Field Coating Heat Exchanger Tubes Reduces Corrosion and Fouling
Shell and tube heat exchanger tubing may suffer accelerated corrosion for many reasons, from process changes, waterside corrosion, or inadequate materials of construction.
The deposition of any undesired material on the heat transfer tubes results in fouling. Fouling may impact significantly the thermal and mechanical performance of the exchanges as it increases the overall thermal resistance and lowers the overall heat transfer coefficient of heat exchangers. Fouling also impedes fluid flow, accelerates corrosion and increases pressure drop across heat exchangers.
Although replacing tubing with an upgraded alloy is often considered the only option, a cost-effective solution may be the application of a thin polymer coating to prolong tube life, and in many cases, improve fouling resistance as well. Both OD and ID tube coating have been performed in the past; however, the most recent technology is focused on internal tube coating applications.
While the majority of users are in the power industry, where surface condenser tubes have been internally coated in the field, there are some chemical companies and oil refineries that have experience with this technology.
The majority of coating applications are with carbon steel tubes. Good coating adhesion is generally more difficult on stainless steel and brass, but experience has been reasonably good. The coating system chosen depends primarily on the operating temperature. For temperatures up to 250°F (121°C), an epoxy coating may be applied. The success of the coating depends on the internal condition of the tube (materials of construction, surface roughening due to corrosion or pitting, etc.). Even with new tubing, insufficient surface preparation, incomplete coating coverage, or inadequate coating thickness (or a combination of the three) can result in poor performance. Understandably, field applications generally on used tubing are that much more demanding.