Fouling with tube inserts

In addition to the use of antifouling chemical agents to mitigate the effect of fouling on the tube-side, twisted tubes can be used rather than plain tubes. These have surface irregularities. Plain tubes can also be fitted on the inside with tube inserts. Twisted tubes and tube inserts promote additional turbulence and pressure drop and reduce the surface temperature of the tube to mitigate fouling. Tube inserts will be dealt with in more detail later. 

The concentric separation-layer micro mixer is constructed as an assembly of stacked plates for feed supply with three tubes, performing lamination for mixing, set into one another (see Figure 1.116) [39, 53, 136-138] (see also [135]). The tubes are inserted into a frit. The three feed lines are each connected to a tube. In this way, a tri-layered concentric fluidic system is achieved. Besides mixing three solutions, a major application for the device is to separate the two fluids to be mixed by a separation layer, usually being the solvent of the two solutions. This is to delay the mixing process in order to avoid unwanted fouling problems at the mixer outlet This is particularly valuable for spontaneous precipitation reactions which are the main field of application of the mixer. 

An alternative insert for tubes is the spiral insert produced by Cal Gavin. The device consists of a matrix of wire loops on a twisted wire core. The diameter of the coils is such that it is a push fit. Laboratory studies using these wire matrix inserts with an Arabian crude oil demonstrate that the steady asymptotic value of the fouling resistance is reached in only 10 hr from start up and only of the order of 2-7% of the recommended TEMA value. The presence of the wires, particularly those in contact with the heat-transfer surface, creates turbulence that is largely responsible for the beneficial effects on the fouling. The benefits are not without the potential penalty of increased pressure drop and hence increased pumping costs. By suitable design for the same duty, this penalty can be substantially reduced. 

The Fixotal inserts have been used in single- and two-phase flow applications— that is, condensation and evaporation. Only the Fixotal inserts are used in air coolers and are intended to reduce fouling, particularly with paraffin coating or water cooler scaling. We also note that Fixotal has been used in U tubes, and can now be left in place during hydroblasting for cleaning. 

The air cooler bundle was then replaced with an ordinary finned tube bundle. The use of wire inserts in fouling service, where extra pressure drop is totally unacceptable, represents poor engineering design practice. This is especially so when the controlling resistance to heat transfer is going to be on the air side of the precondenser cooler. 

The concept is to increase turbulence in the tube-side flow, especially at the tube wall, thus increasing tube-side heat transfer rate and reducing fouling. The additional tube-side pressure drop with the Fixotal insert is about 3 psi per pass at tube-side flow velocities of 3 ft/s. 

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