Heat factor pipe effects

Precise comparisons between packed beds and ceramic foam structures are complex since many factors - activity, effectiveness factors, mass and heat transfer, and pressure drop - are all interdependent. We have simulated the performance of a conventional steam reformer and compared it to one containing a ceramic foam cartridge loaded to achieve equivalent intrinsic activity per gram of catalyst. A 1-D model developed and tested previously for heat-pipe reformers with isothermal walls was used [57]. Pressure drop, mass transfer and heat transfer correlations for the packed bed were known to be accurate for commercial catalysts those used for the foam were determined in the studies described above. Process conditions and results are given in Table 7. 

Fig. 1. Effect of energy use on total cost where total cost is the sum of capital and energy costs for the lifetime of the plant, discounted to present value. Point D corresponds to the design point if the designer uses an energy price that is low by a factor of four in projected energy price. Effects on costs of (a) pressure drop in piping, (b) pressure drop in exchangers, (c) heat loss through insulation, (d) reflux use, and (e) energy recovery through waste-heat boiler...

Isothermal Gas Flow in Pipes and Channels Isothermal compressible flow is often encountered in long transport lines, where there is sufficient heat transfer to maintain constant temperature. Velocities and Mach numbers are usually small, yet compressibihty effects are important when the total pressure drop is a large fraction of the absolute pressure. For an ideal gas with p = pM. JKT, integration of the differential form of the momentum or mechanical energy balance equations, assuming a constant fric tion factor/over a length L of a channel of constant cross section and hydraulic diameter D, yields,... 

Because most applications for micro-channel heat sinks deal with liquids, most of the former studies were focused on micro-channel laminar flows. Several investigators obtained friction factors that were greater than those predicted by the standard theory for conventional size channels, and, as the diameter of the channels decreased, the deviation of the friction factor measurements from theory increased. The early transition to turbulence was also reported. These observations may have been due to the fact that the entrance effects were not appropriately accounted for. Losses from change in tube diameter, bends and tees must be determined and must be considered for any piping between the channel plenums and the pressure transducers. It is necessary to account for the loss coefficients associated with singlephase flow in micro-channels, which are comparable to those for large channels with the same area ratio. 

A comparison with the empty pipe, expression (8,15), shows, that the improvement of the heat transfer indeed amounts to a factor of 2.5 This means, that the laminar boundary layer hardly decreases, but the removal of heat in the radial direction is indeed improved. This also explains, why already from LlD> 30 the effect of this parameter disappears. 

The most prominent characteristic of flow in curved ducts and helicoidal pipes is the secondary flow induced by centrifugal force due to the curvature of the pipe. Consequently, the friction factor is higher in curved pipes than that in straight pipes for the same Reynolds number. The pitch of the helicoidal pipe also has an effect on the flow. As a result, the heat trans-... 

Spiral coils are curved ducts with varying curvature. The friction factor and heat transfer rate for spiral coils are also included in this section. In addition to the dimensionless parameters used in straight pipes, the following parameters are particularly useful in the case of curved ducts or helicoidal pipes the Dean number De the helical number He, and the effective radius of curvature Rc. These are defined as follows ... 

Data for uniform-wall-temperature heating are plotted in Fig. 11.32, and the isothermal friction factors are plotted in Fig. 11.33. Twisted tapes and propellers were used by Koch [4] to heat air (curves a-d). Propellers produce higher heat transfer coefficients than twisted tapes however, this enhancement is at the expense of a rather large increase in friction factor, as seen in Fig. 11.33. Up to Re = 200, the friction factor for the twisted tape is the same as that for the empty half-tube (y = °o). The twisted-tape data of Marner and Bergles [114] with ethylene glycol exhibit an enhancement of about 300 percent above the smooth-tube values. Swirl at the pipe inlet does not produce any effective enhancement [192],... 

We are familiar with the cooling effect of wind and rain on our own exposed skin weather forecasters refer to the wind chill factor . The data in Table 5.1 shows the effectiveness of water sprays in removing heat from the surface of extruded plastic pipes. Consequently, the pipe cooling rate is limited by the transient conduction within the plastic the melt surface can be treated as if kept at the water temperature, and the analysis of the last section be used. 

---