Finned-tube units

The calculation of the film coefficients on the fin side is complex because each unit of surface on the fin is less effective than a unit of surface on the tube wall. This arises because there will be a temperature gradient along the fin so that the temperature difference 

A heat balance over a length dr at distance x from the hot end gives  

In solving this equation, three important cases may be considered  

Tht heat loss from a firmed tube is obtained initially by determining the heat flow into the base of the fin from the tube surface. Thus the heat flow to the root of the fin is  

These expressions are valid provided that the cross-section for heat flow remains constant When it is not constant, as with a radial or tapered fin, for example, the temperature distribution is in the form of a Bessel fimction  

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Compact units with high emissions. Often fitted with damper to reduce output when full emission not required, usually to about 30% of full output. Heat exchangers normally finned tube. Units may be built into wall of building. 

OTHER FORMS OF EQUIPMENT 9.10.1. Finned-tube units Film coefficients... 

Now consider a finned-tube unit with the same I.D. and O.D. tubes. Service conditions are kept identical—i.e., fluid velocities, temperatures, etc., are constant. The increases in ho and over plain-tube exchangers are assumed negligible, especially for phase-change applications, such as condensation or evaporation on the fins. Thus ... 

Since A, > A, it follows from Eq. (3) that < Up—the overall heat-transfer coefficient for finned-tube units is always less than that for plain-tube units. However, the real advantage to using finned tubes is the substantial increase in the surface area. This increase in area more than compensates for the reduction in the overall heat-transfer coefficient, resulting in a greater amount of heat that is transferred in finned-tube units. 

The ratio of heat transferred in a finned-tube unit to that in a plain-tube exchanger working within the same temperature limits is ... 

Table 9.20. Data on surface of finned tube units ...

Finally, an area smaller than 200 sq ft is so expensive that exchangers cannot be justified unless small standard double-pipe or fin-tube units are employed. In general, it is usually economical to bring the temperature of the feedstock by exchange to within about 40 F of the temperature of the hottest stock available. 

This same rate based on fin-side surface is only 104 -f 6 = 17.3, but something is gained by the use of finned surface because the rate in a plain double-pipe exchanger would have been only about 63, and thus about plain-tube units are required to replace one fin-tube unit. 

Forced and Induced Draft The forced-draft unit, which is illustrated in Fig. 11-43 pushes air across the finnedtube surface. The fans are located oelow the tube bundles. The induced-draft design has the fan above the bundle, and the air is pulled across the finned tube surface. In theoiy, a primaiy advantage of the forced-draft unit is that less power is required. This is true when the air-temperature rise exceeds 30°C (54°F). 

Open Tube Sections (Air Cooled) Plain or finned tubes No shell required, only end heaters similar to water units. Condensing, high level heat transfer. Transfer coefficient is low, if natural convection circulation, but is improved with forced air flow across tubes. 0.8-1.8... 

In the interest of energy conversion, process heat can be obtained from a heat recovery unit in which heat is recovered from turbine or reciprocating engine exhaust. In a heat recovery unit, an exhaust gas flows over finned tubes carrying the fluid to be heated. The hot exhaust gas (9()0"F to I.2(K) F) heats the fluid in the tubes in a manner similar to that in which air cools the fluid in an aerial cooler. It is also possible to recover heat from exhausts by routing the exhaust duct directly through a fluid bath. The latter option is relatively inefficient but easy to install and control. 

In the case of circumferentially finned tubes, the tube O.D. shall be the diameter at the root of the fins and the corresponding tabulated or interpolated span shall be reduced in direct proportion to the fourth root of the ratio of the weight per unit length of the tube, if stripped of fins to that of the actual finned tube. 

Figure 10-143 is useful in roughly predicting the relative economic picture for adapting low finned tubes to the heat or cooling of oil on the shell side of conventional shell and tube units. This is not a design chart. 

Figure 10-144. Approximate relationship of the overall coefficient fouled, and the fouling factor of inside tubes for predicting the economical use of finned tubes in shell and tube units. (Used by permission Williams, R. B., and Katz, D. L. Performance of Finned Tubes and Shell and Tube Heat Exchangers, 1951. University of Michigan. Note For reference only, 1950 costs.)...

Follow the procedures oudined for bare tube equipment, substituting the characteristics of finned tubes where appropriate. The presentation of Wolverine recommends this technique over previous methods. The methods of reference 16 have proven acceptable in a wide number of petrochemical hydrocarbon systems. Figure 10-150 is an example unit in summary form. 

Steam and hot-water coils should be constructed of seamless steel tube and preferably be without joints within the tank. These coils can be either plain or finned tube. However, due to their greater surface area, finned tubes generally have a higher rate of heat transfer than plain tubes. On the basis of cost per unit surface area, finned tubes are less expensive than conventional tubes. There is also an advantage of weight saving, and complete coverage of the base area is not necessarily required to achieve specified temperatures. 

Some indication of the performance obtained with transverse finned tubes is given in Table 9.21. The figures show the heat transferred per unit length of pipe when heating air on the fin side with steam or hot water on the tube side, using a temperature difference of 100 deg K. The results are given for three different spacings of the fins. 

Compact (plate and fin) exchangers have 350 sqft/cuft, and about 4 times the heat transfer per cuft of shell-and-tube units. 

Air-cooled exchangers consist of banks of finned tubes over which air is blown or drawn by fans mounted below or above the tubes (forced or induced draft). Typical units are shown in Figure 12.68. Air-cooled exchangers are packaged units, and would normally be selected and specified in consultation with the manufacturers. Some typical overall coefficients are given in Table 12.1. These can be used to make an approximate estimate of the area required for a given duty. The equation for finned tubes given in Section 12.14 can also be used. 

Loss of airflow through a finned tube air cooler bundle is a universal problem. The effect is to reduce the exchanger s cooling efficiency. To restore cooling, you might wish to try the Norm Lieberman method, which consists of reversing the polarity of the fan motor electric leads. The fan will now spin backward. Depending on the nature of the deposits, a portion of the accumulated dirt will be blown off the tubes— but all over the unit. Personnel observe this procedure from a safe distance. 

MHHP sorbers of the second unit are monoblock-type. When installation works, coolant comes in a backlash between an external jacket and a case and then through finned tubes is removed through a branch pipe of a front manifold. 

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