Air fryers work by utilising high velocity air to improve the heat transfer rate at the surface of the foodstuff by breaking down the boundary air layer around the food (like wind chill in reverse). This in effect helps the food to cook quicker.
We know that cooking times (or cooking temperature) for fan ovens are generally shorter/lower than non-fan ovens. An air fryer takes this one step further by greatly increasing the velocity of the air compared to a fan oven therefore further reducing the cook time (to a point*).
All ovens need some sort of vent simply because as food cooks, water is evaporated and expands with the heat and so needs to go somewhere (otherwise you create a pressure cooker). There are some other hidden benefit of the small design too and that is that the small volume will quite quickly become saturated with water vapour at a high temperature. Humid air heats faster than dry air at the same temperature so also improving the heating effect and helps reduce further moisture loss - it's a win-win! What's more there will be less of a browning effect either which is a benefit that higher temperatures can also be used for faster cooking without uncontrolled burning.
Air fryers are also small volume appliances with a low thermal mass which mean they heat-up quicker AND use less energy to get up to temperature and remain at temperature, so saving a lot of energy.
All in all they're fab!
*Air fryers increase heat transfer to the
surface of the food only. Away from the surface, the thermal transmissivity of the food and its thickness determines the overall cooking time. When you apply heat at the surface faster than the food can wick it away you get a burnt outside and a raw inside...see men and BBQs for examples
