Partial condensers coefficients

Condensation of mixed vapours is considered further in Volume 6, Chapter 12, where it is suggested that the local heat transfer coefficient may be expressed in terms of the local gas-film and condensate-film coefficients. For partial condensation where ... 

In partial condensation it is usually better to put the condensing stream on the shell-side, and to select a baffle spacing that will maintain high vapour velocities, and therefore high sensible-heat-transfer coefficients. 

F (2.1 X 10"4 gr am/cm.2 sec.) was calculated from the partial pressures of the three polymers of molybdenum oxide vapor, and Cx (6.8 X 10 3 gram/cm.3) was measured experimentally. For purposes of calculation D was assumed to be 3.0 X 10"7 cm.2/sec. (Normans value of D for M0O3 diffusing in the CAS melt at 1400°C.). Successive estimates of a were tried until a good fit between the calculated and the experimental curves was obtained. The best over-all fit between the two curves was obtained with a = 4 X 10"5 (see Figure 17). This value is an effective condensation coefficient for all three polymers of molybdenum oxide vapor. 

Specific correlations of individual film coefficients necessarily are restricted in scope. Among the distinctions that are made are those of geometry, whether inside or outside of tubes for instance, or the shapes of the heat transfer surfaces free or forced convection laminar or turbulent flow liquids, gases, liquid metals, non-Newtonian fluids pure substances or mixtures completely or partially condensable air, water, refrigerants, or other specific substances fluidized or fixed particles combined convection and radiation and others. In spite of such qualifications, it should be... 

Heat-transfer coefficient in condensation Mean condensation heat-transfer coefficient for a single tube Heat-transfer coefficient for condensation on a horizontal tube bundle Mean condensation heat-transfer coefficient for a tube in a row of tubes Heat-transfer coefficient for condensation on a vertical tube Condensation coefficient from Boko-Kruzhilin correlation Condensation heat transfer coefficient for stratified flow in tubes Local condensing film coefficient, partial condenser Convective boiling-heat transfer coefficient... 

Local effective cooling-condensing heat-transfer coefficient, partial condenser... 

Convective boiling-heat transfer coefficient Local effective cooling-condensing heat transfer coefficient, partial condenser Fouling coefficient based on fin area Heat transfer coefficient based on fin area Film boiling heat transfer coefficient Forced-convection coefficient in equation 12.67 Local sensible-heat transfer coefficient, partial condenser... 

In some services (e.g., refinery fractionators), vapor approaching the chimney tray is hotter than the chimney tray liquid. Heat will be transferred from the vapor to the liquid. If the vapor is condensable, some will condense on the bottom face of the chimney tray. The net result is analogous to leakage. The author is familiar with situations where refractory was installed on the bottom face of the chimney tray. In all these cases, steps were also taken to minimize leakage, making it difficult to independently assess the effectiveness of the refractory. For multicomponent, partially condensable vapor condensing on an uninsulated bottom face of a chimney tray (e.g., in a refinery fractionator), a typical heat transfer coefficient is 15 Btu/(h ft °F) (237). 

Calculation of the required condenser surface is not trivial. In contrast to the common applications where saturated vapors are condensed the permeate is a superheated vapor mixture. For design calculations the selection of appropriate heat-transfer coefficients has to consider the cooling to saturation conditions, the presence of noncondensable gases, and the partial condensation of the components along the respective dew lines. Total condensation of the more volatile components of the permeate vapor will often not be possible, but any losses of permeate vapor through the vacuum pump have to cope with the respective emission control regulations. An important factor is the solubility of the components of the permeate in the liquid phase. An additional condenser at the high-pressure side of the vacuum pump is a feasible option. 

The most realistic option for partial condenser modeling is the LMTD option. The cooling medium is a liquid that enters a counter-current heat exchanger at a specified inlet temperature. The minimum approach differential temperature is specified. The process inlet and outlet temperatures are known, so the log-mean temperature differential driving force is known. With the known condenser duty, the required product of the overall heat-transfer coefficient and the condenser heat-transfer area (UA) is calculated. The required flow rate of the cooling medium can also be calculated. 

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

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. 

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