The figure below shows a typical basic furnace used to raise the temperature of a heat-transfer medium that is used on other equipment in a plant process.
From the figure above, the temperature of the heat-transfer medium is the parameter that sets the demand on the heating system. The amount of heat required is manually set on the temperature controller TC, the output of which forms the set point of the fuel flow controller FCf, which receives its measurement from a flow sensor/transmitter, in this case a vortex flowmeter whose output is directly proportional to the flow. The controller output manipulates the control valve in the fuel line to regulate the amount dictated by the set point, which is in fact related to the temperature of the heat-transfer medium required by the process. One key point to note is in the fuel flow control loop where the measured and the controlled variable are the same. This situation occurs only in flow control loops and in no other. If a vortex flowmeter is not feasible because of cost or otherwise for this application, the figure above shows an alternative option, which is an orifice plate and DP cell. In this case a square root extractor will be needed to linearize the square law signal from the DP cell to make the air-to-fuel ratio “meaningful” because the output of a differential pressure (DP) transmitter used directly in any flow configuration has a square law relationship to the measured flow as demonstrated by Bernoulli.
Related: How to Connect a DP (Differential Pressure) Flow Sensor to a DP Transmitter
A venturi flowmeter measures the airflow, and like the orifice plate, this produces a differential head across the throat – the narrowest part in the middle of the venturi meter. The differential created is measured by a DP cell, which again has a square law relationship to the measured flow. The signal is applied to the square root extractor and is made linear, as a result of which the final measurement is directly proportional to airflow. The airflow controller FCa, sees both this measurement and set point provided by the ratio module and manipulates the air damper accordingly to achieve the desired value (i.e. set point). The ratio module applies a multiplying factor to give the calculated amount of combustion air in relation to the fuel flow. This calculation is trimmed by the amount of oxygen measured in the exhaust gas (flue gas), which alters the ratio module output to ensure complete combustion. An oxygen analyzer measures the oxygen contained in the flue gas using either a katharometer or a paramagnetic oxygen analyzer.
Related: The Operation of a Temperature Control System for lubricating oil
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