Air Cooled Heat Exchangers 2

[DMcMahan]

We have several banks of Air Cooled Heat Exchangers (FinFans). They are used to control the temperature of a closed loop tempered water system. They are designed for 11MMBTU heat removal per bank. They've been in service for over 20 years. They are installed approximately 15 feet above grade and installed over gravel/clay surface. Bottom line is they foul externally very rapidly. The dirt/clay etc gets packed in the fins. In the summer time during the hot weather our ops teams actually spray water from the top down into the fins to increase heat transfer, so we also battle with things like calcium deposits. We have been high pressure water blasting the fins twice per year for the last several years in efforts to clean them. This is not very effective particularly for the internal fin tubes. Does anyone have suggestions for a more effective cleaning method for this type application? Thanks

 

[Montemayor]

DMcMahan:

The use of air-cooled condensers and coolers in the Texas Gulf Coast is very common. They are usually installed 10-15 feet above a gravel/shell or stone grade. Under these conditions and either induced or forced air fans, the fouling is minimal. However, if you have an under-sized or over-loaded cooler bank subjected to cooling water spray in order to makeup for deficiencies or to obtain more duty, then you are creating your own fouling and will pay the tradeoff. The tradeoff is the dissolved solids deposited on your finned tubes that are now aggravating the situation. Of course you have "calcium" deposits. That's from the water that you are not supposed to be applying to the units. These units under no condition were ever designed to handle an external water spray. The solids deposited on the unit are the result of trying to make the unit do what it wasn't designed to do.

There are very few things that will foul an air-cooled unit under normal conditions. This is nothing more than an analogous situation like a home air conditioning condenser unit - except it is not at ground level. The air conditioner unit gets more fouling from grass cuttings and leaves; yet it is routinely cleaned up by blowing air across the finned tubes. Once you've deposited calcium solids on the tubes you're in bad trouble. The manual or chemical cleanup is going to be very expensive - maybe prohibitive. The external water spraying should never have been allowed to take place. After 20 years of service, you may have no justification for attempting to clean them up and may be forced to simply replace the units.

For anyone contemplating the external spraying of cooling water (even treated) on air units, please bear in mind the consequences of removing the inevitable deposits from the water later - you may not be able to do it effectively. Air units should be operated with cooling air only - the way they were designed to operate. Air-cooled units do not normally foul with air to the point where they become inoperable or uncleanable.



Art Montemayor
Spring, TX

 

[DMcMahan]

Agreed 100%. The ops teams are programmed to do what it takes to safely get pounds out the door. This was their solution. A short term fix to sacrifice longer term viability.

 

[speco]

DMcMahan,

Art is right on target, as always.

One point of clarification: Some kinds of finned tubes can be effective cleaned with a fairly high-pressure water spray. Extruded fins are generally much tougher that helically wrapped fins. I have even seen people walking on them, but this is not recommended. They are also better for a salty air environment, since they are much more resistant to corrosive attack, which can eventually cause helical fins to break and loose all contact with the tubes.

If your finned tube bundles have L-footed, helically wrapped fins, then cleaning is a bit of a problem because the fins are rather fragile. If you do attempt to carefully wash them down, it is best to do it against the direction of air flow to keep from forcing the dirt farther into the tube bundle.

Given the age of you coolers, you might want to consider retubing them, especially if you have a very high fin density (say more than 10 fin/inch). You might also want to check the original design for capacity.

Here is a link you might find useful:

http://www.stoneprocess.com/whydoesntmycoolerwork.htm

Regards,

speco

 

[jay165]

This is gong to sound like a smart-ass answer, but I am serious. Have you looked at converting to water cooling with a small cooling tower or packaged evaporative coolers?

Unless your make-up water cost is very expensive, whenever I've looked at true life cycle costs (which don't include waer spraying), the water cooled system beats fin fans every time, especially if you gain economic benefits by having a cooler process stream via lower approach temps.

 

[Montemayor]

jay165:

I would certainly hope everyone reading your post takes it as a serious engineering solution. I highly endorse this type of option analysis when contemplating an air-cooled unit. I failed to mention in my prior post that an evaporative cooler is, in essence, what the existing unit has been "converted" to - except that it doesn't have the proper excess water spray that ensures that solids won't be deposited on the hot tubes' surface.

An evaporative cooler will, indeed, yield a much closer temperature approach and if this is desirable then this becomes a very credible option to an air-cooled unit.

I have a lot of respect for the Operations people in any plant; they often have to do things on their own not because they are a rebellious lot, but rather because their process support team is either not listening to their problems or they are too busy with other things. Operations has to put product out the front gate - it's the fuel that drives everyone's paycheck. They may not have the time or opportunity to communicate at times, but they are trying to do the best they can with limited resources.

Your recommendation of an evaporative cooler type replacement might be just the ticket they need due to a need to cool the tempered water as much as possible. However, I would suggest to DMcMahan to discuss this with Operations in detail, especially noting the trade-offs that exist also with evaporative coolers. Additional water treatment and instrumentation as well as additional pumps will be just some of the immediate changes and added responsibilities for Operations. There are no free lunches, but there certainly is a solution to fouled air coolers and colder tempered water. Depending on economics and the need, this might be the way to go.

Art Montemayor
Spring, TX

 

[rmw]

Air conditioning technicians use a weak acid based fin cleaner for outdoor condenser fin cleaning in residential and commercial applications in my part of the world. If the acid is compatable with the metallurgy of your fins, it might be a way to get some 'deep' cleaning that just water blasting won't reach. I have used it, and it works good in air conditioning applications. Talk to one of them.

rmw

 

[smckennz]

There are countless fin tube coolers that are water sprayed.
For heat dumping duty, fin tubes have normally been selected because cooling water is too expensive.
The price of water may have changed since the installation was originally proposed. In that case a cooling tower may now be viable, but I doubt it.

My suggestions:
asphalt the area beneath the tower and the immediate surroundings. If the capital budget wont stand it, look at using waste oil.
Look at the fin bonding system. If the fins are "L" type tension wound, then the damage is probably already done as they will have separated from the tubes due to thermal shock from the water spray. Your only hope is to keep them wet to provide a thermal linkage between tube and fin. Look at removing Ca by RO; chemical precipitation is normally very expensive. If the fins are the embedded type (groove in tube and fin peened in) then you may be OK by a one-off HP water blast, possibly preceded by a citric presoak, and RO cooling water spray plus dust supression measures.
For the inner banks you will have to make up special spray lances; unless someone knows of an easy way.

At the risk of being shouted down, I would look at an oil wash of the fins; it doesnt help heat transfer but will make subsequent scale removal much easier.
Finally, budget for more cooling surface. If you have more cooling surface, you wont need to wet it, dust wont stick, and the problem will go away.

Cheers

Steve

 

[aberta]

To add to your suggestion, consider heat transfer and cooling as a two way street. The inside of the tubes must be clean as well as the outside. we clean the inside of tubes as well as outside. Our fins are strong enough for chemical cleaning the outside. The inside is HP water blasted.
Regards.

 

[DMcMahan]

Great responses from everyone. Special thanks to rmw. We contacted some HVAC folks about what they use to clean their condensers. They put us in contact with a vendor that came in to evaluate our system. The vendor had never done anything of this magnitude, but we decided to give it a shot. Long story short, we just came out of a maintenance shutdown. Working with the vendor and using a standard pressure washer you could buy at Home Depot we washed the fins from the top, followed by a fire hose rinse. We are back up and running. Comparing before and after Uoa, we have seen a 58% increase in efficiency. Ambient temperatures are up in the 90 deg F range and we still have fans down and are using no water deluge. Thanks.