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Cooling leg for a thermostatic trap questions

RayWohlfarth
RayWohlfarth Member Posts: 1,065
I am doing research for a class and need your opinion. Sometimes a cooling leg is needed before thermostatic steam traps and sometimes it is not. When do you use a cooling leg? My research shows that they are sometimes used but they do not say when one is used. I see plenty of steam radiators with a thermostatic trap and no cooling leg. The other questions is when they are used, what is the length? I am seeing suggestions ranging from 6 to 9 feet. Thanks you in advance for your expertise. The trap manufacturers say "Sometimes a cooling leg is required," LOL
Thanks
Ray
Ray Wohlfarth
Boiler Lessons

Comments

  • clammy
    clammy Member Posts: 2,827
    I think as dan would say it all depends and as for cooling legs it all depends on what it being used for central heating ,heat exchangers ,coils or process . I I ve always done my standard which is a shut off a tee w a drip leg blow off a strainer on inlet of trap weather it s a f n t or a simple thermal trap .Most f and t I ve installed where for mostly light commercial and a 50/50 of process and heating .usually on a thermal disc trap tried to get at least 1 to 2 ft from the outlet ,when it’s to close it will cycle rapid and may cause noise or hammering issues I guess due to the valves body getting to hot and remaining closed until enough condensate cools at which point there may be excess condensate in your emitter.most of the thermal disc traps where on steam to water tube and shell heat exchangers the thermal trap should be sized properly and u should have a vacuum breaker on any heat exchanger that has a supply side control valve just in case u over looked it ,it’s recommend some where out there in manafactures lit that’s been in my head for 20 years or so others wiser then I may chime in .But as I can recall I don’t think I ve every done more then a 18 to 24 inch cooling leg .i think the need for cooling legs become more critical w control valves and heavy load ,strainers one of the least installed and rarely service necessary devices that usually doesn’t get installed and rarely gets regular maintaince lol Hope this helps for what it’s worth Ray peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • Mike_Sheppard
    Mike_Sheppard Member Posts: 691
    From what I’ve read a cooling leg can provide at least two advantages in specific situations. It can decrease the amount of flash steam from F&T traps etc especially on high pressure steam systems. I’ve also seen them used in situations where a thermostatic trap is used and there could be a large amount of condensate - the cooling leg would perform double duty - providing a place for condensate to gather as it cools instead of pooling in the heat exchanger (storage and cooling) which would also reduce the frequency with which the trap has to open and close.

    I’m sure there are more uses for a cooling leg that I am not aware of! Lol
    Never stop learning.
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,065
    Thanks @clammy If I find my answers to my quest I will let you know.
    Thanks @Mike_Sheppard I can understand the need for it and supposed to only be used with thermostatic traps.
    I am just confused as I have never seen a radiator with a cooling leg before the trap. I wonder if the bottom of the radiator acts as a cooling leg. Perhaps it is only needed when the steam pressure is above 7 pounds as the steam temp would be 232 which would make the condensate temp 212
    Ray Wohlfarth
    Boiler Lessons
  • Mike_Sheppard
    Mike_Sheppard Member Posts: 691
    edited August 2019
    @RayWohlfarth I wasn’t aware it was only for thermostatic traps.

    Perhaps it is used for equipment that creates a lot of condensate on a cold start up? A cooling leg would allow space for the water to gather so it doesn’t back up into the heat exchanger or piping and cause water hammer? Just thoughts. I would like to know as well.
    Never stop learning.
  • Pumpguy
    Pumpguy Member Posts: 574
    Attached on page 2 is one example where a cooling leg would be used ahead of a thermostatic trap.
    Dennis Pataki. Former Service Manager and Heating Pump Product Manager for Nash Engineering Company. Phone: 1-888 853 9963
    Website: www.nashjenningspumps.com

    The first step in solving any problem is TO IDENTIFY THE PROBLEM.
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,065
    Thanks @Pumpguy I appreciate it
    Ray Wohlfarth
    Boiler Lessons
  • gerry gill
    gerry gill Member Posts: 3,047
    I use a cooling leg when I have a steam to water heat exchanger because the steam as it continues to enter keeps the condensate to hot, to allow the trap to open, if the trap is to close. obviously I could change styles of traps but if my condensate load is small and I’m concerned about flash steam a thermostatic trap works better.
    gwgillplumbingandheating.com
    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.

  • JUGHNE
    JUGHNE Member Posts: 10,279
    Most of the old prints I see show a 5' cooling leg ahead of F&T's.
    But on the job the actual build does not have them installed.
    The dead men run out of horizontal room or slope drop on the return and did what they could........it is always easier to draw something rather than actually build it that way....and that was hand drafting....it is worse today with CAD plans calling for the impossible.

    I serviced a school with attic main and downfeed drops. There were about 30 drip traps for the drops....simple rad thermostatic traps with the spud screwed into the tee that had the dirt leg on the bottom. Doing all this I was wondering about cooling legs.
    In, perhaps, my imagination the drip leg on the drop before the trap seemed it would give some cooling to the condensate as a 1" X 5" nipple would hold some water that was cooling the incoming drip condensate that would mix with it. FWIW
  • Mike_Sheppard
    Mike_Sheppard Member Posts: 691
    edited August 2019
    I agree with Gerry. That’s the main thing I’ve read about cooling legs. For something like a steam to water heat exchanger that can produce a lot of condensate quickly, the cooling leg gives the water a spot to build up without backing up in the heat exchanger. If it backed up in the heat exchanger it would be kept too hot by the steam entering the heat exchanger, preventing it from draining properly.

    I have never actually seen it in the field though. All the stuff I’ve dealt with has used F&T traps.

    I had a brewery that got a lot of flash steam out of an F&T trap on the condensate line from the kettle. It was right next to the condensate receiver so it didn’t have any time to cool off and they would get some vapor out of the vent. I wonder if a thermostat trap with a cooling leg could have worked in that situation?
    Never stop learning.
  • Bill_Kitsch69
    Bill_Kitsch69 Member Posts: 34
    edited August 2019
    I have only ever needed a cooling leg on high pressure steam. Disc traps always get one. 3 feet min. Mostly on equipment like buck presses in dry cleaning plants. High use, dry steam. Inverted bucket traps are better but significantly more costly. When changing to disc trap from i-bucket, or other, ALWAYS add a cooling leg if none exists in such applications. Steam dry enough for pressing pants eg is correctly 100+ psig, though some cleaners eg, try to cheat to save fuel, and run as low as 85 psig...
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,065
    Thank you all for your input. I think it is clearing up for me now. I spoke with Spirax Sarco yesterday and their tech said a thermostatic trap usually has some condensate back up on the inlet to the trap. A cooling leg is used when the condensate is not desirable in whatever the trap is connected to. It would be similar to to a trap on makeup air preheater when the condensate could freeze inside the coil. When used on a radiator, the condensate does not really affect the heating of the radiator.
    Ray Wohlfarth
    Boiler Lessons
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 13,111
    @RayWohlfarth

    Most of the time you see thermostatics on radiation which the radiation acts as a cooling leg , a little condensate in the bottom of a radiator is usually not an issue. A cooling leg will give the trap some condensate to work with and keep the trap from short cycling if used on a drip for instance
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,065
    Thanks @EBEBRATT-Ed I appreciate it
    Ray Wohlfarth
    Boiler Lessons