I was hoping someone could lend a hand with a question about thermodynamics. It's been years since I've done work on thermodynamics. Let's say I have a plastic 275 gallon liquid container, filled with a mixture of Propylene Glycol and Water (50/50 by volume). The container is placed inside of a cold chamber in order to chill it (to -18 °C). Both the inside of the chamber and the container with liquid start at a temperature of +23 °C. 2 Questions 1. How do I find the rate of heat transfer in order to estimate the time needed to reach thermal equilibrium? 2. If I compare conditioning the room temperature container/liquid for a period of 72 hours to conditioning it for only 24 hours with a room temperature container and pre-chilled mixture, how can I determine if the plastic has reached equilibrium? Several assumptions that I am making: 1. There is no mass exchange between the container and the environmental chamber 2. There is no mass or heat exchange between the inside of the chamber and the lab environment. 3. The container has a uniform wall thickness of 1 inch all around 4. The idealized shape of the container is a cube, with raised corners (think Lego block with 4 pegs) 5. The container has minimal contact with the chamber floor due to its design, therefore heat transfer due to conduction can be considered negligible 6. Speed of air flow from chamber cooling is 20 mph and keeps chamber at a constant -18 °C The main problem I am having is defining the system. Should I use a lump method or determine heat transfer at the level of the container?