Overall heat transfer coefficients typical values

Typical overall heat-transfer coefficients are given in Tables 11-3 through 11-8. Values from these tables may be used for preliminaiy estimating purposes. They should not be used in place of the design methods described elsewhere in this section, although they may serve as a useful check on the results obtained by those design methods. 

Typical values of thermal resistances and individual and overall heat transfer coefficients are given in Tables 9.15-9.18. 

Typical values of the overall heat-transfer coefficient for various types of heat exchanger are given in Table 12.1. More extensive data can be found in the books by Perry et al. (1997), TEMA (1999), and Ludwig (2001). 

Calculation of the overall heat transfer coefficient from Equation 15.13 requires knowledge of the film transfer coefficients. Although Table 15.1 presents typical values,... 

Table 3.4-2 summarizes typical values of the overall heat-transfer coefficient for heat exchangers and high-pressure gases. 

Calculations of the convection heat-transfer coefficients for use in the overall heat-transfer coefficient are made in accordance with the methods described in later chapters. Some typical values of the overall heat-transfer coefficient are given in Table 10-1. 

The overall volumetric heat transfer coefficient is obtained by combining a gas and liquid coefficient. Woods provides a value of 3 kW/m °C for a typical volumetric overall heat transfer coefficient for a packed tower under the above conditions (see Table 3-18). For greater accuracy values for the individual gas and liquid volumetric heat transfer coefficient may be obtained using the relations... 

If scale forms inside and/or outside the tube walls, additional resistance terms (Fig. 4) should be added to Eq. (2). Some typical values of overall heat-transfer coefficients are given in Table 3 for various evaporator designs. 

The value of U was provided in the calculation. Typical values of f/ for new, clean exchangers are available in the literature and are here assigned the symbol C/ iean-After several months of use, the tubes are fouled by scale or dirt. This scale causes a decrease in the overall heat-transfer coefficient, from (/clean t/diny- Th relation between these two C/s is given by... 

Adaptation of the experimental model. During the operation of the cooler the values of the model parameters may change. A typical example is fouling, which reduces the overall heat transfer coefficient, leading to... 

It should also be pointed out that the values of the parameters do not remain constant over long periods of time. Therefore, for effective modeling we need not only accurate values but also some quantitative description of how the parametric values change with time. Typical examples of changing parameters are the activity of a catalyst and the overall heat transfer coefficient of heat transfer systems (heat exchangers, jacketed reactors, etc.). 

Table 7.2 Typical values of overall heat transfer coefficients for shell and tube heat exchangers...

In order to do preliminary estimating of sizes of shell-ind-tube heat exchangers, typical values of overall heat-transfer coefficients are given in Table 4.9-2. These values should be useful as a check on the results of the design methods described in this chapter. 

Table 4.9-2. Typical Values of Overall Heat-Transfer Coefficients in Shell-and-Tiibe Exchangers (HI, P3, WJ)...

Perry and Green (P3) give typical values of overall heat-transfer coefficients U for coils immersed in various liquids in agitated and nonagitated vessels. 

Several approaches have been proposed in the literature to estimate the overall heat transfer coefficient [7-12]. The most common approach implemented in commercial lab scale calorimeters (like the RCl calorimeter from Mettler-Toledo), uses a two point calibration (nsing a calibration heater of a known power) [13]. The calibration is carried out at the beginning and at the end of the polymerization (in absence of reaction) and then the value of U is interpolated. There are two main drawbacks with this approach The first is that the heat of reaction cannot be obtained online because the expected change of U during the polymerization reaction (typically a decrease) cannot be calculated until the end of the reaction, and the second drawback is that the off-line calculated depends on the interpolation of the U values calculated at the beginning and at the end of the experiment. Although commercial equipment allows for different interpolations methods (linear, proportional to conversion, etc.), significant errors can be made in the computation of the... 

The catalyst is a mixture of vanadium and molybdenum oxides on an inert support. Typical inlet reaction tenperatures are in the range of 350°C to 400°C. The catalyst is placed in 25 mm diameter tubes that are 3.2 m long. The catalyst pellet diameter is 5 mm. The maximum tenperature that the catalyst can be exposed to without causing irreversible damage (sintering) is 650°C. The packed-bed reactor should be costed as a shell-and-tube exchanger. The heat transfer area should be calculated based on the total external area of the catalyst-filled tubes required from the simulatioa Because of the high tenperatures involved, both the shell and the tube material should be stainless steel. An overall heat transfer coefficient for the reactor should be set as 100 W/m °C. (This is the value specified in the simulation.)... 

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