A delayed coker unit thermally cracks vacuum residue (VR) and converts it into products like liquefied petroleum gas (LPG), naphtha, gasoil and petroleum coke. A majority of the thermal cracking takes place in the coke drum. Coke formed during the cracking reactions is deposited on the inner tube surface. As the coke layer grows, resistance to heat transfer increases. Result: More fuel firing is needed to maintain the furnace outlet temperature.