Assumed Current Density for Anode Calculation Cooling Water HeatExchgs

[BerkeleyEng]

Friends,
I am putting together some calculations for Zn and Mg anode sizing for heat exchangers in cooling water service. We tend to operate at about 120 F, with flow speeds of 2-7 fps. Some exchangers may eventually use sea, rather than fresh water for cooling.

My question is this: What current density should I be assuming for each of the two (sea and fresh water scenarios)? I was assuming 11 mA/ft^2 and 4 mA/ft^2 assuming conditions would be similar to this paper (

http://www.stoprust.com/pdf/5/(2001...anc-shannon-richards-john-p.-lafontaine.pdf).

I want to be conservative, yet practical, ie not have a calculation that spits out that you need 30 anodes when the exchanger can only fit 9 or 10. Does anyone know of any NACE papers or their own formulas or experience that could help me out?

Thanks,
Paul
Berkeley, CA

 

[fdac]

Hmm, about 48 deg.C. Careful using zinc at higher temperatures than this, 60-70 degrees C is the normal limit for zinc because of inversion of driving potential. Higher temperatures may affect anode efficiency too for other galvanic metals.

Regarding current densities, your numbers sound about right for bare steel. You need to know the water resistivity, presence of coating/lining will of course change things by orders of magnitude.

For the fresh water option you may want to consider a small impressed current system. Make it adjustable enough and it should cope with seawater service in future too without changing anodes etc.

Good luck.

 

[unclesyd]

If my memory is functioning correctly the information is here. don't ask me where.

http://www.corrosion-doctors.org/Alphabet/A.htm

 

[strider6]

in this doc you can find current density for bare and painted steel in fresh water and seawater. (pag 20-21)

the values are higher than yours.

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_570_02n.pdf

S


Corrosion Prevention & Corrosion Control