Why is a man hole door elliptical in shape?

1)     Any opening in a pressure vessel is kept to a minimum and for a man entry an elliptical hole is lesser in size than the corresponding circular hole.

2)     More over it is prime concern to have a smoothed generous radius at the corners to eliminate stress concentration

3)     To compensate for the loss of material in the shell due to opening, a doubler ring has to be provided around the opening. The thickness of the ring depends on the axis length along the direction in which the stresses are maximum and the thickness of the shell.