Expanding Fin-Fan Condenser Capacity

[EmmanuelTop]

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), 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 scenarions 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.

 

[MikeInSWLa]

We have had a similar, but not exactly the same, issue in our plant, where we have two towers- one a distillation column and one a steam stripper, that have fin-fan condensers that have become bottlenecks.

The difference is that yours sounds like it is a partial condenser, and you cannot condense everything you need to (that's what i take the flaring comment to mean). Ours are total condensers, ans we need to achieve a certain degree of subcooling, as we are condensing two liquid phases and we need lower temperatures to reduce the mutual solubility of water and hydrocarbon (actually alcohols).

By forced draft I take it to mean the fan is below the tube bank. We also have forced draft. We went in the direction of your option #3- adding a trim cooler downstream of the fin-fan. Last summer (when we have the most problem, obviously) we tested the idea by renting a unit consisting of a shell and tube exchanger with a water chiller (50 F) water. This unit allowed us to reduce our overhead liquid temperature to improve our phase separation. We may proceed with a permanent installation based on the results of this test.

We tried option 4, but only in a very crude manner. We did not have a very good system for distributing the water (we only used hoses), and so i don't believe we got much of an evaporative effect. As you point out, you need to be careful and make sure you evaporate all of the water so you don't corrode the outside of the tubes.

Another thing we considered was increasing the pitch of the fan blades to increase air flow. Our fin-fan vendor took a quick look at this and indicated he didn't think we'd get more than a few nmore degrees of subcooling, but we haven't tried it. In our case there wasn't much room to increase the pitch. However, in cases where there is more latitiude it might be worth considering.

Regards,

Mike

 

[lastone]

1. Spin your fans faster. Bigger motor with a VFD which controls column pressure.
2. I think I read something once about someone that temporarily reverses the fan direction on some frequency to blow dirt off the outside of tubes (for induced draft fans).
3. You can hire a chemical cleaning company to come out and spray the tubes with a cleaning agent that removes grime and dirt.

 

[reena1957]

Dear EmmanuelTop, There is another option:
I have seen a unit where there is a setup as described by you with the first set of air cooled condensers with a flooded receiver used only for hot reflux and the excess vapors routed through a water cooled condenser and a second receiver where the pressure is controlled for the system. Excess Liquid from the first vessel overflows into the second condenser inlet line along with the vapor. This option reduces the second condenser duty considerably. The hot reflux also reduces the inside pipe fouling of finfans. You have to take care of only the water condenser fouling. You have to consider the cost of additional vessel and reflux pumps vs a full capacity trim condenser and choose the better option.

 

[reena1957]

Dear EmmanuelTop,
There is a good article in Hydrocarbon processing July 2006, Vol 85.No.7. page 89 titled Optimize air-cooled heat exchanger by N.Agius. May give you some ideas to improve performance of the the air condendensers, along with your mechanical counterparts.
Best wishes

 

[0707]

The technique bellow can be applicable to optimise air exchangers cooling in summer times


“Water-Assisted, Air-Cooled Heat Exchangers
In Combin-aire cooling, high temperature ambient air is first pre-cooled by direct contact with water, the cooled air then becoming the low temperature cooling medium for fin-tube elements. Cooling of air, and subsequent use of the cooled air for fin-tube cooling of process streams, are effected in one integrated structure — the Combin-aire.
Cooling with Combin-aire to overcome the occasional limitation of high ambient temperatures has the following advantages:
· Absolute minimum water consumption consistent with attainment of low terminal process stream temperatures.
· Water circulation may be made automatically responsive to air temperatures with no water circulated except at high ambient temperature. No water treatment necessary. Salt water or brackish water may be used.
· No spray carryover or condensation. After passing across the fin-tube elements the heated air leaving the Combin-aire at elevated temperature is under-saturated with water vapor and cannot precipitate condensate or carry water spray.
· Minimum piping. Combin-aire may be installed immediately adjacent to other process equipment.
· Clean air to fin-tube units. During the off season air is washed, removing dust, sand and insects.”

http://www.hudsonproducts.com/products/combinar/index.html