REFRIGERATION AND REFRIGERATOR INSULATION ON BOARD SHIP. 163 
x = Thickness. 
a,— Coefficient of surface flow entering the body. 
ao= Coefficient of surface flow leaving the body. 
k = Coefficient of total heat transmission per degree of difference of air temperature, per 
inch of thickness. 
For the balsa wood, the value of ¢, as determined by the Bureau of Standards, was equal 
to 0.35, expressed in English units and Fahrenheit degrees. 
The value of cork varies with the quality of the cork from 0.32 to 0.42. In practical 
application of cork, cracks or openings are likely to occur and these are usually filled with 
cement. The heat transmission through cement is known to be high, and as a result the 
cork insulation is, in practical application, likely to be less efficient than the balsa wood, 
which can be readily put up in such a way as to avoid air spaces. 
The formula, as stated above, applies to a wall of a single thickness; in the case of a 
wall made up of two different materials with an air space between, then it is obvious the 
formula would become as follows :— 
It is evident that the quantity of heat transmitted through any body is equal to that 
passing in succession each heat-resisting part. For example, if heat passes through a simple 
homogeneous body from a higher to a lower temperature, and from the air on one side to 
that on the other side, it must overcome (1) the resistance of the entering surface, (2) the 
resistance of the material composing the body, and (3) the resistance of the surface from 
which it emerges. The surface resistances, (1) and (3), are overcome by radiation and con- 
vection, the interior resistance, (2), by conduction. 
The surface transmission coefficients have been worked out carefully for numerous 
cases by Rietschel and other German engineers. These are given by Kinealy, in English 
units, as follows :-— 
(40¢ + 30a)¢ 
at c(1 +.0.004/) + d(z + 0.003) 
a@=c+adt 
c = 0.82 for still air. 
c = 1.03 for air with moderate velocity. 
c = 1.23 for air with high velocity. 
VALUES OF d. 
(Per degree diff. of temperature Fah. per hour) 
Gi Ckaw.Or lees ds% Sine ce Siac Siac wersiene sees 0.74 TronPsrustediinssy ek oar as oie 0.69 
ream a ee OMAN niCase: iron.) NeW qe thes tetas huts El 0.65 
plastemonbaris,:.\.casnlscastareertaes: 0.74 Sheet iron, polished ............ ... 0.009 
Stoneiasonry 2)... sists es wae} 0.74 Brass polished ieerea tanita: ceo 0.053 
WOO ee ett s ches tis cise nuts e one etnies 0.74 Copper: :polished eet brs eh Scr 0.033 
HAAG MBNA rics Leila Sim farses oboe sane 0.78 ‘ins polishediin saris wc ae steer © 0.045 
Glasser dinya Serine tus share yas 0.60 Zitic \polishedpiaee yy seers ae rers, 0: 0.049 
Glass, wet surface ................ 1.09 Zines Cla een eee a aee  sec sa 0.10 
