Figure 2. After similar period, coated tubesheet shows minimal fouling.
Chemical treatment. Biocides or oxidizing agents can be used on the tubes to control biological activity. After a biocide treatment, application of biostats, a milder form of chemical control, can control future growth. In recent years, chemical treatment has fallen out of favor due to the toxic nature of the substances used.
Retubing. If severe pitting can’t be alleviated, it may be necessary to resort to complete retubing of the heat exchanger, particularly in older equipment that has run for years. Choosing a more appropriate tube material or accelerating maintenance can stave off recurring fouling and corrosive conditions; however, retubing is extremely costly and may have to be done several times to prolong the working life of the apparatus. The bare tube surfaces will inevitably sustain pitting and bacterial build-up as long as they remain uncoated.
None of these options is as viable or cost-effective as coatings.
What about heat transfer?
Decades of service history and studies have proven that coatings can significantly improve heat transfer and overall performance. While the thermal conductivity of the coating alone is less than that of the metallic parent tube, this is offset by several factors.
The first is normal design margin. Generally, heat exchangers are designed with a fouling factor of 0.001 or 0.002 ft2°F/Btu. Adding a coating to the tube ID impacts the thermal duty by only a factor of 0.0006 – 0.0008 ft2°F/Btu the same at fully dry film thickness. Moreover, the coating can either totally eliminate the subsequent fouling or greatly reduce the accumulation of typical micro- or macro-fouling, thus enabling greater heat transfer.
The second major factor is boundary-layer drag reduction. About 70% of the total heat-transfer resistance of an exchanger is a result of boundary layer drag. Tube wall friction due to fouling reduces designed flow and creates an insulating barrier of low velocity fluid. Polymer coatings cut the friction at the tube wall by a factor of 40 compared to bare metal. Less friction decreases the boundary layer drag and substantially opens up the flow profile.
Data from two refineries showed flow rate improvements of 80% and 100%, respectively, in coated tubes compared to new “bare” tubes in the same fluid train. This increase in flow and the low surface energy of the coating contributes to the improved overall thermal efficiency of the heat exchanger.
Coated tubes have maintained 100% of their heat transfer efficiency over years of service without cleaning cycles. One five-year study that took monthly data on water- and process-side temperature differentials showed that coated heat exchangers remained at optimal thermal duty over that period while bare tubes saw performance drop by more than 50%.
An economical proposition
The cost of coatings is easily only one-quarter that of retubing. Once the tubes are recoated after their first 10-year period, they remain functional in perpetuity, requiring minimal maintenance for the rest of the life of the heat exchanger. Over 12-years, the savings from coating the tubes in a single large heat exchanger could exceed $8 million.
At one refinery, six heat exchangers in a catalytic cracker recovery unit’s refrigeration section required maintenance. They weren’t operating efficiently; examination showed severe tube corrosion and pitting. Two of the six exchangers needed complete retubings due to age and damage over time. The four remaining exchangers had only been operating for three years but still had telltale wear and tear, corrosion and pitting.
Refinery management opted to apply coatings to all six exchangers to prevent further damage and to decrease fouling from sulfate-reducing bacteria. By coating all the equipment, ongoing preventive maintenance would suffice to reduce stoppages, repairs, replacements and the need for any retubings — and the unit would see better performance from the equipment in the refrigeration section.
After retubing and coating, the coolant fluid pressure in the two older heat exchangers dropped 10% from its previous 230 psi to within a steady range of 190 to 200 psi. The additional cooling eliminated all gas recycling and kept the unit at a 96% recovery rate, even in the hottest summer months, which amounts to an extra 1,000-bbl/d output. In contrast, the regular and long-duration cleaning cycles previously required caused lost production of 10,000 bbl/d. During their first three years of “bare” pipe service, the four younger exchangers experienced an increase in pressure drop of 15 psi per year. Once the tubes were coated, the pressure performance stabilized.
The refinery expects a 10-year minimum coating life for the exchangers, given some minor tubesheet touchups during maintenance periods. After a decade, the tube bundles may need to be grit-blasted and possibly recoated, but the life expectancy of the heat transfer equipment is conservatively expected to exceed 20 years and the maintenance required is minimal compared to the bare pipe alternative.
The bottom line
The methodology of tube coating is well proven and many of the world’s largest companies rely on it. There are now more approaches to produce the desired outcomes and reduce the losses incurred through inefficient heat transfer. The best way to start is to consider the various conditions and identify the most efficient method to clean and coat the tubular systems.
By taking care of the small details — paying attention to your tubes — you can eliminate unnecessary maintenance, cut energy costs and enhance operational efficiency over the lifetime of your heat transfer equipment.
Making the most of coatings
The inherent passivation of nickel and chrome/moly alloy materials in cooling water provides significant advantages in corrosion resistance. However, carbon steel often is chosen for exchanger tubing because it’s the least expensive material available, only about a quarter of the cost of materials like admiralty brass, 70/30 or duplex stainless steel.