A support sleeve 100 for supporting a high temperature process tube comprises one or more circumferential channels 120, 126, 124, and 122, each channel connected to either a feed for gas or a vacuum exhaust. One circumferential channel 120 opens to the top surface 108 of the sleeve 100, on which the process tube is supported to provide a gas/vacuum seal between the process tube and support sleeve 100. Another circumferential channel 124 is connected to a gas feed 114 and provided with gas injection holes 134, evenly distributed along the inner surface 106 of the support sleeve 100 to provide a cylindrically symmetrical injection of process gas into the process tube. Another circumferential channel 126 is connected to an exhaust 116 for gas and provided with gas exhaust holes 132, evenly distributed along the circumference of the support sleeve 100, to provide a cylindrically symmetric exhaust of process gases from the process tube.