  We introduce an algorithmic model of heat conduction, the thermodynamic
graph. The thermodynamic graph is analogous to meshes in the finite difference
method in the sense that the calculation of temperature is carried out at the
vertices of the graph, and the edges indicate the direct heat exchange between
the vertices. Recurrence relations of heat conduction in graph are derived
without using of differential equations and based on the coefficients of
thermal conductivity and heat capacity. This approach seems to be more direct
and flexible from the point of view of algorithmic modeling of thermodynamic
process than the derivation of difference schemes from differential equations.
We introduce also the notion of entropy of thermodynamic graph. We find the
maximum length of the time step at which the entropy does not decrease in the
general case. As a result, this give us the accurate boundary of the model
stability.
