Heat exchangers resistance with fouling

It generally is considered that resistance due to fouling is lower with plate heat exchangers than with tubular units. This is the result of four advantages of plate-type exchangers ... 

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... 

The first term depends entirely on the physical properties of the reactor contents and degree of agitation. It represents resistance to heat transfer of the internal film and of eventual deposits at the wall, which may determine the overall heat transfer [3], Therefore, the reactor should be regularly cleaned with a high pressure cleaner. Both last terms depend on the reactor itself and on the heat exchange system, that is, reactor wall, fouling in the jacket, and external liquid film. They are often grouped under one term the equipment heat transfer coefficient (cp) [4, 5],... 

The performance of a heat exchanger depends upon the transfer surfaces being clean and uncorroded. The performance deteriorates with time due to accumulation of deposits on the heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer. In the design of heat exchangers, this added resistance is accounted for by a fouling factor or dirt factor, R. ... 

After a period of operation the heat-transfer surfaces for a heat exchanger may become coated with various deposits present in the flow systems, or the surfaces may become corroded as a result of the interaction between the fluids and the material used for construction of the heat exchanger. In either event, this coating represents an additional resistance to the heat flow, and thus results in decreased performance. The overall effect is usually represented by a fouling... 

The performance of heat exchangers usually deteriorates with time as a result of accumulation of deposits on heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer and causes the rate of heat transfer in a heat exchanger to decrease. The net effect of these accumulations on heat transfer is represented by a fouling factor Rj, which i.s a measure of the thermal resistance introduced by fouling. 

Anaiysis (a) The schematic of the heat exchanger is given in Fig, 11-11. The thermal resistance, for an unfinned shell-and-tube heat exchanger with fouling on both heat Iransfer surfaces is given by Eq. 11-8 as... 

Consider a double-pipe heat exchanger with a lube diameter of 10 cm and negligible tube thickness. The total thermal resistance of the heat exchanger was calculated to be 0.025°CAV when it was first constructed. After some prolonged use, fouling occurs at both the inner and outer surfaces with (he fouling factors 0.000 15 CAV and 0.00015 °C/W, re-... 

It has to be said however, that without other information, these published data are of value in making an assessment of the potential fouling resistance. At the same time data on fouling resistances have to be treated with caution, they can only be regarded as a guide. A further limitation is that these values only apply to shell and tube heat exchangers. Conditions in plate heat exchangers for instance, could be quite different. 

Comparison of plate heat exchanger area requirements with two different cooling water fouling resistances... 

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. 

To determine the heat transfer coefficient ki in Eq. (7-14), see, for example [0.1, 12A-126] covering the field of heat transfer problems in evaporation processes. During solution evaporation and crystallization, fouling and incrustation of the heat exchange areas has to be considered, which may lead to substantially lower evaporation rates in the evaporator. With incrustation on the product side of the evaporator tube wall, the heat transfer resistance is... 

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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