If the percentage of fuel in a fuel—air mixture falls below a certain value called the lower flammability limit (LFL), the mixture cannot be ignited. For example, the LFL of propane in air is 2.05 mole% C₃H₈. If the percentage of propane in a propane—air mixture is greater than 2.05 mole%, the gas mixture can ignite if it is exposed to a flame or spark; if the percentage is lower than the LFL, the mixture will not ignite. (There is also an upper flammability limit, which for propane in air is 11.4%) A mixture of propane in air containing 4.03 mole% C₃H₈ (fuel gas) is the feed to a combustion furnace. If there is a problem in the furnace, a stream of pure air (dilution air) is added to the fuel mixture prior to the furnace in let to make sure that ignition is not possible.

(a) Draw and label a flowchart of the fuel gas-dilution air mixing unit, presuming that the gas entering the furnace contains propane at the LFL, and do the degree-of-freedom analysis.

(b) If propane flows at a rate of 150mol C₃H₈/s in the original fuel—air mixture, what is the minimum molar flow rate of the dilution air?

(c) How would the actual dilution air feed rate probably compare with the value calculated in part (b)? (>,<, =) Explain.