Subcooled condensate

For subcooling, a liquid inventory may be maintained in the bottom end of the shell by means of a weir or a hquid-level-controUer. The subcoohng heat-transfer coefficient is given by the correlations for natural convection on a vertical surface [Eqs. (5-33 ), (5-33Z )], with the pool assumed to be well mixed (isothermal) at the subcooled condensate exit temperature. Pressure drop may be estimated by the shell-side procedure. 

Air vents are most effective when they are fitted at the end of a length of 300 mm or 450 mm of uninsulated pipe that can act as a collecting/cooling leg. Air is an excellent insulating material, having a thermal conductivity about 2200 times less than that of iron. The last place where it can be allowed to collect is in the steam space of heat exchangers. Further, as it contains oxygen or carbon dioxide, which dissolve readily in any subcooled condensate that may be present, the presence of air initiates corrosion of the plant and the condensate return system. 

Figure 24.24a shows the corresponding simple compression cycle, but with subcooled condensate. The cycle is basically the same as before, except that the liquid leaving the condenser is now subcooled rather than being saturated. 

Figure 24.24 Simple compression cycle with subcooled condensate.

The vapor is drawn into a steam jet (discussed in Chap. 16). The steam condensate flows into the boot or hot well. The water in the boot is slightly subcooled. This is accomplished by a pair of baffles that create a small zone of condensate backup. The subcooled condensate, cooled to perhaps 10°F below its boiling or bubbling point, is easier to pump. As the pressure in the hot well is subatmospheric, the hot-well pump typically develops a AP of at least 30 to 50 psi. 

Figure 3.10. Condensers, (a) Condenser on temperature control of the PF condensate. Throttling of the flow of the HTM may make it too hot. (b) Condenser on pressure control of the HTM flow. Throttling of the flow of the HTM may make it too hot. (c) Flow rate of condensate controlled by pressure of PF vapor. If the pressure rises, the condensate flow rate increases and the amount of unflooded surface increases, thereby increasing the rate of condensation and lowering the pressure to the correct value, (d) Condenser with vapor bypass to the accumulator drum. The condenser and drum become partially flooded with subcooled condensate. When the pressure falls, the vapor valve opens, and the vapor flows directly to the drum and heats up the liquid there. The resulting increase in vapor pressure forces some of the liquid back into the condenser so that the rate of condensation is decreased and the pressure consequently is restored to the preset value. With sufficient subcooling, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control by this method.

For a total condenser, the vapor composition used in the equilibrium relations is that determined during a bubble point calculation based on the actual pressure and liquid compositions found in the condenser. These vapor mole fractions are not used in the component mass balances since there is no vapor stream from a total condenser. It often happens that the temperature of the reflux stream is below the bubble point temperature of the condensed liquid (subcooled condenser). In such cases it is necessary to specify either the actual temperature of the reflux stream or the difference in temperature between the reflux stream and the bubble point of the condensate. 

Upon computing the bubble point of the overhead product, we find that the measured reflux temperature is well below the estimated boiling point. Thus, we choose the subcooled condenser model. The steady-state concept of the subcooled condenser often does not exist in practice. Instead, the condenser is in vapor-liquid equilibrium with the vapor augmented by a blanket of noncondensable gas (that has the effect of lowering the dew point of the overhead vapor). The subcooled condenser is a convenient work-around for steady-state models (as is needed here), but not for dynamic models. We assume a partial reboiler. 

Coolant flow is generally not throttled for pressure control, but it is occasionally adjusted for temperature control of the subcooled condensate. Unless there is significant sub-cooling, this latter temperature loop is often ineffectual. At best, it will require loose tuning, or often it will be placed into manual for seasonal adjustment only. 

Design an exchanger to subcool condensate from a methanol condenser from 95 °C to 40°C. Flow rate of methanol 100,000 kg/h. Brackish water will be used as the coolant, with a temperature rise from 25° to 40°C. 

Thus, in order to define the column operation uniquely, two specifications are required, as already concluded using the description rule (Section 17.1.3). These could be the reflux rate and distillate rate, Lg and D. Note that a subcooled condenser is assumed so that no phase equilibrium equation is written for stage 0 and no Foi variables exist. The column pressure profile is assumed fixed or determined independently from hydraulics calculations and is not included in the column variables. Also, the enthalpies and phase equilibrium coefficients are, in general, functions of the temperature, pressure, and composition (Chapter 1) and are therefore not considered as additional unknown variables. 

There is a middle steady state, but it is metastable. The reaction will tend toward either the upper or lower steady states, and a control system is needed to maintain operation around the metastable point. For the styrene polymerization, a common industrial practice is to operate at the metastable point, with temperature control achieved by boiling. A combination of feed preheating and jacket heating ensures a positive heat balance so that the uncontrolled reaction would tend toward the upper, runaway condition. However, the reactor pressure is set so that the system boils when the desired operating temperature is reached. The latent heat of vaporization plus the return of subcooled condensate maintains the temperature at the boiling point. 

Chen [61] conducted a boundary layer analysis of this problem and included the momentum gain of the condensate in dropping from tube to tube and the condensation that takes place directly on the subcooled condensate film between tubes. His numerical results for the average coefficient of N tubes can be approximated to within 1 percent by ... 

SCV - Single-component vapor MCV - Multicomponent vapor SC - Subcooled condensate Ap - Pressure drop C - Coolant. Acceptability G - good F - fair P - poor X - not acceptable or not recommended. 

If liquid accumulates (e.g., due to poor drainage) in the condensing side of the condenser, the flooded surface subcools condensate. This robs the condenser of condensation area and lowers the overall heat transfer rate. This mechanism is responsible for "liquid removal problems in condensers. 

Step 1 Pyrometer measurements must be taken from the inlet (steam) and outlet (condensate) of the steam trap to identify if the trap valve fails to close when steam is present. The temperature difference between inlet and outlet of the steam trap indicates the subcooled degree of condensate. For example, 0 °F subcooled condensate indicates the trap valve sticks in a full open position. 

When carbon dioxide is present, only those traps that discharge condensate and noncondensable gases at or near saturation temperature (within 3°C) should be used. The introduction of new traps that claim to save energy by deliberately subcooling condensate has led to several problems. The new traps do not remove carbon dioxide and corrosion results. Steam leaks develop at threads, uhions, and welds. 

Stripping Tower Theoretical Trays Reboiler Temperature Subcooled Condenser 12 190 °C... 

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