Abstract: In the process for producing a homogeneous and finely crystalline silicon nitride ceramic, ceramic powder is milled, mixed with shaping aids, the mixture is shaped into parts and the shaped parts are subsequently sintered in a nitrogen atmosphere. According to the invention, sintering is carried out in a multistage process at temperatures T.sub.max in the range from 1700.degree. to 1900.degree. C. using gas pressures in the range from 0.3 to 50 MPa. In a first temperature stage, a temperature from 0.9 to 0.96.multidot.T.sub.max is maintained over a time period from 10 to 50 minutes at an N.sub.2 pressure from 0.2 to 1.2 MPa, while in a second temperature stage a temperature from 0.97 to 0.985.multidot.T.sub.max is maintained over a time period from 20 to 80 minutes at an N.sub.2 pressure from 3 to 6 MPa. Then the temperature is increased to T.sub.max and the pressure is increased to from 7 to 50 MPa.

Abstract: In the method of operating heat exchangers by means of hot gas streams containing condensable components, the hot gas stream (1) is divided into substreams (4) and cooled by means of obstacles (3) having a temperature T which is small in comparison with the entry temperature T.sub.I of the hot gas stream (1), in particular less than the dew point temperature T.sub.D of the hot gas stream (1). The substreams (4) are, after leaving the obstacles (3), combined with one another and, in their cores (8), divided into other substreams (6) and cooled by means of further obstacles (3) at the temperature T. This procedure is repeated until the hot gas stream (7) leaving the heat exchanger (2) reaches an exit temperature T.sub.O of from 200.degree. C. to 60.degree. C., preferably from 120.degree. C. to 80.degree. C.