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[Zauberberg]

Deteriorating performance of fin-fan (air) coolers is a common problem in many process plants. In applications where system pressure must be maintained constant, because of required distillate recovery, inability to achieve design condensing duty causes unit capacity reduction and intensive flaring.
According to my experience, there are mainly 3 reasons for poor performance of air coolers:

- Process-side corrosion
- Higher ambient air temperatures, compared to design values (especially in the last decade)
- Airside fouling and loss of fin-to-tube attachment

The problem becomes more interesting by knowing the fact that most of the fin-fan coolers work very well after inside and outside cleaning, which is done during turnaround. But, achieving turnaround cycle of 4-5 years is impossible. So, the questions arrive: if it is not possible to maintain process-side corrosion rates low enough, to keep the performance of fin-fan coolers at minimum acceptable levels, what should be the next step in condensation process improvement?

1) Expanding tube banks is not a solution, in systems where 10 or more tube banks are arranged in parallel. Adding 2, 3 or 4 tube banks actually aggravates the problem, since process fluid takes the path of least resistance to flow. This causes the new tube banks to run cold and dirty. I have seen the installations where this kind of installation actually reduced overhead condensers capacity.

2) If process-side corrosion is beaten by applying continuous waterwash (injecting sufficient amounts of water upstream of the air coolers, to ensure that water is always present in liquid state at the entrance of condensers, which than washes away salt deposits), a significant reduction in LMTD decreases condenser capacity. This reduction of process fluid inlet temperature must be compensated by additional surface area, which brings us back to the problem described at point No.1.

3) Adding a low pressure drop trim-cooler downstream of the existing air coolers. This arrangement poses hydraulic concerns and requires a lower operating pressure in the overhead receiver vessel. If column flash zone must be maintained to achieve the same distillate recovery, the additional pressure drop of new heat exchanger must be compensated by lower operating pressure in the overhead receiver and also a lower operating temperature - if flaring is to be minimized. This kind of installation is very frequent in industrial practice worldwide.

4) Air conditioning. By injecting sprayed water below the tube banks, air temperature approaches to wet bulb temperature, which raises LMTD significantly. If system is designed properly, all amounts of water evaporate before reaching the fin-fan tubes. However, this can be applied only for induced-draft fans (according to process designers). Forced-draft systems do not have sufficient space between the fan and tube banks, to provide required time for complete water evaporation before reaching the tubes.

My question is the following: if you have process stream consisting of 100,000kg/h hydrocarbons and 10,000kg/h steam, at the temperature of 150C and pressure of 1.6barG (fluid is on the dew point - full range naphtha), what should be addressed first in overhead system condensing capacity expansion project - knowing that process-side corrosion rate cannot be lowered further, that air-side fouling is significant and that ambient air temperature is much higher than design? Has anyone of forum members had similar experiences, and what possible scenarios need to be reconsidered when applying design changes in Crude or FCC main fractionator overhead (both are forced-draft) systems?

Any thoughts or guidelines would be appreciated.
Best regards.

[Alikhani]

being confident that hot air recycling is not happening, inceasing the fan motor speed may resolve your problem.