DX Evap Tubes

[socal13]

What are the causes of tube failure in a DX shell and tube evaporator ?

[jimbob73]

Improper water treatment, Excessive water flow, and Freezing are the most common causes of tube failure.

[safa]

Air in your chilled water circuit is also corrosive:eek:

[jogas]

We had a chiller that had the evap tubes expanded beyond the tube sheet.
5.5 years old and they cracked. Six months out of warranty and NO help from the manufacturer.
Jogas

[socal13]

This chiller has a 1/4 " per foot drop. At 18 feet long is this going to cause a problem ? Or is this acceptable ?

[ACCMan]

Are you sure the tubes are the problem?
Many leaks occur at the tube sheet where the tubes are "rolled" to seal at the tube sheet. Another problem area, if you have two circuits, is the gasket between the two circuits leaking as one system runs and the other is off.
If the chiller is a "open system" and has some age you may experience a freeze problem due to sediment/trash build-up in the bottom of the chiller reducing water flow to the bottom tubes. The chiller barrel has weirs built inside to improve heat transfer, but they also trap foreign materials in the water over a period of years.
Leaks in the system while the unit and the pump are off will cause a freeze condition bulging the tubes until they rupture.
Pumping the system down with the water pump off will cause the same problem.

Hope this helps

[RichardL]

Cotrare' on the "Freezing of the tubes will cause them to bulge".....
On all DX chillers the water is circulated "Outside" the tubes....Evaporators of the Acme design also have aluminum star inserts inside the tube that "turbulate the refrigerant flow & aid in the heat transfer.
It is possible on these type evaporators that the copper tubes (only .018-020" wall thickness" can rupture anywhere along the edges of the inserts.
Every manufacturer is "Very" specific in their installation instructions that a certain mesh strainer be installed "Just" prior to the water entering the evaporator....
I would venture a guess that less than 1 out of 20 chillers installed have said strainer....
If yours is one of these that have failed due to the lack of such strainer....why in the hell should said manufacturer warrant such an installation????
Furthermore....I will bet a dollar to a hole in a do-not that you evaporator does "Not" have the "required" strainer installed on the "Evaporator Inlet".....

[jogas]

Cotrare' on the "Freezing of the tubes will cause them to bulge".....
On all DX chillers the water is circulated "Outside" the tubes....Evaporators of the Acme design also have aluminum star inserts inside the tube that "turbulate the refrigerant flow & aid in the heat transfer.
It is possible on these type evaporators that the copper tubes (only .018-020" wall thickness" can rupture anywhere along the edges of the inserts.
Every manufacturer is "Very" specific in their installation instructions that a certain mesh strainer be installed "Just" prior to the water entering the evaporator....
I would venture a guess that less than 1 out of 20 chillers installed have said strainer....
If yours is one of these that have failed due to the lack of such strainer....why in the hell should said manufacturer warrant such an installation????
Furthermore....I will bet a dollar to a hole in a do-not that you evaporator does "Not" have the "required" strainer installed on the "Evaporator Inlet".....

Depending on the amount of freeze up on the water side of the DX evap, the tubed can get "pinched". I served my apprenticeship in a shop that rebuilt compressors, motors, and re-tubed heat exchangers. They would test to find the leaking tube(s) by inserting rubber plugs in both ends of each tube
(end bells removed), they would then pressurize the shell side with Nitrogen and wait for a rubber plug to pop out when pressure built up in the leaking tube.
One time I pulled a large tube in shell and sent it to the shop to be repaired. They did the plug routine, and no plugs popped. I knew there was a leak, so they kept it under test for a week. Still no popped plugs.I was getting a lot of second guessing and then started doubting myself. As it turned out, there was a leak...it was being covered by the rubber plug when it was pushed into the tube.
As for the strainer, how is it that every chiller without the recommended strainer doesn't leak?
jogas

[RichardL]

As for the strainer, how is it that every chiller without the recommended strainer doesn't leak?
jogas[/QUOTE]

Every job is different Jogas..
Example;
A "Completely new" chill water system is farrrrr' more likely to have a serious amount of welding slag, and other errosive materials in the chill water circuit than a simple chiller changeout.
Due to the "Extremely" thin walls of any DX evaporator tube, I personally do not see how any of them make the warranty period with or without a strainer.
It sure don't take much to errode thru a piece of super thin wall copper when being hit by a direct stream of water with trash such as mentioned...
The newer Evaporators with 1/2" or even 3/8" OD tubes do not have the start inserts and that one problem of the tube crushing in on it when freezing does not exist.
However....for some reason the newer ones seem to pull away from the crown sheet in which the tubes are rolled, much more often on the Suction line connection.
My most recent DX re-tube was to replace the 3/8" copper tubes with Stainless-Steel tubes(308 stainless) . we lost about 3°f on the approach temp., but I guarantee that they will not errode thru in a year as the 2 before it did......

[ACCMan]

I stand corrected on the DX versus the "flooded" evaporator freezing difference. The tubes are damaged by the ice, but not from internal pressures of the expanding ice. The line referring to a system leak and the bulging of tubes applies to larger chillers with flooded evaps.
As far as the inlet strainer goes, it would be great if all mechanical contractors followed the book to the letter. I'm sure there are many "unfair" warranty claims due to this problem, and others, in our industry. As a semi-retired Tech with 25 plus years in the Industrial service arena predominately involving chillers, I have seen too many systems fail that were not installed, nor operated, correctly.
Correcting the problems has made life good!

[tex~n~oz]

If your chilled water lines are copper, check for electrolysis.