According to the information provided in (8) Bg may reach a maximum value of about 0.9 (Bs = 0.36) in continuous operation in good "Imbert" type gasifiers. Higher values of Bg give rise to extreme pressure drop over the reduction zone of the equipment.
Minimum values of Bg depend essentially on the heat insulation of the hot zone. Below a certain hearth load the temperature in the hot zone is lowered so much that tar production becomes inevitable.
Normal "Imbert" type generators show minimum values of Bg in the range of 0.30 - 0.35, resulting in power turn-down ratios of a factor 2.5 - 3. Modern gas producers are better insulated, and can operate tar-free at Bg values of 0.15 - 0.18.
Designing an "Imbert" type gasifier now boils down to estimating the maximum amount of gas needed. This is easily done by taking into account the cylinder volume and number of revolutions as well as the volumetric efficiency of an internal combustion engine coupled to the system (see section 2.1 and Appendix 1). From this gas amount as well as from the B maximum value (0.9) the area of the smallest constriction and the diameter of the throat can be calculated.
The Swedish Academy. of Engineering Sciences (43), also presents empirical data with regard to the height of the nozzles above the narrowest constriction, the diameter of nozzle opening ring as well as of suitable nozzles for different capacities.
These data are reproduced in Figures 2.12 to 2.14 and in Table 2.7.
Figure