process and device for cooling a mixture of polymer and monomer from a reactor in which ethylene is polymerized or copolymerized at a pressure greater than 1 , 000 bars between a pressure - reduction valve located downstream of the reactor and a medium pressure separator . the cooling is achieved by injecting monomer at a pressure below that of the separator using a device with a nozzle distance s 1 and corresponding cross - section a 1 the mixture from the reactor passes , a convergent mixing zone in which the mixture is mixed with the monomer injected , which is supplied at a flow rate q , and a diffuser of throat distance s 3 and corresponding cross - section a 3 , which makes it possible to bring the mixture to the pressure of the separator . the dimensions of the device are such that the ratio q / a 1 is between 0 . 20 and 1 . 35 t / hr . mm 2 and the ratio ## equ1 ## is between 0 . 1 and 0 . 3 t / hr . mm 2 .