heat transfer software for tank/digesters

[thickasabrick]

Hello,

I'm looking for a software (freeware if possible)to calculate heat losses from biogas digesters/tanks.

I find it especially hard to estimate the heat loss due to the gas stream out of the digester and the walls in contact with the gas phase, heat losses from the wetted part of the walls is quite straight forward though.

I hope someone has a good tip on where to find something useful.