Steam heat transfer coefficients

L = wind velocity factor, Btu/hr-ft -°F ho = convective heat transfer coefficient, Btu/hr-ft °F hj = steam, heat transfer coefficient, Btu/hr-ft °F ko = thermal conductivity of insulation, Btu/hr-ft-°F L = length of pipe, ft n = number of tracers... 

Heat is flowing from steam on one side of a vertical steel sheet 0.375 in. thick to air on the other side. The steam heat-transfer coefficient is 1700Btu/(h °F) and... 

There are two types of correlations for estimating the heat transfer coefficient for condensation inside vertical tube. In the first type of correlations, the local heat transfer coefficient is expressed in the form of a degradation factor defined as the ratio of the experimental heat transfer coefficient (when noncondensable gas is present) and pure steam heat transfer coefficient (Kuhn et al. [1997]). The correlations in general are the functions of local noncondensable gas mass fraction and mixture Reynolds number (or condensate Reynolds number). In the other type of correlations, the local heat transfer coefficient is expressed in the form of dimensionless numbers. In these correlations, local Nusselt number is expressed as a function of mixed Re5molds number, Jakob number, noncondensable gas mass fraction, condensate Reynolds number, and so on. 

If in the preceding problem the heat transfer area is 14 m and the steam heat transfer coefficient is 10 kW/m °C, what would the rate of heat transfer be Wall thickness is 0.01 m. 

Maintenance of isothermal conditions requires special care. Temperature differences should be minimised and heat-transfer coefficients and surface areas maximized. Electric heaters, steam jackets, or molten salt baths are often used for such purposes. Separate heating or cooling circuits and controls are used with inlet and oudet lines to minimize end effects. Pressure or thermal transients can result in longer Hved transients in the individual catalyst pellets, because concentration and temperature gradients within catalyst pores adjust slowly. 

Internal Regenerator Bed Colls. Internal cods generate high overall heat-transfer coefficients [550 W / (m -K)] and typically produce saturated steam up to 4.6 MPa (667 psi). Lower heat fluxes are attained when producing superheated steam. The tube banks are normally arranged horizontally in rows of three or four, but because of their location in a continuously active bubbling or turbulent bed, they offer limited duty flexibdity with no shutdown or start-up potential. 

Steam. The steam system serves as the integrating energy system in most chemical process plants. Steam holds this unique position because it is an exceUent heat-transfer medium over a wide range of temperatures. Water gives high heat-transfer coefficients whether in Hquid phase, boiling, or in condensation. In addition, water is safe, nonpolluting, and if proper water treatment is maintained, noncorrosive to carbon steel. 

Sindlady, heating surface area needs are not direcdy proportional to the number of effects used. For some types of evaporator, heat-transfer coefficients decline with temperature difference as effects are added the surface needed in each effect increases. On the other hand, heat-transfer coefficients increase with temperature level. In a single effect, all evaporation takes place at a temperature near that of the heat sink, whereas in a double effect half the evaporation takes place at this temperature and the other half at a higher temperature, thereby improving the mean evaporating temperature. Other factors to be considered are the BPR, which is additive in a multiple-effect evaporator and therefore reduces the net AT available for heat transfer as the number of effects is increased, and the reduced demand for steam and cooling water and hence the capital costs of these auxiUaries as the number of effects is increased. 

The heavy-duty jacketed type (Fig. ll-62a) is a special custom-built adaptation of a heavy-duty vibratory conveyor shown in Fig. 11-60. Its apphcation is continuously to cool the crushed materi [from about 177°C (350°F)] produced by the vibratoiy-type caster of Fig. 11-53. It does not have the liqmd dam and is made in longer lengths that employ L, switchback, and S arrangements on one floor. The capacity rate is 27,200 to 31,700 kg/h (30 to 35 tons/h) with heat-transfer coefficients in the order of 142 to 170 W/(m °C) [25 to 30 Btii/(h ft °F)]. For heating or drying applications, it employs steam to 414 kPa (60 IbFin ). 

By far the best application of computers to evaporators is for working up operators data into the basic performance parameters such as heat-transfer coefficients, steam economy, and dilution. 

Heat-transfer coefficients in steam-tube dryers range from 30 to 85 J/(m s K). Coefficients will increasewith increasing steam temperature because of increased heat transfer by radiation. In units carrying saturated steam at 420 to 450 K, the heat flux UAT will range from 6300 J/(m s) for difficult-to-diy and organic solids and to 1890 to 3790 J/(m s) for finely divided inorganic materials. The effect of steam pressure on heat-transfer rates up to 8.6 X 10 Pa is illustrated in Fig. 12-71. 

Design Methods for Calciners In indirect-heated calciners, heat transfer is primarily by radiation from the cyhnder wall to the solids bed. The thermal efficiency ranges from 30 to 65 percent. By utilization of the furnace exhaust gases for preheated combustion air, steam produc tion, or heat for other process steps, the thermal efficiency can be increased considerably. The limiting factors in heat transmission he in the conductivity and radiation constants of the shell metal and solids bed. If the characteristics of these are known, equipment may be accurately sized by employing the Stefan-Boltzmann radiation equation. Apparent heat-transfer coefficients will range from 17 J/(m s K) in low-temperature operations to 8.5 J/(m s K) in high-temperature processes. 

For steam side noncondensibles, a proper vent is required. A small amount of noncondensibles can greatly lower the steam side heat transfer coefficient. The improper removal of condensate is another way to reduce... 

In water-cooled tube-and-shell condensers with shell side condensation, overall heat transfer coefficients for essentially pure steam range from 200 to 800 Btu per hour per square foot per °F. 

Figure 10-53A. Determine the inside heat transfer coefficient for superheated steam. (Used by permission Ganapathy, V. Hydrocarbon Processing, Sept. 1977. Gulf Publishing Company, Houston, Texas. All rights reserved.)...

Figure 10-86A. Influence of air content on the heat transfer coefficient of steam containing air. (Used by permission Edmister, W. C., and Marchello, J. M. Petro/Chem. Engineer, June 1966, p. 48. Petroleum Engineer International.)...

Wall temperatures drop after reaching the maximum in the case of the two highest heat flux levels in Fig. 8, and this is due to increasing convective heat transfer through the steam film, which now completely blankets the surface. The improved heat transfer is caused by the higher flow velocities in the tube as more entrained liquid is evaporated. Finally, at about 100% quality, based on the assumption of thermal equilibrium, only steam is present, and wall temperatures rise once more due to decreasing heat-transfer coefficients as the steam becomes superheated. 

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