Fouling deposits

Fouling—deposition of scale materials on metal surfaces. 

Chemical reaction fouling deposits formed by chemical reaction in the fluid system. 

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. 

Fouling deposition of flora and fauna on metals exposed to natural waters, e.g. sea-water. 

Condenser leaks. Fouling, deposition Chloride depassivation and pitting corrosion of FW lines and boiler... 

As discussed previously, serious fouling, deposition, and corrosion problems may occur in the FW line as a result of the entry of carryover, after-precipitation, corrosion debris pickup, or oxygen and other contaminants, from either the MU or returning condensate. 

Almost all boiler section scale, fouling, deposit, or corrosion problems, however, tend to be associated with additional (and often very similar) problems both upstream and downstream of the boiler itself. Therefore, the presence of, for example, boiler deposits or localized corrosion issues should never be viewed in isolation. Rather, they should be investigated in the light of a likely chain of cause and effect, and the basic causes of the problems should be eliminated wherever possible, rather than simply the noticeable effects. 

We typically use scaling, fouling, deposition, and several other terms in the same context and fairly indiscriminately because there is no universal definition. In general, these physicochemical processes may take... 

Practical methods are now available for diagnostics and monitoring but more research, innovation, and commercial development presents a very great potential to improve the performance of industrial programs. Several new innovations have been recently introduced such as an effective optical fouling-deposit monitor12. 

If fouling (deposition of a solid from the liquid) occurs on a boiling surface, the effect on heat transfer will depend on whether the deposit is a catalyst and whether it is very bulky. If the deposit is thin, the heat transfer may increase. This is an unnatural occurrence to observers not familiar with boiling—a little fouling is sometimes a good thing ... 

Let s assume we have a new heat exchanger. The fluid on the tube side is crude oil. Crude oil, like many process fluids, will eventually foul and plug heat-exchanger tubes. The tendency to lay down fouling deposits is accelerated by... 

Meanwhile, the flow of crude oil through those tubes that started out running faster increases. The crude-oil flow is backed out of the slower tubes by the fouling deposits. The extra flow increases the velocity in the tubes that initially ran faster. The higher velocities retard the buildup of fouling deposits. Also, the greater flow keeps these tubes cooler. This also discourages the accumulation of deposits inside the tubes. 

Why Well, because the doubling of the tube-side velocity has promoted turbulence, which retards the accumulation of fouling deposits. Of course, this is exactly the reason why we exercise. The increased flow of blood through our arteries prevents plaque from sticking to the walls of our blood vessels. The plaque, a fatty deposit derived from cholesterol, restricts the flow of blood, which causes high blood pressure and eventually strokes. 

Actually, retrofitting a tube bundle with low fin tubes often reduces heat-transfer capacity. This happens when the controlling resistance to heat transfer is shell-side fouling. The fouling deposits get trapped between the tiny fins. This acts as an insulator between the shell-side fluid and the surface of the tubes. In severe shell-side fouling services, I have replaced fin tubes with bare tubes, and doubled the heat-transfer duty on the exchanger. 

If this problem—the sudden loss of flow, followed by the premature restoration of flow—occurs repeatedly over a period of a few hours, then layers of fouling deposits or coke are accumulated inside the tubes until a heater shutdown becomes unavoidable. This sort of failure is called a stuttering-feed interruption. 

Eventually, the fouling deposits on the rotor will become so thick that they start to break off, especially if you shut the compressor down for a few hours for minor repairs to the lube-oil system. When the compressor is put back on line, bits and pieces of grayish salt break off, and unbalance the rotor. At 8000 rpm, the high-vibration trip cuts off the fuel to the gas turbine, and the machine is taken off line for repair. 

The adhesive strength of a food fouling deposit may be related to the surface free energy of the substrate. Zhao et al. (2004) have developed a theory that gives the minimum adhesion energy between a deposit and a surface ... 

Consider the typical case of heat transfer between one fluid inside a tube and another fluid outside the tube, shown in cross section in Fig. 11. Heat is transferred by convection from the hot fluid (taken arbitrarily to be the fluid inside the tube) to the fouling deposit (if any) on the inside surface, through the fouling deposits and tube wall by conduction, and then by convection to the fluid outside the tube. At the point where the inside fluid temperature is T and the outside is t. the local heat flux inside the tube is... 

FIGURE 11 Cross section of a heat exchanger tube, with convective heat transfer in the fluids and fouling deposits on the surfaces. 

Void [52] developed a variety of ballistic deposition models to simulate sedimentation processes. Void used ballistic models to determine deposition densities for spherical particles which traveled via vertical paths and were deposited on horizontal surfaces. Recently, Schmitz et al. [53] used a ballistic aggregation model to describe particle aggregation at the surface of a crossflow microfiltration membrane. Schmitz and co-workers were able to account for interfacial forces empirically, and demonstrated the influence of physical and chemical variables on the resulting morphology of the fouling deposits (such as aggregate density variation with depth, and influence of shear flow and re-entrainment properties on fouling deposit density and porosity). 

The samples examined were predominantly lignites from the Pust seam in Montana. However, data for two North Dakota lignites, for slagging and fouling deposits produced by those lignites, and for several subbituminous coals are also included. 

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