Thermal turndown

Fig. 4.13. Integrated ratio actuator controls air/gas ratio by comparing pressure drops across air and gas orifices. It automatically compensates for varying air temperature, thus providing mass flow control. An adjustment allows use of low-fire excess air for thermal turndown. Courtesy of North American Mfg. Co.

Coal burners Type Size range (MW) Fuels Grate thermal loading (mW/m ) Bed thickness Turndown ratio Ashing system Main applications... 

Energy balance thermal mass meters require one heating element located between two temperature sensors as illustrated in Figure 4.21 [24]. Although several design variations exist, their operation is basically similar. As the fluid flows past the heating element, it absorbs heat. This heat is carried downstream where it is transferred to the downstream temperature sensor. The temperature difference between the upstream and downstream sensor is detected. This output signal is then converted into a mass flow rate. These meters typically have a turn down ratio of 10 1 while the constant temperature and constant power meters have a turndown ratio of 1000 1 and 100 1, respectively. 

Long service life, capability for a wide range of turndown to match the thermal load, no flashback potential, and simple maintenance. 

For quasi-static measurements such as illustrated in Figure 8.2, the glass transition temperature, Tg, is often taken at the maximum rate of turndown of the modulus at the elbow, where E = lO Pa. Often the glass transition temperature is defined as the temperature where the thermal expansion coefficient (Section 8.3) undergoes a discontinuity. (Enthalpic and dynamic definitions are given in Section 8.2.9. Other, more precise definitions are given in Section 8.5.)... 

Can minimize weeping good turndown ratio low pressure drop can support internals can undergo thermal expansion without damage ports are easily shrouded well suited to multilevel fluid injection solids can flow from above the grid to below. 

At the termination of the riser it is important to have quick separation of reaction mix from spent catalyst. After the riser, the reaction mix can remain in the reactor vessel for over 20 seconds before it enters the reactor cyclones and is separated from the spent catalyst. Typically, catalyst densities between riser outlet and cyclone inlet average only 1 to 3 Lb/Ft. During that 20+ seconds additional conversion can occur, but since the catalyst is spent the conversion is thermal in nature and not selective to gasoline. Ross (1990) reports commercial information (Figure 15) for a simple riser turndown that shows a 4 LV % FF conversion gain between riser outlet and cyclone inlet. Even though conversion increased, gasoline yield went down. 

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