Vapor velocity, condensation

N,n = Minimum theoretical stages at total reflux Q = Heat transferred, Btu/hr U - Overall heat transfer coefficient, Btu/hrfP"F u = Vapor velocity, ft/sec U d = Velocity under downcomer, ft/sec VD(js = Downcomer design velocity, GPM/fL Vioad = Column vapor load factor W = Condensate rate, Ibs/hr Xhk = Mol fraction of heavy key component Xlk = Mol fraction of the light key component a, = Relative volatility of component i versus the heavy key component... 

In case of excessive carbonization the rate of boiling should be increased or the filament temperature slightly lowered. In general the vapor velocity should be as high as possible without exceeding the capacity of the copper condenser. 

L/D = reflux ratio Pf = packing factor Qc = condenser duty, Btu/hr Or = reboiler duty, Btu/hr Vr = reboiler vapor rate, Ib/hr Vs = superficial vapor velocity, ft/sec Pi = liquid density, Ib/ft ... 

From Figure 26.7 it can be seen that for equal duties and flows the temperature difference for countercurrent flow is lower at the steam inlet than at the outlet, with most of the steam condensation taking place in the lower half of the plate. The reverse holds tme for co-current flow. In this case, most of the steam condenses in the top half of the plate, the mean vapor velocity is lower and a reduction in pressure drop of between 10-40 per cent occurs. This difference in pressure drop becomes lower for duties where the final approach temperature between the steam and process fluid becomes larger. 

The pressure drop across the reactor cyclones, reactor vapor line, main fractionator, and main column overhead condensing/cooling system can be too high. The pressure drop is primarily a function of vapor velocity. Any plugging can increase the pressure drop. 

Carey van P (1992) Liquid-vapor phase-change phenomena. An introduction to the thermophysics of vaporization and condensation processes in heat transfer equipment. Hemisphere, New York Celata GP, Cumo M, Mariani A (1997) Experimental evaluation of the onset of subcooled flow boiling at high liquid velocity and subcoohng. Int J Heat Mass Transfer 40 2979-2885 Celata GP, Cumo M, Mariani A (1993) Burnout in highly subcooled water flow boiling in small diameter tubes. Int J Heat Mass Transfer 36 1269-1285 Chen JC (1966) Correlation for boiling heat transfer to saturated fluids in convective flow. Ind Eng Chem Process Des Develop 5 322-329... 

In an expediently designed plant, one can expect to reach in the pressure range above 8 10-2 mbar a vapor velocity in the cross-section Fk of between 50 and 80 m/s (l/d = 2.5 to 5) However 90 m/s will be reached only, if the design uses special features, e. g. a funnellike connection between the chamber wall and the location of the valve, slow changes in the outline, and smooth surfaces without sharp edges or holes. It is also recommendable, to clarify the maximal amount of vapor transportable at several pressures in a plant specification, e. g. at pch = 1 mbar a minimum of 3 kg/h and at pch = 0.04 mbar a minimum of 25 g/ h flow of water vapor must be demonstrated during the acceptance test, while the condenser temperature is below -30 °C, respectively below -57 °C. Such measurements can be earned out practically with sufficient accuracy (see e. g. Fig. 2.19 and the related text). 

The basic assumptions implied in the homogeneous model, which is most frequently applied to single-component two-phase flow at high velocities (with annular and mist flow-patterns) are that (a) the velocities of the two phases are equal (b) if vaporization or condensation occurs, physical equilibrium is approached at all points and (c) a single-phase friction factor can be applied to the mixture if the Reynolds number is properly defined. The first assumption is true only if the bulk of the liquid is present as a dispersed spray. The second assumption (which is also implied in the Lockhart-Martinelli and Chenoweth-Martin models) seems to be reasonably justified from the very limited evidence available. 

The vapor flowing between trays was at its dew point. A sudden increase in tower pressure caused a rapid condensation of this vapor and a loss in vapor velocity through the tray deck holes. The resulting loss in vapor flow caused the tray decks to dump. 

Figure 8.2 shows a common type of reboiler failure. The steam trap on the condensate drain line has stuck open. A steam trap is a device intended to open when its float is lifted by water. The steam trap remains open until all the water drains out of the trap. Then, when there is no more water to keep the trap open, it shuts. But, if the float sticks open, steam can blow through the steam trap. This is called a blown condensate seal. The average vapor velocity through the tubes... 

The large effective heat capacity of the liquid-solid slurry absorbent enables relatively small slurry flows to absorb the carbon dioxide heat of condensation with only modest absorber temperature rise. This contrasts with other acid gas removal processes in which solvent flows to the carbon dioxide absorber are considerably larger than flows determined by vapor-liquid equilibrium constraints. Large flows are required to provide sensible heat capacity for the large absorber heat effects. Small slurry absorbent flows permit smaller tower diameters because allowable vapor velocities generally increase with reduced liquid loading (8). 

This vapor velocity may lead to flooding of the vapor tube and so requires an active condenser. 

Transition from laminar to turbulent flow within the condensed film can occur when the vapor is condensed on a tall surface or on a tall vertical bank of horizontal tubes [45] to [47]. It has been found that the film Reynolds number, based on the mean velocity in the film, um, and the hydraulic diameter, D, can be used to characterize the conditions under which transition from laminar flow occurs. The mean velocity in the film is given by definition as ... 

In the analysis of film condensation given in the previous sections it was assumed that the shear stress at the outer edge of the film was negligible. In some situations, however, particularly when the vapor velocity is high, this assumption may not be justified, i.e., the shear stress exerted on the outer surface of the condensed liquid film may have a significant influence on the heat transfer rate. The action of the shear stress on the surface of the liquid film is illustrated in Fig. 11.17. 

Carpenter, E.F. and Colbum, A.P.. The Effect of Vapor Velocity on Condensation Inside Tubes," in Proceedings. General Discussion on Heat Transfer. Inst. Mech. Eng.-ASME, New York, pp. 20-26. 1951. 

Chato [38] obtained the following expression for condensation of refrigerants at low vapor velocities inside horizontal tubes ... 

Refrigerant 12 (CCl2F2) is condensed inside a horizontal 12-mm-diameter tube at a low vapor velocity. The condensing temperature is 90°F and the tube wall is at 80°F. Calculate the mass condensed per meter of tube length. h/g = 57.46 Btu/lbm at 90°F. 

MAXIMUM VAPOR VELOCITY FOR CONDENSERS WITH UPFLOW VAPOR 7.56... 

Calculate the heat-transfer coefficient using both mechanisms, and select the higher value calculated as the effective heat-transfer coefficient hL. The vapor-shear effects vary for each typical baffle section. The condenser should be calculated in increments, with the average vapor velocity for each increment used to calculate vapor-shear heat-transfer coefficients. 

Calculate the maximum velocity to avoid flooding for the vapor conditions of Example 7.28. Flooding occurs in upflow condensers when the vapor velocity is too high to permit the condensate to drain. Unstable conditions exist when flooding occurs. 

The Na vapor, traveling at vapor velocities of roughly 8-10 m/s at 1200 K, condenses on the heat exchanger tubes in the dome (the condensing vapor heat exchanger, CVHX) and returns as liquid droplets to the pool, thus completing the cycle. In the dome, the CVHX transfers the thermal energy out of the module to various chemical processors located some distance from the reactor. 

So far we have discussed film condensation on the outer surfaces of tubes and other geometries, which is characterized by negligible vapor velocity and the unrestricted flow of the condensate. Most condensation processes encountered in refrigeration and air-conditioning applications, however, involve condensation on the inner surfaces of horizontal or vertical tubes. Heat transfer analysis of condensation inside tubes is complicated by the fact that it is strongly influenced by the vapor velocity and the rate of liquid accumulation on the walls of the tubes Q ig. 10-34). 

Condensate flow in a horizontal tube with large vapor velocities. 

For low vapor velocities, film condensation heat transfer inside horizontal tubes can be determined from... 

Saturated steam at 270.1 kPa condenses inside a horizontal, 10-m-long, 2.5-cm-inlemal-diameier pipe whose surface is maintained at 110°C. Assuming low vapor velocity,... 

In operation, the sample container is attached to the joint on the Till pot, and the entire column is evacuated down to the stopcock on the sample container. The still pot is cooled in liquid nitrogen, and the sample is allowed to distill into it. The stopcock on the still pot is closed, and a Dewar is placed around the latter. A small amount of heat is then supplied by the heater, and the pressure in the column is allowed to increase. When the desired operating pressure, which should be fairly high in order to decrease the vapor velocity in the column, is reached, the coolant is supplied to the condenser at such... 

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