  We investigate the thermodynamics of general fluids that have the
constriction that their pressure is constant. For example, this happens in the
case of pure dust matter, for which the pressure vanishes and also in the case
of standard dark matter phenomenology. Assuming a finite non-zero pressure, the
corresponding dynamics is richer than one naively would expect. In particular,
it can be considered as a unified description of dark energy and dark matter.
We first consider the more general thermodynamic properties of this class of
fluids finding the important result that for them adiabatic and isothermal
processes should coincide. We therefore study their behaviors in curved
space-times where local thermal equilibrium can be appealed. Thus, we show that
this dark fluid degenerates with the dark sector of the LCDM model only in the
case of adiabatic evolution. We demonstrate that, adding dissipative processes,
a phantom behavior can occur and finally we further highlight that an arbitrary
decomposition of the dark sector, into ad hoc dark matter and dark energy
terms, may give rise to phantom dark energy, whereas the whole dark sector
remains non-phantom.
