A comparison of the magnitudes of the various heat-transfer resistances in Example 6.1 shows that the major resistance is on the fluid side, where the fluid in this case is air. The heat-transfer resistance on the air side is 0.01305 m2·°C/W (0.0741 hr·ft2·°F/Btu) while on the refrigerant side the resistance is 0.000833 m2·°C/W (0.00473 hr·ft2·°F/Btu), so the air-side resistance is 20 times that of the refrigerant side. This comparison suggests immediately where attention should be directed if an increase in the U-value were desired. Certainly not on the refrigerant side, because even if the coefficient could somehow be doubled, the U-value would increase by only 3%. No, the resistance to attack is that on the air side, and this resistance can be decreased only by increasing hf
or increasing the area ratio.
Several methods of increasing hf are to increase the air velocity and to increase the turbulence by introducing irregularities in the heat-transfer surface. Increasing the air velocity will increase hf but at the expense of additional fan power. The coil manufacturer seeks the optimum air velocity which provides a respectable hf but requires a reasonably sized fan and motor. A further consideration affected by the fan power is that the power to the fan motor ultimately appears as refrigeration load, and perhaps 10 to 20% of the heat removed by the air coil was ultimately introduced by the fan and its motor.
The standard approach to reducing the air-side resistance is to increase the area ratio Ao/Ai by the application of extended surface or fins. The air coil shown in Fig. 6.2 is equipped with fins which are formed from flat metal plates that are then punched and the tubes inserted in the holes. When the tubes are in position, they are expanded either hydraulically or mechanically to provide good thermal contact between the tube and fin. The section of fin associated with one tube is shown in Fig. 6.6 in a situation where the tube temperature is 0°C (32°F) and the air temperature is 6°C (42.8°F). If the entire fin were at the same temperature as the tube, namely 0°C (32°F), the resistance on the air side would be as stated in Eq. 6.8, Ai/hfAo. Figure 6.6 indicates, however, that the fin temperature increases at positions progressively further removed from the tube.
So all of the air-side area Ao is not 100% effective. The effectiveness of the fin is usually given the symbol n and the equation for the U-value of the finned coil is:
The values of n for commercial coils generally range between 0.3 and 0.7, and the effectiveness is a function of such factors as the choice of fin material (usually steel or aluminum), fin thickness, and distance from the tube. The
effects of several choices on fin effectiveness and on the overall heat-transfer capacity of the coil are shown in Table. 6.2. The designer for the coil manufacturer spends nights trying to juggle these various decisions to provide the maximum heat transfer rate for a given cost of the coil.
Most of the foregoing discussion has implied that the evaporator is an aircooling coil, and indeed fins are almost universal on air coils, but not always. In some food plant applications the evaporator is composed of unfinned tubes. The reason is that these coils are easier to clean for hygienic purposes, even though the heat-transfer rate suffers. It should be pointed out that while the bare-tube coil is easier to clean, the coils must often be deep with a large number of rows of tubes in the air-flow direction which complicates cleaning.
A further comment to complete the discussion of extended surface is that fins are sometimes used on the refrigerant side in liquid-chilling evaporators. In water-chilling evaporators where refrigerant flows in the tubes, the tubes are often equipped with inner fins that are sometimes rifled. This style is especially adaptable to copper tubes which can be used in halocarbon refrigeration systems, but not ammonia. For ammonia systems, aluminum tubes can be provided with internal and/or external integral fins, and even for steel tubes the possibility of external integral fins exists.