What is a method to check the relative quantity of air being supplied to the furnace?

The correct method to check the relative quantity of air being supplied to the furnace is through flue gas analysis. This process involves measuring the composition of the gases emitted from the furnace after combustion occurs. By analyzing the levels of oxygen, carbon dioxide, and other components in the flue gas, it's possible to determine if there is an appropriate amount of air relative to the fuel being burned.

In an ideal combustion process, there should be a specific ratio of air to fuel to ensure efficient burning and minimal emissions. If the analysis shows too much oxygen, it indicates that there is excess air being supplied, which can lead to lower combustion efficiency. Conversely, if there is too little oxygen and elevated levels of carbon monoxide, this indicates a lack of sufficient air for combustion.

With respect to the other methods listed: pressure drop measurement primarily evaluates airflow, but it doesn’t directly indicate the effectiveness of combustion; fuel flow rate analysis focuses on how much fuel is being delivered rather than the air supply; and heat input calculation assesses the energy content of the fuel being used but doesn't give insights into the air-fuel ratio necessary for optimal combustion. Thus, flue gas analysis is the most accurate method for assessing the relative quantity of air in relation to the combustion process.