Control of Energy Transfer in VS .NET

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The vessel is in a high state of turbulence and naturally exhibits a certain hydraulic resonance, which was described in Chap. 3. But liquid level is affected thermodynamically as well. For example, the sudden introduction of feedwater below its boiling point can momentarily reduce the heat content of the vapor-liquid mixture in the tubes, causing bubbles to collapse, and reducing the apparent liquid level. Thus the response of the level-control loop has a tendency to start in the wrong direction. This property is called phase shifting and is similar to dead time in that it produces phase lag without attenuation. The result is that the period of the liquid-level loop is ordinarily several minutes, although its natural hydraulic resonance may be only a few seconds long. If the boiler must operate under varying steam pressure, the calibration of the liquid-level t ransmitter will vary with steam density.3 But pressure has a transient effect too. If a load increase (withdrawal of steam) is sufficient to cause drum pressure to fall, some of the water in the tubes will flash into steam, temporarily increasing the flow of bot h liquid and vapor into the drum. This effect is called (swell, because it causes a transient rise in liquid level, even though the rate of steam withdrawal may moment,arily exceed that of feedwater flow. Conversely, upon a pressure increase, the liquid level tends to shrink. This effect is more prominent in low-pressure boilers, because of the greater difference between the densities of steam and water. The most favored method of coping wit,h shrink and swell is to ignore them, by letting the forward loop carry the load, while maint aining loose settings on the level controller. Drum-level controllers customarily require a proportional band near 100 percent and several minutes reset time. Pressure in a saturated or even a superheated boiler is a measure of the amount of energy st ored therein. The flow of steam from the plant is usually at the demand of t he user. Pressure can only be maintained, then, if the flow of energy into the boiler equals the rate of withdrawal. Since the drum-level control system admits feedwater at a rate equal to the flow of steam, the pressure-control system is left to manipulate the input of thermal power. To achieve high performance control, a feedforward loop should be used to set firing rate proportional to steam flow. Steam flow is a measure of thermal power and is affected by firing rate. If it alone is used to set firing rate, a positive feedback loop will be formed, a pitfall that was mentioned under decoupling systems in Chap. 7. What is really needed is a steam-flow demand signal. The demanded steam flow IV0 is the measured steam flow less the rate of loss of boiler contents:
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FZG 9.9. The ratio of firing rate to steam flow demand is automatically adjusted to maintain pressure in the boiler.
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where V represents the steam volume of t.he drum. It is more convenient, however, to use pressure error instead of dp/dt, since differentiation is a clumsy operation and t,he error signal is already available at the pressure controller: WD = w + K(PD - p> (9.19) The coefficient K is adjusted to the characteristics of the boiler; pi represents the set point of the pressure controller. In the steady state, p = pD and lYD = 1Y. Sudden opening of a steam valve will raise W but drop p. Until the effect of a firing-rate change is felt, pressure w ill fall to a lower level and steam flow will return to its previous value. But the existence of a pressure error directly proportional to the desired increment in steam flow will maintain the higher level of firing. The pressure is only restored when steam flow is raised to its demanded value. Figure 9.9 outlines the system together with the feedback loop. When steam is superheated, its enthalpy becomes a function of temperature and is relatively independent of pressure. Steam-t,emperature control is often sought by redistributing the combustion gases in the furnace between the saturated and superheated tubes. But redistribution alone will not materially affect the energy content of the steam. So feedwater is sprayed into the steam line between the superheater and the temperature bulb. Control at this point is quite effective because the thermal process is one of mixing.
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