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Category: Heat Transfer Technology
Question: How can I prevent this heat exchanger from fouling during turn down? Do you know of a good information source for phosphate solubility versus temperature and fluid velocity?
Keywords: fouling,phosphate,heat,exchanger,shell,tube
Answer: BACKGROUNDA shell-tube heat exchanger (double pass) has cooling water @ about 110-115 0F and weak nitric acid containing some organics @ about 155-160 0F. We treat our cooling water to prevent both phosphate and carbonate scaling. When we pulled this heat exchanger out of service, we observed severe fouling on the outlet side of the cooling water, but very minimal fouling on the inlet of the cooling water. Although our cooling water chemical treatment could use some improvement, I suspect that most of the fouling is due to the fact that when we operate our plant at reduced production rates, we also operate this exchanger with minimal weak nitric velocities. In order not to under cool the weak nitric, thereby causing any organics in the fluid to solidify, we throttle back our cooling water velocities. Now, because the cooling water is moving at 50-75% less than normal operating velocities, and still heating up, we believe lowers the solubility of phosphates in the cooling water, thereby depositing phosphate scaling at the outlet side of our exchanger.RESPONSEI agree completely with your assessment of the situation (this happens often). I see a description of a calculation method for solubility in a book entitled "Properties of Liquids and Gases" by Robert C. Reid (McGraw-Hill...this is a fairly common book). The calculation procedure requires quite a bit of knowledge of the solid substance and frankly, I'd feel better with experimental or practical data (as is mostly the case), but I don't recall seeing anything like that in the past. I'm assuming that you've already eliminated the possibility of removing the set point for the water flow and allow the flowrate of the water to remain the same during scale-down. This would certainly keep the water at a temperature low enough to prevent the deposits. Is this not an option? Or you could always trial and error the water flow down to a rate where the deposits are minimized and then I'd recommend recording that water outlet temperature for future use.


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