Exchanger Tube Coating
History of Small Tubular Coating Technologies
Interior tube coating, as we know it, began in Europe in the mid-twentieth century, using a “flood and drain” method to coat tubes with phenolic materials. This remained the only option until the mid-1980s, when spray applications were first tried.
Using air pressure to atomize material, tubes were coated with a spray application of engineered polymers. This technique proved to be much simpler and less time-consuming than the fill/drain/rotate method prevalent at the time. With the development of this patented method, Curran International changed the industry standard for tube coatings, applying the polymer to the full circumferential inner diameter (ID) of a small tube with a uniform, homogenous layer of material.
This system was first demonstrated in the U.S. at a Florida power utility, where through wall corrosion in the piping had prematurely shortened the life of a steam condenser. The utility, together with the Electric Power Research Institute (EPRI), began to research epoxy coatings for tube IDs.
The study verified the efficacy of tube coatings, which proved less susceptible to fouling compared to uncoated tubes. Tubes with coatings also held up under sustained heat transfer better than uncoated tubes.
The economic benefits rapidly gained traction when petrochemical refineries applied thin film coatings to the tube id’s in its heat exchangers. Refineries with “bad actor” bundles found that thin-film polymer coating eliminated normal maintenance as a result of reduced foul attachment and corrosion deposits. This in turn improved operational efficiency and favorably impacted profit margins.Curran International patented its spray applied coating system in 1996. Its many clients include the major refineries and power-gen plants around the world.
Thin Film Coatings
Thin film polymer materials, applied at <10 mils or 250 microns, provide a foul release suited for all water services. The application technique achieves a homogenous film, and complete coverage is ensured through 100% inspection according to NACE standards.
Coatings have proven to reduce fouling and boundary layer drag. By promoting flow, backpressure is reduced and the long-term optimal performance of heat transfer equipment is sustained. Coatings are chosen according to the service temperature and conditions in which they are to be applied, as well as where – in the field or in-shop.
Application capabilities include polymerized thermosets and thermoplastics, thick film epoxy cladding, ceramic and glass filled linings, rubber and sheet linings, and inorganic “metallized” linings.
Thermal conductivity
Decades of service history and studies have proven that coatings can significantly improve thermal conductivity and overall performance. In operating service the laminar flow-down tube of cooling water leads to low shear rates, allowing deposits to form on tube wall. With coating, the surface tension of a new carbon steel tube is reduced by 30 times, decreasing micro and macro fouling. Coating also reduces friction that causes boundary-layer drag, and substantially opens up the flow profile. Thermal performance and tube-flow rates are also enhanced as a result of tube ID coating.
Bottom line
Application of polymer coatings to the ID of pipes in heat transfer equipment can increase heat transfer duty, eliminate corrosion, and reduce or eliminate micro and macro fouling. Additional benefits include a reduction of normal maintenance and improving the “useful life” of fixed assets.
Some chemical exposures, elevated temperatures or high fluid/gas velocities can limit coatings, but great benefits can be achieved by applications within acceptable temperature parameters and exposures appropriate to the applied coating.
The many advantages of coating tubular systems include cost savings that can amount to millions of dollars per year, perpetual equipment life, energy savings, throughput efficiency gains and savings in comparative cost of entirely replacing tubing with expensive steel alloy pipes. With coatings, your pipes never need to be replaced again; they last the lifetime of the equipment’s usage.
Thin Film Engineered Polymer Coatings
Phenol Epoxy - Amine catalyzed, ambient cured coating system, shop or field applied, for exchanger tubes in water-side operating environments to 250F (130C); dispersion application method, 8-12 mils (200-300 microns) total DFT. May be used for "in-service" equipment and "in-situ" repair of corroded condenser tube IDs.
Saekaphen (GmbH) - Heat catalyzed, shop applied thin film tubular coatings; offering strong resistance in either acidic or alkaline services, wide Ph, service temperatures to 300F (150C); flood and drain application method, 6-10 mils (150 to 250 microns) total DFT.
Novolac Epoxy - Shop or field applied, primary coating for U-tube exchanger bundles; wide Ph resistance for storage and transport application; suitable for water immersion to 260F (127C); flodd and drain application method, 8-14 mils (200-350 microns) total DFT. Coating may be applied to 60 mils (1500 microns) DFT, for channels and heads.
Curralon ® - PPS proprietary-blended thermoplastic, shop applied and heat cured; wide chemical resistance, wet H2S, HCL; immersion resistant in geothermal fluids, and acids to 400F (200C); requires bond coat pretreatment; contact Curran prior to specifying.