One calls significant heat the quantity of Chaleur which is exchanged, without Transition from phase physical, between two bodies forming an isolated system.

The significant qualifier is justified by the fact that the heat transfer, without physical phase shift, corresponds to a variation of the Température of the body, which can be felt or measured for an observer. In that, significant heat is opposed to the Latent heat, which, it, does not result in a change of temperature, but by a phase shift (for example, under normal conditions, the melting point of the ice (pure water) is 0°C).

Calorimetric relation

Mathematically, the significant quantity of heat Q exchanged by a body which passes from a temperature T1 to a T2 temperature is given by the relation:

where m is the mass of the body (in kg) and C is the specific heat of this body (in J.kg.K).

C is also called mass heat capacity or specific heat which corresponds to the contribution of heat necessary to raise the temperature of the unit of mass of the substance of 1 Kelvin (1 K) at the time of the transformation considered. This size C is characteristic of a given substance.

For the heat transfers uninterrupted, the system works at a stationary normal rate compared to time, but in transitory mode compared to space. Therefore, for the dimensioning of the exchangers of heat, one uses usually the average logarithmic curve of the differences in temperatures (LMTD) such as:

where:

K is the total coefficient of heat transfer (out of W mK)

With the surface (in m)

P thermal power (out of W, therefore in J.s), with: P=Q/t (T in second)

LMTD is the average logarithmic curve of the differences in temperature is

with: ∆TE: the difference in inlet temperature of the fluids and ∆TS: the difference in temperature of exit of the fluids

Heat capacity of the solids

At first approximation, the heat capacity of the solids is described by the Loi of Dulong and Petit

Heat capacity of the liquids

At first approximation, the heat capacity of the liquids is constant. For water: C = 1 cal G K (= 4,18 J kg K). in a general way: 0,5 cal G K < C < 1,0 cal G K

Dependant concepts

• Calorie