Notices
Results 1 to 9 of 9

Thread: Toilet Vent airflow maximization

  1. #1 Toilet Vent airflow maximization 
    New Member
    Join Date
    Jun 2009
    Posts
    1
    I am currently working on a project to improve sanitation in Mongolia but this question is germane to ventilation of a variety of non-flush toilets used around the world.

    We want to know how much, if at all, a vent "cap" (mini roof to keep rain/snow from going down the vent) inhibits airflow in calm conditions and what kind of dynamics the cap may create in shifting wind patterns which either inhance or impeed airflow.

    We have toilet designed to draw clean air in from a low level opening in the door, down the toilet seat and over collected feces (drying on the way) and then out a tall vent pipe. The vent pipe is on the south side of the structure, sheetmetal painted black and we are trying several schemes to improve the solar heating and warm the air in the vent pipe. Those efforts include a double pane window in the access door with black metal backing to collect solar heat and distribute it into the vented area. Also we have installed a meter tall plexiglass sleeve aorund the vent pipe, trapping air for greater solar gain.

    But the local people all insist a vent cap is necessary to keep out the rain, but they largely attribute that to their experience with wood stoves(where the rusting effect is an issue). In our case the rain water would drip into the chamber but would run off into a soak away under the chamber.

    I could really use some help on this as it has become kind of a sticking point. My own "sense" is that the vent slows airflow when things are going good (wind onto the air inlet point, strong solar gain on the vent pipe) but if that is true, I need some good arguments to convince the locals.

    Also, any other ideas on how to enhance the air flow either without electrical devices or with 12 volt fans on solar panels would be appreciated.

    I am loath to mount a fan in the actual tube of the vent because maintenance on solar panels, etc. is poor here, so far, and if the fan isn't actively running then it will slow the natural convection up the pipe. Has anhyone got ideas on how to have a fan mounted inside a smaller pipe and attached at an angle to the main vent pipe so that when the fan runs, it is pushing air up the vent but if it stops, it doesn't occlude the main vent?

    Kent Madin


    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Ph.D.
    Join Date
    Jan 2009
    Posts
    951
    Put a 180 degree u bend on the top pointing down -no rain -no obstruction, problem solved.


    Reply With Quote  
     

  4. #3  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,822
    What kind of sheet metal? If it's galvanized, that stuff will rust eventually, and I'd agree with the locals it could benefit from a cap to keep the water out of it. If you raise the cap up enough off the end of the pipe, it shouldn't obstruct the flow too much.
    Reply With Quote  
     

  5. #4  
    Forum Isotope Bunbury's Avatar
    Join Date
    Sep 2007
    Location
    Colorado
    Posts
    2,590
    I'd agree with the need for a vent cap, and don't forget a fly screen. The design you describe sounds a lot like the pit toilets found at campsites in our national forests, fondly known as honey pits.

    Most of the ventilation in my opinion comes from the effect of wind blowing across the top of the vent stack, which reduces the pressure (variation on Bernoulli effect) and sucks air out of the pit. I'm not convinced the plexigalss shroud will help that much but it might. If you use a flat plate for a cap and locate it at least 1/4 of the diameter above the opening, (i.e. for an 8" pipe the plate must be at least 2" above the opening) the circumferential flow area will be equal to or greater than the cross section area of the pipe and the resistance will be small.
    Reply With Quote  
     

  6. #5  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    I'd just like to throw my two bits in and say that while normal rain might drain away without a vent cap, you wouldn't want to deal with the 1 in a million rainstorm (or other conditions) that could flood such a device. (And as everyone knows 1 in a million chances happen 9 times out of 10. )
    Reply With Quote  
     

  7. #6 Re: Toilet Vent airflow maximization 
    . DrRocket's Avatar
    Join Date
    Aug 2008
    Posts
    5,486
    Quote Originally Posted by kentmadin
    I am currently working on a project to improve sanitation in Mongolia but this question is germane to ventilation of a variety of non-flush toilets used around the world.

    We want to know how much, if at all, a vent "cap" (mini roof to keep rain/snow from going down the vent) inhibits airflow in calm conditions and what kind of dynamics the cap may create in shifting wind patterns which either inhance or impeed airflow.

    We have toilet designed to draw clean air in from a low level opening in the door, down the toilet seat and over collected feces (drying on the way) and then out a tall vent pipe. The vent pipe is on the south side of the structure, sheetmetal painted black and we are trying several schemes to improve the solar heating and warm the air in the vent pipe. Those efforts include a double pane window in the access door with black metal backing to collect solar heat and distribute it into the vented area. Also we have installed a meter tall plexiglass sleeve aorund the vent pipe, trapping air for greater solar gain.

    But the local people all insist a vent cap is necessary to keep out the rain, but they largely attribute that to their experience with wood stoves(where the rusting effect is an issue). In our case the rain water would drip into the chamber but would run off into a soak away under the chamber.

    I could really use some help on this as it has become kind of a sticking point. My own "sense" is that the vent slows airflow when things are going good (wind onto the air inlet point, strong solar gain on the vent pipe) but if that is true, I need some good arguments to convince the locals.

    Also, any other ideas on how to enhance the air flow either without electrical devices or with 12 volt fans on solar panels would be appreciated.

    I am loath to mount a fan in the actual tube of the vent because maintenance on solar panels, etc. is poor here, so far, and if the fan isn't actively running then it will slow the natural convection up the pipe. Has anhyone got ideas on how to have a fan mounted inside a smaller pipe and attached at an angle to the main vent pipe so that when the fan runs, it is pushing air up the vent but if it stops, it doesn't occlude the main vent?

    Kent Madin
    At the flow velocities that you will get with natural convection, including any solar heating effects, I doubt that a vent cap at a reasonable height above the top of the stack will affect flow very much. I also doubt that rain falling into a hole full of crap will do much damage.

    It you are going to spend money on a fan, I would use the fan to draw fresh air into the outhouse itself, providing higher pressure than ambient in the outhouse with the vent pipe being used to relieve that pressure. That should not occlude any flow at all and ought to make the building smell better (less bad is the best that you can do). It might also help with what is probably quite a heat problem in a small enclosed structure in the Mongolian desert. Even without a fan, vents near the ceiling level might help.

    If it were me I would not spend resources arguing over the pros and cons of a vent cap. I would put the available resources into digging a deep hole, the deeper the better.
    Reply With Quote  
     

  8. #7  
    Forum Isotope Bunbury's Avatar
    Join Date
    Sep 2007
    Location
    Colorado
    Posts
    2,590
    I thought I'd post a pic of a pit toilet I visited on a snowshoe hike this winter, but it turns out the vent stack isn't visible. There were instructions inside the building to keep the window open. The cold air enters and the exothermic reaction generates heat in the pit so you get a thermosiphon effect.

    I'm sure the vent stack could bridge over with snow, but it's not obvious that having a cap would prevent this. I never looked up to see if the vent was clear, but it didn't smell too bad. This one was in the trees so soalr heating of the stack was negligible.

    I do think rain or snow could kill the thermosiphon effect by cooling the rising air so I would still recommend using a cap.

    Reply With Quote  
     

  9. #8  
    Time Lord
    Join Date
    Apr 2008
    Posts
    5,305
    A passive turbine vent could work here. These spin a bit with light wind from any direction, and the centrifugal force throws out air. More wind means more suction. But they also vent alright without wind.

    Turbine vents are very common on North American housetops; they're cheap and well developed.
    A pong by any other name is still a pong. -williampinn
    Reply With Quote  
     

  10. #9  
    Forum Bachelors Degree
    Join Date
    Jul 2009
    Posts
    403
    Quote Originally Posted by Pong
    A passive turbine vent could work here. These spin a bit with light wind from any direction, and the centrifugal force throws out air. More wind means more suction. But they also vent alright without wind.

    Turbine vents are very common on North American housetops; they're cheap and well developed.
    sounds like a good idea - i'd stay away form any electronics - it is only going to to wrong and is expensive really. try to use natural convection if you can, its the only thing you can rely upon!!

    I don't know enough of the science of this stuff as to how the air will affect what is inside - so carn't offer a greater opinion on its construction but if you want a vent i'd put a cap on it with a half decent gap - if its all made of metal with the 'crap' thats beneath it surely it will get higher than ambient in both sun or otherwise so natural convection will occur.

    like the hut pictures you may need another vent to draw air in which in the right place i'm sure could help remove the smell - you may need to create a vacuum effect which in simple terms is cold air against the warmer one inside!
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •