I am setting up a hydraulic system on a cable laying machine. We dont have much space for the tank, and I am thinking of constructing it on the "wall" og a big 1m^3 stainless steel sand holder. That would make the tank With 5 faces in open air, 1 face With sand on the opposite side. I am wanting to calculate the heat loss I can expect though the steel wall to the sand mass. Specifically if this will be better or worse than being Oil/wall/air I have heat transfer values for the materials, I just dont know how to set up the an Equation for multiple layers.. Thanks in advance.