An up-flow hot wall hydride vapour phase epitaxy (HVPE) reactor with a single stationary substrate was designed for the growth of thick free-standing GaN substrates. The influences of (i) the reactor chamber geometry and aspect ratio, (ii) the orientation of the reaction chamber along the gravity field, (iii) the reactants' composition, (iv) the total flow through reactor, and (v) the pressure were systematically investigated using a computational fluid dynamics (CFD) model. The relative importance of the geometry vs the operating conditions is discussed. The reactor optimisation was performed against specific design objectives of high growth rate, high uniformity of the reactants' concentrations near the substrate and the efficiency of utilisation of the metal precursor. The results show that it is possible to achieve good uniformity for both ammonia and gallium chloride, with the V/III molar ratio around 10 near substrate at the absolute reactor chamber pressure 1 bar. The GaN growth rate was estimated to be similar to 400 mu m h(-1) assuming 100% conversion of molecules reaching the substrate. These performance criteria allow practical realisation of an efficient reactor system.