Tube-side coefficients, heat exchangers

In a shell-and-tube type of heat exchanger with horizontal tubes 25 mm external diameter and 22 mm internal diameter, benzene is condensed on the outside of the tubes by means of water flowing through the tubes at the rate of 0.03 m3/s. If the water enters at 290 K and leaves at 300 K and the heat transfer coefficient on the water side is 850 W/m2 K, what total length of tubing will be required ... 

A shell-and-tube heat exchanger with four tube passes is used to heat 2.3 kg/s of water from 25 to 70°C. Hot water at 93°C is available for the heating process, and a flow rate of 5 kg/s may be used. The cooler fluid is used on the tube side of the exchanger. The overall heat-transfer coefficient is 800 W/m2 °C. Assuming that the flow rate of the hot fluid and the overall heat-transfer coefficient remain constant, plot the percentage reduction in heat transfer as a function.of the mass-flow rate of the cooler fluid. 

A shell-aod-tube heal exchanger vvilb 2-sheII passes and 12 lube passes is used lo heal waicr (c = 41801/kg "Q in the lubes from 20°C to 70°C at a rate of 4.5 kg/s. Heat is supplied by hot oil (c, = 2300 JAcg "C) that enters the shell side at I70°C at a rate of 10 kg/s. For a tube-side overall heat transfer coefficient of 350 W/m °C, determine the heat transfer surface area on the lube. side. Answer 25.7 rn ... 

Standard construction for shell-and-tube exchangers includes nickel tubes and carbon steel in cooling water service on the shell side. Compact heat exchangers also are widely used in caustic service. They have found a growing market in chlor-alkali plants. Their high heat-transfer coefficients and resistance to fouling reduce the surface area required. This is especially valuable when using expensive materials such as nickel, titanium, and the Hastelloys. 

The overall heat transfer coefficient of the designed heat exchanger must be calculated to be compared with the guessed corrected value for this purpose, the tube side h) and shell side (hj film heat transfer coefficient must be determined. The tube side film heat transfer coefficient h, is calculated using the following equation [6] ... 

Some processes have large heat transfer requirements. This may result in large inventories of material within the heat transfer equipment. If the material is thermally unstable it would be inherently safer to reduce the residence time in the heat exchanger. Options to minimize heat exchanger inventory include the use of different types of heat exchangers. Inventories in shell and tube heat exchangers can be reduced by the use of turbulators in the tubes to enhance heat transfer coefficients, and by placing the more hazardous material on the tube side. 

For sheU-aiKl-l-ube heat exchangers with shell-side balile. die shell-side fluid flow is perpendicular to the tubes. In this arrangement, the outside film coefficient can be calculated from the following equation ... 

Higher overall heat transfer coefficients are obtained with the plate heat exchanger compared with a tubular for a similar loss of pressure because the shell side of a tubular exchanger is basically a poor design from a thermal point of view. Considerable pressure drop is used without much benefit in heat transfer efficiency. This is due to the turbulence in the separated region at the rear of the tube. Additionally, large areas of tubes even in a well-designed tubular unit are partially bypassed by liquid and low heat transfer areas are thus created. 

TWo tubular heat exchangers are available each with a 0.44 m i.d. shell fitted with 166 tubes, 19.0 mm o.d. and 15.0 mm i.d., each 5.0 m long. The tubes are arranged in two passes on 25 mm square pitch with a baffle spacing of 150 mm. There are two passes on the shell side and operation is to be countercurrent. With benzene passing through the tubes, the anticipated film coefficient on the tube side is 1000 W/m2K. 

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 complex flow pattern on the shell-side, and the great number of variables involved, make it difficult to predict the shell-side coefficient and pressure drop with complete assurance. In methods used for the design of exchangers prior to about 1960 no attempt was made to account for the leakage and bypass streams. Correlations were based on the total stream flow, and empirical methods were used to account for the performance of real exchangers compared with that for cross flow over ideal tube banks. Typical of these bulk-flow methods are those of Kern (1950) and Donohue (1955). Reliable predictions can only be achieved by comprehensive analysis of the contribution to heat transfer and pressure drop made by the individual streams shown in Figure 12.26. Tinker (1951, 1958) published the first detailed stream-analysis method for predicting shell-side heat-transfer coefficients and pressure drop, and the methods subsequently developed... 

Emerson, W. H. (1973) Conference on Advances in Thermal and Mechanical Design of Shell and Tube Exchangers, NEL Report No. 590. (National Engineering Laboratory, East Kilbride, Glasgow, UK). Effective tube-side temperature in multi-pass heat exchangers with non-uniform heat-transfer coefficients and specific heats. 

The detailed allocation of fluids to tube-side or shell-side can only be made later in the heat exchanger network design. Also, the area targeting formula does not recognize fluids to be allocated to the tube-side or shell-side. Area targeting only recognizes the individual film heat transfer coefficients. All that can be done in network area targeting... 

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