want to read about : Controls and instrumentation - Engine speed
Turbine gas temperature
27. The temperature of the exhaust gases is always indicated to ensure that the temperature of the turbine assembly can be checked at any specific operating condition. In addition, an automatic gas temperature control system is usually provided, to ensure that the maximum gas temperature is not exceeded (Part 10).
|Fig. 12-7 Turbine thermocouple installation.|
29. The thermocouple probes used to transmit the temperature signal to the indicator consist of two wires of dissimilar metals that are joined together inside a metal guard tube. Transfer holes in the tube allow the exhaust gas to flow across the junction. The materials from which the thermocouples wires are made are usually nickel-chromium and nickel- aluminium alloys.
30. The probes are positioned in the gas stream so as to obtain a good average temperature reading and are normally connected to form a parallel circuit. An indicator, which is basically a millivoltmeter calibrated to read in degrees centigrade, is connected into the circuit (fig. 12-8).
31. The junction of the two wires at the thermocou- ple probe is known as the 'hot' or 'measuring' junction and that at the indicator as the 'cold' or 'reference' junction. If the cold junction is at a constant temperature and the hot junction is sensing the exhaust gas temperature, an electromotive force (E.M.F.), proportional to the temperature difference of the two junctions is created in the circuit and this causes the indicator pointer to move. To prevent variations of cold junction temperature affecting the ndicated temperature, an automatic temperature compensating device is incorporated in the indicator or in the circuit.
|Fig. 12-8 A typical double element thermocouple system.|
33. The output to the temperature control system can also be used to provide a signal, in the form of short pulses, which, when coupled to an indicator, will digitally record the life of the engine. During engine operation in the higher temperature ranges, the pulse frequency increases progressively causing the cyclic-type indicator to record at a higher rate, thus relating engine or unit life directly to operating temperatures.
34. Thermocouples may also be positioned to transmit a signal of air intake temperature into the exhaust gas temperature indicating and control systems, thus giving a reading of gas temperature that is compensated for variations of intake temperature. A typical double-element thermocouple system with air intake probes is shown in fig. 12-8.