We study a system of disks with inelastic collisions. Energy is given to the system via a force of constant modulus, applied on each particle in the direction of its velocity. This system shows a dynamical phase transition from a disordered system towards a system organized into a vortex when the dissipation is increased. The state equation of the disordered system is obtained by using kinetic theory arguments. We also show that the transition is similar to the kinetic-cluster transition observed in the cooling of a gas of inelastic particles