The 100mm gap between columns and the enclosure is to aid convection cooling, also make sure that any internal ducting installed does not impact on this distance as well by obstructing the module ventilation slots. If you spread the 10mm shortfall across the cubicle it is only 3.5mm per gap, but that's a design decision for you to make.
Also, I am not sure if it's important to your application but your layout would appear to be only good for a 40degC ambient, not 55degC.
a.) The TB840A is only rated to 40degC when fitted on a vertical rail (3BSE020923-510, p171, Note 11). There is a kit to rotate the TU by 90deg to achieve 55degC if required or alternatively, just add a horizontal DIN rail for the TU840/841. Either solution will require additional Module Bus Cable Adapters and module bus cables to connect the TU to the first MTU.
b.) I understand that using a horizontal redundant MTU in a vertical configuration also limits the maximum temperature to 40degC. I can't find a reference to this, however I seem to remember that the 55degC rating is only achieved if the IO module is installed vertically when MTUs are stacked vertically. Your installation layout (in particular, the spacing between modules) is similar to using compact MTUs, which has a 40degC limit in vertical service. Maybe someone else can clarify.
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Thanks a Lot for reply.We are ensuring all things with respect to maintenance point of view however ABB catalog recommening (3BSE020923-510 B@page 103)100mm from all side of Enclosure as well as between two cluster.We are getting 100mm from everywhere expect left side where we are geting 89mm.Is this make any issue in functionality point of view? Please suggest.