Once-through Boilers in .NET framework

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Once-through Boilers
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The operation of a once-through boiler is more readily analyzed because Feedwater it is dynamically continuous, not broken in half by a drum. is pumped into the tubes at one end and superheated steam withdrawn at the other. There is no recirculation. There is also no liquid level to measure. In subcritical boilers, there is a transition from liquid to vapor somewhere in the tubes, but exactly where is of little consequence. In
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supercritical boilers, there is no phase change, hence no point of transition. A once-through boiler is shown schematically in Fig. 9.10. Under normal operation, three controlled variables are of primary importance: steam pressure,. steam temperature, and thermal power. The first two are to be regulated, the third is the heat load on the plant. They are controlled by the manipulation of firing rate, feedwater flow, and steam-valve position. These variables interact with each other to the extent that poor performance will result if the three loops are operated independently. To appreciate the mechanism of this interaction, consider how the controlled variables would respond to step changes in each of the manipulated variables:4 1. An increase in firing rate would increase both t hermal power and steam temperature. With the steam valve in a fixed position, upstream pressure would increase, because thermal power has been shown to vary with pressure and differential across an orifice. 2. An increase in feedwater flow will cause an increase in steam flow, but thermal power will not change. Thus steam pressure will not change either. Since steam enthalpy is thermal power divided by flow, an increase in steam flow will cause steam temperature to fall. 3. As the st,eam valve is opened farther, pressure will fall to a new equilibrium value, during which a certain amount of steam and energy will have been released. But when the new steady state is reached, steam flow and thermal power must return to their original values, since feedwater flow and firing rate have not changed. The above responses can be simply represented by a dimensionless matrix, without going into a detailed calculation of process gains. Let Q represent thermal power; p, steam pressure; and T, steam temperature. WF will be firing rate; W,V, feedwater flow; and HZ, position of the steam valve :
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FIG 9.10. In a once-through boiler, feedwater is conducted through tubes all the way to the steam valve.
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(Note that the sum of the columns for the regulated variables is zero, and for the command variable is 1.0.) This is a matrix of open-loop gains whose actual values have not been inserted. The intent is to show that half-coupling exists between Q, p, and T. But normalization according to the procedure outlined in Chap. 7 yields a unit diagonal, with all other elements zero. This proves the absence of full coupling, and makes pairing obvious. The detailed equations for decoupling of the half-coupled loops follow. Thermal efficiency E and heat of combustion Hc determine the firing rate required to satisfy a given load:
wF = HcE
(9.20)
Feedwater enthalpy Hw must be included as well as steam enthalpy Hs to determine the feedwater requirements:
Q Ww=Hs-Hw
Feedwater temperature its enthalpy:
Tlrr and specific heat CW may be substituted for (9.21)
Q ww = Hs - CwTw
Because the points of steam temperature measurement and feedwater flow manipulation are widely separated, control is somewhat difficult. To ease the situation, a small amount, of feedwater is bypassed around the furnace and admitted as spray between the superheater and t he temperature bulb. If this stream is taken as a part of, and not in addition to, the set flow of feedwater, its variat,ion will not change the long-term steam enthalpy, and is only capable of producing a transient effect. Since steam pressure is to manipulate the position of a valve immediately downstream of its measurement, control is rather easy. Consequently feedforward will not be used here because it is not really warranted. The unknown parameters in the feedforward equations are Hc and E. These must be found by means of feedback. All other terms are either measurable or constant. A control system designed according to the rules given in Chaps. 7 and 8 is shown in Fig. 9.11. It is not possible to use reset in two controllers which operate on t he same input. So proportional plus derivative is used to manipulate the spray valve. In this way, the valve will operate around a position determined by the bias set into t.he controller. The lead-lag unit is used t o
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