Exchange length

When points for 20-ft-long tubes do not appear in Fig. 11-41, use 0.95 times the cost of the equivalent 16-ft-Iong exchanger. Length variation of steel heat exchangers affects costs by approximately 1 per square foot. Shell diameters for a given surface are approximately equal for U-tube and floating-head construc tion. 

Fig. 4.26 shows the temperature changes over a small differential element of exchanger length AZ. 

Heat balance equations on the element of heat exchanger length AZ, according to enthalpy balance relationship. 

Figure 4.27. Finite-differencing of heat exchanger length.

Analyzing this expression in terms of the exchange length reveals that imperfections are harmful if their size is comparable to or larger than a few nanometers. 

We are particularly interested in the diameter of the tubes we should use, the length of the tubes, and the total cost of the installed unit. In case the exchanger length is unreasonable for one unit, what would you recommend ... 

The heat exchanger length for a given conduction loss on each side is expressed as... 

The procedure is similar to that outlined in Example 14-9. Based on average temperatures on both sides of the exchanger, an estimate of the heat load can be obtained. With an estimated overall heat transfer coefficient, a preliminary area can be established to minimize the number of heat exchanger lengths that will need to be examined. With a heat load of iq>proximately 530 kJ/s, it will require a heat exchanger of at least several meters to meet the heat load. Thus, the... 

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