Coefficients transfer

The problem with Eq. (7.5) is that the overall heat transfer coefficient is not constant throughout the process. Is there some way to extend this model to deal with the individual heat transfer coefficients ... 

The effect of individual stream film transfer coefficients can be included using the following expression, which is derived in App. 

FIgur 7.4 If film transfer coefficients difier significantly, then nonvertical h t transfer is necessary to achieve the minimum area. (Reprinted from Linnhoff and Ahmad, Cost Optimum Heat Exchanger Networks I. Minimum Energy and Capital Using Simple Models for Capital Cost," Computers Chem. Engg., 7 729, 1990 with permission from Elsevier Science, Ltd.)... 

By constrast, Fig. 7.46 shows a diflFerent arrangement. Hot stream A with a low coefficient is matched with cold stream D, which also has a low coefficient but uses temperature diflferences greater than vertical separation. Hot stream B is matched with cold stream C, both with high heat transfer coefficients but with temperature differences less than vertical. This arrangement requires 1250 m of area overall, less than the vertical arrangement. 

Thus, if film transfer coefficients vary significantly, then Eq. (7.6) does not predict the true minimum network area. The true minimum area must be predicted using linear programming. However, Eq. (7.6) is still a useful basis to calculate the network area for the purposes of capital cost estimation for the following reasons ... 

One problem remains where to get the film transfer coefficients hi and hj. There are three possibilities ... 

Stream Supply temp. T, rc) Target temp. Tr rC) AH (MW) Heat capacity flow rate CP (WN C- ) Heat transfer coefficient h(MW... 

Thus, for a given exchanger duty and overall heat transfer coefficient, the 1-2 design needs a larger area than the 1-1 design. However, the 1-2 design offers many practical advantages. These... 

These small positive and negative errors partially cancel each other. The result is that capital cost targets predicted by the methods described in this chapter are usually within 5 percent of the final design, providing heat transfer coefficients vary by less than one order of magnitude. If heat transfer coefficients vary by more than one order of magnitude, then a more sophisticated approach can sometimes be justified. ... 

Example 16.4 The stream data for a process are given in Table 16.5. Steam is available condensing between 180 and 179°C and cooling water between 20 and SO C. All film transfer coefficients are 200Wm C For lO C, the... 

Now consider the heat transfer area required by enthalpy interval k, in which the overall heat transfer coefficient is allowed to vary... 

Uij = overall heat transfer coefficient between hot stream i and cold stream j... 

Equation (F.l) shows that each stream makes a contribution to total heat transfer area defined only by its duty, position in the composite curves, and its h value. This contribution to area means also a contribution to capital cost. If, for example, a corrosive stream requires special materials of construction, it will have a greater contribution to capital cost than a similar noncorrosive stream. If only one cost law is to be used for a network comprising mixed materials of construction, the area contribution of streams requiring special materials must somehow increase. One way this may be done is by weighting the heat transfer coefficients to reflect the cost of the material the stream requires. 

The relationship between heat exchanger area and overall heat transfer coefficient U is given by... 

Liquid viscosity is one of the most difficult properties to calculate with accuracy, yet it has an important role in the calculation of heat transfer coefficients and pressure drop. No single method is satisfactory for all temperature and viscosity ranges. We will distinguish three cases for pure hydrocarbons and petroleum fractions ... 

Kovats retention indices RI Mass transfer coefficient h... 

If solvent recovery is maximized by minimizing the temperature approach, the overall heat-transfer coefficient in the condenser will be reduced. This is due to the fact that a large fraction of the heat transfer area is now utilized for cooling a gas rather than condensing a Hquid. Depending on the desired temperature approach the overall heat-transfer coefficients in vent condensers usually range between 85 and 170 W/m K (ca 15 and 30 Btu/h-ft. °F). 

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