In relation to combustion, what does the term "draft" describe?

Draft refers to the difference in pressure that causes the movement of air and flue gases through a combustion system. It is crucial to maintaining proper air flow into the combustion chamber and ensuring that the products of combustion are effectively vented out.

Draft helps to sustain combustion by ensuring that there is enough oxygen available for the fuel to burn efficiently. A negative draft, for example, occurs when the pressure inside the combustion chamber is lower than the atmospheric pressure, thus drawing in air for combustion through combustion air inlets. This process is vital for optimizing the combustion efficiency and minimizing emissions.

The other options describe different aspects of combustion but do not accurately define "draft." Understanding draft is essential for those in power engineering, as it directly impacts the efficiency and safety of combustion systems.