Heat exchange, cross section

Figure 27.1 Spiral heat exchanger cross-sectional flow diagram.

Very strong stirring equipment is needed for mixing because of the high viscosity, and long tubular reactors with low cross-sectional area are needed for heat exchange. 

Favored locations for erosion-corrosion are areas exposed to high-flow velocities or turbulence. Tees, bends, elbows (Fig. 11.5), pumps, valves (Fig. 11.6), and inlet and outlet tube ends of heat exchangers (Fig. 11.7) can be affected. Turbulence may be created downstream of crevices, ledges (Fig. 11.8), abrupt cross-section changes, deposits, corrosion products, and other obstructions that change laminar flow to turbulent flow. 

From Tolmin s theory and experimental data (e.g., Reichardtthe relationship between velocity profile and temperature profile in the jet cross-section can be expressed using an overall turbulent Prandtl number Pr = v /a, where Vf is a turbulent momentum exchange coefficient and a, is a turbulent heat exchange coefficient ... 

A = total exchanger bare tube heat transfer, ft or, net external surface area of tubes exposed to fluid heat transfer, ft or, area available for heat transfer, ft (for conduction heat transfer, A is a cross-sectional area, taken normally in the direction of heat flow, ft2). 

It is shown in Section 9.9.5 that, with the existence of various bypass and leakage streams in practical heat exchangers, the flow patterns of the shell-side fluid, as shown in Figure 9.79, are complex in the extreme and far removed from the idealised cross-flow situation discussed in Section 9.4.4. One simple way of using the equations for cross-flow presented in Section 9.4.4, however, is to multiply the shell-side coefficient obtained from these equations by the factor 0.6 in order to obtain at least an estimate of the shell-side coefficient in a practical situation. The pioneering work of Kern(28) and DoNOHUE(lll who used correlations based on the total stream flow and empirical methods to allow for the performance of real exchangers compared with that for cross-flow over ideal tube banks, went much further and. 

The ventilation capacity required for drying amounts to 0.4-0.5 m3/hen/h at a backpressure of 500-1000 Pa in the heat exchanger tubes. The pressure is determined from the volume of air, the length and cross-section of the perforated ducts. For good drying, the pressure at the end of the perforated duct must be at least 300-350 Pa. This is the lowest pressure at which the air can be blown out of the holes at a speed of ca 20/m/sec., so that it can be distributed over the manure on the belt at a speed of 0.5-2.0 m/sec. 

PAFC systems are commercially available from UTC Power as 200-kW stationary power sources operating on natural gas. The stack cross section is 1 m (10.8 ft ). It is about 2.5 m (8.2 ft) tall and rated for a 40,000-h life. It is cooled with water/steam in a closed loop with secondary heat exchangers. Fuel processing is similar to that in a PEFC system, but a preferential oxidizer is not needed. These systems are intended for on-site power and heat generation for hospitals, hotels, and small businesses. 

All types of catalytic reactors with the catalyst in a fixed bed have some common drawbacks, which are characteristic of stationary beds (Mukhlyonov et al., 1979). First, only comparatively large-grain catalysts, not less that 4 mm in diameter, can be used in a filtering bed, since smaller particles cause increased pressure drop. Second, the area of the inner surface of large particles is utilized poorly and this results in a decrease in the utilization (capacity) of the catalyst. Moreover, the particles of a stationary bed tend to sinter and cake, which results in an increased pressure drop, uneven distribution of the gas, and lower catalyst activity. Finally, porous catalyst pellets exhibit low heat conductivity and as a result the rate of heat transfer from the bed to the heat exchanger surface is very low. Intensive heat removal and a uniform temperature distribution over the cross-section of a stationary bed cannot, therefore, be achieved. The poor conditions of heat transfer within... 

The existence of a heat sink owes a debt to the heat exchange with the external environment on the lateral surface of the bar. The quantity is equal to the amount of heat being supplied to the segment xi. l/2 < x < xi+1/2 through the cross-section x = xi 1 j2, while wi+1/2 refers in a similar fashion to the heat transfer from this segment through the cut x = xi+1/2. The third member on the left-hand side of (11) reflects the amount of... 

In a heat exchanger, heat is transferred between hot and cold fluids through a solid wall. The fluids may be process streams or independent sources of heat such as the fluids of Table 8.2 or sources of refrigeration. Figure 8.2 shows such a process with inlet and outlet streams, but with the internal flow pattern unidentified because it varies from case to case. At any cross section, the differential rate of heat transfer is... 

Figure 8.8. Plate and spiral compact exchangers, (a) Plate heat exchanger with corrugated plates, gaskets, frame, and corner portals to control flow paths, (b) Flow patterns in plate exchangers, (i) parallel-counter flows (ii) countercurrent flows (iii) parallel flows throughout, (c) Spiral exchanger, vertical, and horizontal cross sections.

If flow is cocurrent the lower sign is used if countercurrent the upper sign is used. Since the mass flowrate of the cooling fluid is based upon the cross-sectional area of the reactor tube the ratio G Ip Gq SpC(= H is a measure of the capacities of the two streams to exchange heat. In terms of the limitations imposed by the onedimensional model, the system is fully described by equations 3.9S and 3.96 together with the mass balance equation ... 

G is the amount of heat given off to the walls over a unit tube length in unit time with respect to a unit cross-section. If the heat exchange rate is determined by the heat transfer from the gas to the walls,... 

Numerous fin corrugations have been developed, each with its own special characteristics (Figure 15). Straight fins and straight perforated fins act like parallel tubes with a rectangular cross section. Convective heat exchange occurs due to the friction of the fluid in contact with the surface of the fin. The channels of serrated fins are discontinuous, and the walls of the fins are offset. For air flows,... 

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