690 MECHANICAL DESIGN AND PACKAGING 



As stated above, the objective of a cooling system is to keep the tempera- 

 ture of each component low enough to allow its satisfactory operation 

 during the required period of time. While Equation 13-2 gives the necessary 

 condition for the steady state, i.e., that the heat lost equal the heat gained, 

 it is not a sufficient condition for the proper cooling of each component. 

 The latter depends upon achieving satisfactory paths for the heat flow from 

 the sources (including environment) to the cooling fluid which acts as a heat 

 sink. The means for accomplishing this varies with the type of cooling used. 

 For example, in systems similar to those in Fig. 13-3a and b, the problem 

 essentially is to meter the correct proportion of the available air to each 

 heat-producing component. The provision of a good conductive path is 

 essential in systems similar to Fig. 13-3c and would be helpful to systems 

 such as in Fig. 13-3a and b. In a system such as Fig. 13-3d, the natural con- 

 vection is negligible; the flow of heat to the cooling fluid depends upon 

 conduction, forced fluid circulation, or a combination of the two. 



The circulating cooling fluid in a sealed or closed-cycle system or portion 

 of a cooling system must be chosen by the designer. In principle, a variety 

 of gases, liquids, and mixtures is available. Air is most often used, since it 

 has adequate thermodynamic and dielectric characteristics for most 

 purposes, aside from being the most available fluid. Helium and hydrogen 

 have superior heat-transfer properties which may justify their consideration 

 in specific cases. Various liquids have favorable characteristics; their 

 weight, however, restricts airborne cooling applications. Although there 

 are evident disadvantages to such a scheme, the use of air with entrained 

 liquid has been seriously advocated for the cooling of airborne equipment. 

 This would combine the light weight and low viscosity of a gas with the 

 high latent heat of evaporation of a liquid, producing the effect of a gas 

 with exceedingly high specific heat. 



Since entrained liquid is usually not desired in cooling air, it is necessary, 

 whenever refrigerated ram air is to be used, for a designer to ascertain 

 whether water vapor in the air can be expected to condense. It is shown in 

 various treatises on the thermodynamics of moist air that the maximum 

 amount of water vapor which will remain in a stable mixture with air is 

 given by the following expression : 



W. = 18/29(P.//> -/>.). (13-5) 



In this equation, W^ is the number of pounds of water vapor per pound 

 of dry air, in a saturated mixture. The coefficient 18/29 is the ratio of the 

 molecular weight of water to the "apparent molecular weight" of air. The 

 quantity p is the total atmospheric pressure. The quantity ps is the 

 saturation pressure of water and is a function of temperature which is 

 tabulated in engineering handbooks. To determine whether condensation 

 may take place, the cooling-system designer calculates the worst tempera- 



