the exposure period. For example, copper tube/copper fin exchangers 

 coated with an electrostatic polyester coating were transferring 56% 

 more heat than the uncoated all copper unit after 2 years; specification 

 alkyd and zinc inorganic silicate coatings showed improvements of 45% 

 and 63%, respectively, over uncoated surfaces. Each of these curves 

 with its data points is shown in Figures 9 through 11. 



Data plotted in Figure 8 allow comparisons of the performances of 

 the various primary surfaces, both uncoated and coated. For uncoated 

 exchangers, Figure 8a shows that after 24 months aluminum tube/aluminum 

 fin exchangers are performing 57% better than copper tube/copper fin 

 exchangers and 32% better than copper tube/aluminum fin units. When the 

 units are coated with an electrostatic polyester coating (Figure 8b) , 

 the copper tube/copper fin units are superior to all the others. Much 

 smaller differences occurred when the exchangers were coated with the 

 specification alkyd or the zinc inorganic silicate materials (Figures -8c 

 and 8d). 



In summary, after a 2-year operation in a temperate marine environ- 

 ment at Port Hueneme, Calif., copper tube/copper fin heat exchangers 

 coated with the three different systems were found to have a higher heat 

 transfer capacity than the same unit in the uncoated condition. The 

 zinc inorganic silicate and the specification alkyd coatings were found 

 to be in better condition than was the electrostatically applied polyester 

 coating. Conversely, the application of these coatings to aluminum 

 tube/aluminum fin and copper tube/aluminum fin heat exchangers resulted 

 in a lower heat exchange capacity than that of the uncoated units. 

 Again, the zinc inorganic silicate and specification alkyd coatings were 

 in better condition than the electrostatically applied polyester coatings. 



DISCUSSION 



Although the findings give an indication of the effect of coatings 

 on the corrosion resistance and the heat transfer capacity of the three 

 types of heat exchangers, variables such as bonding of the coating and 

 coating thickness should be considered. For example, Guaranteed Products 

 Division cleans and coats only aluminum extrusions for the building 

 industry. They ran the heat exchangers with copper components in their 

 cleaning baths in a minimum time because of a fear of possible copper 

 contamination to the baths. As a result, the surface preparation of the 

 copper was apparently not sufficient to assure proper bonding of the 

 coatings. Also, the coatings applied at Guaranteed Products and at CEL 

 were not modified for use on the heat exchangers. Thus, the effect of a 

 thicker electrostatically applied polyester coating and thinner specifi- 

 cation alkyd and zinc inorganic silicate coatings is not known, and 

 needs to be investigated. In addition, an in-service evaluation of 

 coated heat exchangers in a very aggressive environment should be carried 

 out to determine their cost effectiveness in a relatively short time. 



Any in-service evaluation should take into account the fact that a 

 heat exchanger is probably overdesigned in order to compensate for loss 

 of thermal efficiency as a result of corrosion. Thus, an exchanger 



