Heat-transfer efficiency exchangers

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. 

Steam traps are automatic mechanisms that remove low heat-content air and condensate from the steam delivery system. The lack of steam traps or use of traps that fail to function properly leads to a gradual decline in heat-transfer efficiency, waterlogged heat exchangers, and water hammer (which may in turn result in ruptured pipes). When adequate maintenance of steam traps is neglected, this ultimately leads to a serious overall loss of operating efficiency. There are various types of steam traps, each designed for a specific function. Some common variations are discussed in the following sections. 

Heat exchange surfaces must be kept clean deposits reduce heat transfer efficiency and promote various forms of under-deposit corrosion. It also is easier to keep a clean system clean than to prevent a dirty system from getting dirtier, so measurement of the dirt loading or deposit loading on a heat transfer surface is an important part of determining when a boiler needs cleaning. 

The engineering specification and water consumption figures provided are correct. No other information is readily available for heat exchangers or other equipment. However, customer believes that heat-transfer efficiency has gradually decreased over the last 2 to 3 years. Treated river water is used as makeup. 

By evaluating the performance of micro heat exchangers, the temperature changes of heat transfer fluids were used, and the heat transfer efficiency of devices made of stainless steel or glass were found to be higher than that of devices made of copper (see Table 4.2). 

FIGURE 6.2 Forced circulation evaporator such as used for simultaneous brine evaporation-crystallization. Mechanical movement of brine past the heat-exchange surface avoids decreased heat transfer efficiency from crystallization on this surface. Constructed of Monel or Monel-clad steel for parts contacting brine. (Reprinted from Kirk-Othmer [10], with permission.)... 

Although reduced heat transfer efficiency is of prime importance there may also be pressure drop problems. The presence of the foulant will restrict flow that results in increased pressure drop. In severe examples of fouling the exchanger may become inoperable because of the back pressure. Indeed the pressure drop problems may have a more pronounced effect than the loss of thermal efficiency. 

In addition to heat transfer, of concern to designers and operators of heat exchangers is the pressure drop experienced in the fluid as it passes through the exchanger. Often it is the increased pressure drop brought about by the presence of deposits, rather than the reduced heat transfer efficiency that forces the shut down of a heat exchanger for maintenance and cleaning (see Chapter 3). 

Accumulation of undesirable corrosion products on heat exchanger tubing and pipelines decrease the heat transfer efficiency and reduce the pumping capacity. Soluble corrosion products can contaminate a system, and decontamination of the system results in additional cost. An example of this is the expensive shutdowns of nuclear reactors during the decontamination process. 

Heat transfer coefficients have been calculated for studied devices (Figure 2.43) using thermal balance equations and considering the equations describing liquid media counter flow (heat exchange surface value F = 0.044 m for the diffnser-confnsor channel, F = 0,05 for the cylindrical charmel). Calculated results demonstrate that the heat transfer coefficient value is 1.4-1.7 times higher in diffnser-confnsor channels, than in cylindrical ones. The heat transfer efficiency of a cylindrical device shows a slight decrease in the transition area (Re = (4-10)10 ). 

You may have seen bundles constructed with serrated, or very small, fins covering the exterior of the tubes. These are called low-fin-tube bundles. These fins increase the outside surface area of the tubes by a factor of 2.5. However, this does not mean that the heat-transfer efficiency of the exchanger will increase by 250 percent. Two factors curtail this improvement ... 

As a consequence of the channel head tube-side inlet and outlet being located on the same end of the exchanger, the lower half of the shell is in co-current flow. Depending on the temperature profile, this typically reduces the LMTD by 5 to 25 percent. To calculate this loss in heat-transfer efficiency due to this problem, we use the F-factor correction factor as presented in your TEMA Data Book. 

Sizing of heat exchangers assumes a certain heat-transfer efficiency between the bulk fluid and metal wall. Because biofilms more or less behave like gels on the metal surface, heat transfer can occur only by conduction through the biofilm. The thermal conductivity of biofilms is similar to that of water but much less than that of metals. On the basis of relative thermal conductivities (Table 2.37), a biofilm layer 41 p,m thick offers the same resistance to heat transfer as a titanium tube wall 1000 p,m thick. 

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