What is the term for the difference in pressure between two points that causes air or gases to flow?

The term that specifically describes the difference in pressure between two points that causes air or gases to flow is known as "draft." Draft is a crucial concept in the context of fluid dynamics, particularly in systems involving combustion or ventilation. It is the pressure difference that drives the movement of air or gases through a system, such as the flow of combustion gases up a chimney or the intake of fresh air into a combustion chamber.

This pressure difference can occur due to various factors, including temperature variations, the design of the ventilation system, or the height of vertical ducts. In efficient boiler operations, maintaining proper draft is essential for effective combustion, ensuring that sufficient air enters the burner while allowing flue gases to exit properly.

Understanding draft helps boiler operators control the combustion environment, optimize fuel usage, and minimize emissions, directly impacting overall system performance and safety. The other options, such as pressure drop and flow rate, are related concepts but do not specifically describe the effect of pressure differences that lead to the movement of air or gases in the same way that draft does.