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The physics behind condensate

Hey guys,
As an instructor, I am continually searching for new ways to teach or describe what happens inside the piping. Holohan always did a great job in many of his books. I am looking for some help in explaining in simple, yet physical terms how condensate can counterflow against steam under pressure. In other words, I would like to be able to easily answer the question: How does the condensate flow by gravity back to the steam main or boiler when the inside of the pipe is under pressure? Wouldn't the steam flow under pressure prevent that from happening? I realize there's a lot behind this as heat loss, temperature drop, velocity, etc., etc., can come into play; but I'm still struggling to try a simplify the physics in my head in order to be able to write and speak about this most important part of steam heating. I sometimes


  • Ironman
    Ironman Member Posts: 7,336
    A simplified answer:
    Once the vents begin to close down because steam has reached them, the flow of steam is greatly reduced or will even periodically cease. In that case, the pressure in the pipe is exerting force equally in every direction and would not effect the direction that the condensate flows. All it then needs is a little bit of gravity to induce it to flow. The gauge glass on the boiler is an example of this: because it has equal pressure at the top and bottom, gravity causes the water to go to the bottom.

    Imagine that if you took some form of clear tubing and capped it on both ends with it partially filled with water. The water will always flow towards the end that is lower. If you had a fitting on it that would allow you to add 2psi of air pressure, the water would still flow to the lower end by gravity. The pressure is irrelevant once the loop is sealed. This is always true no matter what the pressure is as long as the pipe or loop is sealed.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Jamie Hall
    Jamie Hall Member Posts: 22,877
    Furthermore... keep in mind that the condensate (if the piping is right!) is in a layer at the bottom of the pipe. The drag from the steam whizzing along in one direction isn't enough to overcome gravity acting on the condensate. If The Piping is Pitched Properly and the pipe is Big Enough!!! Which is why we pitch counterflow pipes more steeply, and use bigger ones!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Dave in QCA
    Dave in QCA Member Posts: 1,784
    edited September 2015
    Your question is specific to steam flow under pressure and the effect it would have on condensate flow in the same pipe. Of course, the condensate is under the same pressure, if it is in the same pipe.

    For example, if you have a 50 sq ft radiator connected to steam at 2 psi via a lateral runout, which will have counterflow conditions in that runout. Let's say the radiator is fully heated, the vent is closed, and the radiator is sitting in a 70F room. In a static situation like this, there is still a LOT of flowing going on. The radiator will be giving off roughly 12,000 BTU of heat per hour. In doing so, it will be cooling and condensing steam. The condensation will be flowing out of the radiator and new steam will be flowing back in at the same time. Roughly speaking, if I have interpreted the tables correctly, the flow will be around 8 cfm of steam.

    Now, the potential issues with steam and condensate in the same pipe have probably more to do with the volume of the condensate and the velocity of the steam. For example, if you try to connect the above describe radiator to a 1/2" steam lateral, the required velocity will be high as well as the relative volume of the condensate. The steam moving at high velocity will pickup up the condensate and cause it to flow backwards in the pipe, creating pockets of condensate and steam....and loud banging water hammer. If you connect the same describe radiator to a 1 1/2" pipe, the condensate will be only a trickle in the large diameter pipe, and the large area left for the steam will allow it to move slowing while delivering the cfm that the radiator is converting to water.

    Pressure has little effect on the dynamics compared to velocity of the steam and volume of the condensate. But, on the other hand, increasing the pressure will reduce the volume of the steam. Therefore, since the radiator can only condense a given amount of (pounds of) steam, increasing the pressure reduces the volume, thereby also reducing the velocity. Of course, steam at increased pressure will be somewhat hotter and therefor the radiator will give off slightly more heat and thereby requiring a higher flow of steam, but the reduced velocity because of reduced volume will be the most pronounced change.

    It stands to reason that counterflow arrangements require a larger sized pipe that parallel flow.

    Apologies for the length....but I hope this serves to help you visualize what is going on.
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
  • steamfitter
    steamfitter Member Posts: 156
    I sometimes get caught up in scientific explanations. I appreciate your thoughts on this! Thanks!
  • steamfitter
    steamfitter Member Posts: 156
    Thank you all for your great explanations! It is always amazing how different people, with much knowledge about a subject, can explain ideas and concepts in many different ways.
    As I read your explanations I now better understand what is happening inside the piping and how the steam and condensate behave in relation to velocity, flow, temperature change and pressure.
    The size of the piping (space) and its pitch are crucial to create the proper flow of condensate as it moves in a opposite direction of the steam. The dynamics of steam is so interesting!
    Thanks again for all of your input!!!