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Heat Exchanger Tube Coating

The fouling of heat transfer tubes is a never ending problem for almost every industry requiring heating or cooling.  This demands regular maintenance in order to physically remove deposit build-up, to restore lost heat transfer efficiency, and to prevent high energy costs.  For large refrigeration chillers, tube cleaning is performed annually on the condenser water side, while the evaporator side is typically cleaned once every five years.

Even though refrigeration manufacturers typically specify a 0.0005 in. maximum permissible fouling factor or build-up of deposits at the tube surface, deposits of 20 times that thickness and greater are common.  In its worst examples, tubes may be totally clogged – requiring their cleaning once or twice mid-season at high labor and downtime costs.  For many process applications handling difficult compounds, tube cleaning may be a regularly scheduled event.

Documented in many studies, a loss of heat transfer efficiency and flow can often be measured within only a few weeks after a tube cleaned refrigeration machine is placed back in service – this under even the best chemical treatment program and most well maintained operating conditions.

• Deterioration From Mild To Severe

In some examples, standard tube brushing may not adequately remove heavily bonded rust, scale and fouling products from the tubes – thereby requiring specialty cutting equipment.  A heavily fouled piping system, in fact, may contain many thousands of pounds of iron oxide deposits capable of dislodging and then settling into the heat exchanger.

• Deposits Equal Metal Lost

Considered more important than the loss of transfer efficiency, however, is the loss of tube metal itself.  A fouled tube will immediately suffer higher corrosion and pitting rates at its relatively thin wall surfaces – thereby dramatically limiting service life.

Today’s enhanced tubes, since they contain internal grooves or “rifling” for greater heat transfer performance, are even more susceptible to fouling and premature failure from particulate and biological attachment.  We are aware of many examples of enhanced tube failures after only 3 – 4 years of service.

For process and industrial applications, cleaning requirements are often more demanding.  Heat transfer equipment may carry extremely corrosive products and by-products which may have an affinity to either corrode, etch, or attach to the tube surface.  The failure of a critical heat transfer process may interrupt an entire production run resulting in not only downtime, but in the loss of the end product as well – often with high financial loss.  Even the use of stainless steel is no guarantee to eliminate such corrosion and deposit problems.

• New Coating Options

New advances in coatings technology and application procedures now provide a viable alternative to chiller tube replacement.  Used worldwide in the petroleum and power generation industry for over a two decades, it is possible to actually bond a thin layer of an epoxy polymer to almost any chiller or heat exchanger tube surface.

The epoxy polymer is applied on site for chiller tubes, and is an industrial process requiring approximately 5-7 days of work, depending upon equipment tonnage.  For smaller applications such as heat exchangers, it is often more economical to ship the unit to the factory for reconditioning.

• Surface Preparation Critical

Critical to this procedure is the need to first sandblast the entire length of the tube using a suitable abrasive and proprietary application system.  Done properly, sandblasting removes a negligible amount of the actual tube wall, and leaves a perfect base for securely bonding the epoxy polymer.  Sandblasting removes embedded debris and reaches the base of the deepest pitted areas otherwise untouched by any other cleaning method.

Shown in the photographs below, sandblasting removes all foreign materials to reveal underlying surface pits and deterioration.  Once prepared, two layers of the epoxy coating are high pressure applied to the inside diameter of each individual tube in order to provide a final 4-6 mil layer.  The self-leveling nature of the coating will fill the more shallow pits, although deeper pits will still exist.

• Many Applications

The benefits of such equipment reconditioning are immediate. All further corrosion activity at the tubes is eliminated as the water is then isolated entirely from the tube metal.  In most applications, the tube sheet, water box, and heads are also sandblasted and coated with epoxy.  This eliminates any corrosion activity in the area of the heads, and future maintenance concerns.

Stages Of Chiller Tube Coating And Re-Conditioning

Original Condition Borescope internal view of an original 7/8 in. refrigeration chiller tube showing deposited material and corrosion product throughout its entire length.		Cleaning Sandblasting the entire tube to an SP-5 white metal finish reveals the actual surface profile and depth of pitting into the tube surface.		Rehabilitation After coating, smaller indentations are filled, though some of the larger pits still exist.  Interior surface texture is noticeably smoother – thereby providing greater flow.  The tube metal is now completely isolated from any further corrosive effects from the water, and free from the threat of biological or particulate fouling.

• Multiple Benefits

Tube coating offers an obvious advantage to re-tubing in cost alone, and a substantial number of secondary advantages over bare copper nickel tubes. Advantages such as:

• Eliminating any further corrosive influence of the water upon the typically copper or copper nickel tube surface.  Therefore, tubes having up to an 80% loss of metal can be successfully coated, re-conditioned, and returned to service knowing that no further wall loss will occur.
• Offering a very economical alternative to conventional re-tubing. Total costs are approximately 35% – 45% that of new tubes, including coating the tubesheet, waterbox, and heads.
• The addition of a thin 4-6 mil coating produces no significant deterioration in heat transfer efficiency.  In fact, for most applications, the increase in water flow produces a net increase in heat transfer or BTU output across the tube.
• A well documented side by side evaluation of two identical chillers, one coated vs. one re-tubed, showed only a negligible loss of heat transfer by the coated chiller.  Yet within a short time period, normal deposits at the conventionally re-tubed chiller produced a heat transfer loss far greater than the coated tubes – and with the heat transfer of the coated tubes remaining constant over time.
• The service life of the epoxy polymer coating generally exceeds 10 – 12 years, depending upon application and fluid properties.  This often equals the service life of conventional tubes under certain conditions.  After 10 years, the coating can be reapplied – thereby extending tube life indefinitely.
• Tube coating seals the entire tube to tube sheet interface against the potential for galvanic corrosion due to dissimilar metals.
• The epoxy polymer will reduce the surface tension of even a new copper tube by a factor of fifty.  This inhibits the attachment of foreign debris, iron oxide corrosion products, and microbiological growths to the tube wall.
• The need to regularly brush clean or “punch” the tubes is eliminated or greatly reduced in frequency.  Maintenance is no longer necessary at the tube sheet, water box and heads.  Should it be necessary however, tube brushing will not impact or damage the coating.  Having non-fouling coated tubes means lower maintenance, uninterrupted operation, and increased savings or production related profits.
• Except for suspected metal fatigue or tube deterioration at the refrigerant side or at the internal tube supports, further eddy current testing is unnecessary – as the tube’s base metal is now totally isolated from any corrosive effects of the water.
• The slippery properties of the epoxy polymer substantially reduces tube wall friction or boundary layer drag. This results in increased flow rates through the tube by as much as 80%, and reduced pressure differentials by as much as 50%.  Greater water flow equals greater overall BTU transfer.
• In addition to refrigeration chillers, condensers and surface condensers, epoxy polymers can be applied to all types of tube and shell heat exchangers.  The coating can be applied to both sides of the tube, as well as inside the shell itself.  Formulations and application methods exist to handle high temperatures of up to 500o F.
• Approximately 70% of total heat transfer resistance exists due to fouling and to the boundary layer fluid film.  Coated tubes generally show an increase in BTU transfer rate due to the elimination of this fouling effect.
• For those applications where the installation of new chiller tubes by conventional rolling methods is either preferred or required, pre-coated tubes can be supplied to specification.
• Epoxy polymers, and other appropriate materials, can also be applied to domestic water steel house tanks, fire reserve tanks, cooling tower pans, and relatively straight runs of piping – again isolating the underlying metal from any further corrosive deterioration.  The critical step of sandblasting to an SP-5 white metal finish is always required.

• Prerequisites Necessary

Refrigeration machines of any size can be re-conditioned using this process, and it is applicable to the condenser, evaporator and surface condenser sections.  A pre-requisite is often a recent eddy current test in order to ensure that the maximum wall loss limits of the tubes have not been exceeded.  A high capacity source of compressed air is required on site. Containment of the air and supplemental dust collection is mandatory.

© Copyright 2023 – William P. Duncan, CorrView International, LLC