Electrocoating Tubular Heat Exchangers

Question:

We have experienced some corrosion on tubular heat exchangers used for swimming pools. We tried to Teflon coat the coils but could not get a complete or uniform coating. Could electrocoat be the answer? G.A.

Answer:

All types of heat exchangers are excellent candidates for electrocoat because it provides corrosion resistance. Even though the coating is an organic finish and has a thermal exchange coefficient of itself, the coating thickness can be controlled to a thin film that can be built into the design of the exchanger or will fall within the safety factor of the design. The completeness of the coating should eliminate corrosion moving to uncoated areas. All-tubular exchangers should be able to get a uniform coating on 100% of the surface, unless the tubes are wound or placed very tightly. Tube-to-tube contact must be avoided. Fin-and-tube exchangers (like radiators) require some special considerations to achieve uniform coating thickness, which will be covered next.

The design of all types of heat exchangers needs to be looked at before even having samples electrocoated. The following are design considerations:

1. What is the internal volume of the tubes? Electrocoating is an immersion process and exchangers can float if the volume of coating displaced exceeds the weight of the exchangers.
2. Can the tubular ends be sealed during coating? Accumulation of pretreatment chemicals and/or electrocoat in the inside of the tubes can cause several problems, including bath contamination.
3. If any part of the heat exchanger is soldered, what temperature can the solder withstand? Most electrocoats require at least 350F for proper curing.
4. What type of pressure is the exchanger designed for? During the curing process, the air inside the exchanger will try to expand and will produce pressure.

If the design fits the above considerations, then the surface area to be coated is the next concern for electrocoating. Heat exchangers usually have a very high ratio of surface area (in ft2) to cubic area of the system design. Several equipment design consideration can be altered or adjusted to accommodate heat exchangers, if the system is not dedicated to heat exchangers. Some of those equipment design considerations are as follows:

1. Rectifier design must be adequate to coat the square footage in the immersion time allowed.
2. If the rectifier capability is not adequate, can slowing down the line (monorail) or decreasing cycle time (square transfer), while heat exchangers are coated, be used to increase time?
3. Anode quantity and/or location can also help improve throwing power on exchangers with closely spaced fins or tightly wound coils.
4. Additional bath circulation by eductor quantity and/or location can also help.

Working with your equipment supplier or a consultant can help with these considerations. Most custom coaters have used some of these “tricks of the trade” to accommodate heat exchangers. An article on PF ONLINE talks about a shop that electrocoats heat exchangers. You can find it here.