I want to smoke a pork shoulder low and slow: 240 degrees at around 15-20 hours. I'm using an offset smoker, Oklahoma Joe Highland.
What is a good placement of water pan(s) and why?
According to Meathead Goldwyn, water pans can go over the heat source or under the meat, depending on the style of cooker you are using, and the desired effect. They can aid in maintaining temperature or adding moisture to the environment. I would say, in your case, to avoid placing it under the meat, as there will be quite a bit of air circulation in your offset smoker.