One frequently raised question is how designers determine the amount of recirculating air to use in a particular oven. The question usually will be be phrased "Why did they use that number of air changes?" or "How was the recirculating fan cfm figured?" (Air changes, for the uninitiated, are the recirculating fan flow divided by the internal volume of the oven.)
Every designer has guidelines and reasons for the fan capacity he or she chooses, and generalizing about them is risky business. However, the fundamentals are pretty much the same everywhere, and they're worth understanding. These descriptions are simplified because of space limitations, but they illustrate the point.