Chimney Height: does it matter?

[Reep]

Is the height of a chimney in a pizza oven matter?  Aside from initial smoke, does the height change the amount of draw?  If so, is shorter better or is it variable depending on the rest of the circumstances (wind, ambient temperature, door height (j/k))?

[Tscarborough]

For an oven sitting in the middle of a field on a windless day, it doesn't matter at all, you don't even need one.  Environmental and location issues can make it very important.

[TXCraig1]

Yes, height affects draw, and if you don't have enough of it, the front of your oven will be black.

[Reep]

Yes, height affects draw, and if you don't have enough of it, the front of your oven will be black.

Although I do like the look of some of the rock ovens with some char on the outside, probably not the best for my design.  Taller chimney = better draw, right? 

The prevailing coastal winds to blow from the back side of the oven toward the house, so probably not a bad idea to get the chimney up relatively high for start-up anyway.

[shuboyje]

Reep,

You seem like a pretty smart guy, so I'll give you the technical version.

Draw is the result of a pressure differential caused by temperature difference.  The volume of air in the flue is at a lower pressure then the cooler ambient air.  This pressure differential drives the flues draw.  The more air in the flue, the more draw.  The hotter the air in the flue, the more draw.  So, to increase draw you can increase the height leading to more volume, increase the flues cross section leading to more volume, or do something to increase the temperature of air in the flue like insulate it.

edited to add:

And of course by limiting anything in the flues design that will lead to friction loses, like drastic transitions, elbows, ext....

[TXCraig1]

Here is a link to a flow rate calculator spreadsheet that will let you see how the changes in the different variables change the flow.

http://www.pizzamaking.com/forum/index.php/topic,20117.msg201190.html#msg201190

[Reep]

Sounds good.  My current designs all have a strait shot vertically.  I was considering chimney sizes (most of the design candidates are tall) and wanted to make sure there was nothing bad about having a large chimney--like you could have too much draw that would then negatively impact the oven function.