Deposit thickness change with time

As the fouling process is dynamic, the thickness of the deposit will change with time, as indicated in the earlier discussion. As a result, the temperature distribution will change, and if heat passes from the heat transfer surface to the bulk fluid, the surface/deposit... 

The potential effect of deposit age on the qualities of the deposit, where the character of the deposit may change with time or thickness. For instance the changing temperature on the outside of deposits in a high temperature system (combustion) is more than likely to result in differences of porosity for example, within the deposit. 

Fig. 15.16 (a) Change with time of the copper deposit thicknesses at bottom (Tbottom) and top (Ttop) of trenches, and (b) change with time of deposition rate at the bottom of trenches... 

Another real-time study of the reaction of M-FA films with H2S utilized ellip-sometry to monitor changes in film thickness concurrent with metal sulfide formation (53). The reactions appeared to reach equilibrium within the same period of time (within 2 h), with a change per monolayer of 0.2 nm for CdBe and 0.9 nm for both CuBe and ZnBe. Their ellipsometry results, in agreement with Peng et al. (66), also show a dependence of the reaction rate on the H2S pressure and the surface pressure at which the films were deposited. 

The °Th and Pa methods are not used directly for absolute dating of individual sedimentary horizons, because the assumption that No is constant over time does not hold exactly but can be upset by fluctuations due to changes in sediment-deposition rate and other factors. Instead, the common practice is to use the decreasing activity with depth to derive an average rate of sediment accumulation over the length of a core or some other long interval. If t is the age (time since deposition) of a sediment horizon at depth z, and if S is the average sediment accumulation rate (thickness per unit time), then from Equation (1)... 

FIGURE 1.2. The change in deposit thickness with time... 

It is probable that the temperature of the deposit near the original surface will approach the pour point temperature. Under these conditions a steady state will exist and there will be little change in deposit thickness with time. Fig. 8.18 illustrates the steady state condition. 

As an alternative to direct heat transfer measurements it is possible to use changes in pressure drop brought about by the presence of the deposit. The pressure drop is increased for a given flow rate by virtue of the reduced flow area in the fouled condition and the rough character of the deposit. The shape of the curve relating pressure drop with time will in general, follow an asymptotic shape so that the time to reach the asymptotic fouling resistance may be determined. The method is often combined with the direct measurement of thickness of the deposit layer. 

The reduction in j with time therefore represents an apparent reduction in a (by about 20% from the initial value), but in reality the relatively low value of a associated with gas streams will not be affected greatly by the presence of the deposit unless the deposit becomes thick enough to change substantially the flow pattern across the fins. The presence of the fouling layer however interposes a thermal resistance between the air flow and the metal fins and core tube of the model heat exchanger, thereby reducing the effective heat transfer and giving the impression that the heat transfer coefficient has diminished. 

The first method is an optical method based on the use of a laser sheet at grazing incidence. With this method, it is possible to follow the deposit thickness and kinetics in situ and real-time. The second method is an acoustic method. The analysis of the acoustic wave of the deposit with time enables its thickness to be followed. Moreover, variations of the amplitude of the acoustic wave are linked to possible changes in the structural properties of the deposit. Nonetheless, only qualitative variations are available. The use of both methods on the same apparatus will enable a complete characterization to be obtained (Figure 11.1). 

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