Lutz Posted August 14, 2021 Share Posted August 14, 2021 The final significant project in my shop is the chimney for the coal forge. I am planning to do a Super Sucker style hood. I have some (about 10’) of stainless double wall pipe that has an 8” ID, which I understand is a bit undersized as most recommend 10-12” to ensure good airflow. My question is, if I use that 8” ID pipe until I transition through the roof and then go to 10” single wall, how will that impact my airflow? Will there be a benefit to using the 10” for the final run, or would it make little difference if the initial section is only 8” ID? I’d appreciate any input. Thank you! Quote Link to comment Share on other sites More sharing options...
Glenn Posted August 15, 2021 Share Posted August 15, 2021 The area of a 8 inch circle is 50.27 square inches. The area of a 10 inch circle is 78.54 square inches. 10 inch is 1.56 times larger. You can not get 10 inches of draft through a 8 inch pipe. Quote Link to comment Share on other sites More sharing options...
Lutz Posted August 15, 2021 Author Share Posted August 15, 2021 14 hours ago, Glenn said: The area of a 8 inch circle is 50.27 square inches. The area of a 10 inch circle is 78.54 square inches. 10 inch is 1.56 times larger. You can not get 10 inches of draft through a 8 inch pipe. That part makes sense to me, but what I wasn’t sure about is whether the fact that the chimney opens to 10” inches increases the pressure differential such that there is some advantage to having that occur to as opposed to just sticking with 8” all the way. I know the 8” portion will not truly function as though it were 10”, but I wasn’t sure if there might be some improvement in performance. Quote Link to comment Share on other sites More sharing options...
Irondragon Forge ClayWorks Posted August 15, 2021 Share Posted August 15, 2021 (edited) My chimney does go from 8in to 10 double wall then up through the roof and it works well for me. I just found a thread about the stack size and I have my pipe sizes here wrong. It's 6 in from the hood to 8in. This may help. https://www.iforgeiron.com/topic/66496-new-coal-forge-any-luck-with-8-chimney-pipe-and-half-hood/ Edited August 15, 2021 by Irondragon ForgeClay Works add picture and thought Quote Link to comment Share on other sites More sharing options...
Frazer Posted August 16, 2021 Share Posted August 16, 2021 I agree with Glenn. You wont get the full performance of a 10 inch stack no matter what you do. However, if you go this route, make sure the opening on your side draft is sized for the smallest diameter of pipe in your stack. (i.e. less than 50.25 sq. in) Quote Link to comment Share on other sites More sharing options...
Lutz Posted August 17, 2021 Author Share Posted August 17, 2021 Thank you all for the input! I'll share my results once everything is operational. I'm looking forward to being able to have a coal forge again! Quote Link to comment Share on other sites More sharing options...
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