Thermal Driving Head

Thermal driving head is the force which causes natural circulation to occur. It is caused by the difference in density among two bodies or regions of fluid.

Consider two equal volumes of similar kind of fluid. When the two volumes are not of the similar temperature, then the volume with the higher temperature will also have a lower density and, hence, less mass. As the volume at the higher temperature will have a lower mass, it will also have less force applied on it by gravity. Such difference in the force of gravity applied on the fluid will tend to cause the hotter fluid to increase and the colder fluid to sink.

This effect is observed in many places. One illustration of this is a hot air balloon. The force causing a hot air balloon to go up is an outcome of a difference in density among the hot air within the balloon and the cooler air enclosed it.

Heats added to the air in the balloon add energy to the molecules of air. The movement of the air molecules rises and the air molecules take up more space. The air molecules within the balloon take up more space than the similar quantity of air molecules exterior the balloon. This means that the hot air is less dense and lighter than the surrounding air. As the air in the balloon is less dense, gravity has less effect on it. The outcome is that the balloon weighs less than the surrounding air. The gravity pulls cooler air down into the space engaged by the balloon. Movement of the cooler air in downward direction forces the balloon out of the space formerly occupied and the balloon get higher.