F@B coil hammering

kuz

I recently installed a new airhandler and started the steam that night during a shutdown. Everything was working fine. I came back a few days later and as I walked up I could hear what sounded like popcorn poping in the intake louver.The sound is definatly coming from the face and bypass coil.

1. The system is designed for 15# steam and the owner is only delivering 7# steam.

2. The night we installed the unit it was 40 degrees.

3. the next time I came by it was 20 degrees.

4.The unit is 100% outside air.

5. The control valve is full open or closed and the heat is modulated by the <a href="mailto:F@B">F@B.

My coworker suspects the steam is condensing in the coil and if we raise the pressure to the design spec we will solve our problem.I am concerned since I installed it and do not want to inconvience the customer.Any thoughts?

5. I have a 14"  cooling leg and a 6" dirt pocket.</a>

Brad White

Vacuum Breaker and Tube Sheets

One thing I would check is the presence of a vacuum breaker after the control valve. I know you said the valve is normally open but on the off-chance that the valve modulates, the condensing steam can cause a vacuum and that in turn causes a ticking or banging sound.



If the noise seems external to the coil, what about the tube sheets, are they binding?



I would not worry about the steam condensing in the coil- that is exactly what it should be doing. Stop that and there is no point in having the coil.



I do have a question, regarding the steam pressure. Are you sure the selection is for 15 psi for performance or is this a pressure rating limit? Reason I ask is, unless this is an industrial building, the presence of steam over 15 psi is high pressure by code (the source would be higher than 15 just to deliver 15 at the coil). Depending on your jurisdiction that may require a fireman (stationary engineer) as an operator on duty all shifts. That is the law here in MA for example, not all states.  In some jurisdictions you cannot run piping over that pressure in an occupied building without special separation, I had heard.



That said, I do not see 7 psi being an operational difference, but it would reduce the capacity and may affect downstream portions such as traps, lift fittings and so-on.

kuz

F@B coil hammering

Thank You, Mr. White , You are correct this is a commercial building and the design spec of the air handler calls for 15# steam which is fed from a steam pressure reducing station. The steam is stepped down from 125PSI to 60PSI and then to 15psi.I did install a vacum breaker at the top of the header after the control valve and another at the top of the drip leg since various manufactures of the airhandlers show different placement I put one in each location.

In speaking with my coworker his thought was that the steam was condensing more rapidly than it should be since we were not at the design pressure.This makes some sense to me since the 7# steam was less then the engineer had spec'd. I have not had this problem before, I have experienced limited amounts of this with 100% outside air units but not to this degree.

The condensate goes to a pump after the trap and is pumped into a 1-1/2" line that enventually reduces to 1-1/4" shortly after this another trap a bucket trap ties into this condensate line and eventually to a condensate sump.could this have anything to do with it?

I will double check the fins are not hitting the clams shells on the F@B on moday.

I just had another thought I dont know if we verified that the condensate pump was wired correctly. I will have this checked also.

Brad White

Actually

and this is a bit academic, but the higher the steam pressure, the higher the temperature at which it condenses. (Higher pressure steam has less usable heat -heat of evaporation- per pound.)



What kind of a trap is there on the F&B Coil?  I am also gathering that you have two traps in series on the way to the condensate pump? One trap is all you need. I use Float and Thermostatic traps for steam coils, with a 2.0 factor to allow for warm-up. Once you are past the first trap, you may not have enough lift to get through the second (bucket) trap, is one other thought.



Not sure if the condensate pump wiring is at issue- if backwards it will still pump but weakly, so if a check valve in the discharge as there should be, it may rattle a bit. Possible?

Charlie from wmass

I was just reading in an old ASHRAE book I got

about configuring a fitting if you will be flashing condensate to steam by dumping into a low pressure area. I will try and scan and up load the page of the book. It uses a pipe bend with 1/8" holes drilled into it to dissipate the condensate so it stil flashes to steam but over a larger area thus reducing the hammer. I mention the pressure drop because maybe your trap is weak and not allowing any back pressure as the system runs letting the condensate flash back to steam as it runs through the coil. In effect "venting" too fast.

kuz

F@B coil hammering

Gentlemen, Thank you for your response's.The owner increased the steam pressure to the required 15# and the problem still exists.I am at a loss.I stopped by today and looked at the the condensate pump and it seems to be working correctly.

I am haveing the office check with the manufacturer this is not normal and I fear that if it continues for any length of time we could damage the coil.

The condensate line originates in this mechanical room and has several pieces of equipment tied into it,but none are gang trapped.Each has a valve, strainer, union, trap,union, test tee, check valve and gate valve.In the case of the coil since the ceiling was so low and the AHU so big for the room we had to put it on rails sitting on the floor. This left the outlet of the condensate at eleven inches from the floor.This was not enough to get the required cooling leg and with the F@T trap haveing a 4" c-c dimension we ended up below grade.So we installed a pit with a pump , yes the pump has a spring check on it and is lifted and then it is gravity return the remainder of the way.It goes through the wall of the Mech room and into a furred out wall for forty feet and down a chase into a steam tunnel and into the basement , at this point there is another trap from something that dumps into the line ,The line continues another 100' to a condensate sump pit and from there is sent back to the heating plant.

I installed the coil and related piping per the spec,print and details. I am missing something , and I can't quite figuire it out.

I have used the items that were described with 1/8th inch hloes we called them spurgers.If I remember correctly they were installed on high pressure pumped condensate and then flashed off into the low pressure steam.

Thank you fellas for your suggestions , I am truly appreciative.

Brad White

I would focus

on the potential for having two traps in-series, at least that is what I think you were describing. After the condensate passes through the first trap it would be held captive between that first trap outlet and the second trap inlet.



If the second trap is a bucket trap, only volume (flow) will operate the trap and by that time there is little pressure left to lift it.  Just the weight of the condensate and what residual vapor pressure the hot condensate can muster, that is all you have to work with.



Lifting condensate is best left to higher pressures or best of all, vacuum systems. If you have to get something down there to complete the lift, a pressure powered pump (Tunstall, Spirax-Sarco and others), make sense. Essentially two check valves in series with an injection of steam between them to force out the condensate in one direction. Some use compressed air.



(P.S.: The "sparger tube" is and works as you described it, inserted in a tank below waterline to allow any flash to take place below the water and, by default, pre-heat the boiler feed water. As you noted, sparger tubes are typically limited to higher pressure steam systems where "flash" steam is generated in appreciable and useful amounts.)



My $0.02



Brad