Steam Coil - Cold Section(s)

TNEng21

There's a plethora of things a facility engineer doesn't want to hear, and one of those is "a freeze stat has tripped"……

This is the same unit I posted about February 2024……..

Recap: 100% OA AHU (~20 kCFM), (3) vertically orientated steam distribution preheat coils (i.e.: supply/return headers @ bottom of coils) with dedicated F&T per coil and (1) modulating control valve (CV) serving preheat coil bank.

Since last years post, the F&Ts have been replaced, Vaccum Breakers (VBs) installed, and the CV was replaced with a correctly sized unit.

The recent freeze stat trip happened when the OAT was ~18^F. Reset the freeze stat, and the unit ran for ~5 minutes and tripped again.

Upon the second reset, the coil faces where measured with an infrared thermometer gun with peculiar readings: 120^F, 100^F & 31^F. The hottest coil section being closest to the steam supply and the coldest section being the last on the piping header. The F&Ts measured between 160-170^F.

So it gets even more peculiar, at times the temperature of the coil faces seem to oscillate, or appear to being doing the wave as you'd see at sporting event.

Of course there aren't any pressure gauges installed, so the outlet pressure of the steam from the CV is unknown. CV trend data shows its modulating between 50-60%.

This unit is the last unit in the fan gallery, thus sitting furthest from the dedicated Pressure Reducing Station serving the gallery.

I've been leaning towards a supply pressure issue and/ or an air bound coil section(s).

Any insight and/ or thoughts would be greatly appreciated.

pedmec

Sounds and feels like you have steam in the returns that are not allowing the air to escape. You might have a failed F&T in the system that is preventing the air from leaving the coil. the oscillating is the steam collapsing and being replace with more steam.

Do you have an H pattern F&T.

TNEng21

The F&Ts installed are Spirax Sarco FT-75.

Along with chasing down potentially failed traps, would a thermostatic vent on the coils and/ or supply header be of any benefit?

pedmec

Might hurt. Once you get rid of the air thru the air vent on the steam coil the steam from the failed trap (if it is a failed trap) steam will try and come thru the return because the steam coil is a low pressure zone with the air vent now bleeding air. It could cause water hammer with steam trying to come in thru the return. The return is only sized to drain the condensate not steam. Remember, the F&T is one part air vent so the air should be going out thru the F&T. all indications from your numbers seem to point to an air bound coil.

you should test this theory by removing a plug from the F&T and see if the coil heats up with the trap removed. If you have an H-pattern F&T i used to remove a plug and put a ball valve in as a test port.

EBEBRATT-Ed

@TNEng21

I have had this exact same problem several times.

Many people run around in circles before they figure it out.

I will save you the time.

Your coil has frozen internally and needs replacement.

The symptom you will get are:

The far end of the steam tubes will be cold on the far end (the ones that froze) Good tubes will be steam hot.

No apparent steam or water leaks

Steam line into coil hot

Condensate out hot

Coil will not heat or only partially heats

The sections of the coil that are bad will feel cold, the ones that are good will heat (if any)

Its a one row steam distributing coil. That's the coil they use for 100% outside air.

To avoid future problems:

Install a freeze stat in accordance with the mfg instructions. The cap tube in the freeze stat should run in the direction the mfg wants (I think you are supposed to zig zag it horizontally, not vertically) can't recall.

Freeze stat should:

Shut fan down

Turn steam valve to 100%open and close all dampers

Use a replacement coil with 1" tubes not 5/8" tubes

Try not to lift condensate from trap. Return line or condensate pump should be below the trap.

This is what happens.

The coil froze with some condensate trapped inside. Those coils have a tube within a tube. Outer tube is condensate and steam. Small tube inside is steam only. It freezes internally, no visible leaks. This causes steam to bypass to the condensate and not "distribute down the coil"

EBEBRATT-Ed

Hear

iom-1-Coils.pdf

s some steam coil information

TNEng21

Thank you for the insight and suggestions. I'll be tackling the issue(s) this week and will be utilizing the noted suggestions/ guidance to hopefully resolve the ongoing issue(s). Stay tuned.

EBEBRATT-Ed

@TNEng21

In my 46 years working I think I remember 4 jobs where this happened. The first one was a real stinker that I just couldn't figure it out.

This one was a Trane coil with a cast iron header that had threaded access plugs in the header. This coil had about 8 tubes in it. I finally cut an access hole in the duct and feeling the coil, you had hot sections and cold sections and checking the tubes on the far end of the coil is the easiest way to check the coil. Anyhow we removed the threaded plugs with difficulty and after realizing how the coil is constructed figured it out. The customer had 3 or 4 people look at it before I got there.

It's one of those things that once you find it you will never forget.

pedmec

We had a steam coil that one of our guys was working on that drained into a condensate pump, District steam. he shut down the outlet of the coil and never open the gate valve back up. As you can imagine what happened next. Coil froze in so many places there was no chance of a repair. You could walk in the ductwork it was so large.

EBEBRATT-Ed

@pedmec That was a costly mistake

TNEng21

Update:

Was able to use a FLIR Thermo Camera on the coil bank in question this morning. Temps measured are as follows: 140^F, 60^F & 130^F. The hottest coil section being closest to the steam supply and the coldest section being the middle section this time around…… The CV was ~55%.

The coil section closest to the CV had a nice uniform glow, the other two didn't.

The steam header into each coil measured ~203^F, and each condensate leg was ~165^F; F&Ts measured ~160^F.

The very tops of the two hot coils were ~140^F, and the cold section was ~60^F.

Placed the F&T of the cold section into bypass, where it heated up to 100^F very quickly and dissipated just as quick. I believe this indicates the F&T is function correctly?

With the CV closed, the bypass line was opened and all three sections stared to glow; the two warmer coils were ~170^F, while the cold section was ~185^F. Line pressure is 30 PSIG.

With the bypass closed, the CV was then put to 100%, the two warmer sections were ~160^F, and the cold section was ~140^F.

So is this leading to a potential mass flow issue or internally compromised coils?

Defiantly scratching my head…..

EBEBRATT-Ed

What type of coil is it? Does it have return bends on the ends or is it a steam distributing coil with straight pipe going across the coil? It should be steam distributing with a 100% OA application.

Can you post some pictures of the coil and the piping?

Also how many individual coils do you have? I had assumed you had 1 coils with parts of that coil not heating well. Is that the case or not? Or do you have multiple individual coils piped together?

Need to know what you have to be able to find the problem.

Attached is a picture of a steam distributing coil.

Note that the tubes run straight through the coil with 1 tube inside the other and the steam supply and return are on the same header.

Is this whatt you have?

ACT-Steam-Distributing.pdf

pedmec

i think a picture of what you have would help.

It sounds like you have multiple coils being fed thru one control valve?

The system is running at 30 psi? if that's the case do you have the correct F&T? it has to open against a 30 psi differential

TNEng21

Good evening, and thank you for the quick responses.

The coil bank consists of (3) three vertical style steam distribution coils. The supply/ condensate header is located at the bottom of the coil.

The coil bank is served by a single modulating control valve. Supply pressure to the CV is 30 PSIG. Per the CV manufacture data and Cv valve tag, the outlet pressure should range between 6-15 PSIG based upon valve position.

The supply header is piped from the CV and only serves these (3) three coils; there is a F&T at the end of the header.

Each coil section has a F&T piped to a common gravity condensate main; the main is ~18" below the F&Ts, and the F&Ts are ~24" below the condensate outlet of the coils.

Unfortunately this facility doesn't allow cameras or recording devices inside the facility….. it makes troubleshooting and field work quite difficult……

EBEBRATT-Ed

Sounds like your hooked up correctly.

My only comment is that with 3 coils the steam pressure in each coil when the valve is open will very greatly. There is no way to get exact airflow across each coil. When the freezing cold air comes in the coil that gets the most air flow will have the lowest steam pressure. I would make sure each coil has a steam rated vacuum breaker or reverse mounted Y pattern check valve on it to act as a vacuum breaker.

Your control valve is looking at (most likely) the common discharge temp from all 3 coils mixed together. 3 control valve might be a better choice. There is a lot going on there. Does the system have face and bypass dampers?? When you go to face and bypass if you have it the control valve goes wide open and the dampers mix the air temp

In any event climb up on top and check the tube ends of the coils (opposite of the supply/return) if the tube end are hot (steam hot) then the coils are good and not frozen internally. I would check this with the control valve wide open

EBEBRATT-Ed

I would also make sure the traps are sized correctly. The coil MFG or unit specs should give you a LB/hour rating of the coils.

I would take the LB/ hour for each coil x 2 (for a safety factor). Traps are sized for the lb/hour you get at the correct pressure differential. I would use a trap rated for 30 psi steam even though the trap should never see that pressure.

With a modulating control valve you size the trap at a 0 inlet pressure +24" trap below coil so call it 3/4 psi inlet- whatever outlet pressure the trap sees (hopefully 0psi)

The reason is when the control valve goes closed you could end up with a vacuum in the coils handling that cold air. (hence the vacuum breaker). Steam condensing into water will collapse 1700x in volume. A vacuum in the coil will hold condensate and maybe freeze. You want free draining. A vacuum will also reduce the trap capacity.

I had the exact same install where the dumb installer used 1 F & T for 2 or 3 coils (I forgot how many) they didn't think it could freeze a coil but it did because of unequal air flow across the coils. 1 coil had steam pressure and 1 was in a vacuum. Because the condensate was tied together with 1 trap the coil in the vacuum sucked up the condensate and froze