Vol. II · Heat Plate 3 of 11convection, heat-transfer coefficient, air, fan oven, water, steam, boundary layerHT

Convection.

A fluid moving past food carries heat to it. Water and steam carry far more than air, and moving air more than still air.

Outside the food, heat arrives by convection: a fluid — air, water, oil, steam — flows past the surface, gives up some of its heat and is replaced by more. Right at the surface sits a thin layer of fluid that hardly moves, the boundary layer, and heat must conduct across it. Fast-moving fluids keep it thin; dense fluids like water carry far more heat in it. The result is summed up in one number, the heat-transfer coefficient h.

The workshop puts the same chicken breast, 25 mm thick, into five media all at 100 °C — still air, fan-forced air, simmering water, boiling water and steam — and times how long its centre takes to reach 70 °C. Only the medium changes.

Below: centre temperature of a 25 mm chicken breast in each medium at 100 °C, and the time each takes to bring the centre to 70 °C.

What you will learn
Workshop
A 25 mm chicken breast from 4 °C in five media at the same temperature, 100 °C (the air cases with a dry surface that can evaporate). Heat-transfer coefficients are typical values: still air 12, fan air 28, simmering water 500, boiling water 800, condensing steam 1500 W/m² K.

Same temperature, different speeds

At 100 °C, steam brings the breast's centre to 70 °C in 11 min and boiling water in 11 min, while still air takes 1 h 39 min and fan-forced air 60 min. The air cases are slowed twice over: air carries little heat per degree, and the wet surface of the meat keeps evaporating, which cools it (III.5). Water and steam are fast because they are dense, they wet the whole surface and — in the case of steam — they release their latent heat as they condense on the cooler food (III.6).

Notice that the water and steam curves are close together. Once the heat-transfer coefficient at the surface is large, the surface sits almost at the medium's temperature, and the speed is set by conduction inside the food (II.1). Raising h further changes little. Raising it from air to water changes everything. That is why the fan in a fan oven matters, why a lid that traps steam speeds a braise, and why a pot of simmering stock cooks meat faster than a hotter oven.

At the stove

Set a fan oven and a still (conventional) oven, or one oven with and without its fan, to the same temperature, and roast two identical potatoes for the same time. The fan-oven potato will be further along. Many recipes say to lower the setting by about 20 °C for a fan oven for this reason.

Isaac Newton described convective cooling in 1701: the rate at which a hot body cools is proportional to its temperature excess over its surroundings — the relation behind every heat-transfer coefficient.

Terms

Convection
Heat carried to a surface by a moving fluid: air, water, oil or steam.
Boundary layer
The thin, slow-moving layer of fluid next to a surface, across which heat must conduct.
Heat-transfer coefficient
Watts per square metre per degree carried from a fluid to a surface; larger for denser and faster fluids.
Forced convection
Convection in which a fan or pump drives the fluid, thinning the boundary layer.
At the same temperature, water and steam cook several times faster than air, because they carry far more heat to the surface.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.