Flooded Condensers

Pressure can also be controlled by variable heat transfer coefficient in the condenser. In this type of control, the condenser must have excess surface. This excess surface becomes part of the control system. One example of this is a total condenser with the accumulator running full and the level up in the condenser. If the pressure is too high, the level is lowered to provide additional cooling, and vice versa. This works on the principle of a slow moving liquid film having poorer heat transfer than a condensing vapor film. Sometimes it is necessary to put a partially flooded condenser at a steep angle rather than horizontal for proper control response. 

Flooded condensers and flooded reboilers are sometimes used on distillation columns. In the sketch below, a hquid level is held in the condenser, covering some of the tubes. Thus a variable amount of heat transfer area is available to condense the vapor. Column pressure can be controlled by changing the distillate (or reflux) drawoff rate. 

Derive a dynamic mathematical model of the flooded-condenser system. Calculate the transfer function relating steam flow rate to condensate flow rate. Using a PI controller with tj = 0.1 minute, calculate the closedloop time constant of the steam flow control loop when a closedloop damping coefTident of 0.3 is used. Compare this with the result found in (u). 

I well remember one pentane-hexane splitter in Toronto. The tower simply could not make a decent split, regardless of the feed or reflux rate selected. The tower-top pressure was swinging between 12 and 20 psig. The flooded condenser pressure control valve, shown in Fig. 3.1, was operating between 5 and 15 percent open, and hence it was responding in a nonlinear fashion (most control valves work properly only at 20 to 75 percent open). The problem may be explained as follows. 

Figure 13.8 Flooded condenser pressure control the preferred method.

In general, flooded condenser pressure control is the preferred method to control a tower s pressure. This is so because it is simpler and cheaper than hot-vapor bypass pressure control. Also, the potential problem of a leaking hot-vapor bypass control valve cannot occur. Many thousands of hot-vapor bypass designs have eventually been converted—at no cost—to flooded condenser pressure control. 

The function of the reflux drum in a flooded condenser design is to... 

Sometimes we see tower pressure control based on feeding a small amount of inert or natural gas into the reflux drum. This is bad. The natural gas dissolves in the overhead liquid product and typically flashes out of the product storage tanks. The correct way to control tower pressure in the absence of noncondensable vapors is to employ flooded condenser pressure control. If, for some external reason, a variable level in the reflux drum is required, then the correct design for tower pressure control is a hot-vapor bypass. 

Figure 5.5 Heat transfer control by variable heat transfer area in flooded condenser.

A fourth degree of freedom is consumed to control column pressure. The valves available are condenser cooling (by far the most commonly7 used), reboiler heat input, and feed (if the feed is partially vapor). If a flooded condenser is used, the cooling water valve is wide open and an additional valve, typically located between the condenser and the reflux drum, is used to cover or expose heat-transfer area in the condenser. 

Step 2. This process has 14 control degrees of freedom. They include fresh feed valve DIB column steam, cooling water, reflux, distillate, and bottoms valves purge column steam, cooling water, reflux, distillate, and bottoms valves furnace fuel valve flooded condenser cooling water valve and DIB column feed valve. 

The same problem occurs on the hot side of the FEHE. As the hot reactor effluent is cooled, it starts to condense at some temperature. And this dewpoint temperature could occur in the heat exchanger and not in the flooded condenser if a large area is used. 

To avoid the high-pressure safety constraint, we must control reactor pressure. We can use the distillate valve from the purge column, the flooded condenser cooling water valve, or the DIB column feed valve. The most logical variable to use for control of the flooded condenser (.reactor) pressure is the DIB column feed valve (as shown in Fig. 5.5). Based upon the discussion in Step 3, we would then use the flooded condenser cooling water valve to keep the liquid level in a good control range. 

In the HYSYS simulation, the flooded condenser is simulated as a cooler with a temperature control loop manipulating cooling water rate. The reactor inlet pressure is set by the exit of the liquid recycle pump on the purge column distillate stream. 

For column pressure control there are Ihrea genaial approaches vent bleed (to arraosphere or to vacuum system), hot vapor bypass, and flooded condenser. These approaches are illustrated in Fig. 5.11-2.3 For an atmospheric column, the vent approach is quite simple. The vapor bypass represents a temperature bleading method. Partial flooding of the condenser suiface adjusts the bent transfer capability or the condenser. The schemes are generally self-explanatory. 

When the condenser contains excess heat transfer area, the fraction of condenser area that is flooded can be manipulated to vary the rate of condensation. This principle is used in "flooded condenser control schemes (Sec. 17.2.2). 

Flooded reflux drums. These are used as part of flooded condenser control schemes (Sec. 17.2.2). A flooded drum is only suitable when substantial fluctuations in product rate can be tolerated (e.g., product goes to storage). A flooded drum provides no surge for controlling and smoothing out product fluctuations, but it maintains surge for the reflux pump and reflux circuit. 

Figure 16.2 (Continued) Initial pairing of variables and manipulated streams, (d) Flooded condenser (e) internal condenser, vapor product.

Control techniques. Pressure and condensation control techniques are classified into four categories vapor flow variations, flooded condensers, coolant flow variations, and miscellaneous methods. These techniques are described below. 

Figure 17.5a illustrates a flooded condenser with the control valve located at the condenser outlet. The control valve required is small... 

Figure 17.5/ shows a flooded condenser scheme similar to that of Fig. 17.5a, but with the control valve located at the condenser inlet. This method is inferior compared to Fig. 17.5a (77). It requires a larger control valve, is more difficult to understand, and it affects condensation at a lower temperature. The condenser outlet line must enter the reflux drum well below the liquid level. A pressure-equalizing line as in the method shown in Fig. 17.5a is also required.

Flooded condenser schemes shown in Fig. 17.5a to c and f can be used instead of coolant flow variations. In such cases, the inerts normally leave finm the top of the condenser instead of the reflux drum (Fig. 17.8e). If the reflux drum is not flooded, a pressure balance line must be included otherwise, a stable pressure will be impossible to keep in the reflux drum. An overflow line should also be included in this arrangement. 

Column pressure was controlled using a hot vapor bypass scheme (partially flooded condenser). Severe pressure and reflux drum level upsets occurred whenever the reflux drum surface was inadvertently agitated. 

Condenser controlled using a hot vsqwr bypass (partially flooded condenser). bcooM liquid entered the reflux drum vsqwr qiace (presumably due to unflooding the liquid inlet), and contacted drum vapor that was 100°F hotter. The rapid condensation sucked the liquid leg between the condenser and drum in seconds. 

Vapor recompression evaporators Evaporation rate < design fouled heat transfer surface/uneven movement of liquid over heat transfer surface/feed property changes/excessive noncondensibles from leaks or present in feed/flooded condensate, trap malfunction. Section 5.1/feed temperature < design/water leakage into the system/lower compressor suction pressure, see also Section 2.1. 

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