6 Loss of Heat from Covered Steam Pipes 



ones, give the values of the heat loss by air contact per square 

 foot per hour per one degree. 



Figure i gives the coefficients by which these values are to 

 be multiplied for any given difference of temperature between the 

 surface of the body and the air. 



Assume that the outside diameter of the covering in the ex- 

 ample used above for radiation loss is six inches, and that the pipe 

 is horizontal. Then under the same conditions, a difference of 

 temperature of 40 degrees, the air contact loss is 

 A X 40= (.52 X 1.13) (40) = 23.5- 



The combined loss for this square foot of surface of covering 

 under the conditions given is therefore, 



39.2 + 23.5 = 62.7 B. T. U. per hour. 



In a test the temperature of the air should be measured be- 

 fore the air reaches the heated covering and the thermometer 

 should be protected from radiation from the covering. 



We may now consider the conduction of the heat through the 

 covering. 



The law for a flat plate of insulating material is very simple ; 

 the quantity of heat transmitted per square foot per hour varies 

 directly as the conductivity of the material, inversely as its thick- 

 ness, and directly as the difference of temperature between the 

 two surfaces of the plate. 



Note that it is the temperatures of the surfaces of the plate, 

 not of the air in contact with the surfaces. 



The formula for a flat plate is then, 



. 



E 

 where / and ( are the surface temperatures, 



C the coefficient of conductivity, 



E the thickness in inches, and 



M the heat transmitted in B. T. U. per square foot per hour. 



For a cylinder, the principle is the same but the expression 

 changes. Consider a section one foot long. 



lyet R and R be the inside and outside radii, in feet, of the 

 cylinder of insulating material, 



/ and f the respective surface temperatures, and 



the temperature of the surrounding air. 



