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Air Bound Steam Systems

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SteamNow
SteamNow Member Posts: 56
in Strictly Steam

TLDR:

I know in The Lost Art of Steam Heating Dan says that replacing the traps in a system won't necessarily lead to a significant increase in efficiency. But if the traps are broken, then there's a good chance there is also no venting. And based on the information below, without a vent the air only compresses 12% at 2psig, and it could take more than an hour for the steam to diffuse into the trapped air. How would replacing traps not help with efficiency? I suppose if the system was running with a vent, but bad traps it could still get the air out and heat ok. To be clear I am in not questioning Dan's knowledge or expertise. I only would like to understand a little better because I'm lost right now.

Math Below:

Now if a system has no way to get air out of the system the air must be stuck inside. Now if I model the air as an ideal gas(it's not mixed with the steam yet), we can use PV=nRT to predict the change in volume of the air based on the pressure the steam is putting on it. We can simplify this equation because we are only using one gas(air) and get the following equation: P1*V1*T1=P2*V2*T2.

Based on this equation if we start at 0psig(14.7psia) and finish at 2psig(16.7psig) with constant temp of 70f the volume only decreases by 12%. This means the steam can only get into 12% of the system? How does it heat the building AT ALL? I've seen a couple places that have had zero vent, but it still had some heat. Based on my estimates the air only takes 34btu/lb to raise the temp from 70 to 212. But using Fick's first law of diffusion and calculating using a 6in main that is 30 ft long(I'm being generous), it will take 147 minutes for the air and steam to diffuse. But again that's very generous given this isn't in a large single volume and is in a long piping system with radiators.

So with only 12% of the air compressing and allowing the steam to enter the system and the air and steam not diffusing into each other very quickly, how do systems which have capped off vents heat at all? Even if the boiler is running at full capacity, that power isn't all going into heating, it starting going into compression of the steam, and that doesn't help.

Owner

Steamnowrockford.com

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 20,742

    It's simple. If the trap(s) don't work the system won't work right. How good or how bad depends on the # of bad traps.

    In a commercial job with a condensate tank or boiler feed tank you waste a lot of steam, add a lot of water and potentially rot out the boiler

  • mattmia2
    mattmia2 Member Posts: 17,804
    https://forum.heatinghelp.com/discussion/comment/1901481#Comment_1901481

    and if they are failed closed or open and where they are in relation to other things.

    i'd have to find the quite that it won't change efficiency because it can have a huge effect on efficiency. if the air can't get out the emitter will never heat no matter how long you wait. if you crank up the pressure you can sometimes compress some of that air and get a little steam in the radiator and get some heat. if it is because the trap is failed closed the radiator will fill with water and you won't even get that little bit of heat for long. as this is happening the t-stat won't satisfy and most of the heat will go up the vent and in to the basement. the boiler will cycle on the vaporstat/pressuretrol because the system can't consume all of the steam it is producing.

    failed closed traps directly prevent radiators from heating. failed open traps prevent other radiators from heating by allowing steam in to the returns. this either closes the vent on the returns and prevents the radiators with good traps from venting, reaches the return on radiators with good traps and closes the trap from the return side and stops it from venting in to the return or the steam itself blocks the flow of air.

    once steam starts condensing the reduction of volume pulls in more steam. the condensing steam is what causes the steam flow in both a vented and non-vented system, the vented system just gets much more surface in contact with the steam. diffusion isn't a significant factor or you would get drips filling with steam and hammering.

    you really just need boyle's law, the volume change of the air due to temp isn't significant.

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