Heat transfer coefficients fouled conditions

Ganapathy, V, Nomograph Relates Clean and Dirty Heat Transfer Coefficients, Fouling Factor, Heating/Piping/Air Conditioning, Jan. (1979) p. 127. 

U, Uq = overall heat transfer coefficient corrected for fouling conditions, Btu/ (hr) (fL) (°F). 

The difficulty with these synthesis methods is that they generate HENs for fixed nominal values of the stream supply temperatures and flow rates and for assumed nominal values of the heat transfer coefficients. In an industrial HEN, the supply temperatures and flow rates will vary (because of unpredictable environmental disturbances or because of predictable feedstock and throughput changes), and the heat transfer coefficients are highly uncertain (due to fouling, etc.). The HEN synthesized for nominal conditions must be resilient (flexible) to changes in supply temperatures and flow rates and to uncertainties in heat transfer coefficients. 

Calculate the heat-transfer coefficient for a fluid with the properties listed in Example 7.18 if the fluid is flowing across a tube bundle with the following geometry. The fluid flows at a rate of 50,000 lb/h (22,679.5 kg/h). Calculate the heat-transfer coefficient for both clean and fouled conditions. 

Design an air-cooled heat exchanger to cool water under the following conditions. The design ambient air temperature is 35°C (95°F). The tubes to be used are steel tubes [thermal conductivity = 25 Btu/ (h)(ft2)(°F), or 43 W/(m)(K)] with aluminum fins. The steel tube is 1 in (0.0254 m) outside diameter and 0.834 in (0.0212 m) inside diameter. The inside heat-transfer coefficient and the fouling coefficient hs are each 1000 Btu/(h)(ft2)(°F) [5680 W/(m2)(K)]. Heat-exchanger purchase cost is 22 per square foot for four-tube-row units, 20 per square foot for five-tube-row units, and 18 per square foot for six-tube-row units. 

The objective of a rating problem is to determine if an existing process unit will satisfy process conditions. To arrive at an approximate calculation procedure for rating a heat exchanger, first define a clean overall heat-transfer coefficient, i.e., in the absence of any fouling. Therefore, Rfj andRfo = 0 in Equation 4.15. 

Agitated horizontal film (votator) usually to condition foodsmffs, crystallize, and react. Especially for viscous feed. OK for foaming, fouling, crystal formation, and suspended solids. Viscosities >2000 mPa s. Relative to agitated film retention time of 1 1 and volume 1 1. Overall heat transfer coefficient 2 kW/m °C, decreasing with increasing viscosity 3 to 12 kW/m. See also evaporation, Section 16.11.4.1. 

Overall heat transfer coefficient for fouled conditions... 

The first surface selected will be either the same or smaller than the final selection to eliminate the possibility of making a selection that satisfies all conditions but is oversurfaced, a possibility if we started with a larger surface than the final selection. Starting with a smaller surface is accomplished by specifying in the input a which is the maximum acceptable overall heat transfer coefficient. If in doubt about what to specify, use a U that is abnormally high or use the reciprocal of the total fouling factor. 

In large jacketed reactors used for suspension polymerization of vinyl chloride, the reaction rate is limited by the rate of heat removal [42,43]. Thick-walled reactors are needed because of the high pressure, and a very smooth surface is desired to minimize fouling. Compare the overall heat transfer coefficients for the following conditions, assuming hi = 300 and hj = 500 Btu/hr ft ... 

Moreover, scale up of the data obtained is virtually impossible, mainly due to different hydrodynamic conditions in the pi-lot-scale plant and the technical plant, impurities in the technical crystallizer which influence nucleation and crystal growth, and different heat transfer coefficients caused by different degrees of incrustation (or fouling) in pilot-scale and technical plants. Different models plug flow, macromixing, etc.) are discussed in [7.38] which consider deviations from ideal behavior in an MSMPR crystallizer. Results of scale up for this type of crystallizer are given in [7.48]. 

Furthermore, state estimation techniques such as the boot-strapping method developed by BenAmor et al. [14] allow one to account for fouling in real time. The method assumes that the overall heat transfer coefficient (multiplied by the surface) changes very little in the course of the time required to measure the reactor temperature. Thus, the reactor temperature is used to calculate first a heat generation rate over the course of 1-2 s, assuming constant heat transfer conditions, then a heat transfer coefficient assuming constant rate for a very short time. If fouling occurs over a reasonable timescale, it can be accounted for by the software sensor in-line. 

Given the uncertainties associated with the calculations, especially those on the shell-side, a sensible design basis for the heat transfer area specification would be the shell-side flow characterized by the clean condition. Of course, the fouling coefficients for the shell-side and tube-side should be included to account for the surface fouling resistance. 

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