First principle of thermodynamics

Considering its most simplified version, it establishes the relationship between :

• the thermal power P absorbed by the fluid in contact with the solid
• the difference between the temperature at the output T_o and at the input T_i of the cooler (hsf or wb)
• the volumetric flow rate D (in m ³ / s)
• the specific heat of fluid c
• the density of the fluid r

The thermal power P is proportional to all the remainder, and that gives :

P and T_i are given and do not depend on the cooler considered (P depends on the CPU and T_i from what occurs before the input). We are looking for the coldest solid surface, therefore the fluid temperature should also be the coldest possible along its course.
Thus it is necessary to look for T_o closest to T_i. Writing this first principle of thermodynamic like that :

then a better flow rate D, a better density r and a better specific heat c is desired (actually the product of both). Concerning flow rate, this joins what we saw thanks to the interpretation with the Fourier's Law, i.e. the more it is, the more velocity also is. With regard to the two remainders, density and specific heat, it depends entirely on the choice of the fluid, since they are physical properties.

The last thing in connection with this principle, let us compute T_o - T_i and you will notice that this difference is rather small, generally smaller than 1 °C for water with the usual flow rate of a water cooling system.