Why Most Public Buildings Install Two sets of Front Doors?

The front doors here means the swing door or sliding door .

The answer to the question Why Most Public Buildings Install Two sets of Front Doors? is to stop the air exchange from in and out of the building. A office building like a mall or a bank can count on the fact that tons of people are going to be going in and out all day. For a big building already paying lots of money for energy that could lead to huge additional, unnecessary costs. Having two doors where both are open at the same time relatively rarely is a simple solution to this problem. 

Or we could explain that creating a lobby could stop the internal air out of the building or the outer air go into the building every time someone open the front door. 

This is also the reason why the revolving doors exist. TSTC could provide all kinds of revolving doors to save the energy of the building, including the normal 3/4 wing revolving door, all glass revolving door or luxury 2 wing revolving door . 

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