Example

A person wearing a white shirt and pants (e = 0.9) at room temperature will radiate energy.

In SI units:

q = (5.6697)(10_8)(0.9)(W/m2K4)(293)4(K)t = 376 W/m2

In English units:

q = (0.1713)(10~8)(0.9)[Btu/(h)(fl?)(°R)4](528)4(oR)4 = 119.8 Btu/ihXft2)

Checking units:

where

Since there is an interchange between two bodies, the net amount of radiation received from a source is the radiation received minus the radiation returned or emitted. The combination of the heat being absorbed by a food and the amount being emitted gives an equation or the net heat transfer to a food from a radiation source:

q = <r([(%aiiFs.Je)(rs)4] - \_(eRasFR_s)(TRft) (24)

where a is the Stefan-Botzmann constant, es is the source emissivity, aR is the receiver absorptivity, FS.R is the view factor, source to receiver, Ts is the temperature of source, eR is the receiver emissivity, as is the source absorptivity, FRS is the view factor, receiver to source, and TR is the temperature of receiver.

The amount of energy absorbed from a body depends on how much of the radiating body can be "seen" by the receiver. In most situations involving the processing of food by radiant heat transfer, the view factor is equal to unity since the source is directly below or adjacent to the receiver. Also, the emissivity is essentially the same as the absorptivity in a food processing operation. The emissivities for many surfaces have been determined experimentally and are available in standard data tables. The emissivity of many foods is approximately 0.9, while hot metal (as in the case of oven heating units) has a wide range emissivity values, from as low as 0.1 to 0.6 or higher.

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