications, particulate matter can clump and clog, and in pharmaceutical applications, cosmetic and pharmaceutical particles produced during processing can stick and accumulate. To help control fouling, operators pay attention to four key processing factors:

• Fluid velocity
• Fluid temperature
• Fluid chemistry
• Materials of fabrication

FLUID VELOCITY

In most cases, fouling decreases at higher fluid velocities because increasing flow velocity increases the fluid shear stress, which causes more removal of deposits.

For more substantial deposits, increasing the flow velocity beyond a particular point may not decrease fouling significantly. In the case of very strong deposits, increasing flow velocity may not have any effect.

FLUID TEMPERATURE

Water can produce scaling from minerals such as calcium carbonate (CaCO3). Salts deposit on the heat exchanger surface increases with an increase in temperature.

Similarly, biological growth can occur with an increase in temperature during food processing. Finally, proteins and fats in food products can burn if the temperature difference between the hot product outlet and the hot medial inlet is too significant.

FLUID CHEMISTRY

During milk processing, calcium phosphate and whey protein can build up on heat exchanger surfaces. The fouling causes pressure drop increases by reducing the area in which the product can flow. In dairy products generally, proteins, fats, sugars, and minerals can come out of solution and deposit on heat exchanger surfaces and foul channels.

If chemical changes occur within a fluid, the difference can result in a layer of fouling on tube or plate surfaces. For example, suspended salts may harden with an increase in temperature.

MATERIALS OF FABRICATION

To prevent corrosion fouling from layers of thermal-resistant material, select units fabricated with corrosion-resistant stainless steels and alloys. Plates, tubes, and entire heat exchanger units may be fabricated with AISI 304 or AISI 316L stainless steel.

For more corrosive, high-salt products, parts of units may be manufactured with titanium, stainless steel alloys, or Super Alloys™ AL-6XN® and Hastelloy® C-22

Credits: 

Melissa Fryer is the Sanitary Heat Transfer Business Development Manager for Alfa Laval’s Food Heat Transfer team. Melissa received her B.S. degree in Chemical Engineering from The State University of New York at Buffalo, and brings over 25 years’ experience in specifying, calculating and troubleshooting heat exchangers in the food, beverage and dairy market. Melissa focuses on engineering solutions to meet customer needs and possesses extensive knowledge of and passion for this industry.