Hot gas flows into a cylinder. A pump pushes the gas into a smaller cylinder. The pressure and the temperature of the gas rises. The gas is released into a larger cylinder. The pressure and the temperature falls. The cold gas goes past a cabinet. The gas cools the cabinet. Food in the cabinet gets colder. The gas gets hotter.
b.
Gas flames heat the bottom of a cylinder. The cold water at the bottom of the cylinder takes heat from the metal. The heat puts more energy into the water. The hot water rises to the top of the cylinder. A pump pulls the hot water out of the cylinder Cold water flows into the bottom of the cylinder.