Failing air handler steam coils

rlne

Problem is damaged tubes. Coil manufacturer dissection of failed coil says wet steam/carbon/high velocity high temp. Pin hole leaks in tubes. Water hammer noise.

Background Unit 1
-Building modifications...2 units added doubling steam load on 3" main @ 120 PSIG. Load calcs say we are on the edge of not enough on a design day (Northern Ohio). Have not been there yet this winter.
-5 units 100% outside air with their own Pressure Reduction Stations reducing to 15- 25 PSIG
-Coil keeps failing about every 2 months even when ambient is higher

Attempts to correct the issue
-Added vacuum breaker on coil condensate outlet
-Added air vent at steam supply coil inlet
-Verified with trap specialists that we are using the appropriate type and size trap (a couple of times) trap is new
-Verified controls operation
-Installed steam drier on main 120 PSIG prior to any reducing station (they are all in a rooftop penthouse area)
-Internal engineering department is out of ideas
-Installed Pressure and temperature sensor between control valve and coil. Typically steam shows about 3°F superheat.

Second unit coil failing
Background Unit 2
-Old coil developed leaks and it was replaced with OEM recommended coil
-Coil keeps failing about every 2 months even when ambient is higher same signs as Unit 1
-No other modifications have been made

What do they have in common?
-Supplied by central boiler plant serving multiples of this same application and these are the only two that fail
-Pressure reducing Station piping

------Unit 1 the Pressure Reducing Station is at the end of a main header. In other words the pipe does not come off of the top of the header. The header has a 90 degree elbow that comes down into the reducing station. This is why the drier was added. Is this drier for real? Does it actually work?

------Unit 2 the Pressure Reducing Station comes off the bottom of a main steam header. No attempts have been made to even operate this system. Once the replacement coil failed it was replaced a 2nd time and left off using Unit 1 as our Ginny Pig.

Our engineering group wants to install electric heat as they have given up. God help us all. Anyone have any ideas on this one?

Jamie Hall

How much superheat, if any, do you have after the pressure reducing valves? The piping geometry isn't helping, but if you really have superheat the steam should be quite dry. It may not be, after the pressure change.

One thought which occurs would be to add reheat after the pressure reduction. But that would help only if you are saturated or nearly so at that point.

Kahooli

Are there drips and traps at the entrances to the reducing stations? You don't want condensate carrying with the steam

https://www.armstronginternational.com/files/common/allproductscatalog/c-14.pdf

Takeoff should come off the top of a main, strained and dripped.
if yours come off the bottom, and have no drips, it's no mystery why they're failing.

EBEBRATT-Ed

Also how far is the steam trap located below the coil outlet? Most drawings are 12-14" minimum. Not enough in my opinion. I would drop the trap as low as possible. Make sure the trap has a 3-1 safety factor (3 x coil condensing rate)

The two clues to me are #1 water hammer. I would put an additional trap with a drip installed immediately at the inlet of the steam control valve. Make this drip full size of the pipe before any reduction into the control valve and make the drip as deep as possible with a trap at the bottom. Do you have eccentric reducers on the control valve?

#2High velocity, are you overloading the coil? Trying to heat more air volume, or colder air (higher temp rise) than the coil is designed for??

#3 higher temp. Your main steam is 125 dropping to 15-25 psi. That means superheated steam as @Jamie Hall mentioned

Is the mfg of the coil aware of the superheat hotter than 15-25lb saturated??

Had a job myself with superheated steam a few years back 30 psi steam. Coil kept failing. Don't know if it ever got resolved (I left that company) carrier unit. (I am trying to remember who the coil mfg was)???? Old age

rlne

Thanks for the comments.
Drip traps before reducing station.

Control valve set up is 1/3 - 2/3 with drip trap on control valve manifold. This comes off a strainer located before the 2/3 valve. Understood this is not ideal.

Piping leaves the control valves and goes to a strainer (orientation is lateral) and drip trap prior to piping going vertical to the coil inlet (4 FT rise).

Drop from coil outlet to the main trap is 24".

Coil is rated for 78°F delta at 22 PSIG 16,000 CFM. Air flow verified at 15,535 CFM.

I have not been able to achieve a delta higher than 40°F.

Condensate load 1,425 and trap is rated at 4,590.

Disconnected the condensate line after the trap and check valve and collected the condensate in a bucket. Collected approximately 1 GPM.

I have not removed insulation around the control valves but I do not believe there are eccentrics on them.

Russell Nobles coil also a Carrier unit. Have not contacted the manufacturer since the original autopsy.

Thank you for the link @Kahooli. This is pretty much what we have installed. The only variable is the take off on the header as previously mentioned and the vacuum breaker on the supply side for us is an air vent and breaker.

Attached a couple pics.

nicholas bonham-carter

There is sometimes a condition in the returns of high pressure systems, called condensate grooving, caused by the formation of carbonic acid.
Maybe this problem is from that.--NBC

EBEBRATT-Ed

Boy, that unit is very similar to the one we had nothing but problems with. The coil pulls out through the back of the unit??

Is this unit 100% outside air? I am guessing it is with a 78 degree rise. the coil on the one I worked on failed at least 3 times that I know of.

The only difference was our coils failed, we had no capacity issue. Seems you have a major capacity issue AND coil failures.

What I wanted to do was buy a better quality coil like an Armstrong big $$.

Is your coil standard copper tube, copper header and aluminum fins one row steam coil??

Wish I had better news.

rlne

Super heat reading is directly at the coil inlet. Heavier wall thickness on the newest coil. It lasted 2 months just like the other ones. I am going to try convincing them to re-pipe the main. I will let you know what happens. Thanks for the input.

EBEBRATT-Ed

I believe "Aerofinn" manufactured the coils we had that were failing