As derived in section 2.1.1, the rate of change of the enthalpy of a vial equals the heat exchange with its surroundings, i.e. the heat flow Q(m,n,p) , and is therefore governing the evolution of the process. In terms of arrangement and geometry, we assume that vials are of cubic shape, completely filled, and surrounded by six direct neighbors, i.e., two in each dimension. For such system, the surface area of all sides of a vial, A, is the same. We note, however, that this model is generally applicable for any arbitrary shape and arrangement. The heat flow for a vial in position (m, n, p) is given by