Total air flow rate needs to be considered when making total heat dissipation calculation. If VFDs will be installed with no space in between, then the maximum allowed ambient temperature limit will be lower. If there will be multiple rows of VFDs, a metal plate should be installed between the rows, together with a proper space between the plate and upper and lower row. Please see the related section in user manuals for exact dimensions and calculations. ie. Installation and Mounting for more details.