Monday, August 9, 2010

Air-Cooler Condensers

In many process plants, the pump alleys are covered with forceddraft, air-cooled condensers. Dozens of coolers are arranged in parallel. I have seen services where 300 mm Btu/h of condensation duty was easily handled by aerial cooling. All these large systems had one problem in common. They all tended to have higher flows through cooling banks connected closest to the inlet and/or outlet headers. The higher relative flows were indicated by both higher air outlet and higher process outlet temperatures.

The mechanism that causes this often-severe flow maldistribution is based on low-temperature dew-point corrosion, as explained below. Here is a rather common example, assuming that the main corrodants are chlorides, in a hydrogen sulfide–rich, condensing hydrocarbon vapor:
1. A small amount of ammonia is injected into the overhead vapor line to control the pH of a downstream water draw.
2. Vapor-phase ammonium chloride is formed.
3. Tube bundles nearest the inlet header tend to see perhaps 1 percent more flow than the tube bundles farthest from the inlet header.
4. Bundles seeing the lower flows have slightly lower outlet temperatures.
5. Lower temperature favors the sublimation of ammonium chloride vapor to a white saltlike solid.
6. This salt is very hydroscopic, meaning that it will absorb and condense water vapor from the flowing hydrocarbon vapor stream at unexpectedly high temperatures.

7. The resulting wet chloride salts are very corrosive, especially to carbon steel tubes.
8. A ferric chloride corrosion product is formed.
9. This metallic chloride salt then reacts with the abundant molecules of hydrogen sulfide to produce hydrochloric acid and iron sulfide.
10. The hydrochloric acid may then continue to promote corrosion.
11. The iron sulfide (or pyrophorric iron) accumulates as a blackish-gray deposit inside the tubes.
12. This deposit further restricts vapor flow through the lowflow tubes.
13. The reduced flow causes a lower tube outlet temperature.
14. The lower tube outlet temperature promotes higher rates of salt sublimation from vapor to a corrosive fouling solid.
Meanwhile, the air-cooler bundles nearest the inlet header tend to see a greater and greater percentage of the total flow as the cooler bundles foul and plug. They tend to stay hot and clean, and those bundles farthest from the inlet header tend to run cool and dirty.
It is a general principle of heat exchange that low flows tend to promote fouling and fouling promotes corrosion. The corroded, fouled heat-exchanger surface retards flow and creates a vicious cycle. We will see this problem again in shell-and-tube heat exchangers, as discussed in Chap. 22.