TABLES 286-287. 

 TABLE 286. Conduction of Heat across Air Spaces (Ordinary Temperatures). 



253 



Loss of heat by air from surfaces takes place by radiation (dependent upon radiating power of surface; for small 

 temperature differences proportional to temperature difference; follows Stefan-Boltzmann formula, see p. 247) 

 conduction, and convection. The two latter are generally inextricably mixed. For horizontal air spaces, upper surface' 

 warm, the loss is all radiation and conduction; with warm lower surface the loss is greater than for similar vertical 

 space. 



Vertical spaces: The following table shows that for spaces of less than i cm width the loss is nearly proportional 

 to the space width, when the radiation is allowed for; for greater widths the increase is less rapid, then reaches a maxi- 

 mum, and for yet greater widths is slightly less. The following table is from Dickinson and van Dusen, A. S. Refrigerat- 

 ing Engineers J. 3, 1916. 



HEAT CONDUCTION AND THERMAL RESISTANCES, RADIATION ELIMINATED, 

 AIR SPACE 20 CM HIGH. 



Variation with height of air space: Max. thermal resistance 

 20 cm high; 8.9 at 2.5 cm, 60 cm high. 



4.0 at 1.4 cm air space, 10 cm high; 6.0 at 1.6 cm, 



TABLE 287. Heat Convection in Air at Ordinary Temperatures. 



In very narrow layers of air between vertical surfaces at different temperatures the convection currents, in the 

 main, flow up one side and down the other, with eddyless (stream-line) motion. It follows that these currents trans- 

 port heat to or from the surfaces only when they turn and flow horizontally, from which fact it follows, in turn, that 

 the convective heat transfer is independent of the height of the surface. It is, according to the laws of eddyless 

 flow, proportional to the square of the temperature difference, and to the cube of the distance between the surfaces. 

 As the flow becomes more rapid (e.g., for a 20 difference and a distance of 1.2 cm) turbulence enters, and the above 

 relations begin to change. For the dimensions tested, convection in horizontal layers was a little over twice that in 

 vertical. 



Taken from White, Physical Review, 10, 743, 1917. 



Heat Transfer, in the Usual C.G.S. Unit, i.e., Calories per Second per Degree of Thermal Head per Square Cm of 

 Flat Surface, at 22.8 Mean Temperature. 



Where two values are given, they show the range among determinations with different methods of getting the tem- 

 perature of the outer plate. It will be seen that the value of the convection is practically unaffected by this difference 

 of method. 



SMITHSONIAN TABLES. 



