Page 328 - Mechanical Engineers' Handbook (Volume 4)
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3 Rating Methods  317

                           Heat-Transfer Coefficients
                           For a first approximation of the surface required, the bare-surface-based overall heat-transfer
                           coefficients recommended by Smith 35  may be used. A list of these values from Ref. 3 is
                           abstracted in Table 6. The values in Table 6 were based on performance of finned tubes,
                                                                                  5
                                                                3
                           having a 1 in. outside diameter base tube on 2 ⁄8-in. triangular pitch, ⁄8-in.-high aluminum
                               1
                           fins ( ⁄8-in. spacing between fin tips), with eight fins per inch. However, the values may be
                           used as first approximations for other finned types.
                              As stated by Mueller, air-cooled heat exchanger tubes have had approximately the pre-
                           ceding dimensions in the past, but fin densities have tended to increase and now more
                           typically range from 10 to 12 fins/in. For a more detailed estimate of the overall heat-
                           transfer coefficient, the tubeside coefficients are calculated by methods given in the preceding
                           sections and the airside coefficients are obtained as functions of fin geometry and air velocity
                           from empirical relationships such as given by Gnielinski et al. 36  Rating at this level of
                           sophistication is now done mostly by computer.

                           Temperature Difference
                           Air-cooled heat exchangers are normally ‘‘cross-flow’’ arrangements with respect to the type
                           of temperature profile calculation. Charts for determination of the F-factor for such arrange-
                                                     9
                           ments are presented by Taborek. Charts for a number of arrangements are also given by
                                2
                           Paikert based on the ‘‘NTU method.’’ According to Paikert, optimum design normally re-
                           quires NTU to be in the range of 0.8–1.5, where
                                                                t   t
                                                         NTU    2   1                           (56)
                                                                MTD
                           For first approximations, a reasonable air-temperature rise (t   t ) may be assumed, MTD
                                                                              1
                                                                          2
                           calculated from Eq. (4) using F   0.9–1.0, and NTU checked from Eq. (56). It is assumed
                           that if the air-temperature rise is adjusted so that NTU is about 1, the resulting preliminary
                           size estimation will be reasonable. Another design criterion often used is that the face ve-
                           locity V should be in the range of 300–700 ft/min (1.5–3.5 m/sec):
                                 ƒ

                           Table 6 Typical Overall Heat-Transfer Coefficients (U o ), Based on Bare Tube Surface, for
                           Air-Cooled Heat Exchangers
                                                                                     U o
                                                                               2
                                                                                                2
                           Service                                      Btu/hr ft   F       W/m  K
                           Sensible Cooling
                           Process water                                  105–120            600–680
                           Light hydrocarbons                              75–95             425–540
                           Fuel oil                                        20–30             114–170
                           Flue gas, 10 psig                                10                 57
                           Condensation
                           Steam, 0–20 psig                               130–140            740–795
                           Ammonia                                        100–200            570–680
                           Light hydrocarbons                              80–95             455–540
                           Refrigerant 12                                  60–80             340–455
                           Mixed hydrocarbons, steam, and noncondensables  60–70             340–397
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