Effect of Altitude on Temperature Rise

The problem is to determine the effect of altitude in increasing the temperature rise of electrical apparatus. Stated in detail, heat is dissipated from the various surfaces of apparatus principally by convection and radiation. The former is a function both of temperature rise and of air density, that is, of altitude; the latter, of temperature rise only. The energy loss in the apparatus, that is, the heat to be dissipated, is not changed significantly by altitude. Thus, lower air density at higher altitude means decreased convection. Hence, higher temperature, which increases both convection and radiation, is required to carry off the same heat. Specifically, then, the problem is to relate these several factors so that quantitative calculations can be made for various types of apparatus. An investigation was made in 1921 to determine how dissipation by convection varies with temperature, air density, and also with respect to the air movement-i. e., whether it is a blast or merely the natural movement created by temperature difference: in other words, whether it is ''forced'' or ''free'' convection. The results check reasonably with those obtained by Montsinger
¹
, Rice
²
and others. The law of radiation, of course, has long since been well known. The tested apparatus, consisting of electrically heated plates, was enclosed in a large wooden tank or drum, in which the air pressure could be controlled.