Optimum amount of holes for windscreen

Lose the holes Don. I mean, all of them. They are a cute affectation. In my experience so little oxygen is required you don’t need holes, and I actually have found them to decrease the effectiveness of the windscreen in very windy situations.

Don, I would think you already have enough air intake holes at the bottom of the wind screen. You won’t need any “exhaust holes” at the top, as the chimney effect should vent nicely, assuming that you have a space of .375″ to .50″ between your pot and the wind screen.

I make all my own wind screens, and also some MYOG cone clones. With the cones, there must be exhaust holes all along the top, and I usually do 2 or 3 rows of vent holes. I’m not a fan of having the bottom air intake holes be placed all around the bottom of the screen, as I feel a strong wind can sneak in and mess with the alcohol or Esbit flame.  So I place holes along 1/3 of the bottom of the screen, 3 rows of them below the slot for my pot handle, and they are positioned so that they are on the lee side of any breeze. That way, any stiff breeze can’t easily get through the vent holes or the pot handle slot to affect the flames.

For cones, I’ve learned that I need more exhaust holes than air intake holes, because hot exhaust gases escape faster than the intake air comes in. Or, better stated, the hot exhaust air must be able to properly escape, and air intake will passively be sucked in to replace it (I think this is how it works, anyway).

However, for a simple wind screen, all you need is a proper screen-to-pot space to allow the hot gases to escape upward. If you find that the wind gets inside on the windward side, you might need to figure out a way to partially block some of the intake holes there.

Edit: Looks like Cameron and I are thinking pretty much the same way. However, I like to have some intake holes on the lee side to ensure proper air intake. What you might do is to use the wind screen inverted, such that the stock holes are at the top , and see how it works for you. You could always place some holes to enhance the intake (but the original holes will always be there, not affecting anything at all).

My windscreen is a rickety affair so its leakiness may account for not needing any holes :)

But seriously in a strong wind that little Esbit needs all the help it can get.

Gary is more correct. Given extremely windy conditions I would lean more to Cameron’s description (around 25mph+ with no wind break.)

Technically, you need twice as many outlet vents as inlet vents. This is not counting expansion due to heating.

At the most rough level you have a fuel combining with oxygen to produce a gas and water. The water is hot enough to be called water vapor, a second gaseous combustion product. So, ignoring a lot of the technical stuff, moles, condensation, expansion due to heating, etc. you have about 2+ outlets for 1 inlet. A standard hole punch is 1/4″ in diameter. Figure the area of the holes and calculate the area above as the exhaust port. Just use the standard circle areas, all calculations will be the same format: (pi*radius)squared. A bunch of addition and subtraction, but not hard at all. (You can actually run area calculations on the web.)