Application Notes

EAP-030025

What is a process diagram of a typical temperature control system for a furnace?

The figure shows a simplified temperature control system of a gas-burning furnace which uses a thermocouple for sensing the furnace temperature (The actual system has more control elements).

The furnace is heated with the gas burner to which fuel (gas) is fed through the control valve. The thermocouple senses the furnace temperature. The thermocouple emf is converted to the standard 4-20mAdc signal with the thermocouple transmitter. This is transmitted to the controller as the measured temperature or the process variable (PV). It is compared with the set point (SP). When there is deviation of PV from SP, the controller changes its control output (MV), which is sent to the control valve. MV determines the valve opening, in turn changing the fuel flow to the furnace.

We recommend using the thermocouple transmitter with burn-out function. The effect of the burn-out function is as follows:

The output of a thermocouple transmitter with the burn-out function increases its output signal, if the thermocouple burns out. The controller interprets the signal increase as if the furnace temperature became higher. It changes the control output to close the control valve. The burn-out function reduces fuel (gas) when the thermocouple burns out.

Assume that the thermocouple burns out and the transmitter has no burn-out function. Since the transmitter loses the input signal, its output is suddenly reduced as if the furnace temperature is lowered. In response, the controller changes its output to open the control valve wider. Even though the furnace temperature becomes high, the emf from the thermocouple remains zero, signaling to continue heating. If the failure is undetected, the furnace overheats and may be damaged.