What is the purpose of selecting a low temperature at the furnace outlet and a small temperature difference between the inlet and outlet of the furnace?

I’ve never seen such a control scheme; usually, the amount of gas is controlled based on the temperature at the furnace outlet.

Generally, there is a cascade control scheme for the furnace outlet temperature and the pressure difference before and after the main gas burner

Is it a concern that the process will be shut down, the medium inside the furnace tubes will stop flowing, and it will be heated under confinement? Provide the logical relationship for low selection and the low selection value.

What parameters are associated with low selection? Is it the amount of fuel?

The furnace outlet temperature is controlled via a low-value selection based on the temperature difference between the inlet and outlet, followed by series control with the mass flow rate of the fuel gas; subsequently, the pressure of the fuel gas is used for high-value selection to control the furnace temperature.

The lower of the furnace outlet temperature and the temperature difference between inlet and outlet is selected. Is it the low temperature difference between the inlet and outlet that causes the interlock to shut off the gas?

The furnace outlet temperature is controlled via a low-value selection based on the temperature difference between the inlet and outlet, followed by series control with the mass flow rate of the fuel gas; subsequently, the pressure of the fuel gas is used for high-value selection to control the furnace temperature. The temperature difference between the inlet and outlet of the furnace is clearly lower than the outlet temperature, as the inlet temperature cannot be negative. In other words, the lower value chosen is always the temperature difference between the inlet and outlet, which seems to be a nonsensical setting. Is there any operating condition under which the temperature difference between the inlet and outlet could be lower than the outlet temperature? Other than instrument failure, I can’t think of any other possibility. For general selection control, the control parameters chosen are those required by the process; another set of parameters relates to safety interlocks. For example, in this control scheme, the mass flow rate and pressure of the fuel gas are subject to selection control, with pressure control serving as a safety measure to prevent the furnace from shutting down due to low gas pressure resulting from adjustments only to the gas mass flow rate. But in conventional designs, the secondary circuit can be set to the gas pressure, as pressure is much more stable than flow rate. Additionally, the interlock based on the low pressure of the pilot light and the main fuel gas is sufficient, so such a control option doesn’t hold much significance. For low medium flow rate in the furnace tube, a low circulating hydrogen flow rate interlock is sufficient. For the outlet temperature, two options were provided along with cascade control, which complicates what should have been a simple issue. The original poster could ask about the design approach; I’d like to learn from it as well.

Thank you for participating. I’ll go ask the designer later; I really don’t understand it.