Heat transfer coefficients coils

There are 12 equations in all (overall material and energy balances side A and B energy balances coil 1 to 8 energy balances) and 36 variables. However, the heat transfer coefficients are not known with any great accuracy. Further, both the side and coil heat transfer coefficients depend on the fire-box temperature. It is therefore necessary to calculate values for the heat transfer coefficients from the data. This effectively reduces the number of independent equations to 11. 

The coil heat transfer coefficient had a value of 0.1159 and a x2 statistic of 27.8. This value suggests that a gross error is present. Some clues can be found from examination of the residuals of the balances. They are presented in Table 7. There appears to be a problem with the balances for coils 2 and 3 their residuals are different from the others (especially coil 2). The reconciled value for the coil 2 cracking temperature is also significantly different from the measured value, thus suggesting an abnormal situation associated with both coils 2 and 3. 

Thermal mass flowrate Coil heat transfer coefficient Deviation from side average Deviation student s t value... 

Interesting results are obtained if 160 kg/h is deducted from the measured hydrocarbon flow for coil 2, and 210 kg/h added to the hydrocarbon flow for coil 3. The anomaly between the coil heat transfer coefficients for coils 2 and 3 has disappeared. Figure 7 shows the effect that the adjustment to the coil flow has on the crossover... 

The heat transfer rates in bubble columns are much higher than that anticipated from single phase flow considerations. This enhancement is ascribed solely to the bubble-induced turbulence and liquid circulation. Little work has been reported on heat transfer, both at wall and to/from immersed surfaces, in bubble columns employing non-Newtonian media. Nishikawa et al. reported the first set of data on the effect of shearthinning viscosity of CMC solutions on jacket and coil heat transfer coefficients [7]. They reconciled their results for Newtonian and power law liquids by introducing the notion of an effective viscosity estimated via Equation 3, provided the gas velocity was greater than 40 mm/s. For superficial gas velocity lower than this value, the effective shear rate varies as for coil heat transfer... 

Helical Coils Heat transfer coefficients for fluids flowing inside helical coils can be calculated with modifications of the equations for straight tubes. The equations for straight tubes should be corrected as below ... 

Heat-transfer coefficients in clean coiled-tube evaporators for seawater are shown in Fig. 11-24 [Hillier, Proc. Jn.st. Mech. Eng. (London ), 1B(7), 295 (1953)]. The tubes were of copper. 

TABLE 11-2 Overall Heat-Transfer Coefficients for Coils Immersed in Liquids... 

In general, 50.8- ana 63.4-mm (2- and 2V 2.- n) coils are the most economical for shop fabrication and 38.1- and 50.8-mm Wi- and 2-in) for field fabrication. The tube-side heat-transfer coefficient, high-pressure, or layout problems may lead to the use of smaller-size pipe. 

Panel Coils Immersed in Liquid Overall Average Heat-Transfer Coefficients ... 

Description These exchangers are typically a series of stacked helical-coiled tubes connected to manifolds, then inserted into a casing or shell. They have many advantages hke spiral-plate designs, such as avoiding differential expansion problems, acceleration effects of the helical flow increasing the heat transfer coefficient, and compactness of plot area. They are typically selected because of their economical design. 

FIG. 11 91 Effect of circulation ratio on the overall heat-transfer coefficient of an air-cooling coil. 

U = Overall heat-transfer coefficient of heating coil Btu (ft ) ( F) (hr)... 

At a given Reynolds number, heat transfer coefficients of coils, particularly with turbulent flow, are higher than those of long, straight pipes, due to friction. This also applies to flow through an annular jacket with spiral baffling. 

Dg = Mean or centerline diameter of internal coil helix, mm (ft) hj = heat transfer coefficient on inside surface of jacket = viscosity at bulk fluid temperature, [(N s)/m ][kg/(m sec)] = viscosity at die wall temperature, [(N s)/m ][kg/(m sec)]... 

The following equation can be used to predict heat transfer coefficients from coils to tank walls in agitated tanks. 

Heat transfer coefficient to fluids in a vessel using mechanical agitated coils or jacket... 

Heating or cooling of process fluids in a batch-operated vessel is common in the chemical process industries. The process is unsteady state in nature because the heat flow and/or the temperature vary with time at a fixed point. The time required for the heat transfer can be modified, by increasing the agitation of the batch fluid, the rate of circulation of the heat transfer medium in a jacket and/or coil, or the heat transfer area. Bondy and Lippa [45] and Dream [46] have compiled a collection of correlations of heat transfer coefficients in agitated vessels. Batch processes are sometimes disadvantageous because ... 

The essential feature of a Jluidized-bed reactor is that the solids are held in suspension by the upward flow of the reacting fluid this promotes high mass and heat transfer rates and good mixing. Heat transfer coefficients in the order of 200 W/m-°C between jackets and internal coils are typically obtained. The solids may be a catalyst, a reactant (in some fluidized combustion processes), or an inert powder added to promote heat transfer. 

Improve heat transfer coefficient by forcing flow past coil surfaces. 

Tank diameter, ft, or L (consistent units). Figure 5-34 = Residence or holding time, sec, or time of mixing = Overall heat transfer coefficient, bulk mixing liquid to transfer fluid on opposite side of heat transfer wall (coil, plate, jacket), Btti/hr/sq ft/°F = Velocity of mixed fluids through mixer, ft/sec = Volume, consistent units... 

Figure 10-94. Heat transfer coefficients for jackets and coils with fluid agitation. (Used by permission Engineering Manual Dowtherm " Heat Transfer Fluids, 1971. The Dow Chemical Co.)...

Toluene is continuously nitrated to niononitrotoluene in a cast-iron vessel of 1 m diameter lined with a propeller agitator of 0.3 m diameter driven at 2 Hz. The temperature is maintained at 310 K by circulating cooling water at 0.5 kg/s through a stainless steel coil of 25 mm outside diameter and 22 mm inside diameter wound in the form of a helix of 0.81 in diameter. The conditions are such that the reacting material may be considered to have the same physical properties as 75% sulphuric acid. If the mean water tcmpcralure is 290 K, what is the overall heat transfer coefficient ... 

A stirred reactor contains a batch of 700 kg reactants of specific heat 3.8 kJ/kg K initially at 290 K, which is heated by dry saturated steam at 170 kN/m2 fed to a helical coil. During the heating period the steam supply rate is constant at 0.1 kg/s and condensate leaves at the temperature of the steam. If heat losses arc neglected, calculate the true temperature of the reactants when a thermometer immersed in the material reads 360 K. The bulb of the thermometer is approximately cylindrical and is 100 mm long by 10 mm diameter with a water equivalent of 15 g, and the overall heat transfer coefficient to the thermometer is 300 W/m2 K. What would a thermometer with a similar bulb of half the length and half the heat capacity indicate under these conditions ... 

A batch of reactants of specific heat 3.8 kJ/kg K and of mass 1000 kg is heated by means of a submerged slearn coil of area 1 m2 fed with steam at 390 K. If the overall heat transfer coefficient is 600 W/m2 K, calculate the time taken to heat the material from 290 to 360 K, if heat losses to the surroundings are neglected. 

Overall heat transfer coefficient in coil — 2000 W/rn2 K Heat transfer coefficient to surroundings — 10 W/nr K... 

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