Combustion chamber pressure depends only on downstream equipment pressure. Usually there is an exhaust fan or tall stack at the tail end of the process that provides draft (negative to ambient atmospheric pressure), which keeps the flame from going backwards out through the combustion air supply. Natural gas pressure is always higher than either air inlet pressure or combustion chamber pressure, otherwise it wouldn't flow and the fuel supply source could become contaminated with a backflow of air which could become explosive.
Natural gas is a fuel; it requires oxygen (present in Air) for combustion chamber; direction of flow is towards the combustion chamber; rate of flow of air depends on the rate of flow of natural gas or any kind of fuel. ID & FD fan capacity depends upon these basic factors.
It should also be an excess air for complete combustion; therefore if the air pressure is low no complete combustion will occur. At constant Volume (of combustion chamber) pressure is directly proportional to temperature.
Usually there is some pressure drop across the burner, between the air inlet and the combustion chamber. This is the result of mixing of fuel and air in a localized high velocity zone, which could be reacting or not.
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