Anytime there is a significant change to the feed slate, a review of the operating variables is needed. Light feed normally means a low concarbon value. Those types of feeds can require more energy per unit mass than heavier feeds to fully convert the feed to lighter products and coke. This can be done by increasing the heater outlet temperature or by extending the cycle time or some combination of both. The drum outlet temperature is an indicator if the heater outlet temperature is high enough. A drum outlet temperature of 440 C (825 F) is needed to fully thermally decompose the feed into lighter products and to cross link the coke to make it hard and mechanically stable.
With a light feed, the temperature drop from the drum inlet to the drum outlet is higher than with heavy feed. This requires the heater outlet temperature to be higher with light feed than with heavy feed.
Anytime there is a significant feed change, the first few drums should be closely monitored to ensure the operating conditions are suitable for the feed.