Dick Bennett addresses the question, how much recirculation air should you use?
In Air Changes, Part 1: Recirculation Air Review, I looked at the criteria for determining how much recirculating airflow an oven should have. I also mentioned that all the airflow calculations are based on the assumption that you can get the heat transfer rate you need. Maintaining sufficient air velocity at the load surface is a big part of this. Another, often overlooked -- or even abused -- part of heat transfer is getting proper exposure of the load to the heat source.
One of the cardinal rules of heat transfer, whether it's by convection, radiation, conduction or any combination of these, is that the more load surface you expose to the heat source, the faster you'll be able to heat it. Common sense? Sure, but you'd be surprised how often people lose sight of it.
The proper design of an oven or furnace takes this rule into account. Loading patterns and conveyor systems will be designed for the best possible compromise between heating speed (and efficiency) and floor space requirements. The trouble often starts when someone, in the hope of increasing productivity, upsets these loading and space relationships by trying to pack more product into the available space (figure 1).