Air is flowing through a steam-heated tubular heater under such conditions that the steam and wall resistances are negligible in comparison with the air-side resistance. Assuming that each of the following factors is changed in turn but that all other original factors remain constant, calculate the percentage variation in q/Δ̅T̅_L that accompanies each change 

(a) Double the pressure on the gas, but keep fixed the mass flow rate of the air. 

(b) Double the mass flow rate of the air. 

(c) Double the number of tubes in the heater. 

(d) Halve the diameter of the tubes.