Overall fouling resistance

Atkins perception of the fouling process was that a primary tarry deposit was subject to further chemical reaction (or decomposition) resulting in the hard coke layer. Vapours or gases resulting from the decomposition process left a porous deposit structure remote from the tube wall. The overall fouling resistance Rj on the tubeside under these circumstances would be given by ... 

The heat transferred is obtained from a heat balance between the two fluids flowing through the equipment. By estimating the temperature driving force between the two fluids, see Equations 2.11 and 2.12, it is possible using Equation 2.13, to estimate the overall fouling resistance. 

In other plant heat exchangers for example where the two fluids produce deposits on both sides of the heat exchanger, it may only be possible to estimate changes in overall fouling resistance. It is not possible to separate the individual components that make up the overall fouling resistance. These data are then only strictly applicable to conditions similar to those for which the data were obtained. 

Overall fouling resistance Fouling resistance at time t... 

By adding the overall fouling resistance to Uq, actual Ua is dehned as... 

In reality, heat exchangers operate under fouled conditions with dirt, scale, and particulates deposit on the inside and outside of tubes. The overall fouling resistance is defined in equation (6.3) as... 

Fouling resistances for streams are based on the physical properties of the streams and the average fouling factors are documented in TEMA (2007). For illustration purposes. Table 6.3 shows the typical overall fouling resistances for hydrocarbon liquids based on the API gravity of the streams. 

The fouling deposits build up very quiekly in the first few months of serviee until a terminal veloeity is reaehed. After this point, fouling continues but develops at a slower pace for most fouling eases. Thus, the overall fouling resistance behaves in the manner as depieted in Figure 7.1. Similarly, pressure drop increases as fouling deposits build up. The fouling deposits make the tube flow diameter smaller over time and pressure drop continues to inerease as tube flow diameter reduces. 

Work in connection with desahnation of seawater has shown that specially modified surfaces can have a profound effect on heat-transfer coefficients in evaporators. Figure 11-26 (Alexander and Hoffman, Oak Ridge National Laboratory TM-2203) compares overall coefficients for some of these surfaces when boiling fresh water in 0.051-m (2-in) tubes 2.44-m (8-ft) long at atmospheric pressure in both upflow and downflow. The area basis used was the nominal outside area. Tube 20 was a smooth 0.0016-m- (0.062-in-) wall aluminum brass tube that had accumulated about 6 years of fouhng in seawater service and exhibited a fouling resistance of about (2.6)(10 ) (m s K)/ J [0.00015 (fF -h-°F)/Btu]. Tube 23 was a clean aluminum tube with 20 spiral corrugations of 0.0032-m (lA-in) radius on a 0.254-m (10 -in)... 

Assume fouling resistances for shell side and tube side. Calculate the overall resistance, less shell-side film resistance ... 

From these correlations it is possible to calculate the film heat transfer coefficient and the pressure loss for laminar flow. This coefficient, combined with that of the metal and the calculated coefficient for the service fluid together with the fouling resistance, is then used to produce the overall coefficient. As with turbulent flow, an... 

U = overall heat transfer coefficient h0, hi = heat transfer coefficients for the outside and inside of plain tubes including the fouling resistance... 

Thermal resistance due to fouling has to be included in the calculation of the overall heat transfer coefficient. By definition, the overall heat transfer coefficient is the reciprocal of the overall thermal resistance. Thus, including the thermal resistance due to fouling, it follows... 

The overall coefficient (including a dirt or fouling resistance) can be related to the individual coefficients or resistances by the following equation ... 

Use Table 4.3 to obtain approximate values of the individual heat-transfer coefficients and fouling resistances. Then, calculate the overall heat-transfer coefficient from Equation 4.5.9 after selecting a conservative heat-transfer coefficient of 5000 W/m -K for water on both the shell and tube sides. Also, select a high... 

Substitute Equation 4.16 into Equation 4.15 and let Rf j and Rfo equal (Rf )a and (Rfo)A> the available fouling resistances. Then, the overall heat-transfer coefficient. 

Note that the fouling resistances have been subtracted from the fouled individual heat transfer coefficients to obtain the overall clean coefficient, compared with a normal value of... 

In order to enhance the overall performance of the membrane, it is necessary to modify the membrane material or the structure (41). The objectives for modification of the existing membranes are to increase flux, selectivity, and chemical resistance (solvent resistance, swelling resistance, and fouling resistance). Some of the most commonly practiced membrane modification methods are listed in Table 3. 

The scale or fouling resistances represent a necessary safety factor that increases the surface of the heat exchanger. This enables the full process duty requirements to be attained between cleaning periods. When an exchanger is first placed in operation there is no dirt or scale on the tubes consequently, the overall resistance consists of the two film and the tube wall resistances. During operation, dirt or scale accumulates on the surface of the tubes and the overall heat transfer rate decreases as the dirt buildup increases. The rate of this scale or dirt depends on the cleanliness or fouling tendencies of the process fluids. 

In these equations, and h are the inside and outside lihn heat-transfer coefficients. A, and A are the inside and outside surface areas of the clean tube, and Rf are the inside and outside surface fouling resistances, is the interface temperature between the fluid and the surface of the fouling deposit inside the tube, tf is the interface temperature between the fluid and fouling surface on the outside, and are the inside and outside tube wall temperatures, and is the thermal conductivity of the tube material. Each of the terms in the denominator is a resistance to the heat flow, and their sum is the total resistance to heat transfer between the two fluids. These equations can be combined to eliminate the intermediate temperatures and find the heat-transfer rate using only the overall temperature difference (T- t) ... 

These resistances are illustrated in Figure 9.51. The subscript i in Equation (9.83) refers to the coefficient at the inside wall of the mixing vessel the subscript j refers to the jacket side. The other terms are the wall resistance and the fouling resistances for either side. A similar equation can be written for an internal coil or other device. In situations where both a jacket and an internal device are used, the overall coefficients for each type of surface should be calculated separately, and the two g s should be added to obtain the overall heat-transfer capability. 

Also calculate the wall thermal resistance R , = 8IA kw. Finally, compute overall thermal conductance UA from Eq. 17.6, knowing the individual convective film resistances, wall thermal resistances, and fouling resistances, if any. 

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