Properties of steam exhaust

[aeviaanah]

If a fairly large kettle say 40" diameter is completely sealed with the exception of a 6" vertical vent to roof (approx 15') would installing an adjustable port opposite end of the exhaust port assist with increasing the draft through the exhaust port? Does the cool air coming in assist the hot steam to exhaust?

 

[Smiling Frog]

If you have a sealed kettle with a 6" diameter pipe rising 15 feet to the roof, when the kettle is boiling, steam will rise up the pipe. I would suspect most of the steam would condense on the sides of the pipe and drain back into the kettle. If the pipe is insulated, it will reduce the amount of condensation, but it will take a long time for the entire pipe to heat above the boiling point of water (15 feet is a long way), which is what would have to happen to eliminate the condensation. If you introduce cool air into the pipe, it will increase the condensation.

If you want to eliminate the condensation from falling back into the kettle, you might consider building a dog-leg offset close to the kettle and put in a condensate trap.

 

[Vale71]

Steam does not need any assistance in leaving a sealed kettle through a vent hole, the positive pressure that develops from the vaporization of water will propel it outside very efficiently. If it didn't your kettle would become a pressurised boiler and it's clearly impossible to pressurise a vessel that has a big hole in it.

That sad, as the jolly frog already mentioned condensation could be an issue with the kind of setup you have and you should definitely do something to prevent it from dripping back into the kettle.

 

[Bobby_M]

Installing a room air intake into the kettle would have the opposite effect. Steam would likely come out of that hole and air from the stack would suck outside air in. The whole vertical vent tube would also drip condensate back into the kettle the whole time. You want to run up a bit, make a 90 degree turn, then go back upwards. That Z bend would collect any condensate so you'd need a hose barb attached so you can run that down into a bucket of water.

If that pipe has to go straight up, You'd want to install an interior trough so that any condensate dripped down the walls would get caught and you'd drain it away.

 

[aeviaanah]

I have a tee installed just below the vertical stack to collect condensate. A cap with ferrule allows for condensate to go to trench drain. I am still experiencing lower than desired boil off rates. With the main access lid installed, boil over is an issue. My thoughts were the air intake would help cool the surface of the boil and reduce boil over. I also understand steam will come out of this new port but I didnt realize it would "rob" from the main exhaust. My gut is that the 6" port is a bit undersized. The new port at the very lease should increase boil off rate, unfortunately indoors rather than vent to roof?

 

[bkboiler]

I'd try to use a blower somewhere to motivate the steam to leave...
Either into that new port you added or downstream, feeding into a wye so that you get a Venturi effect along your exhaust pipe.

 

[aeviaanah]

I'd try to use a blower somewhere to motivate the steam to leave...
Either into that new port you added or downstream, feeding into a wye so that you get a Venturi effect along your exhaust pipe.

Can you explain further what you mean by feeding into a wye?

 

[Vale71]

Again, you don't need to motivate steam to leave. It has to leave or pressure will increase instantly and that's impossible in a vessel that is clearly anything but sealed.

Increasing the exhaust diameter would also have as only effect to increase the surface are of the pipe which will lead to even more steam condensing and ,without appropriate countermeasures, dripping back into the kettle. If anything you'd want to reduce the diameter to decrease surface area and reduce condensation. Steam will leave the kettle at the same rate, it will just flow faster. With gases to get significant resistance from pipe friction you'd have to go down to ridiculously small diameters, at least with the low flow rate we are dealing with here.

 

[aeviaanah]

Again, you don't need to motivate steam to leave. It has to leave or pressure will increase instantly and that's impossible in a vessel that is clearly anything but sealed.

Increasing the exhaust diameter would also have as only effect to increase the surface are of the pipe which will lead to even more steam condensing and ,without appropriate countermeasures, dripping back into the kettle. If anything you'd want to reduce the diameter to decrease surface area and reduce condensation. Steam will leave the kettle at the same rate, it will just flow faster. With gases to get significant resistance from pipe friction you'd have to go down to ridiculously small diameters, at least with the low flow rate we are dealing with here.

Ok. Maybe I'll resort to turning down the heat, using ferm cap and dealing with 3-4% evap rate. Is it possible that excessive foaming in the kettle is reducing evap rate?

 

[Vale71]

No, not really.

 

[bkboiler]

Deleted: double post

 

[bkboiler]

sounds like a range hood or something similar is what you need....would suggest hiring a professional plumber to get you dialed in.

 

[aeviaanah]

Thanks guys