I have a cuboid shaped box. It is being used as a chilling tank in my refrigeration system based on Evan Perkins cycle. For heat load calculation I tried to calculate the heat leakage in the sysyem. So I considered conduction and natural convection. I am in preliminary stage, so no use of softwares and simulations. Can anyone help me using approximations and comparing with simple geometric shapes like flat plates or any kind of characteristics length concept.