Air movement and heat loss from sheep. III. Components of insulation in a controlled environment

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Abstract
The rate of sensible heat loss from a Clun Forest ewe was studied at several fleece depths in a temperature-controlled chamber. A simple resistance analog was used to describe the heat flow from different body regions. Heat loss from the trunk depends largely on the mean fleece depth .cxa.l. The fleece resistance was about 1.5 s/cm per centimeter depth. Heat transfer through the fleece was accounted for by molecular conduction, thermal radiation and free convection. The fleece conductivity .hivin.kb attributed to free convection depends on the mean temperature difference (.hivin.Tst-.hivin.Tct) across the fleece according to the relation .hivin.kb = 8.9 (Tst-Tct)0.53. Estimates of the sensible heat flux from the trunk at environmental temperatures, Ta of 0-30.degree. C range from about 8 W (.cxa.l = 7.0 cm, Ta = 30.degree. C) to about 160 W (.cxa.l = 0.1 cm, Ta = 0.degree. C). The sensible heat loss from the legs depends mainly on the local tissue resistance. For environmental temperatures of 0-30.degree. C, the calculated tissue resistance for this region of the body varied from about 8 to 1 s/cm. The corresponding heat loss from the legs was 10-20 W, compared with 3-7 W from the head. The fastest heat loss from the legs occurred at an environmental temperature of about 12.degree. C. Although the proportion of the heat loss from the extremities depends on environmental temperature, the total heat loss (sensible or latent) was closely related to the mean skin temperature of the trunk.

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