A gas-fired furnace has a temperature measurement of the outlet temperature of the process fluid, a flow rate measurement on the process fluid, a flow measurement for the fuel to the furnace, and a control valve on the fuel line to the furnace.

a. Draw a schematic diagram of a control system that adjusts the ratio of fuel flow to the flow rate of the process fluid to control the outlet temperature of the process fluid. A flow control loop is used to maintain the measured flow rate at the level specified by the ratio controller.

b. A high tube-skin temperature overrides the set point for the flow controller on the fuel to the furnace (must clearly label the override controller and provide an override diagram).