16 is joined to an external pump 8 by a pipe 18 and the opening 17 is also joined to the said pump 8 by a pipe as described below in the embodiments shown by way of non limiting examples in fig1 and 8 a heat exchanger 6a 6d is mounted in the pipe 19 which joins the side opening 17 made in the bottom 5 of the autoclave 1 to the pump 8 however whereas in the embodiments shown in fig1 and 2 the heat exchanger 6a is arranged in the pipe 19 outside the autoclave 1 in the embodiment shown in fig8 the heat exchanger 6d is housed in a pipe 35 which joins the pump 8 to the treatment tank 10 and more precisely inside the expansion zone 35 which is located inside the expansion zone 13 the inner end of the said pipe 35 coming out in the partition 9 in the embodiments shown by way of non limiting examples in fig3 to 5 the opening 17 is joined directly to the pump 8 by a pipe 21 and the heat exchanger is placed in a housing provided in the expansion zone 13 in the lower part of the latter facing the opening 17 in the embodiment shown in fig3 the heat exchanger 6b is placed in a non leaktight housing 22 which joins the treatment tank 10 to the expansion zone 13 in the embodiment shown in fig4 and 5 the heat exchanger 6c is placed in the housing 23 which is leaktight in relation to the treatment tank 10 and.