The boiler is very effective for the distillation column because it prevents disturbance of the distillation column in the heating environment. Reboiler controls include boiler level control, reboiler duty control, reboiler steam flow control and so on.
The most suitable variable for setting a column is boil-up. Boil-Up is normally controlled at the bottom to achieve the desired product quality.
When the boiling rate is kept constant, the boiler control valve is usually manipulated by a heating current controller. When the welding is set to achieve the purity of the desired product, the boiler control valve is directly or indirectly manipulated by the temperature of the tray, a product analyzer or the base surface. Indirect manipulation is performed by a cascade controller that changes the adjustment point of the heating controller flow. The flow controller, in turn, manipulates the boiler control valve.
Boil again with the compacting liquid
Examples of this type of reboilers are steam boilers and refrigeration reboilers. As shown in Figure 1, the control valve may be located at the inlet line or at the condensate outlet line.
Figure 1: Position of the control valve in the boiler system
When the control valve is located at the boiler inlet, the heat transfer rate is controlled by changing the boiler density pressure and thus the condensate temperature. When more welding is required, the valve opens and increases the pressure of the boiler, which increases the temperature difference of the boiler, which in turn increases the boiling speed.
When the condensate flow rate is manipulated, the steam always condenses essentially at the source header pressure. The rate of heat transfer changes with the partial filling of the boiler with condensate, resulting in a change in the surface area of the boiler for condensation.
Control valve location (upstream in front of downstream boiler)
The location of the control valve has a major impact on the efficiency and performance of the entire column. There are several advantages and disadvantages to each location, as follows:
1. The inlet valve control immediately changes the steam flow, which in turn changes the boiler pressure and heat transfer rate. On the other hand, condensate outlet valve does not have a direct effect on steam flow rate. The condensate flow determines the level of the condensate and this level changes slowly. Because of this slow response, brewing the steam flow of beer is a much better means of control than controlling the flow of condensate.
۲. Condenser outlet control scheme, sometimes the control valve in the condensate line can not withstand the amount of condensate produced by the boiler, the maximum steam flow is possible while the condensate still covers part of the pipes and the pressure difference between the pipes cover. Boiler and condensate system is small A condensate boiler with pump may be needed to overcome the problem.
Conversely, this problem can also be troublesome when the boiler fails to condense the steam as quickly as the condensate valve expels, the liquid seal in the boiler may be lost and steam may enter the condensate system, leading to The casualties are significant. Heat transfer also causes a hammer in the condensate system. This problem can be solved with the above control scheme (Figure 2-A) without pump. The other best arrangement is like Figure 2-B.
Figure 2: Gas condensate boilers in the boiler
Here the flow controller (Figure 2B) typically controls the condensate valve, and the level is ignored whenever the level drops too low.
3. The condensate outlet design allows the boiler to operate at higher pressures because it eliminates the pressure drop in the inlet control valve. When refrigerant steam is a heating device, it is a major advantage. Since the intermediate pressures of the refrigeration compressor are usually set to the boiler compression pressure. The higher the pressure, the lower the refrigerant compressor power consumption.
4. In a steam-heated boiler, the steam inlet control scheme minimizes the boiler tube wall temperature. This reduces boiler deposition (process side) and reduces thermal stresses at the boiler head.
If scale build-up becomes a serious concern, it is often advisable to keep the boiler wall temperature as low as possible. The layout in Figure 2C uses the entire boiler area to automatically minimize the compression temperature.
5. A smaller control valve with a condensate outlet is required.
6.The steam inlet design may be problematic when an additional surface is present in the boiler. During the initial operation, the inlet control valve closes to reduce the condensing temperature (Q = UA ΔTlm) UA is large, where Q is the heat duty, U is the total heat transfer coefficient, A is the boiler area, ΔTlm is the average temperature difference) And pressure condensation. It will be impossible to remove the condensate if the condensing pressure is less than the condenser header pressure (for example in a 15 to 35-PSIG steam welding machine). Unconducted condensate collects in the weld and sinks some of the pipe surfaces. The point at which condensate begins to form can be calculated. By submerging the gas condensate pipes to some extent, any further change in the steam-to-boiler flow will affect both the delta T in the boiler and the fraction of pipe surface until a new equilibrium is reached. The two often interact and cause slow and sometimes erratic responses. The steam trap offers little help in controlling the condensate level. In addition, if the boiler load changes are sudden, it will be difficult to maintain or maintain the manipulated balance above as well as hunting the control valve.
To overcome this problem, a submerged gas condensate pot is usually installed instead of a flow trap, as described earlier. An alternative solution is to replace the steam trap with a surface compression boiler (see Figure 2D). By changing the level control point, the level in the boiler can be adjusted so that the boiler operates at a sufficiently high pressure to ensure that it always removes condensate without a pump.
Note: The bottom of this drum is located under the condenser of the boiler, otherwise the operation of the dry boiler at high speed will not be possible and the capacity of the boiler will be reduced.
7. Steam traps are generally considered problematic because they are prone to opening or closing. Using a steam trap is a disadvantage. The steam inlet design in Figure 6 can overcome this problem.
8. In the steam control scheme, the flow rate across the valve changes from non-critical to critical as the boiler is reduced again. As the boiling point decreases, the absolute downstream pressure of the valve also decreases. When the pressure ratio u / s & d / s exceeds a critical value, a critical flow is established through the valve.
To avoid this problem, it is best to design the system to operate within its normal range in one stream or another. The surface condensate boiler can keep the d / s pressure down. Or installing a pressure regulator u / s The flow controller can reduce the u / s inlet valve pressure.
Figure 3: Different controls of the boiler
9. With the gas condensate outlet design (Figure 3A), the accumulation of condensate in horizontal shells in downward conditions can flood most of the converter baffle windows and restrict the passage of steam through the window. This may lead to a liquid hammer.
According to the above points, it was observed that the steam inlet design is more effective than the condensate outlet design in reducing steam supply disturbances.
10. The condensate outlet design can destabilize the operation of the thermosyphon boiler.
۱۱ . Corrosion due to the level of condensate retained in the boiler often occurs with the condensate outlet design. The rust layer on the steam side of the boiler clearly indicates the surface on which the condensate is flowing.
In some low-pressure boilers the gas condensate boiler is replaced with a ring seal (see figure below). Increasing the flow of steam to the boiler increases the pressure in the boiler shell, which in turn reduces the liquid level in the boiler and exposes the surface of the pipe more. The behaviors of this system are similar to the condensate output design. The height of the liquid in the ring is usually 5 to 10 feet. This arrangement (Figure 3B) can be problematic when the boiler heat load or boiler grid pressure tends to fluctuate and is usually best avoided.
Reboil with noticeable heat:
Hot oil, hot gas is sometimes used for boiling purposes. The inlet heat is regulated by changing the flow of the heating medium through the boiler, which in turn changes both the heat transfer coefficient and the temperature difference throughout the boiler. Boiler responses depend on whether or not a change in heat transfer coefficient is a dominant factor influencing the heat input. In general, in the high boiler ΔT, the effect of the heat transfer coefficient is dominant, while in the low boiler, the ΔT effect is more dominant.
This type of boiler (Figure 3C) is mainly used in refineries. These fires are usually a mixture of gases emitted from different units that are supplemented by natural gas. A problem that often occurs in direct fuel boilers is the incoming thermal changes with the fuel gas composition. For this reason, it may be difficult to control the flow of fuel to the furnace, and it may be necessary to control the inlet temperature.
Combination correction can be performed using a factor (مقدارSG heating value), known as the Wobbe index, in which SG is the specific gravity of the gas. For hydrocarbon mixtures, the Wobbe index is linearly different from SG, and SG measurements are sufficient to modify the composition. Shows the order of the control subsystem using the Wobbe Index.
The densitometer must be installed directly in the fuel gas line, downstream of the exhaust drum, and at any point where the fuel gas flow is increased and is not affected by vibrations.
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