Heat-Exchanger Design Considerations

3 Why does a mixed or unmixed fluid arrangement influence heat-exchanger performance  

4 When is the LMTD method most applicable to heat-exchanger calculations  

6 What advantage does the effectiveness-NTU method have over the LMTD method  

8 Why is a counterflow exchanger more effective than a parallel-flow exchanger  

10-1 A long steel pipe with a 5-cm ID and 3.2-mm wall thickness passes through a large room maintained at 30°C and atmospheric pressure 0.6 kg/s of hot water enters one end of the pipe at 82°C. If the pipe is 15 m long, calculate the exit water temperature, considering both free convection and radiation heat loss from the outside of the pipe. 

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C.F.McDonald, et al. Heat exchanger design considerations for HTGR plants, the American Society of Mechanical Engineering, July 27-30 1980. 

Considerable interest has been generated in turbulence promoters for both RO and UF. Equations 4 and 5 show considerable improvements in the mass-transfer coefficient when operating UF in turbulent flow. Of course the penalty in pressure drop incurred in a turbulent flow system is much higher than in laminar flow. Another way to increase the mass-transfer is by introducing turbulence promoters in laminar flow. This procedure is practiced extensively in enhanced heat-exchanger design and is now exploited in membrane hardware design. 

From the standpoint of heat-exchanger design the plane wall is of infrequent application a more important case for consideration would be that of a doublepipe heat exchanger, as shown in Fig. 10-2. In this application one fluid flows on the inside of the smaller tube while the other fluid flows in the annular space between the two tubes. The convection coefficients are calculated by the methods described in previous chapters, and the overall heat transfer is obtained from the thermal network of Fig. 10-2h as... 

In the preceding analysis, consideration has been given to the general case in heat-exchanger design in which the following conditions apply ... 

For traditional cascaded plants, consisting mainly of process units connected in series, the controllability properties can easily be deduced from consideration of the individual units. The disturbance sensitivity is simply a product of the disturbance sensitivities of the units cormecting the disturbance and output under consideration, and any fundamental control limitation can be attributed to some individual unit. Since there exist a wealth of knowledge on how to improve the disturbance sensitivity, and avoid control limitations, through design of common units like reactors, distillation columns and heat-exchangers, design for controllability is fully feasible for cascaded plants. 

The geometry of coiled-tube heat exchangers can be varied widely to obtain optimum flow conditions for all streams and still meet heat transfer and pressure drop requirements. However, optimization of a coiled-tube heat exchanger design is very involved and complex. There are numerous variables, such as tube and shell flow velocities, tube diameter, tube pitch, and layer spacer diameter. Other considerations include single-phase and two-phase flow, condensation on either the tube or shell side, and boiling or evaporation on either the tube or shell side. Additional complications come into play when multicomponent streams are present, as in natural gas liquefaction, since mass transfer accompanies the heat transfer in the two-phase region. 

Friction Coefficient. In the design of a heat exchanger, the pumping requirement is an important consideration. For a fully developed laminar flow, the pressure drop inside a tube is inversely proportional to the fourth power of the inside tube diameter. For a turbulent flow, the pressure drop is inversely proportional to D where n Hes between 4.8 and 5. In general, the internal tube diameter, plays the most important role in the deterrnination of the pumping requirement. It can be calculated using the Darcy friction coefficient,, defined as... 

Design considerations should be examined by process design engineers when designing heat exchangers for stage 1 tube rupture transient effects, which includes the following ... 

For the first kind of application, the focus is on certain elements of the HVAC component under consideration. The simulation is used to study and optimize design-specific aspects such as the pipe size and spacing or wetted area and fin geometry in a heat exchanger. This kind of modeling requires detailed knowledge on many input parameters and the related physical processes. 

Collins, G. E. and R. T. Matthews, Climatic Considerations in the Design of Air-Cooled Heat Exchangers, Heat Transfer Div. Paper No. 59-A-255, ASME, presented at annual meeting Atlantic City, N.J. (1959). 

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. 

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