Fouling Resistance Calculations

For the exchangers with too low velocity, it was recommended to increase tube passes. The message from this example is that the fouling mechanisms could be different even for the same process unit, and thus different mitigation methods should be employed. 

The overall heat transfer coefficient Uq for a clean heat exchanger can be described as 

When a heat exchanger becomes fouled, fouling resistances inside and outside of tubes prohibit the heat exchanger from performing at the level as clean heat transfer coefficient. In practice, these two fouling resistances are lumped together into a total fouling resistance, R. The dirty heat transfer coefficient (C/d) can be expressed as 

The fouling factor has to be determined from actual heat exchanger performance based on online measurements taken from a process unit test run. Heat exchanger clean performance is obtained from process flowsheet simulation software (e.g., Hysys by Aspen Tech or Unisim by Honeywell), while dirty performance from exchanger rating software (e.g., HTRI by Heat Transfer Research Institute). 

heat exchanger heat balance calculations are conducted in a flowsheet simulation software, which has adequate thermal data and can describe process streams according to their physical properties and operating conditions. By providing measured temperatures, the simulation can determine the heat transfer duty from Q = m Cp AT. At the same time, the simulation calculates transfer capability by lumping overall heat transfer coefficient and surface area together as U - A = 2/ATlm where ATlm is defined in equation (6.7) in Chapter 6. 

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The Nomograph Part 3 (Figure I0-43C) may be used in a number of ways. For example, what will the fouling resistance, R(, be after an arbitrarily chosen time, t, or it can calculate the thickness of a fouling deposit after an arbitrarily chosen time t, providing the thermal conductivity of the deposited material is known. It can calculate thermal conductivity of a deposit, providing thickness is known, or estimated. 

The simplest calculation is to find the unknown fouling resistance after a time t. For this purpose, the nomograph is used as follows ... 

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

The values of /"fh and ffc (the film resistances for the hot and cold fluids, respectively) can be calculated from the Dittus-Boelter equations previously described and the wall metal resistance / from the average metal thickness and thermal conductivity. The fouling resistances of the hot and cold fluids /"dh and are often based on experience, but a more detailed discussion of this will be presented later in this chapter. 

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... 

Equation (20-80) requires a mass transfer coefficient k to calculate Cu, and a relation between protein concentration and osmotic pressure. Pure water flux obtained from a plot of flux versus pressure is used to calculate membrane resistance (t ically small). The LMH/psi slope is referred to as the NWP (normal water permeability). The membrane plus fouling resistances are determined after removing the reversible polarization layer through a buffer flush. To illustrate the components of the osmotic flux model. Fig. 20-63 shows flux versus TMP curves corresponding to just the membrane in buffer (Rfouimg = 0, = 0),... 

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. 

The calculated tube skin temperature is mainly a function of the fouling resistance assumed inside the tube. The greater the assumed fouling resistance, the higher the design tube skin temperature, and the thicker the tube wall. In a sense, then, we partially assume the design tube thickness, on the basis of experience, for a particular plant service. 

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... 

Calculate the available fouling resistance, Roa, from Equation 4.7.10 and the require fouling resistance, Ror, from Equation 4.7.9. 

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. 

In general the design of heat exchangers involves the determination of the required area A. The necessary heat transfer, the temperatures and the fluids are generally known from the process specification, the individual heat transfer coefficients of the fluids may be calculated, and values of the fouling resistances on either side of the heat exchanger would have to be estimated. It is the latter that can be difficult and if the resistances are incorrectly estimated difficulties in subsequent operation may be manifest. 

At first sight it may be thought possible to calculate the fouling resistance, i.e. 

The choice of the individual fouling resistances for the calculation of C/p can have a marked influence on the size of the heat exchanger and hence the capital cost. 

For very large values of t and constant operating conditions of water quality, flow velocity and surface temperature. Equation 8.21 can be used to calculate the asymptotic fouling resistance, i.e. 

The clean membrane resistance Rm is calculated for pure water flux, while the fouling resistance Rf is the total resistance minus the membrane resistance (see equations (8.1) and (8.2)). AP is the... 

Independent of the required /based on thermodynamics stated as in equation (6.1), U value can be calculated based on transport considerations without taking into account the fouling resistances. In other words, transport-based / is a function of film coefficients hi for tube side and h for shell side in Btu/h ft F) as expressed in equation (6.2) ... 

This U value is called as clean U value because fouling resistances (/ j, / o) are not taken into account in equation (6.2). The film coefficients, hi and h, can be calculated based on the fluids physical properties and the geometry of the heat exchanger. For example, for U-tube exchangers with streams all liquid or aU vapor (no boiling and condensing), the correlation (Dittus and Boelter, 1930) is used to estimate the tube side Nusselt... 

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